OIE 2019
The 13th Japan-Finland Joint Symposium on Optics in Engineering
26-30 August 2019, in Espoo and Tallinn

Proceedings of OIE 2019
Edited by: Petri Kärhä, Erkki Ikonen, Yoshio Hayasaki, and Kai
Peiponen

Abstracts of the 13th Japan-Finland Joint Symposium on Optics
in Engineering

Table of Contents
Prefacevi

Committee

vii

International Organizing Committee

vii

Local Organizing Committee

viii

Chairmen of the sessionsviii

Author Index (end of document)ix

Session 1: Holography & Nanostructures
Ville A. Kaikkonen, Eero O. Molkoselkä and Anssi J. Mäkynen, “ICEMET – asensor for in-cloud icing condition monitoring”… 1

Yasuo Tomita, Toshi Aoi, Shuma Hasegawa, Kento Kaseda, Juro Oshima and
Keisuke Odoi, “Hyperbranched polymer nanocomposite gratings for volume
holographic optical elements”… 3

Filipp Temerov and Jarkko J. Saarinen, “Enhanced photocatalytic activity of
TiO2 inverse opal structures with gold-silver nanoparticles”… 5

Masayuki Yokota, Tatsuya Ishikawa and Yoshinobu Aoki, “Polarization state of
the object wave analyzed by using FMCW-digital holography”… 7

Session 2: Imaging science & Optical computing
Yoshio Hayasaki, Ryo Sato and Yujiro Ito, “Single-pixel imaging with hole-array
coding masks”… 9Nathan Hagen, “High throughput spectral imaging with snapshot sensing”… 11

Jun Tanida, Suguru Shimomura, Takahiro Nishimura, Yuki Miyata, Naoya Tateand Yusuke Ogura, “A quantum dot reservoir based on Förster resonanceenergy transfer for optical reservoir computing”… 13

Shuji Taue, “Visualizing AC magnetic fields by using atomic magnetometer andmicro-mirror device”… 15

Yukitoshi Otani, Shuhei Shibata, Masayuki Suzuki and Nathan Hagen, “Videorate imaging for Stokes parameters by polarization cameras”… 17

Session 3: Posters
Mikko J. Huttunen, Radu Hristu, Adrian Dumitru, Mariana Costache and Stefan
C. Stanciu, “Deep learning enhanced multiphoton microscopy for investigating
the dermoepidermal junction in human skin”… 18

Katri Kukkola, Anssi Mäkynen, Ville Kaikkonen, Gleb Bulygin, Eero Molkoselkä,
Niklas Pikkarainen, Nanni Nielikäinen, Aleksi Rantanen, Antti Kämppi, Tarja
Tanner and Vuokko Anttonen, “Full arch dental imaging using an extraoral
camera system”

… 20

ii

Elnaz Fazeli, Takahiro Deguchi, Sami Koho, Juha Peltonen, Tuomas Näreoja
and Pekka Hänninen, “Live cell imaging with integrated STED-AFM”… 22

Kaustav Das, Yuta Kobori, Tomonori Yuasa, Hideki Funamizu and Yoshihisa
Aizu, “Monte Carlo based investigation of spectral reflectance and point spreadfunction in skin tissue model”… 24

Benjamin O. Asamoah, James Amoani, Matthieu Roussey and Kai-E.
Peiponen, “On optical sensing of microplastics in water”… 26

A. Pokatilov, G. Porrovecchio and T. Kübarsepp, “Development of self-adjusting
photodiode arrangement for accurate laser optical power measurements”… 28

Ilpo Niskanen, Hiroshi Hasegawa, Jukka Räty, Hariyadi Soetedjo, Kenichi
Hibino, Hiroshi Oohashi, Rauno Heikkilä, Kiyofumi Matsuda and Yukitoshi
Otani, “Polarimeter analysis of the birefringence of thermally modified Scots
pine wood”… 29

Kenji Harada, Sho Sakurai and Yutaro Shibata, “How to control polarization
color and its applications”… 31

Prince Bawuah, Mohammed Al-Sharabi, Cathy Ridgway, Anssi-Pekka
Karttunen, Ossi Korhonen, Patrick Gane, Jarkko Ketolainen, Kai-Erik Peiponen,
J. Axel Zeitler and Daniel Markl, “Predicting the refractive index of porous
powder samples during compaction using terahertz time-domain spectroscopy”… 33

Jeremias Seppä and Tuomas Helojärvi, “Ambient air spectroscopy andthermometry for accurate distance measurement”… 35

Natsuki Suzuki and Katsuhiro Ishii, “Label-free and vibration-free imaging using
low-coherent quantitative phase microscope”… 37

Oskari Pekkala, Tomi Pulli, Alexander Kokka and Erkki Ikonen, “Setup forcharacterising the spectral responsivity of Fabry-Perot-interferometer-based
hyperspectral cameras”… 39

Harri J. Juttula, Matti Törmänen and Anssi J. Mäkynen, “Comparison ofexperimental and the Mie scattering coefficients of bulk suspensions of idealand deformed cylindrical particles”… 41

Virpi Korpelainen, Johan Nysten, Jeremias Seppä and Antti Lassila,“Development of metrological atomic force microscope”… 43

Eero O. Molkoselkä, Gleb A. Bulygin, Justus F. Kleine and Anssi J. Mäkynen,“3D printed lens project for bachelor’s education”… 44

Yasuhiro Matsuda, Hiroshi Kigami, Noriyuki Unno, Jun Taniguchi and Shin-ichi… 46
Satake, “Three-dimensional flow measurements around micro-pillars in water by
micro-digital holographic particle tracking velocimetry”
Janne Lauri, Antti Koponen, Sanna Haavisto and Tapio Fabritius, “Evaluation ofyield stress from velocity fluctuations of microfibrillated cellulose suspension
flow by optical coherence tomography”… 48

Petri Kärhä, Hans Baumgartner, Kasperi Kylmänen, Benjamin Oksanen andErkki Ikonen, “Measurement setup for differential spectral responsivity of solar
cells”… 50

iii

Päivi Heimala, Mikko Harjanne, Ari Hokkanen, Matteo Cherchi, Tapani Vehmas,
Srivathsa Bhat and Timo Aalto, “VTT’s versatile 3 µm silicon photonics platform”… 52

M. Baah, T. Kaplas and Y. Svirko, “Effect of nanometrically thin nickel catalyst
on the properties of CVD synthesized graphitic films”… 54

Sampson S. Andoh, Kenneth Nyave, Boniphace Kanyathare, BenjaminAsamoah, Tarmo Nuutinen, Cheetham Mingle, Kai-E. Peiponen and Matthieu
Roussey, “Qualitative analysis for presence of Sudan IV in edible palm oil”

… 56

Andris Slavinskis, Antti Nasila, Mihkel Pajusalu, Jaan Praks, Anu Reinart,
Iaroslav Iakubivskyi, Tomas Kohout, Colin Snodgrass and Geraint Jones,“OPtical Imager for Comets (OPIC) for ESA’s F-class mission Comet
Interceptor”… 58

Markku Pekkarinen, Petri Karvinen and Jyrki Saarinen, “Direct 3D-printing of
optical components at the University of Eastern Finland”… 60

Session 4: Biomedical optics & nonlinear optics
Yusuke Ogura, Daiki Shinkawa, Takahiro Nishimura, Yosuke Tamada and Jun
Tanida, “Super-resolution imaging by scanning subdiffraction-limit optical
pattern”… 61

Matti Huotari, Kari Määttä and Juha Röning, “The beneficial effects of the sauna
exposures measured by PPG”

… 63

Léo Turquet, Xiaorun Zang, Joona-Pekko Kakko, Harri Lipsanen, Godofredo
Bautista and Martti Kauranen, “Creation and observation of a longitudinallypolarized optical needle”… 64

Jussi Hiltunen, Sanna Uusitalo, Sanna Aikio, Marianne Hiltunen, Olli-Heikki
Huttunen, Johanna Hiitola-Keinänen, Christina Liedert and Pentti Karioja,“Opto-fluidic biosensors by roll-to-roll fabrication”… 66

Yelena Kan, Sapun Parekh, Erik M. Vartiainen and Lasse Lensu, “Quantitative
CARS spectral imaging in studying protein droplets”… 68

Session 5: Interferometry 1
Kenichi Hibino, Toshiki Kumagai and Yasunari Nagaike, “Synthetic aperturephase-shifting measurement for high numerical-aperture spherical surfaces”… 70

Ville Heikkinen, Johan Nysten, Ville Byman, Björn Hemming and Antti Lassila,“3D measurements of freeform samples using multi sensor optical profilometer”… 72

Takamasa Suzuki, Bin Liu, Samuel Choi and Osami Sasaki, “Thickness
distribution measurement with a full-field pulsed SD-OCT”… 73

Katsuhiro Ishii, Masaharu Hoshikawa, Takeshi Makino, Takahiro Hashimoto,
Hideaki Furukawa and Naoya Wada, “Low-coherence interferometer with
10MHz repetition rate”… 75

Session 6: Interferometry 2
Jun Uozumi, “Plenary talk: Analyzing and applying light with randomness; fromspeckles to fractals and to spectra”

iv

… 77

Yukihiro Ishii, Shunpei Yukita, Kosuke Kiyohara, Jun Chen and Eiji Tokunaga,“Wavelength-tunable common-path interferometer with a diode source”… 79

Ryohei Hanayama and Katsuhiro Ishii, “Speckle shearing interferometricvibrometer for heartbeat monitoring”… 81

M. Roussey, R. Rao and S. Pélisset, “Multimode interferometers on strip-loaded
waveguide for sensing”… 83

Juha Toivonen, “Finnish flagship program on photonics research andinnovation”… 85

Session 7: Detectors, lasers, and integrated & printed optics
Takashi Okamoto and Keigo Imamura, “Designing random structures for
random laser media”… 87

Jussi Rossi, Juho Uotila, Toni Laurila, Erkki Ikonen and Markku Vainio,“Broadband electromagnetic radiation detector based on photoacoustic effect”… 89

Meelis-Mait Sildoja, Charles W. Stark, Jüri Pahapill, Matt Rammo, Katrin
Petritsenko and Aleksander Rebane, “Measurement facility for high accuracy
multiphoton absorption spectra”… 91

Marja Vilkman, Kaisa-Leena Väisänen, Marja Välimäki, Mari Ylikunnari andThomas Kraft, “Printed electronics in pilot scale processing and production oforganic photovoltaics, CIGS solar cells and OLEDs”… 93

Timo Dönsberg, Santeri Porrasmaa, Farshid Manoocheri and Erkki Ikonen,“Predictable quantum efficient detector (PQED) based on n-type silicon induced
junction photodiodes”… 95

Session 8: Optical spectroscopy, spectral imaging and lidars
Kim Patokoski, Joona Rissanen, Teppo Noronen, Regina Gumenyuk, Yuri
… 97
Chamrovskii and Valery Filippov and Juha Toivonen, “Tapered fiber amplifier forcoherent lidar”
Tommi Mikkonen, Caroline Amiot, Antti Aalto, Kim Patokoski, Goëry Genty andJuha Toivonen, “Fourier transform photoacoustic spectroscopy withsupercontinuum light source”… 99

Heikki Saari, Harri Ojanen and Ingmar Stuns, “Novel hyperspectral imager
based on angle-tuned multi pass band filter, Leds and RGB image sensor”… 101

Antti Näsilä, Heikki Saari, Kai Viherkanto, Christer Holmlund, Rami Mannila,
Roberts Trops, Endija Briede, Akseli Miranto, Altti Akujärvi, Harri Ojanen, Ismo
Näkki, Ingmar Stuns, Hans Toivanen, Tahvo Havia, Martti Blomberg, Sari
Elomaa and Tapani Antila, “Miniaturized spectral imaging technologies at VTT”

… 103

v

Preface
The 13th Japan-Finland Joint Symposium on Optics in Engineering (OIE) will take place on
26-30 August 2019 in two locations: The Otaniemi campus of Aalto University in Finland andthe historical city of Tallinn in Estonia. The first two days of the symposium will be held on thecampus of Aalto University in Finland. The campus is designed by the renowned Finnish
architect Alvar Aalto. The latter half of the symposium will be held in Tallinn, Estonia. The oldtown of Tallinn is listed as UNESCO World Heritage Site.
We will have a tightly packed scientific program which is designed to allow many possibilities
for detailed discussions between the participants. This international symposium gathers
together the best scientists working in the field of optics in Japan, Finland and Estonia.
On behalf of the OIE Committee I welcome all of you to the Symposium!

Erkki Ikonen
Aalto University
General Chair of the OIE 2019

OIE 2019 sponsored by Aalto University, City of Espoo, and Optical Society of Japanvi

Committee
General Chair
Erkki Ikonen (Aalto University)
Co-Chairs
Yoshio Hayasaki (Utsunomiya University)
Pekka Hänninen (University of Turku)

International Organizing Committee (Program Committee)
Chair
Kai-Erik Peiponen (University of Eastern Finland)Members
Takamasa Suzuki (Niigata University)
Katsuhiro Ishii (The Graduate School for the Creation of New Photonics Industries)
Yukitoshi Otani (Utsunomiya University)
Yusuke Ogura (Osaka University)
Shuji Taue (Kochi University of Technology)
Yoshihisa Aizu (Muroran Institute of Technology)
Kenji Harada (Kitami Institute of Technology)
Yasuo Tomita (The University of Electro-Communications)
Jun Uozumi (Hokkai-Gakuen University)
Ken-ich Hibino (National Institute of Advanced Industrial Science and Technology)
Toshiaki Iwai (Tokyo University of Agriculture and Technology)
Nathan Hagen (Utsunomiya University)
Masayuki Yokota (Shimane University)
Tapio Fabritius (University of Oulu)
Jukka Hast (VTT Technical Research Centre of Finland)
Seppo Honkanen (Microsoft)
Jarkko Saarinen (University of Eastern Finland)
Zhipei Sun (Aalto University)
Juha Toivonen (Tampere University of Technology)
Erik Vartiainen (Lappeenranta University of Technology)vii

Local Organizing Committee
Petri Kärhä (Aalto University)
Toni Laurila (Aalto University)
Tomi Pulli (Aalto University)
Aigar Vaigu (VTT Technical Research Centre of Finland)

Chairmen of the sessions
Session 1: Holography & Nanostructures
Prof. Yoshio Hayasaki (Utsunomiya University)
Session 2: Imaging science & Optical computing
Prof. Jukka Hast (VTT Technical Research Centre of Finland)
Session 3: Posters
Doc. Petri Kärhä (Aalto University)
Session 4: Biomedical optics & nonlinear optics
Prof. Jun Tanida (Osaka University)
Session 5: Interferometry 1
Prof. Yukihiro Ishii (Tokyo University of Science)
Session 6: Interferometry 2
Prof. Yasuo Tomita (University of Electro-Communications)
Session 7: Detectors, lasers, and integrated & printed optics
Prof. Tapio Fabritius (University of Oulu)
Session 8: Optical spectroscopy, spectral imaging and lidars
Prof. Erik Vartiainen (Lappeenranta University of Technology)viii

ICEMET – a sensor for in-cloud icing condition monitoring
Ville A. Kaikkonen1, Eero O. Molkoselkä2 and Anssi J. Mäkynen212

Unit of Measurement Technology, University of Oulu, Kajaani
Optoelectronics and Measurement Techniques, University of Oulu, Oulu
Corresponding author: ville.kaikkonen@oulu.fi

Keywords: digital holography, clouds, imaging systems, optical instruments
Monitoring of icing conditions in cold regions has gained more interest in recent years, mainly due to the rapid
increase in wind power [1]. To model atmospheric icing, one would need to know liquid water content (LWC),
droplet concentration and the droplet size distributions (DSD), but now there is no standard way of measuring
these parameters for the low-level clouds [2]. In this paper, we present a novel instrument for measuring droplet
size distributions (DSD) and liquid water content (LWC) of icing clouds [3]. The measurement principle ofthe instrument is lensless point source digital holographic imaging. In this paper, we present the ICEMETsensor and show results from field measurements.
The ICEMET-sensor is based on capturing the digital in-line holograms of cloud droplets and ice crystals. The
in-line configuration is the simplest of holographic imaging geometries to implement. It produces diffracted
shadow images from the objects located between
the coherent light source and the detector screen.
A holographic in-line imaging system can be implemented as a lensless imaging system, making
the design simple and very robust, the latter an important factor when designing optical instruments
for harsh environmental conditions. By using avisible wavelength laser diode as a coherent anddivergent point source of light, the geometricallymagnified holograms of the droplets can be recorded using a conventional and cost effective digital camera detector. The magnification of thistype of hologram recording geometry at different
distances from the laser can be simply calculatedusing the parameters in equation 1. The basic imaging geometry including the protective windows
and droplets inside the imaging volume is shownin Fig. 1.
Fig. 1. A lensless in-line holographic imaging geometry.

𝑀=

𝑍𝑃𝑆 𝑡𝑜 𝐷𝐸𝑇⁄𝑍
𝑃𝑆 𝑡𝑜 𝑂𝐵𝐽

(1)

The measurement volume, where the droplets are measured by the sensor, is located in the upper part of thesensor, see Fig. 2. The ICEMET-sensor was designed as a freely rotating construction. By the wing on thebackside, it aligns itself against the wind so that the cloud droplets would always pass the sensor in optimally– orthogonally in the direction of the laser light. The sensor is mounted on a support arm from the bottom, theheight of the sensor is 53 cm without the support arm. The weight of the sensor is 8 kg. The sensor is kept freefrom ice with heating elements with the total power of 500 W. Heating of the sensor is controlled by an adjustable digital thermostat controller and additional overheating protection thermostats are installed. Two symmetrical heads with knife-edge rims house the laser and the image sensor. The measurement volume is defined
between the two protective windows in the middle of the two housing. The volume imaged in a single hologramis 0.5 cm3. The maximum frame rate of the sensor is 6 fps, resulting in a maximum sampling of approx. 3
cm3/s. The diameters of cloud droplets that can be measured with the sensor are 5 to 200 microns.

1

The sizes of the droplets are measured by first numerically reconstructing image slices at different positions between the laser and the image
detector, then segmenting droplets and finding the best focus of each
droplet in the segmented sub-windows. Then the reconstructed shadow
images of the droplets are binarized and the diameters and other shapeparameters are calculated from the binary images. After processing astatistically valid amount of particles, typically a period of one minute,the LWC is calculated. From the acquired DSD, the median volume
diameter (MVD) typically used in icing modelling is calculated. MVD
is a non-real droplet diameter, which defines the middle point of thewater content of the whole measured DSD. The result from an icing
event measurement on a wind turbine nacelle in Kivivaara-Peuravaara
Wind Park, located in Suomussalmi, Finland, is shown in Fig. 3. Theinterval between the measurement points is 3 minutes. The rising of
the LWC and MVD can be clearly seen from the plot as the icing cloud
passes by the wind turbine.

Measurement volume

The ICEMET-sensor is an instrument designed for evaluating present
icing conditions in real time. In future, we aim to deploy more sensors
in the field to gather enough data to help making weather forecasts,
especially icing forecasts, more accurate. In industrial applications, theaim is to use the data provided by the sensor to improve the productionof wind turbines in icing conditions. Monitoring present icing conditions near structures prone to icing, such as power line networks, would
also benefit from the early warnings of hazardous icing conditions.
Fig. 2. An unmounted ICEMET-sensor

Fig. 3. LWC and MVD measured from Kivivaara-Peuravaara wind park on 20th and 21th of March 2017
References
1. Parent, Olivier, and Adrian Ilinca. "Anti-icing and de-icing techniques for wind turbines: Critical review." Cold regions science and technology 65.1 (2011): 88-96.
2. Krenn, A., et al. "Available technologies for wind energy in cold climates." International Energy Agency Wind Task
19 (2016).
3. Kaikkonen, V., Mäkynen, A., Arstila T., Kananen, T. 2018, Freezing of structures caused by cloud droplets,
WO2018220268, viewed 29 April 2019, retrieved from Espacenet

2Hyperbranched polymer nanocomposite gratingsfor volume holographic optical elements
Yasuo Tomita,1 Toshi Aoi,1 Shuma Hasegawa,1 Kento Kaseda,2 Juro Oshima,2 and Keisuke Odoi2
1

Department of Engineering Science, University of Electro-Communications
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
2
Materials Research Laboratories, Nissan Chemical Corporation, 722-1, Tsuboi, Funabashi. Chiba 274-8507, Japan
Corresponding author: ytomita@uec.ac.jp

Keywords: nanocomposite materials, photopolymer, volume gratings, holography
Holographic photopolymers have been investigated extensively for their use in photonics and information
display such as holographic data storage, holographic optical elements, narrowband optical filters, GRIN
lens, sensors, electrically switchable Bragg gratings, and three-dimensional and headup/headmount displays.
These applications generally require a large saturated refractive index modulation amplitude (Dnsat) toachieve high diffraction efficiencies (h) near 100%, high recording sensitivity and high dimensional stability(i.e., mitigated polymerization shrinkage and low thermal changes in film thickness and refractive index).
Other conditions such as wide acceptable angles (i.e., wide Bragg apertures) are also demanded for somedisplay and diffractive device applications. Since 2002 we have developed a new type of nanocomposite material system, the so-called photopolymerizable nanoparticle-polymer composite (NPC), in which either inorganic or organic nanoparticles having a large refractive-index difference between nanoparticles and theformed polymer are dispersed in photopolymer [1]. In this summary we report on the development of a new
photopolymerizable NPC material that consists of ultrahigh refractive index hyperbranched polymer (HBP)
acting as organic nanoparticles, together with an electron-donor/acceptor photo-initiator system, dispersed ina monomer blend including an alkyl substituted acrylate. We also investigate the volume holographic recording properties of photopolymerizable HBP-dispersed NPC films at a recording wavelength of 532 nm.
Our newly synthesized HBP containing triazine and aromatic ring units was prepared by the polycondensation of a diamine monomer with 2, 4, 6-trichloro-1, 3, 5-triazine in N, N-dimethylacetamide, followed by theend-capping reaction with aniline [2]. The average size of the HBP was estimated to be approximately 12 nm
by a small angle X-ray scattering method. The heterogeneity index was 4.4 that was estimated by the gelpermeation chromatography. The glass transition and decomposition temperatures were 200ºC and 430ºC,
respectively. The refractive index (n) of the HBP was found to be 1.82 at a wavelength of 532 nm. The ultrahigh value of 1.82 is attributed to the incorporation of triazine and aromatic ring units to the HBP structure
and is much higher than those of our previously reported poly(ethyl methacrylate) HBP (n=1.51) and polystyrene HBP (n=1.61) [3]. Its molecular structure is shown in Fig. 1. We employed a monomer blend consist-

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 1. (a) The synthesized HBP, (b) 4HBA, (c) A-DPH, (d) RB, (e) NPG, (f) THF-A, (g) NVP and (h) Irgacure784.

3ing of a single functional alkyl substituted acrylate as host monomer (4HBA) and a multifunctional dipentaerythritol polyacrylate (A-DPH) as crosslinking monomer. We also added an electron-donor/acceptor
photoinitiator system consisting of Rose Bengal (RB) and N-phenyl glycine (NPG) at their appropriate ratio
to the monomer blend. These chemical structures are also shown in Fig. 1. It is known that a photoexcitedRB acting as an electron acceptor forms an encounter complex with NPG acting as an electron donor. Sucha complex in the excited state finally dissociates into one ionic radical pair, an anionic RB radical (RB•-) anda cationic NPG radical (NPG•+). The former acts as an inhibition radical, while the latter is continuously decomposed into the phenyl-amino-alkyl radical (PhNHCh2•) as well as a proton and a carbon dioxide by decarboxylation [4-6]. This phenyl-amino-alkyl radical acts as an initiation radical for chain-growth polymerization under green light illumination. The mixed syrup was cast on a glass substrate loaded with a 5-µm
spacer and was covered with another glass substrate to prepare NPC film samples. It is referred to as Sample#1. A similar NPC film sample was also prepared, consisting of the same HBP and a monomer blend consisting of N-vinylpyrrolidone (NVP), a tetrahydrofurfuryl acrylate (THF-A) and A-DPH with titanocen photoinitiator (Irgacure784) [2], as referred to as Sample #2, to compare their holographic recording performancewith that of Sample #1.
We employed a two-beam interference setup to write an unslanted and plane-wave transmission volume grating at grating spacing of 1µm with two mutually coherent s-polarized beams of equal intensities from a diode-pumped frequency-doubled Nd:YVO4 laser operating at a wavelength of 532 nm. A low-intensity spolarized He-Ne laser beam operating at a wavelength of 633 nm was employed as a photoinsensitive
readout beam to monitor the buildup dynamics of an NPC grating being recorded. We defined h was definedas the ratio of the 1st-order diffracted signal power to the sum of the transmitted 0th- and the 1st-order diffracted beam powers. The effective thickness of an NPC film sample was estimated by curve fitting of a
Bragg-angle detuning dependence of the saturated h to Kogelnik's formula, by which the buildup dynamics
of Dn was determined. Figure 2 shows a photograph of a recorded NPC grating of Sample #1 at HBP concentration of 23 vol.%. Figure 3 shows the buildup dynamics of Dn for Samples #1 (HBP 23 vol.%) and #2(HBP 27 vol.%) at 532 nm. It can be seen that Sample #1 possesses the largest value for Dnsat as high as
0.045 and the lowest optimum recording intensity of 5 mW/cm2. It is approximately a two-fold increase in
Dnsat and a fifteen-fold reduction in recording intensity as compared with those of Sample #2. Our obtained
result is promising for an application to wearable glasses and head-mounted display for augmented andmixed reality [7].

Fig. 3. Buildup dynamics of Dn, where optimumrecording intensities are shown in the parenthesis.

Fig. 2. Photograph of a recorded plane-wave
transmission grating.

References
1. Y. Tomita, E. Hata, K. Momose, S. Takayama, X. Liu, K. Chikama, J. Klepp, C. Pruner, and M. Faly, J. Mod. Opt.
63, S1 (2016).
2. Y. Tomita, H. Urano, T. Fukamizu, Y. Kametani, N. Nishimura, and K. Odoi, Opt. Lett. 41, 1281(2016).
3. Y. Tomita, K. Furushima, K. Ochi, K. Ishizu, A. Tanaka, M. Ozawa, M. Hidaka, and K. Chikama, Appl. Phys. Lett.
88, 071103 (2006).
4. S. Ikeda and S. Murata, J. Photochem. Photobiol. A: Chem. 149, 121 (2002).
5. J.-P. Fouassier, F. Morlet-Savary, J. Lalevée, X. Allonas, and C. Ley, Materials 2010, 5130 (2010).
6. G. Chen, M. Ni, H. Peng, F. Huang, Y. Liao, M. Wang, J. Zhu, V. A. Roy, and X. Xie, ACS Appl. Mater. Interfaces
9, 1810 (2017).
7. M.-L. Piao and N. Kim, Appl. Opt. 53, 2180 (2014).
4

Enhanced photocatalytic activity of TiO2 inverse opal structures with
gold-silver nanoparticles
Filipp Temerov and Jarkko J. Saarinen
Department of Chemistry, University of Eastern Finland, Joensuu
Corresponding author: filipp.temerov@uef.fi

Keywords: TiO2, inverse opal structure, metal nanoparticles, photocatalytic activity
Titanium dioxide (TiO2) is a well-known semiconductor material with photocatalytic activity. Fujishima andHonda [1] demonstrated water splitting into hydrogen and oxygen in a seminal work in 1972 by irradiating
TiO2 electrode with an ultraviolet A (UVA) light that initiated a vast research of photocatalysis. TiO2 is nontoxic and has a good stability but the anatase crystalline form has a rather large bandgap of 3.2eV (~387 nm)
in the UVA range. Moreover, TiO2 has a high refractive index and limited light absorption due to reflection
with a fast charge carrier recombination [2]. Therefore, a trend in recent research has been to shift the lightabsorption into the visible range. Enhanced photocatalytic activity in visible range can be used for solardriven artificial photosynthesis and solar cells with a higher efficiency that may provide solutions for substitution from the current fossil fuel based economy into a more sustainable solar economy.
There are different approaches for solving this problem. For example, TiO2 doping with noble metal nanoparticles (Au, Ag, Pt) or preparation of TiO2 based nanostructures (nanotubes, nanorods, nanobelts) havebeen suggested. Recently, TiO2 inverse opal (TiO2 IO) photonic crystal structures have attracted attention due
to its excellent properties such as photonic band gap, slow light photons, and localized photons [3]. TiO2 IO
has a three-dimensional ordered porous structure with a large specific surface area and optical properties associated with photonic crystals. Furthermore, TiO2 IO photocatalytic activity can be enhanced by dopingwith noble metals such as silver or gold together with long interaction times due to slow light in photonic
crystal.
In this study, TiO2 IO structures were prepared and loaded with different nanoparticles for enhanced photocatalytic activity. TiO2 IO was prepared using three well-known steps: first, self-assembly of polystyrene(PS) microspheres followed by infiltration of TiO2 precursor. Finally, by calcination the PS particles were
removed that also converted TiO2 into the anatase crystalline form [4]. Three types of nanoparticles wereprepared by hydrothermal method; gold, silver and gold-silver core-shell nanoparticles (AuNPs, AgNPs and
Au-AgNPs) [5]. Nanoparticles were deposited into TiO2 IO on glass substrate by casting on three solutions
containing the same amount of above-mentioned nanoparticles. By slow evaporation in the oven and capillary forces, nanoparticles were successfully deposited into TiO2 IO structure.
Historically photocatalytic activity has been characterized using an indirect method based on a color transformation of an optical dye such as methylene blue (MB) [6]. However, this method should not be used forphotocatalytic activity measurements (especially at a visible range), as the incident light itself can bleach theused dye and therefore, induce an erroneous result [7]. Therefore, in this work the photocatalytic activity ismeasured using an in-house built detector for gas-phase oxidation of organic compounds such as acetylene(C2H2). The mineralization of carbon compounds into CO2 can be directly and simply measured by monitoring the rate of CO2 increase in the reaction chamber. Additionally, the gas-phase detection removes all mechanical stresses induced by liquid phase methods.
TiO2 IO has a relatively uniform structure as shown in Fig. 1(A). However, the sample surface had some defects and cracks, which may affect the photocatalytic activity. The observed pore size was around 350 nm.
The used PS spheres had a diameter of 400 nm, which indicates a shrinkage of approximately 12 – 13% during the calcination process. The crack formation mainly originates from shrinking and infiltration process. It
should be noted that overlayers were not observed from the top view of the structures. The loading of noble
metal nanoparticles was performed successfully, and the size distribution of loaded NPs is rather narrow.
Figure 1 A inset shows the SEM image of TiO2 IO structure with Au-AgNPs. It can be clearly seen that therims of hexagonal structure are covered with small Au-Ag NPs with diameter of 30 – 50 nm. Agglomerated
nanoparticles were found inside the honeycomb structure with diameter of 70 – 90 nm. Similar nanoparticles
coverage was also observed for TiO2 IO loaded with Au and Ag NPs.

5

Fig. 1. (A) SEM image of TiO2 IO structure with Au-Ag nanoparticles, (B) oxidation of C2H2 into TiO2 IO
structures with various noble metal nanoparticles loading.
The nanoparticle formation was confirmed by energy-dispersive X-ray spectroscopy (EDS) and ultravioletvisible (UV-vis) spectroscopy. EDS mapping technique as well as characteristic absorption band confirmed
the nanoparticle formation.
The photocatalytic activity was measured using an in-house built photocatalytic reactor. TiO2 IO with nanoparticles were exposed to the UVA light (λex=365nm, 100W lamp intensity) in reactor. Acetylene undergoesa photocatalytic oxidation on TiO2 IO surface, and CO2 produces. The photocatalytic activity can be characterized from the observed CO2 increase. Figure 1(B) displays the measured photocatalytic activity of TiO2 IO
loaded with different metal nanoparticles. The lowest activity (22 ppm) was observed with pure TiO2 IO
without nanoparticle loading while TiO2 IO with nanoparticles have an increased activity. The highest activity (36 ppm) was observed with TiO2 IO loaded with Au-AgNPs with an increase of 60% in the activity. Thisis probably due to a higher capability of Au-AgNPs on TiO2 IO for generating electrons and holes.
As a summary, in this work an infiltration approach was used for preparation of TiO2 IO. However, a disadvantage of the method is the difficulty of forming large-area crack free films as during the infiltration step
the probability for crack formation is very high. Our future research work will focus on crack-free method
for inverse opal formation on large (100 µm × 100 µm) area including heterostructures such as TiO2/ZrO2 IO
structures. Furthermore, visible light activated photocatalysis will also be studied in the presence of plasmonic metal nanoparticles.
FT wishes to thank the Finnish Cultural Foundation for a research grant. JJS acknowledges the UEF Facultyof Science and Forestry 184 (Grant No. 579/2017) for the financial support.
References
1. A. Fujishima and K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238, 37–38(1972).
2. F. Sordello, V. Maurino, and C. Minero, Improved photochemistry of TiO2 inverse opals and some examples, Molecular Photochemistry - Various Aspects, InTech (2012).
3. J.Yu, J. Lei, L. Wang, J. Zhang, and Y. Liu, TiO2 inverse opal photonic crystals: Synthesis, modification, and applications, J. Alloy. Compd. 769, 740–757 (2018).
4. L. Liu, S. K. Karuturi, L. T. Su, and A. I. Y. Tok, TiO2 inverse-opal electrode fabricated by atomic layer deposition
for dye-sensitized solar cell applications. Energ. Environ. Sci. 4, 209–215 (2011).
5 Q. Han, C. Zhang, W. Gao, Z. Han, T. Liu, C. Li, Z. Wang, E. He, and H. Zheng, Ag-Au alloy nanoparticles: Synthesis and in situ monitoring SERS of plasmonic catalysis, Sensor. Actuat. B: Chem. 231, 609–614 (2016).
6 C. Xu, G.P. Rangaiah, and X.S. Zhao, Photocatalytic degradation of methylene blue by titanium dioxide: Experimental and modeling study, Ind. Eng. Chem. Res. 53, 14641–14649 (2014).
7 X. Yan, T. Ohno, K. Nishijima, R. Abe, and B. Ohtani, Is methylene blue an appropriate substrate for a photocatalytic activity test? A study with visible-light responsive titania, Chem. Phys. Lett. 429, 606–610 (2006).

6

Polarization state of the object wave analyzed by using FMCW-digitalholography
Masayuki Yokota1, Tatsuya Ishikawa1 and Yoshinobu Aoki1
1

Graduate School of Natural Science and Technology, Shimane University
Corresponding author: yokota@ecs.shimane-u.ac.jp

Keywords: digital holography, FMCW, polarization state
Frequency-Modulated Continuous-Wave (FMCW) technique has been introduced into digital holography(DH) using an injection-current-induced frequency modulation of a commercial laser diode (LD). Since thefrequency of beat signals observed in the hologram intensity variation recorded by a high speed camera
depends on the optical path length difference between the reference and the object light waves, a polarization
state of object wave transmitting through a transparent sample was investigated using two orthogonallylinearly polarized reference waves having the different optical path lengths.
Recently, the FMCW technique has been introduced into digital holography (DH)[1,2]. In the method, aseries of digital holograms are recorded during the frequency modulation period of a LD and the temporal
Fourier analysis is applied to extract the object wave from the series of the recorded holograms. The imageof a coin having a diffusely reflecting surface can be obtained based on the FMCW technique [1]. The
position resolution of digital holography based on the FMCW technique depends on the frequency
modulation bandwidth in the light source. The digital holographic microscope with a swept light source
having the modulation bandwidth of more than 70 nm has been successfully demonstrated to image a swine
sperm cell [2]. In this paper, to record and analyze the polarization state of the object wave transmitting
through a transparent object, a Jones vector of a polarized object wave is determined by using two
orthogonal linearly polarized reference waves having different optical path lengths.
Figure 1 shows the optical configuration for the polarization analysis. A LD with a wavelength of 782.4
nm whose optical frequency was modulated with a sawtooth function was used. The frequency modulation
bandwidth of f = 19.6 GHz was achieve by changing the injection current of 7.0 mA. A period of themodulation signal was set at 1.0 s. Temperature of the LD was held to be constant at 25.0 by using atemperature controller. By using a polarization beam splitter (PBS), the frequency-modulated laser beam was
split into two orthogonally linearly polarized beams traveling different optical paths: 368 mm for thehorizontally and 445 mm for the vertically polarized beams. The difference in their optical path length
generates the different beat frequencies in their holograms. Holograms between the object and the two
reference beams were recorded for 1.0 s by using a high speed CMOS camera with 1024 × 1024 pixels at
500 fps. The pixel size of the CMOS camera was 13.68 × 13.68 m2. A series of holograms were subjectedto the further signal processing for the reconstruction. To observe the variation in the polarization state ofobject beam, the transparent PMMA ring having the outer (inner) diameters of 14.0 (3.0) mm and thethickness of 5.0 mm was set after QWP3 in the object optical path. The distance between the object and theCMOS camera was 215 mm.

Object(PMMA ring)10.0 mmFig. 1 Optical configuration for the polarization analysis: ISO, optical isolator; NE, laser power controller; H(Q)WP,
half (quarter) wave plate; SF, spatial filter; BS, beam splitter; Obj, object (PMMA ring); Pol, polarizer; CMOS, highspeed CMOS camera.
7

Figure 2 shows the reconstructed intensity and the phase images of the object wave recorded with thehorizontally polarized reference wave before and after tightening a vice. By tightening the vice andcompressed the PMMA ring, the patterns of the phase distributions are different between before and aftertightening the vice. This is due to the induced birefringence of PMMA by applying stress to the sample. To
see the variation in the polarization states before and after tightening the vice, the polarization state of theobject wave was obtained and the polarization states of the typical points are shown in Fig. 3. As seen in Fig.
3, the polarization states of the object wave changes after tightening the vice. By compressing the PMMA
ring with the vice, the polarization states were varied due to the stress induced birefringence in the PMMA.
The results show the stress-induced variation of polarization states in the object wave by using the proposed
FMCW-DH method.

Ix(X,Y)

ox(X,Y)

Ix(X,Y)(a) Before tightened.

ox(X,Y)(b) After tightened.

Fig.2 Reconstructed intensity and phase images of the object recorded for the horizontally polarized reference wave: (a)before and (b) after tightened with a vice.

(b) After tightened.

(a) Before tightened.

Fig. 3 Polarization states of the object wave before and after tightened by the vice.

Two orthogonally linearly polarized reference waves having the different optical paths have been
introduced into the FMCW-DH system. By using the Jones matrix imaging [3-5], the polarization state of theobject wave has been obtained. The technique was also applied to investigate the photoelastic effect in the
stressed PMMA sample. It has been shown that the proposed technique can be used to investigate thepolarization state of the object wave and to analyze the photoelastic analysis
References
1. A. Wada, Y. Miyamaoto, 14th Workshop on Information Optics Th-P9 (2015).
2. S. Chen, J. Ryu and Y. Zhu, Opt. Lett., 41(4), 665-668 (2016).
3. T. Colomb, P. Dahlgre, D. Beghuim, E. Cuche, P. Marquet, and C. Depeursinge, Appl. Opt., 41(1), 27-37 (2002).
4. M. Yokota, Y. Terui and I. Yamaguchi, Opt. Eng., 46(5), 0558011-0558017 (2007).
5. M. Yokota, Appl. Opt., 47(34), 6325-6333 (2008).

8

Single-pixel imaging with hole-array coding masks
Yoshio Hayasaki, Ryo Sato, and Yujiro ItoCenter for Optical Research and Education, Utsunomiya University*Corresponding author: hayasaki@cc.utsunomiya-u.ac.jp

Keywords: Single-pixel camera, spatial light modulators, spectroscopy, computational imaging
1. Introduction
The single-pixel imaging architecture [1-2] is composed of optical coding masks, a photodetector, and a decoding calculation. The biggest feature is simple optics and simple electronics because it can perform an imaging without an image
sensor. The implementation with a simple photodetector for the light detection gives wide opportunity for developing a
new imaging system. Especially, it is very effective when an image sensor with a sufficient performance cannot be available for a given budget or in a frequency region that an image sensor is not existed at present. Recently it was extended
to a complex amplitude to get a complex amplitude imaging [3]. The spectral imaging [4, 5] can be performed without aspecial technique only by replacing from the photodetector to a spectrometer. The coding masks are mostly implemented
by a liquid crystal spatial light modulator (LCSLM) and a digital micromirror device (DMD). The spectral property ofthese devices gives a limit in the spectral imaging. The key idea of this study is that the coding mask is made of holearrays drilled in a plate. It has no wavelength dependence except for an optical absorption of air. The hole arrays areplaced on a disk that is rotated to change the cording mask. It is also possible to operate the rotation with a hand.
The coding masks are made of holes arrays drilled in a plate, which is the key idea in this research. The most importantfeature of the light modulation using holes is no wavelength dependence except for an optical absorption of air. In addition,it has an easy and low-cost fabrication. The hole arrays are placed on the circumference of a disk that is rotated to change
the cording masks.
2. Experimental setup
Figure 1 shows an experimental setup for the single-pixel spectral imaging. The experimental setup was composed of anillumination light source, coding masks on a rotational disk, a spectrometer, a set of relay lenses, a laser, and a computer.
A transmissive object was illuminated with white light generated by a halogen lamp (MHAB-150W-100V，Moritex).
The light passing through the object was imaged on a coding mask and spatially encoded by the coding mask. The disk
was rotated with a motorized rotation stage to evaluate the system performance. The arrangements of the holes were thepseudo-Hadamard matrix that the value of -1 was changed to the value of 0. Therefore, the first pixel had a lot of noises.
The disk had another hole at the symmetric position of each mask for indicating the position of the light detection (aposition indicator) illuminated by a He-Ne laser. The encoded image was detected by a fiber-based spectroscope (C7473,
Hamamatsu) through the relay lenses. The spectral range was from 200 nm to 950 nm with the resolution of < 2 nm. The
spectrum was continuously detected at the minimum sampling period of 19 ms while the disk was rotating, and the intensity for each mask was extracted by referring the position indicator signal. Finally, the inverse matrix calculations were
performed at each wavelength.

Fig. 1 Experimental setup.

9Figure 2 shows an object and the reconstructed spectral images. The object shown in Fig. 4(a) was made of plastic color
films with different spectral transparencies, which were put on the frame. The spectral images fat 500, 600, and 700 nm
are shown in Fig. 4(b). Figure 5 shows the spectrum of the red film from 300 nm to 750 nm. The red curve was obtainedby an ordinary spectroscopy. The dots were obtained by the single pixel spectral camera. They were well agreed. The
measurement RMS errors were 2.82% on the red film, 5.12% on the blue film, and 2.28% on the blue film.
We developed a new implementation method of a single-pixel camera for spectral imaging. The optical coding masks
were composed of holes on a substrate, and arranged on the circumference of a disk enables us to switch with a rotationof the disk. The main features are small wavelength dependence other than air for wideband spectral imaging, and thesimple structure. The spectral imaging for a sample composed of three color films was demonstrated, and the measurement accuracy of the present system was less than 3% at red channels. The spectral range from 300 to 750nm depended
on the performance of the spectrometer we used. The next subjects of our single-pixel spectral camera will be to improvethe detections speed and the spatial resolution.

(a)

(b)
Figure 2 (a) Object made of plastic color films with different spectral transparencies. (b) The spectral images at 500, 600,
and 700 nm.
References
1. M. F. Duarte, M. A. Davenport, D. Takhar, J. N. Laska, T. Sun, K. F. Kelly, and R. G. Baraniuk, “Single-pixel
imaging via compressive sampling,” IEEE Signal Process. Mag. 25, 83–91 (2008).
2. D. L. Donoho, “Compressed sensing,” IEEE Trans. Inf. Theory 52, 1289–1306 (2006).
3. K. Ota and Y. Hayasaki, “Complex-amplitude single-pixel imaging,” Opt. Lett. 43, 3682-3685 (2018).
4. Y. Garini, I. T. Young, and G. McNamara, “Spectral imaging: principles and applications,” Cytometry Part A
69A,735–747 (2006).
5. Q. Li, X. He, Y. Wang, H. Liu, D. Xu, and F. Guo, “Review of spectral imaging technology in biomedical engineering: achievements and challenges,” J. Biomed. Opt. 18, 100901 (2013).
6. W. L. Chan, K. Charan, D. Takhar, K. F. Kelly, R. G. Baraniuk, and D. M. Mittleman, “A single-pixel terahertz
imaging system based on compressed sensing,” Appl. Phys. Lett. 93, 121105 (2008).

10

High throughput spectral imaging with snapshot sensing
Nathan Hagen
Utsunomiya University, Utsunomiya, Japanemail contact: nh.hagenlab.org

Keywords: spectral imaging, throughput, snapshot, remote sensing
The classic literature on spectral imaging talks about the “efficiency” of different architectures, such that theFourier transform and Fabry-Perot spectrometers are mentioned as having the Jacquinot (throughput) advantage(or freedom from an exit slit), and the Fourier transform and Hadamard transform spectrometers as having the
Fellgett (multiplex) advantage. However, it is not widely realized that modern detector technology has obsoleted
both of these advantages, at least in their classic form. Using large arrays of detector elements instead of single
detector elements has eliminated the Jacquinot advantage, while working with detectors that are limited by shotnoise rather than detector noise has eliminated the Fellgett advantage. However, if we consider the geometryof light collection among the different spectral imaging architectures (Fig. 1), we can see that an important
advantage remains. We can call this the snapshot advantage.[1]Scanning imaging spectrometers collect a subregionof the datacube at a time, meanwhile using a slit ora filter to reject any light from the object from outside
this subregion. By scanning this subregion across theobject (or across the spectrum), the entire datacube isassembled by collecting together all of the scans. In
contrast, snapshot imaging spectrometers collect theentire 3D datacube in a single integration period without scanning. While the existing literature cites advantages for snapshot instruments such as the lack ofscanning artifacts and the increased robustness or compactness due to the lack of moving components, thesequalities are actually secondary to snapshot advantage
in light collection efficiency.

line-sspec canning
trom
eter

point-scanningspectrometer

wavelengthscanning(filteredcamera)snapshot imaging
spectrometer

y
x

y
x

Figure 1: Scanning and snapshot data collection.

The snapshot advantage can be dramatic for larger datacubes. For a datacube of dimensions (Nx , Ny , Nw ) =(500, 500, 500), for example, it requires 500 scans in order to complete the dataset. As a result, any individual
scan must reject light from 499/500ths of the cube at any given time, so that its light collection efficiency cannever exceed 0.2% even under ideal conditions. A datacube (500,500,25) in size is best scanned along the
wavelength dimension, so that it only rejects 24/25ths of the light, for a maximum efficiency of 4%. For these
two examples, the snapshot advantage will be a factor of 500 and 25, respectively.
In order for snapshot spectral imagers methods to work, systems need to be designed with much larger pixel
counts than are typically used in scanning systems. In order to collect a datacube of dimensions (Nx , Ny , Nw ) =(500, 500, 500), a 125 MPix detector array is needed, assuming that none of the pixels are wasted. Some wasteof detector real-estate, however, is inevitable. Thus, while the geometry of the datacube (Fig. 1) illustrates how
scanning architectures are inefficient at light collection, another important consideration is how efficiently eacharchitecture uses its pixels.
Figure 2 shows an idealized view of how the various 2D “slices” of the datacube are viewed on the detector
array, for different snapshot architectures. What we can see is that, due to the need to provide some minimum spacing between slices, filtered cameras architectures are efficient when Nx Ny  Nw (i.e. fluorescencemicroscopy); image slicers are efficient when Ny Nw  Nx , and fiber-based and lenslet-based methods are
inefficient for every datacube aspect. Figure 3 shows the optical layout for an image slicing architecture, illustrating how the 3D volume of the datacube cane be sliced into 2D subvolumes so that they can be simultaneously
measured on a 2D detector array.

11

Nw

Nx

Nw
Ny

ﬁber-based,
lenslet-based
integral ﬁeld
spectrometers

image
slicingNyﬁlteredcamerasFigure 2: How each snapshot architecture utilizes detector array real estate, assuming (Nx , Ny , Nw ) =(5, 5, 16), and 1 pixel spacing. [2]objective lens

mapping
mirrorobject(image of lorikeet)(a)pplaupilnelen
arr slet
ay

de
te

cto

pr
ar ism
ra
y
ra

rra
y

(b)

(c)
(image without disperser)(image of uniform object)

Figure 3: Optical layout for image slicing spectrometry. [2]While the advantages of snapshot systems are many, they have serious drawbacks as well. They are generally
difficult and expensive to build, requiring high-precision custom optics and mechanics, and high pixel-countdetector arrays. They also require an extremely high rate of data flow in order to take full advantage of their
speed and light collection. Thus it is only recently that detector arrays have achieved the combined speed andsize to meet these challenges.
References[1] N. Hagen, R. T. Kester, L. Gao, and T. S. Tkaczyk, “Snapshot advantage: a review of the light collection improvement
for parallel high-dimensional measurement systems,” Opt. Eng. 51, 111702 (2012).
[2] N. Hagen and M. W. Kudenov, “Review of snapshot spectral imaging technologies,” Opt. Eng. 52, 090901 (2013).

12

A Quantum Dot Reservoir Based on Förster Resonance
Energy Transfer for Optical Reservoir Computing
Jun Tanida1 , Suguru Shimomura1 , Takahiro Nishimura2 , Yuki Miyata3 , Naoya Tate3 , and Yusuke Ogura1
1

Graduate School of Information Science and Technology, Osaka University, Suita
2
Graduate School of Engineering, Osaka University, Suita
3
Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka
Corresponding author: tanida@ist.osaka-u.ac.jp

Keywords: optical neural systems, fluorescence, energy transfer, neural network
Reservoir computing is a kind of computational architecture categorized in a recurrent neural network[1]. The
recurrent neural network has an internal feedback structure, so that applications on temporal information suchas speech recognition and dynamic prediction are expected to be processed. Owing to simplicity and flexibility
in construction, reservoir computing is a promising architecture for optical neural computing system. To date,
several reservoir computing systems were proposed based on an optical fiber[2] and memristor[3]. As a fundamental drawback of these implementations, internal optical/electric signals are guided, so that their hardware isdifficult to be reduced. In this research, an optical reservoir computing system using Förster resonance energy
transfer (FRET) between quantum dots is investigated. Some preliminary experimental results are presented toshow diverse response generated by ensemble of quantum dots.
Excitation
input

全結合層

Fluorescence
outputs

・時間的応答Spectral signals

入力・空間的応答Spacial signals
Temporal signals

Quantum dots

・波長応答

FRET

Figure 1: Schematic diagram of a quantum dot reservoir for optical reservoir computing.
Figure 1 shows a schematic diagram of an optical reservoir implemented by a quantum dot network. Several
kinds of quantum dots are dispersed in a small space and an exciting light is induced to them. The excitedquantum dots either emits a light or transfers the energy to adjacent quantum dots. This energy transfer is
FRET and quite sensitive to the distance between the donor and the acceptor. With an adequate density ofquantum dots, an energy transfer network can be constructed in the dispersed space. When the excitation lightis induced to some part of the quantum dots, energy transfer through the network occurs and fluorescencelights are emitted from the bunch of the quantum dots. The output response reflects the distribution of theindividual quantum dots and the position of the excitation light. Sort and composition of the quantum dots has
a large amount of freedom and the excitation light can be modulated in the spatial and/or temporal domains.
The observation position and the wavelength of the fluorescence light provides diversity of the output signals.
Considering the flexibility and diversity of the configuration, the quantum dot network is expected to be aneffective reservoir for optical reservoir computing.
The features of the quantum dot reservoir are compactness, low power consumption, and potential easy implementation. Because quantum dots distributed in a small space are the core component, tremendous hardware
compaction is expected even if peripheral components are necessary for the operation. Ideally, a quantum dot
is excited by a single photon and energy dissipation in each component during their operations is necessarily small. Even the output interface requires larger energy consumption, total consumption of the quantum
dot-based device can be theoretically estimated as 100 eV order, which 10−2 -10−3 smaller than the typical13

Intensity [a.u.]electronic device. As a result, very low-power computation is expected. In terms of fabrication, it is enoughthat arbitrary composition of quantum dots are dispersed without precise arrangement. In the reservoir computing, weighted-sum of the output signals from the reservoir provides the processing result. Only the connecting
weights of the output signals are variable and configured by the training for desired response. This process isperformed after capturing the output signals by a computer, which is easy to implement.
A

Excitation
light

Ensumble
QD
A
B
C

Intensity [a.u.]

Wavelength [nm]B

Figure 2: Experimental setup for exciting
the quantum dot ensemble. Excitation position was changed by translation of the slide
glass.

Intensity [a.u.]

Wavelength [nm]C

Wavelength [nm]Figure 3: Spectra of the output signals observed at three different
positions on the quantum dot ensemble.
To investigate the characteristics of the quantum dot reservoir, the response of quantum-dot aggregates containing three kinds of quantum dots, Q490, Q525, and Q575 was measured. The center peaks of their fluorescencewavelengths are 490nm, 525nm, and 575nm, respectively. To prepare the ensemble of the quantum dots, theirsolutions were dropped on a slide glass, and then dried at a room temperature. Figure 2 shows the experimental
setup for the measurement. An excitation light of 405nm is focused on the prepared sample and the output lightsignal was measured by a spectroscope (Hamamatsu, C10083CAH). The spot size and the light power density
at the focused point were 225µm and 34mW/cm2 , respectively. Figure 3 shows the spectra observed at threedifferent positions on the sample with 100ms of the measurement time. As seen from the signals, spatial variation of the quantum dot ensemble was observed. But further verification is required to confirm the evidence ofenergy transfer between the quantum dot network.
In this study, a quantum dot reservoir for optical reservoir computing was presented and an experimental evaluation of quantum dot samples was performed. The result indicates promising characteristics of the quantum
dot reservoir. We will perform detailed evaluation on temporal characteristics and ensemble composition. This
work was supported by JST CREST Grant Number JPMJCR18K2, Japan.
References
1. H. Jaeger, et al., “Harnessing nonlinearity: predicting chaotic systems and saving energy in wireless communication,” Science 304, 78–80 (2004).
2. Y. Paquot, et al., “Optoelectronic Reservoir Computing,” Sci. Rep. 2, 287 (2012).
3. C. Du, et al. “Reservoir computing using dynamic memristors for temporal information processing.” Nat.
Commun. 8, 2204 (2017).

14

Visualizing AC magnetic fields by using atomic magnetometerand micro-mirror device
Shuji Taue
School of system engineering, Kochi University of Technology, Kochi, Japan
Corresponding author: shuji.taue@kochi-tech.ac.jp

Keywords: Magneto-optic systems, Image detection systems, Zeeman effectA spatial evaluation of an electromagnetic field is valuable to estimate positions of signal or noise sources.
We propose an optical measurement system to detect spatial distributions of the AC magnetic field. The alkali-metal atoms and a micro-mirror device enable us to obtain field intensity distributions with a sub-mm
spatial-resolution. Obtained field images from a fine metal-wire clearly indicated the wire position. This imaging technique has the potential for spatial evaluations of the AC field from electrical and electronic devices.
Electromagnetic waves have been already utilized in measurements, information-communications,
identification, and electric power transmissions. In the meanwhile, electromagnetic interference (EMI)problems are often caused by integrated circuits in electronic devices. To visualize electromagnetic fields
allows us to evaluate spatial distributions as signal or noise components with a pattern recognition manner.
The well-known conventional methods to obtain AC field distributions are scanning a coil as a sensing probe
or collocating a number of small probes. However, a spatial resolution is limited by the probe size. And theprobes and signal wires attenuate and distort the magnetic field.
An atomic magnetometer is an optical detection method utilizing interactions between magnetic fields andelectron’s spins of vaporized alkali-metal atoms [1]. However well-controlled environmental DC-field is required to operate the magnetometer with high sensitivity, the optical detection of the AC field has advantagesof accurate and low-invasive sensing because of sensing probe and signal wire containing less metal. We
have already developed the atomic magnetometer with using cesium-133 to detect 70 kHz AC magnetic field[2]. An imaging technique for AC magnetic field distribution was performed by using a micro-mirror device[3]. As a demonstration, intensity distributions of an AC field generated from a thin metal-wire were obtained.
Figure 1 shows the schematic of experimental setups to obtain the AC field distributions. The sensor head isa glass cell with a 10 mm light path length and including cesium and buffer gases. The wavelength of the laser is turned to the cesium D2 line (852 nm), and the light transmits through the cell. The cell is placed at thecenter of a 300 mm diameter 3D Helmholtz coils which generate a DC field to control the environmental DC
field. The signal source of the AC field is a fine metal-wire connected to the function generator (FG) applying a 70 kHz AC current. The wire is attached to the side of the glass cell as shown in Fig. 2. The transmitted
light forms the image in the cell onto a digital micro-mirror device (DMD) by using lenses L2 and L3. The
mirrors of the DMD are divided into 50 × 50 image-elements, and the light reflected from each element is
detected by a photodetector (PD). The detected light signals are grabbed by a PC and the images are reconstructed.
Figure 3 shows a contour-plot image of the output signal intensity. At the left side in the figure, a white
dotted circle indicates the position of the metal wire. Two dashed lines in x-direction indicate inner edges ofFig. 1. Experimental setups for AC field imaging

15

Fig. 2. Photo of the metal wire attachedto the glass cellthe 10 mm-width glass cell. From the result,about 30 image-elements resolves the 10 mm
cell region in y-direction. The circle-shaped
intensity profile observed inside of the cellregion represented the light transmitted area
where was working as the magnetometer. In addition, the wire position in x-direction was easily recognized from the output intensity distribution. The intensity profile matches well with thetheoretical field intensity profile. The signal
source position in y-direction was estimated byusing experimental and theoretical results. The
estimated depth position (1.41 mm) was very
close to the real position (1.25 mm).

Fig. 3. Contour map of the output signal intentisy

We constructed the AC field distribution imaging system. The atomic magnetometer and the DMD realized
to visualize the field distribution with a sub-mm resolution. The position estimation of the signal source was
performed by using field distributions, and the result is very close to the real position. The obtained field image has good potential for circuit evaluations and signal-source estimations.
References
1.
2.
3.

E.B. Alexandrov and A.K. Vershovskiy, “Mx and Mz magneotmeter,” in Optical Magnetometry, D. Budker and
D.F.J. Kimball, Eds. 60-84, (Cambridge University Press, New York, 2013).
S. Taue, M. Shinohara, Y. Toyota et al. IEICE Trans. Commun. (Japanese Edition), J100-B(3), pp. 158-165 (2017).
S. Taue and Y. Toyota, Progress In Electromagnetics Research Symposium (PIERS2018), 2P15b-1 (2018).

16Video-rate imaging for Stokes parameters by polarization cameras
Yukitoshi Otani, Shuhei Shibata, Masayuki Suzuki and Nathan Hagen
Department of Optical Engineering / Center for Optical Research and Education (CORE)
Utsunomiya University, Japan
Corresponding author: otani@cc.utsunomiya-u.ac.jp

Keywords: polarization imaging, Stokes parameters, polarization camera
Polarization technology has become an important area in the field of optical science and engineering, especiallyin display technology, optical data storage, semiconductor technology, nano-material research, bio-technology
and interferometry. We have proposed some new technologies for measuring states of polarizations by apolarization camera whose adjacent 4 pixels are attached on an image sensor and aligned to 0, 45, 90 and 135°of azimuthal direction of linear micro-polarizer arrays [1]. A polarized camera is often used for video-rate
measuring partial Stokes parameters but it is not so easy to measure full Stokes parameters. We have alreadyproposed a full-Stokes polarimeter [2] but it required for mechanical mechanism.
We propose a video-rate imager for Stokes parameters by polarization cameras. Figure 1shows its opticalarrangement. An object with Stokes parameters is imaged on to polarization cameras by a camera lens. The
incident light is divided into two optical paths by a non polarizing beam splitter (NPBS). Both transmitted andreflected light pass through retarders. The Stokes parameters is analyzed 8 intensities with different azimuthal
angles of two polarization cameras as shown in Fig.1. In addition, we compensate error of retardance and outof alignment of pixelated polarizers in advance. The measurement accuracy is affected by birefringence phasedifference error of the non polarizing beam splitter so that we also need to calibrate it. Figure 2 shows a pictureof video-rate full-Stokes imager.
We demonstrate tomeasureacirculardichroism(circular
diattenuation) of a beetle.
Albert Michelson firstviewed scarab beetles
throughcircularpolarizers in 1911. Figure
3 shows one of themeasuredresultsofcirculardiattenuation
image calculated by S3parameter. We succeeded
to identify the beetle
tracking by polarization
parameters.
We proposed avideo-rate imaging for
full-Stokes parameters bypolarization cameras. It is
succeeded to reconstructa full Stokes imaging in
the video-rate.
References

sampleStokes parameters

!!
!!

!!
!!

iris NPBS

Camera lens



0°

45°

135°

90°

polarization
camera 1

BF

reterderpolarization
camera 2

Fig. 1 A schematic diagram of video-rate Stokes imager

Fig.2 Picture of video-rate full-Stokes
imager

Fig.3 Circular diattenuation image

1. Nathan Hagen, Shuhei Shibata, Yukitoshi Otani: Calibration and performance assessment of microgrid polarization
cameras Optical Engineering, 58,8, 082408 (2019).
2. Shuhei Shibata,Nathan Hagen, Yukitoshi Otani : Robust full Stokes imaging polarimeter with dynamic calibration,
Optics Letters,44, 4, 891-894 (2019).

17

Deep Learning Enhanced Multiphoton Microscopy for Investigating the
Dermoepidermal Junction in Human Skin
Mikko J. Huttunen1, Radu Hristu2, Adrian Dumitru3, Mariana Costache3 and Stefan C. Stanciu2
1

2

Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
Center for Microscopy-Microanalysis and Information Processing, University Politehnica of Bucharest, Bucharest,
Romania3
Department of Pathology, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
Corresponding author: mikko.huttunen@tuni.fi

Keywords: Label-free, nonlinear microscopy, machine learning, deep learning, tissue characterization
Histopathological image analysis of stained tissue slides is routinely used to diagnose many pathologies, suchas cancer [1]. A highly trained pathologist performs the analysis, which is labour-intensive, time consuming
and even contains a risk for bias or human error. Therefore, a need exists for faster, more accurate and lessinvasive approaches. Ideally, the approaches should also be in-vivo compatible that could remove the timeconsuming bottleneck of performing tissue biopsy.
Multiphoton microscopy (MPM) and emergence of convolutional neural networks (CNNs) have recently attracted interest for medical diagnosis [2,3]. First, MPM allows label-free and minimally invasive imaging oftissues. For example, texture analysis of second-harmonic generation (SHG) images from unstained ovariantissues has been used to quantify remodeling of tissues surrounding cancer and to classify ovarian cancer types[4]. Furthermore, MPM is compatible with in-vivo tissue investigations showing potential to dramatically
speed-up the diagnosis [5]. Second, CNNs have led to a revolution in computer vision, where many challenging image-processing tasks are now routinely and automatically performed promising to dramatically reduce
the need for human intervention in diagnosis [3]. Despite rapid progress, so far only a few demonstrations
exists where deep learning enhanced MPM has been utilized for medical diagnosis [6,7].
Here, we demonstrate how by fine-training pre-trained CNNs we can perform automated classification of
MPM images of the dermoepidermal junction (DEJ) in human skin. We wanted to study the potential of theapproach for skin cancer screening and early diagnosis, because successful diagnosis facilitates timely intervention ultimately reducing the morbidity rates. As a proof of principle, we investigated fixed but unstainedtransversal tissue sections, which were prepared and imaged using intrinsic and nonlinear two-photon fluorescence (TPEF) and SHG modalities. In total 358 MPM images were recorded, and a binary CNN-based classifier was trained using a training/validation scheme [3]. Due to the relatively small size of our data set (250training images), we systematically studied how data augmentation by using image reflections, rotations, contrast enhancement and filtering affects the training results. We found out that the fine-trained CNN (GoogLeNet) can achieve an outstanding 93.5 % classification accuracy alongside with ~94 % and ~92 % specificity
and sensitivity, respectively.
We performed combined SHG and TPEF microscopyof unstained samples by using a modified confocal laser-scanning microscope (Leica TCS-SP). Representative MPM images from healthy and dysplastic
tissues are shown in Fig. 1. Ti:Sapphire femtosecond
laser (Chameleon Ultra II, Coherent) outputting ~140
fs pulses at the incident wavelength of 860 nm wasused for excitation through an objective lens (40 0.75
NA). We recorded backward-generated MPM signals
and kept the average powers reaching the samples be- Fig. 1. Representative MPM images from (a) healthy
low 15 mW in order to avoid possible sample damage. and (b) dysplastic tissues near the DEJ of human skin.
TPEF signals (red) originate from the epidermis,
Three spectral channels were simultaneously detected,
while SHG emission (blue) is due to fibrous collagen
where the first channel (425–435 nm) corresponded toand elastin fibers present in the dermis. The DEJ is
SHG emission. The second channel collected TPEF
the area joining the dermis and the epidermis of thesignals emitting at 440–490 nm (TPEF1), which corresponded to autofluorescence of reduced nicotina- skin. Scale bars are 50 m.
mide adenine dinucleotide phosphate (NAD(P)H). The third channel collected TPEF signals emitting at 510–
600 nm (TPEF2) corresponding to autofluorescence mainly due to flavin adenine dinucleotides (FAD). These18

TPEF1 and TPEF2 signals were then used to calculate the so-called redox ratio often written as
FAD/(NAD(P)H + FAD), which has been associated with changes in the cellular level metabolic activity [8].
In addition, combined TPEF images were formed by averaging the TPEF1 and TPEF2 images. The final MPM
images were transformed into composite RGB images, where each color channel corresponded to either of the
MPM signals (SHG, TPEF1, TPEF2 or redox ratio).
Processed MPM images were then randomly divided into validation and training sets using a ratio of 30/70,
respectively. Supervised learning scheme was used for training where the ground truth was provided by apathologist inspecting adjacent H&E stained tissue sections. In total, N = 14 healthy and N = 14 dysplastic
tissue sections were imaged. Because the MPM data set size was relatively small, the amount of training data
was increased by performing data augmentation [9]. Here we used horizontal and vertical reflections alongsidewith 4-level Gaussian blur pyramid in order to augment the data set resulting in a 20-fold increase in the
training set size (5000 training images). We repeated the training process 25 times due to the random natureof the set selection, and report here the mean sensitivities, specificities and accuracies along with their standard
deviations. In order to investigate the role of each collected MPM signal (SHG, TPEF1, TPEF2 and redox
ratio) for overall image classification performance, the network training was repeated for three different data
set permutations (see Fig. 2).

Fig. 2. Calculated classification sensitivity (a), specificity (b) and accuracy (c). (a)-(c) Performance improvement is seen when data augmentation includes also a 4-level Gaussian blur pyramid (red markers), compared
to data augmentation not utilizing the pyramid (blue markers). (c) Highest classification accuracy (93.5 %) isachieved by including the redox ratio into the data set.
In the future, it would be interesting to study whether polarization-sensitive or super-resolution MPM could
be used to provide data sets resulting in even better performance [10,11]. In addition, we intend to investigate
whether coupling traditional computer vision tools, such as invariant feature descriptors, to CNNs could further
improve the performance and robustness of the approach for varying acquisition conditions [12].
To conclude, we have performed automated classification of combined second-harmonic generation and twophoton excitation fluorescence multiphoton microscopy images collected from the dermoepidermal junctionof human skin. Our approach is based on convolutional neural networks (GoogLeNet) and provides real-time
image classification sensitivity, specificity and accuracy all exceeding 92 %. The demonstrated methodology
is label-free and compatible for in-vivo investigations. Our results show great promise for speeding up andimproving the current methodologies used in skin cancer screening and diagnosis.
References
1. M. N. Gurcan et al., IEEE Rev. Biomed. Eng., 2, 147–171 (2009).
2. W. R. Zipfel, R. M. Williams, and W. W. Webb, Nat. Biotechnol., 21, 1369–1377 (2003).
3. C. Szegedy et al., in Proc. of IEEE Conf. on Computer Vision and Pattern Recognition, 1–9 (2015).
4. B. L. Wen et al., Sci. Rep., 6, 35734 (2016).
5. R. B. Saager et al., J. Biomed. Opt., 20, 066005 (2015).
6. S. Weng, X. Xu and S. T. C. Wong, J. Biomed. Opt., 22, 106017 (2017).
7. M. J. Huttunen et al., J. Biomed. Opt., 23, 066002 (2018).
8. M. C. Skala et al., Proc. Natl. Acad. Sci. U.S.A., 104, 19494–19499 (2007).
9. L. Perez and J. Wang, arXiv preprint:1712.04621 (2017).
10. M. J. Huttunen et al., J. Opt., 19, 085504 (2017).
11. H. Lee et al., Biomed. Opt. Express, 4, 909–916 (2013).
12. D. Ünay and S. G. Stanciu, IEEE Access, 6, 40154–40164 (2018).

19

Full arch dental imaging using an extraoral camera system
Katri Kukkola1, Anssi Mäkynen1, Ville Kaikkonen1, Gleb Bulygin1, Eero Molkoselkä1, Niklas Pikkarainen1,
Nanni Nielikäinen1, Aleksi Rantanen1, Antti Kämppi2,3, Tarja Tanner2, Vuokko Anttonen2
1

Optoelectronics and Measurement Techniques Unit, Faculty of Information Technology and Electrical Engineering,
University of Oulu, Oulu
2
Oral Health Sciences Unit, Faculty of Medicine, University of Oulu, Oulu
3
Department of oral and Maxillofacial Diseases, Faculty of medicine, University of Helsinki, Helsinki
Corresponding author: katri.kukkola@oulu.fi

Keywords: intraoral, extraoral, dental, imaging, caries screening, oral healthCaries is a common oral disease, which affects quality of life by causing pain and discomfort. [1]. Caries canbe arrested if it is detected early and restoring can be conducted using Minimally Invasive techniques (MI) [2].
Usually caries is detected on visual inspection conducted by dentists, but optical imaging is also successfully
used for caries screening [3]. Caries is a lifestyle and behavior related disease, so the patient has a key role incaries prevention [4]. Easy caries screening and information of the personal situation could help motivate themaintaining of oral health, in which visualization and images are effective ways to convey information. Thus,
we started to examine optical imaging tools for dentition photographing that could be used in easy access oral
health services. The target was to find an imaging system which would enable both caries screening and informing and motivating patients to adopt and maintain habits that support oral health.
There are several intraoral cameras commercially available whose performance against visual inspection has
been reported by several authors [5,6,7,8]. However, the equipment is mainly targeted for imaging a single
tooth as in Fig1 which shows pictures of a tooth with different intraoral cameras (DIAGNOcam, SoproCare
and VistaCam iX ). There are issues when applied for imaging whole dentition: the usability of handpieces
and controls was challenging in some positions, which also affected image quality. The included image softwares were either unstable or the storing and naming of images was slow. The most remarkable hinderance tothe use any of these intraoral cameras was the time spent on imaging: taking images of all teeth took between
11 to 15 minutes, which is uncomfortably long for the patient and unacceptable for easy access type of services.

Fig. 1. Images of same tooth with different intraoral cameras a) DIAGNOcam, b) SoproCare andc)VistaCam iX.
In addition to intraoral cameras other cameras and smartphones have been used for dental photographing andteledentistry [9,10,11]. In order to get an image of the full dental arch, typically mirrors are placed into themouth for the duration of photographing. Our target was to make an imaging solution which is pleasant for thepatient, meaning no mirrors. Additionally our target was to make the photographing fast and minimize needed
procedures for infection prevention.
The main challenge of extraoral full arch imaging is to optimize camera and flash position so that the image
area unobscured and covers the full dental arch, and that the arch is properly illuminated. There is variation on
how patients can open their mouths and the individual adjustment of imaging angle. Our solution uses multiple
cameras and optics with large depth of focus. Their set up and positioning is based on assessments of camera
angles and mouth positions that are feasible for various patient sizes and mouth opening angles. The camera
system consists of board level cameras (Basler daA2500-14uc) with 6 mm optics (C 6 mm F1.8/ 1/2""). It is
possible to use different kinds of filters on the cameras and flashlights that also enable fluorescent imaging.
Anti-reflection coated plastic plates protect the cameras and filters, while also enabling easy cleaning after
each photographing. Miniature fans blow away the mist that forms on the plates from the patients breathing.
The frames of the plates have mechanical guides that guide patient positioning for photographing and live
video that shows to the patient how their teeth are visible for assisting the cameras in final adjustments to the20image. Fig. 2 shows the fully constructed camera system and fig 3. shows dental arch images taken with it. To
have fascial tooth surfaces visible a cheek retractor is needed, we have used the Optragate [12], which is softand flexible and retracts cheeks and lips evenly around the mouth.

Fig. 2. Our camera system consists of five separate cameras a) upper arch camera, b) lower arch camera, c)frontal teeth camera, d) fluorescent upper arch camera, e) fluorescent lower arch camera, figure on the rightshows parts f) flash, g) protective plate and h) mechanical positioning guide and the fan on its right side.

Fig. 3. System produces five images a) upper arch, b) lower arch, c) frontal teeth,, d) fluorescent upper arch,e) fluorescent lower arch.
In the initial evaluation dentists have been able to identify and distinguish different caries levels: dentin caries,
active enamel caries and inactive enamel caries. The images enable detection of plaque and dental calculus.
Furthermore, dentists have made some other observations such as abrasions and attrition. However further
analysis and validation is required.
References
1. P.E. Petersen, and H. Qgawa, Community Dental Health, 33, 66–68 (2016).
2. A. Banerjee and S. Doméjean, Primary dental journal, 2 (3), 30-37(2013).
3. M. Estai, S. Bunt, Y. Kanagasingam, E. Kruger and M. Tennat, The Journal of Evidence-Based Dental Practice, 16(3), 161-172 (2016).
4. J.T. Newton and K. Asimakopoulou, British Dental Journal, 223 (3), 147-150 (2017).
5. G. Shaefer, V. Pitchika, F. Litzenburger, R. Hickel and J. Kühnisch, Clinical Oral Investigations, 22 (7) 2431-2438(2018). (DiagnoCam)
6. M. Melo, A. Pascual, I. Camps, A. del Campo and J. Ata-Ali, Odontology, 105, 283–290 (2017) (Vistaproof)
7. U. Boye, A. Willasey, T. Walsh, M. Tickle and IA. Pretty, Community Dentistry and Oral Epidemiology Journal, 41,526-533 (2013). (SoproCare)
8. A. Jablonski-Momeni, F. Liebegall, R. Stoll, M. Heinzel-Gutenbrunner and K. Pieper, Lasers in Medical Science, 2881), 101-109 (2013). (VistaCam)
9. P. Bottenberg, W. Jacquet, C. Behrens, V. Stachniss and A. Jablonski-Momeni, BMC Oral Health, 16 (93),https://doi.org/10.1186/s12903-016-0291-z (2016).
10. C. Chossengros, L. Guyot, B. Mantout, F. Cheynet, P. Olivi and JL. Blanc, Revue de stomatologie et de chirurgie
maxillo-faciale, 111, 79-83 (2010).
11. M. Estai, Y. Kanagasingam, H. Boyen, J. Shiikha, E. Kruger, S. Bunt and M. Tennat, Telemedicine and e-Health, 23(5), 435-440 (2017).
12. More information can be found from http://www.ivoclarvivadent.com/en/p/all/products/clinical-accessories-instruments/application-aids/optragate

21

Live Cell imaging with integrated STED-AFM
1*Elnaz Fazeli , Takahiro Deguchi2*, Sami Koho2, Juha Peltonen3, Tuomas Näreoja4 and Pekka Hänninen1
1

Biophysics Laboratory, University of Turku, Turku, Finland2
Istituto Italiano di Tecnologia, Genoa, Italy
3
Laboratory of Cell Biology, University of Turku, Turku, Finland
4
Karolinska Institute, Stockholm, Sweden
Corresponding author: elnfaz@utu.fi* equal contribution

Keywords: Microscopy, osteoclast, super-resolution, AFM, STED
Multi-modal correlative microscopy allows combining the strengths of several imaging modalities while
overcoming the shortcoming of each individual technique. Furthermore, combining different techniques will
help gain better insight of the specimen. There are various super resolution techniques available to study nanoand sub-cellular organisms in great details. Atomic Force Microscopy (AFM) can provide, with atomic
resolution, nanomechanical properties of cells and material and play as a manipulator in order to get specificcellular response [1]. In addition, stimulated emission depletion (STED) super-resolution microscopy
technique has been used widely to study structures close to 10-20 nm size [2]. By using STED one can acquire
fluorescent images of labelled organisms in live, or fixed cells.
In this study we used a combination of these two techniques in order to study osteoclasts (OCs) adhesion. OCs
are bone resorbing cells, which play an important role in the bone remodeling process. They adhere to thesubstrate using specific actin structures called podosomes and sealing zone, if they are cultured on glass andbone, respectively [3]. If the OCs’ activity is disturbed, for example by using inhibitors of actin binding or
recycling, or if OCs are taken from diseased individuals with hyperactive osteoclasts, the behavior of cells andactin structures will change dramatically. We use the OCs from healthy donors and donors withneurofibromatosis type 1 (NF1) disease. NF1 is a progressive autosomal dominant disease cause by a genetic
mutation in the tumor suppressor Nf1 gene [4]. NF1 hallmark includes neurofibroma tumors, learning
deficiencies, skeletal abnormalities and disturbed bone growth starting from childhood [5]. it was presented
that NF1 preosteoclasts hyperactively migrate and adhere, leading to osteoclasts, which are bigger and have ahigher resorption capacity.
In this study we plan to fulfill two aims. Firstly, we use AFM as a cell manipulator to measure the adhesion
forces of healthy and NF1 OCs. We show that the hyperactivity of NF1 OCs is due to stronger attachment tothe bone, and therefore, due to differences in actin structures. We also compare attachment strength of OCs tothe bone and glass substrates. We do STED imaging of actin structures while manipulating the cells using
AFM microscopy to look into the formation of podosomes. Secondly, we use STED imaging to compare thedifferent actin structures; we characterize podosome and sealing zone size and distances to provide further
proof to the role of actin organization in overall cellular activity. We are currently analyzing the distance andthe structure of podosomes in comparison to sealing zone. Our result would possibly lead to the conclusionthat sealing zone is the mature form of podosomes and is formed by joining podosomes in order to provide
better attachment to the bone and more efficient resorption.
References
1. Raman, A, S Trigueros, A Cartagena, A P Z Stevenson, M Susilo, E Nauman, and S Antoranz Contera. 2011.
“Mapping Nanomechanical Properties of Live Cells Using Multi-Harmonic Atomic Force Microscopy.” Nat.
Nanotechnol. 6 (12): 809–14.M. Mainio, J. Juonio, and T. T. Tutkija, Optics Journal 12, 123-129 (1998).
2. Wildanger, Dominik, Brian R Patton, Heiko Schill, Luca Marseglia, J P Hadden, Sebastian Knauer, Andreas Schönle,et al. 2012. “Solid Immersion Facilitates Fluorescence Microscopy with Nanometer Resolution and Sub-Ångström
Emitter Localization.” Adv. Mater. 24 (44): OP309–OP313.
3. Teitelbaum, S. L. (2011). The osteoclast and its unique cytoskeleton. Annals of the New York Academy of Sciences,
1240, 14–17. https://doi.org/10.1111/j.1749-6632.2011.06283.x

22

4. Wallace, M R, D A Marchuk, L B Andersen, R Letcher, H M Odeh, A M Saulino, J W Fountain, A Brereton, J
Nicholson, and A L Mitchell. 1990. “Type 1 neurofibromatosis gene: identification of a large transcript disrupted inthree NF1 patients.” Science (New York, N.Y.) 249 (4965): 181–86.
5. Brunetti-Pierri, Nicola, Stephen B Doty, John Hicks, Kelly Phan, Roberto Mendoza-Londono, Maria Blazo, Alyssa
Tran, et al. 2008. “Generalized metabolic bone disease in Neurofibromatosis type I.” Molecular Genetics andMetabolism 94 (1): 105–11. https://doi.org/10.1016/j.ymgme.2007.12.004.

23

Monte Carlo based investigation of spectral reflectance and point
spread function in skin tissue model
Kaustav Das, Yuta Kobori, Tomonori Yuasa, Hideki Funamizu, and Yoshihisa Aizu
Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan
Corresponding author: aizu@mmm.muroran-it.ac.jp

Keywords: skin tissue, spectral reflectance, point spread function, Monte Carlo simulation
Spectral reflectance is used for estimating absorption and scattering of light in skin tissue, for example, byusing multi-variate analysis, and data mining from spectral reflectance database. However, the similar spectralreflectance curves can be obtained from some different combinations of absorption and scattering conditions.
In such a case, the estimation of absorption and scattering coefficients suffers from uncertainty. To treat thisproblem, we expect use of point spread function (PSF) observed in the skin surface under the point-like illumination. In this paper, we investigated properties of both spectral reflectance and PSF on the basis of Monte
Carlo simulation (MCS) [1] in the nine-layered skin tissue model [2]. Results potentially indicate that theabsorption coefficient in dermal and subcutaneous layers and the scattering coefficient in the upper dermal
layer predominantly affect patterns of spectra and PSFs.
A typical structure of human skin tissue can usually be classified into three layers such as epidermis, dermis,and subcutaneous layer in Fig.1(c). In the more detailed
and definite way, skin tissue is classified into nine layers as
it can be demonstrated in Fig. 1(b). These layers withparticular thickness are; layer 1 (L1): stratum corneum(0.015mm), layer 2 (L2): stratum granulosum and stratum
spinosum (0.060mm), layer 3 (L3): stratum basale(0.015mm), layer 4 (L4): papillary dermis (0.045mm), layer
5 (L5): subpapillary dermis (0.175mm), layer 6 (L6): upper
blood net dermis (0.080mm), layer 7 (L7): reticular dermis(0.600mm), layer 8 (L8): deep blood net dermis (0.100mm)
and layer 9 (L9): subcutaneous tissue (6.000mm). In MCS
Fig. 1 Skin tissue model of nine-layered
with the help of quite accurate and organized coding, theand three-layered types.
phenomenon of light propagation in human skin tissue has
been mimicked as genuinely as possible.
Table 1 Different magnification of M
aand Ms.
To carry out MCS, five parameters were required for each layer:
scattering coefficient µs, absorption coefficient µa, anisotropy scatLayers
MaMs
tering parameter g, refractive index n and thickness t. The fiveparameters for each of nine layers result in totally 45 values to beµa L1-3µs L1-3
1~3
specified for MCS execution. This is quite complicated and0.6~1.6
0.6~1.4
impractical for investigation of the spectral reflectance and PSF withµs L4-6
respect to changes in values of the five parameters. We propose a4~6
0.3~9.3µa L4-9moderate approach in which the two main parameters µa and µs are
0.2~3.6
varied by magnification while values of g, n, and t are fixed. Valuesµs L7-97~9of µa and µs are specified by magnification Ma and Ms with respect
0.1~1.0
to the initially-set values, respectively, as given in Table 1. Here wechange the magnification value Ma for the two groups of layers 1-3 and 4-9, and Ms for the thress groups oflayers 1-3, 4-6, and 7-9.
Figure 2 shows spectra and PSFs in 580 nm and 700 nm with different magnification values of Ma whichindicates different conditions of µa in dermal and subcutaneous layers (L4 – L9), keeping all other parameters
fixed to be the initial values [2]. With the change in Ma value from the small to large, the spectral reflectanceclearly decreases, especially in a wavelength range of 500-600nm. In PSFs, there was no change in the patternwith different values of Ma while difference of broadening is found between the two wavelengths. Figure 3
demonstrates spectra and PSFs in 580 nm and 700 nm with different magnification values of Ms which refers
to different condition of µs in the upper dermal layer (L4 – L6), keeping all other parameters fixed to be the initialvalues [2]. With an increase of Ms-value, the spectral reflectance becomes higher, especially in a wavelength24range of 500-600nm. In PSFs, low values of Ms produce spread out of intensity decaying curves and the higher
Ms values result in the narrow spread of curves.
Comparison of Fig. 2(a) and Fig. 3(a) indicates that it is difficult to discriminate in the cause of spectral variation between a change of absorption and a change of scattering. In such a case, observation of PSFs gives
useful information. When the scattering condition changes, we expect to observe changes in both spectralreflectance curve and PSF, while a change in the absorbing condition may be reflected on spectral reflectancecurve only.

We have investigated spectral reflectance and PSFs for various values of µa and µs. Change in the absorbing
coefficient in the dermal and subcutaneous layers had contribution to change in the pattern of spectral reflectance curves but had no contribution to the pattern of PSFs. Change in the scattering coefficient in the upper
dermal layer had contribution to change in the patterns of both spectral reflectance curves and PSFs. In thisway, use of both spectral reflectance and PSF is reasonable for estimation of absorbing and scattering conditions in skin tissue. We are developing a Monte Carlo based library of set of spectral reflectance curves and
PSF curves for various values of of µa and µs, and further investigate their properties to enrich these library
data.
References
1. L. Wang, S.L. Jacques, and L. Zheng, Comput. Methods Programs Biomed. 47, 131-146 (1995).
2. T. Maeda, N. Arakawa, M. Takahashi, and Y. Aizu, Optical Review 17, 223-229 (2010).

25

On optical sensing of microplastics in water
Benjamin O. Asamoah, James Amoani, Matthieu Roussey, and Kai-E. Peiponen
Department of Physics and Mathematics, University of Eastern Finland, P. O. Box 111, FI-80101 Joensuu, Finland
Corresponding author: benjamin.asamoah@uef.fi

Keywords: microplastics, surface roughness, specular reflection, laser speckle pattern, speckle contrast.
Despite the huge demand for plastics in today’s world, they largely contribute to water pollution [1]. Large
plastics in water bodies can easily be collected. However, for micro- and nanoplastics, their identification inwater bodies become challenging as a result of the varying shape, size, surface and volume properties, transparency and the surrounding ambient conditions. Advanced methods of plastic detection such as Raman spectroscopy, Fourier transform infra-red (FTIR) spectroscopy and the combination of spectroscopy and chemical
imaging exist [2]. However, such methods are expensive, not portable, and usually, require sediments fromwater bodies instead of direct implementation in water bodies. We present a prototype of an optical sensor forthe detection of microplastics in water. The sensor detects the specular reflection and the speckle pattern, fromwhich we can estimate the speckle contrast, to discriminate microplastics based on the smoothness, transparency, translucency, surface roughness, and their curvature.
The optical sensor, which is connected to a laptop, is a combinationof a portable handheld glossmeter (MGM device, Finland), formeasuring the specular reflection, and a charge-coupled device(CCD) camera (DCC1465C-HQ, Thorlabs), for capturing the
speckle pattern [3]. The schematics of the sensor is shown in Fig. 1.
The handheld sensor consists of a stable red (635 nm) semiconductor laser light focused to 50 µm at 6° angle of incidence. It uniquely
obtains the specular component of light reflection by filtering the
diffusely reflected light using a diffractive optical element (DOE)[4]. Moreover, to the sensor head, a black cup with a glass (disk)
base can be attached to create 800 µl volume compartment for holding liquid. The CCD camera was positioned 11 cm from the glass
disk. The commercial plastics studied are transparent polyethylene
terephthalate (PET) and translucent low-density polyethylene(LDPE). The plastics were cut into 5×5 mm2 to represent the upperbound of microplastic (MP) definition [5]. Additionally, to simulate
the case of MPs in water having different roughness on the two surfaces, some PET samples were roughened, before cutting, on bothsides with sandpaper having grits of 80, 180, 320, 600 and 1200.
Fig. 1. Schematics of the experimental setup.
The respective measured average surface roughness was 4.12 µm, 2.52 µm, 1.15 µm, 0.63 µm, and 0.33 µm
using Mitutoyo stylus profilometer (SJ-210, Japan). For the roughened MPs, they are accordingly referred toas G80D, G320D, and G600D, as an example, indicating which rough surface of the MP is toward the incident
light source. Next, pristine LDPE, PET and PET bottle (B MP), with regular curvature, were also cut to thesame size.

The volume compartment was filled with 700 µl of fresh water (FW) and the samples were added.
The MPs were studied with a glass disk having one of its surfaces roughened. In this case, with thecoherent light source, a speckle pattern, which is grainy structure of bright and dark spots, is formedin the far-field region as a result of the random phase originating from the rough glass surface. The
specular reflection and the speckle patterns were simultaneously recorded with the sensor. From the
speckle pattern, we calculate the normalized standard deviation to obtain the speckle contrast (C),which has a minimum and maximum value of 0 and 1, respectively, using Eq. (1) [3],1𝐶=⁄[〈𝐼2 〉−〈𝐼〉2 ] 2
,
〈𝐼〉

(1)

where I is the light intensity and <…> is the ensemble average. Fig. 2 (a) and (b) show the mean specularreflection and the speckle pattern for the pristine MPs with that of water subtracted, respectively. From thespecular reflection, which depends on the refractive index (RI) mismatch between the MPs and water (at the26

probe wavelength), we observe that the PET (RI = 1.5570) gives a higher average reflection signal than FW.
On the contrary, LDPE (RI = 1.4920) with volume inhomogeneity, we obtain a lower average reflection signal
as it scatters more of the light. Moreover, the B MP with inner curvature (concave) towards the incident (BMP CONC) light shows a higher signal than when the outer curvature (convex) is toward the incident light (BMP CONV). Thus, the B MP CONC tends to act as a converging mirror focusing the reflected light whilst the
B MP CONV, on the other hand, diverges the incident light beam. We also observe clear variation from the
speckle patterns of the pristine MPs when the speckle pattern from water (and glass) is subtracted. The PET,
in comparison, shows an almost dark pattern as expected for a transparent material whereas the LDPE showsa much brighter pattern. Interestingly, for the B MP (CONC and CONV), which is the same MP, we observe
clearly how the curvature contributes to the significant modification of the speckle pattern.

(a)

(b)

Fig. 2. Average specular reflection (a) and speckle patterns (b) water (FW) and pristine MPs of PET, LDPE and PET
bottle (B MPs) with that of water subtracted. (Conv)ex and (Conc)ave.
Tab 1. Speckle contrast of water and pristine MPs.
SampleFW
PET
LDPE
B MP CONVB MP CONCSpeckle contrast (C)
0.34
0.33
0.37
0.42
0.49

The speckle contrast, shown in Fig. 3, presents a
powerful tool in discriminating between the
different pristine MPs and their curvatures aswell as the average surface of the MPs. In Tab. 1,
the translucent LDPE MPs show a higher speckle
contrast than the transparent PET PET MPs as
the volume inhomogeneities act as scattering
Fig. 3. Speckle contrast of PET MPs with both sides roughened.
sources for the incident light. Thus, the pristine MPs further modify the original speckle pattern of the roughglass. As an example, we show, in Fig. 3, how the upper surface roughness, with reference to the incident lightsource, influences light and MP interaction for each specific lower surface roughness. We observe that the
speckle contrast decreases with increasing upper surface roughness. Moreover, decreasing the average surface
roughness of the lower surface correspondingly increases the speckle contrast especially for roughness larger
than the probe wavelength of the incident light source.
This study is a step toward the practical implementation of an optical sensor for in-situ detection of MPs in an
aquatic environment.
References
1. J. R. Jambeck, et al., Science, 347, 768-771 (2015).
2. M. G. J. Löder and G. Gerdts, Marine Anthropogenic Litter, (Springer, Cham, 2015).
3. R. K. Erf, Speckle Metrology, (Academic Press Inc, New York, 1978).
4. R. Silvennoinen, K.-E. Peiponen, and K. Myller, Specular Gloss, (Elsevier Science, Amsterdam, 2008).
5. More information can be found in: http://mcc.jrc.ec.europa.eu/documents/201702074014.pdf, valid 2/201927

Development of self-adjusting photodiode arrangement for accurate
laser optical power measurements
A. Pokatilov1, G. Porrovecchio2 and T. Kübarsepp1
1

AS Metrosert
Central Office for Metrology, Tallinn, Estonia
2
CMI
Czech Metrology Institute, Prague, Czech Republic
Corresponding author: tkubarsepp@metrosert.ee

Keywords: optics, semiconductor physics, photodetectors
One of the tasks in the EURAMET/EMPIR project 17FUN06 SIQUST ’Single‐photon sources as new quantum standards’ [1] is to develop the measurement devices for the traceable characterisation of single-photon
sources. To reduce noise in such a low optical flux detection, a photodetector with improved performance,
e.g. zero back-reflection and decreased noise, has been designed.
The zero-back reflection is achieved by aligning six
photodiodes in such a way that nonabsorbed lightreflected from one photodiode is directed to the next
photodiode. In the device six commercial photodiodes having small active area (2.4 mm x 2.4 mm
type S1227 33BR from Hamamatsu) are aligned in apolarization-independent configuration (Fig. 1). The
careful design provides collinear input and output
beams of so-called transmission type detector. The
noise is reduced by cooling (down to 12 ºC) thephotodetector compartment in conjunction with high Fig 1. Schematic view of the six photodiodes assemaccuracy readout electronics. The aimed operation bled in the 6-photodiode detector. The red line folrange is in the wavelength range from 650 nm to lows the beam between the photodiodes.
750 nm.
The collinear input and output beams to better than ± 0.5º are provided by the use of a piezoactuator connected to the feedback sensor at the photodetector optical output. The device with improved performance
will be described and presented.
This work has received funding from the European Union Horizon 2020 and the EMPIR Participating Statesin the context of the projects EMPIR-14IND05 ‘MIQC2’ and EMPIR-17FUN06 “SIQUST”.

References
1. SIQUST ‘Single‐photon sources as new quantum standards’, https://www.siqust.eu/

28

Polarimeter analysis of the birefringence of thermally modified Scots
pine wood
Ilpo Niskanen1*, Hiroshi Hasegawa2, Jukka Räty3, Hariyadi Soetedjo4, Kenichi Hibino5, Hiroshi Oohashi6,
Rauno Heikkilä1, Kiyofumi Matsuda7, and Yukitoshi Otani2
1

Faculty of Technology, Structures and Construction Technology, University of Oulu, Finland
Department of Optical Engineering / Center for Optical Research and Education, Utsunomiya University, Japan
3
Unit of Measurement Technology (MITY), University of Oulu, Kajaani, Finland
4
Center for Integrated Research and Innovation (CIRNOV), University of Ahmad Dahlan, Yogyakarta, Indonesia
5
National Institute of Advanced Industrial Science and Technology/National Metrology Institute of Japan, Tsukuba,Japan
6
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
7
Optical Information Processing and Systems Engineering Division, Hamamatsu, Japan.
*Corresponding author: email ilpo.niskanen@oulu.fi
2

Keywords: light polarization, wood heat treatment, birefringence, imaging Stokes polarimeter, Mueller matrix
Wood is environmentally friendly material which can use for many purposes. However, some of its properties
such as the dimensional stability change along with ambient condition. The characteristics of wood can be
modified by different ways. Thermal treatment (at temperatures 185-215 0C) is one of the methods that has
been used to improve wood’s durability, resistance against mold and decay fungi, dimensional stability,
weather resistance, and visual appearance [1-2]. The method is commonly used by wood industry and, forexample, applied for c. 194,000 m3 of sawn timber in Finland in 2017 (Thermowood® production statistics
2017) [3]. There is a practical need for easy-to-use, inexpensive and rapid analysis method that could be applied for defining the treatment degree from wood specimens after the thermal modification. This would help
monitoring the quality of thermally modified wood in the markets.

Fig.1 Experimental setup of the Stokes Polarimeter.

Here we assume that the thermal modification alters chemical composition of the wood cells and thus change
and decrease the birefringence of molecular structure. This paper describes a light polarization method to determine the loss of birefringence of the thermally modified wood material. The polarized light reflected fromthe surface of thermally modified Scots pine wood was measured by using an imaging Stokes polarimeter anddata analysis utilized the Mueller matrix method. The polarization state of light was characterized by fourmeasurable parameters called the Stokes polarization parameters (S0-S3). [4]. Our detection system consists
29

of a dual rotating speed of retarder and analyzer at a rotating ratio of 1:3 in Fig.1 [5]. The CCD camera was
Prosilicga GS1380 which resolution 1360 x 1024 pixels. The wavelength of the light source (He-Ne laser) was
632.8 nm.
The degree of polarization as a function of temperature for untreated and thermally modified Scots pine samples are presented in Fig. 2. Light and dark stripes in Fig. 2 represent annual growth rings. The density of theearlywood (thinner, lighter stripes) is higher than the density of the latewood (wider, darker stripes). Furthermore, from the figures we can observe that the degree of polarization (DOP) increases (lighten) when thetreatment temperature rises. This means that the wood fiber loses birefringence property. The loss of birefringence could be due to crystal to amorphous deformation.

Fig. 2. Stokes image (DOP) of Scots pine wood a) untreated, b) treatment at160 °C c) treatment at 200 °C, andd) treatment at 220 °C).
The present method could provide useful information that, in principle, can be applied in the optimization of
the thermal modification process and inspection of the quality of thermally modified wood products. Major
advantages of imaging Stokes polarimeter are that the method is sensitive and nondestructive for measuring
the optical properties exhibited by thermal modified wood or untreated wood. We believe that the imaging
Stokes parameter method and the polarimeter are useful in the basic studies of optical properties of thermallymodified wood. Using the polarization information it is possible to assess changes in the crystalline forms ofcellulose of thermally modified wood as a function of temperature and the treatment time.

References
1. M. Romagnoli V. Vinciguerra and A. Silvestri, J. Wood Chem. Technol. 38, 57–70 (2018).
2. B. E. B. Uribe and A. Ayala, J. Indian Academy of Wood Sci 12, 54-62 (2015).
3. Thermowood® production statisticsInternational Thermowood® Association. 2017 https://asiakas.kotisivukone.com/files/en.thermowood.palvelee.fi/uutiset/Productionstatistics2017.pdf, Accessed 15 February 2019
4. M. Ginya, Y. Mizutani , T. Iwata and Y. Otani, Sens Actuators A, 200, 37–43 (2013).
5. M. Tanaka, M. Chujo and Y. Otani, Kogaku 40, 430-438 (2011).

30

How to control polarization color and its applications
Kenji Harada, Sho Sakurai, and Yutaro Shibata
Department of Computer Science, Kitami Institute of Technology,
165 Koen-cho, Kitami, Hokkaido 090-8507, Japan
Corresponding author: haraken@cs.kitami-it.ac.jp

Keywords: Birefringence, polarization, polarization color
A birefringent cellophane tape or a polymer sheet exhibits polarization color (interference color) by polarizer.
We propose a novel technique for viewing images using polarization [1, 2]. The polarization color is controlled by changing the total retardation of the birefringent materials. Various colors can be displayed without color filters, which can be applied to color imaging device. There are various ways to control the polarization color. In this presentation, we show how to control the polarization color, and its applications for artsand entertainments.
Figure 1(a) shows the conventional optical arrangement. The angle between the transmission axis of a polarizer and the retarder axis of a retarder film is 45°or 135° because the maximum transmittance is obtained atthese angles. Figure 1(b) shows the proposed optical arrangement. The angle between the transmission axisof a polarizer and the retarder axis of a retarder film can be varied, and up to four retarder films are stacked.
Using this proposed setup, we can display a high chroma image.

(a) General optical composition(b) Proposed optical composition

Fig. 1. Optical composition for viewing polarization color image.
Figure 2 shows a color chart of sixty colors using polarization color. The color chart is fabricated by sandwiching up to four retarder films between two polarizing films. We used two types of retarder film with retardations of 140 and 565 nm. Conventional /4 retarder films are used for the 140 nm retarder films, andconventional  retarder films are used for the 565 nm retarder films. In this way, we can display various highchroma colors. Figure 3 shows a prototype of active polarization color filter. This device consists of four retardation films (two /4 retarder films and two  retarder films), four stepping motors and a microcomputer
to control them.

Fig. 2. Color chart using polarization color.

31

Fig. 3. Prototype of active polarization color filter.

In recent research, we found that we can control not only the color but also the brightness using optical composition shown in Fig.1 (b). Figure 4 shows a calculation result of brightness control of polarization color(red, green and blue). We calculated about arrangement of all combinations of retardation films, and sorted
in order of brightness by color. The brightness was calculated using the L*a*b* color system.

Fig.4. Brightness control (simulated value).

In this way, we can control the polarization color and brightness. This technique can be also applied for art
and entertainment. Figure 5 shows a high chroma polarization art. This is the original design art created withthe hint of the Japanese floral playing cards “Hanafuda”. Figure 6 shows another type of polarization art.
This device is composed of retarder films, Aluminum rolls and polarizers. Aluminum rolls make interestinggeometric patterns. Figure 7 shows a transparent jigsaw puzzle for entertainment. This technique is very
simple and can be applied to color filter, educational tools, security, art and entertainment.

(a) Without polarizer(b) With polarizer

Fig.6 Polarization color art 2.

Fig.5 Polarization color art 1.

Fig.7 Transparent puzzle using polarization color.
References
1. S. Sugawara, K. Harada, and D. Sakai, “High-Chroma Visual Cryptography Using Interference Color of High-Order
Retarder Films”, Optical Review, 22 (2015) 544-552.
2. K. Harada, T. Matsuzaki, H. Qin, “Stereoscopic Imaging System Using Interference Color of Retarder Films”, Optik
156 (2018) 914–920.

32

Predicting the refractive index of porous powder samples during
compaction using terahertz time-domain spectroscopy
Prince Bawuah1, Mohammed Al-Sharabi1, Cathy Ridgway2, Anssi-Pekka Karttunen3, Ossi Korhonen3,
Patrick Gane4, Jarkko Ketolainen3, Kai-Erik Peiponen5, J. Axel Zeitler1, Daniel Markl6,7
1

Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, UK
2
Omya International AG, Oftringen, Switzerland
3
School of Pharmacy, Promis Centre, University of Eastern Finland, Kuopio, Finland
4
Chemical Engineering, Bioproducts and Biosystems, Aalto University, Helsinki, Finland
5
Institute of Photonics, University of Eastern Finland, Joensuu, Finland
6
University of Strathclyde, Strathclyde Institute of Pharmacy and Biomedical Sciences, Glasgow, UK
7
EPSRC Future Manufacturing Research Hub for Continuous Manufacturing and Advanced Crystallisation (CMAC),
University of Strathclyde, Glasgow, UK
Corresponding author: pb612@cam.ac.uk

Keywords: terahertz, refractive index, spectroscopy, compaction
Abstract: In this study, a terahertz (THz) based method for accurate prediction of the refractive index during
the compaction of porous compacts is introduced. By using training sets of porous particle functionalised
calcium carbonate (FCC) based and microcrystalline cellulose (MCC) based compacts, the refractive index of
the powder in the compaction die (henceforth referred to as in-die refractive index) was accurately predicted
and compared to the measured out-of-die refractive index. Since the refractive index is a fundamental physicalproperty of a material, the ability to track how it evolves during powder compaction will increase ourunderstanding of the overall compaction process and inform the development of compaction models.
Background: Recent studies have shown that terahertz time domain spectroscopy (THz-TDS) coupled with
Bruggeman’s effective medium theory is a powerful tool for measuring the refractive index of porous materials[1–2]. The ability to accurately measure the refractive index, which is a basic optical property of a material,
serves as the basis for model development that can be used for non-destructive and non-invasive processmonitoring and material characterisation.
This study uses a THz-TDS method [2] that facilitates the estimation of the in-die refractive index similar to
the experimental setup proposed in [3]. This was possible by, firstly, measuring the intrinsic refractive index,
nsolid, of calibration compacts using the anisotropic Bruggeman effective medium approximation (AB-EMA)
followed by the estimation of the in-die effective refractive index.
Materials and Method: Eight batches of porous compacts were compressed from porous particle
functionalised calcium carbonate (FCC) and microcrystalline cellulose (MCC) based formulations as shownin Table 1. The compacted samples were flat-faced with a diameter and thickness of 10 mm and 1.6 mm,
respectively. The four formulations were compacted to two different porosities. By following the wellestablished THz porosity measurement approach [2], the intrinsic refractive index of the solid matrix was
estimated for both materials. The intrinsic refractive index, nsolid, coupled with the known in-die porosity, findie, were used to extract the in-die effective refractive index based on the AB-EMA. The in-die porosity, fin-die,
was calculated from the known mass, measured volume change and known true density of the solid material.
Table 1: Material composition of the formulation used in the direct compression of all the batches of compacts.
The target compact porosities, f, and the intrinsic refractive index, nsolid, are given for each formulation.
Formulation
Material
nsolidf
FP
Functionalised calcium carbonate (88 %), croscarmellose 45%, 60%
2.697
sodium (2 %) and paracetamol (10 %)FTFunctionalised calcium carbonate (88 %), croscarmellose 45%, 60%
2.714
sodium (2 %) and theophylline (10 %)MP
MT

Microcrystalline cellulose (88 %), croscarmellose sodium 10%, 25%(2 %) and paracetamol (10 %)Microcrystalline cellulose (88 %), croscarmellose sodium 10%, 25%(2 %) and theophylline (10 %)

33

1.869
1.864

Results: The extracted in-die effective refractive index, neff.in-die, plotted against the compression pressure andthe in-die porosity, fin-die, respectively, for both the FCC based and MCC based compacts is shown in Fig. 1.
For the sake of comparison, the neff.in-die at maximum pressure (neff.in.max) and the measured out-of-die effective
refractive index (neff.out) are also shown. It is interesting to observe that the out-of-die refractive indices showed
an average decrease in magnitude of 0.004 compared to the in-die refractive indices at maximum pressure. The
decrease in magnitude of the out-of-die refractive indices can be attributed to after-compaction elastic
relaxation of the compacts. Another intriguing observation is the Heckel-like nature of the refractive index
curves shown in (a), which buttresses the concept of optical Heckel analyses that was introduced in ourprevious study [3].
2.2

(a)

2

2

1.8

1.8

n eff.in.max

1.6

n eff.out

1.4

FP 45%FP 60%FT 45%FT 60%

1.2
1n eff.in-die (-)

n eff.indie (-)

2.2

0

100

200

300

MP 10%MP 25%MT 10%MT 25%
400

500

Compression pressure (MPa)(b)

FP 45%FP 60%FT 45%FT 60%MP 10%MP 25%MT 10%MT 25%n eff.in.max
n eff.out

1.6
1.4
1.2
1

0

0.2

0.4

0.6

0.8

1

fin-die (-)

Fig. 1. The evolution of the effective refractive index of porous compacts during compression. FP and MP
indicate FCC based and MCC based compacts containing paracetamol as the active ingredient whereas FT
and MT represent similar compacts with theophylline as the active ingredient.
Conclusions: This study highlights that it is possible to track how the refractive index evolves during powder
compression. By using a prepared training set of powder compacts, the in-die refractive index was extracted
and compared to the measured out-of-die refractive index. The relatively low out-of-die refractive index values
compared to the in-die counterpart has manifested the effect of elastic relaxation on the optical properties ofcompacts. The ability to detect the in-die refractive index during powder compaction will increase ourunderstanding of the overall compaction process by tracking the pore shape and orientation in real time basedon the well-established shape and structure parameter as described previously [2, 4].
References
1. D. Markl, P. Wang, C. Ridgway, A.-P. Karttunen, P. Bawuah, J. Ketolainen, P. Gane, K.-E. Peiponen, J.A. Zeitler,
Chem. Eng. Res. Des. 132,1082–1090 (2018).
2. D. Markl, P. Wang, C. Ridgway, A.-P. Karttunen, M. Chakraborty, P. Bawuah, P. Pääkkönen, P. Gane, J. Ketolainen,
K.-E. Peiponen, J.A. Zeitler, J. Pharm. Sci. 106, 1586–1595 (2017).
3. P. Bawuah, A.-P. Karttunen, D. Markl, C. Ridgway, O. Korhonen, P. Gane, J. A. Zeitler, J. Ketolainen, K.-E. Peiponen,Int. J. Pharm. 544, 278–284 (2018).
4. D. Markl, P. Bawuah, C. Ridgway, S. van den Ban D.J. Goodwin, J. Ketolainen, P. Gane, K.-E. Peiponen, J. A. Zeitler,
Int. J. Pharm. 537, 102–110 (2018).

34

Ambient air spectroscopy and thermometry for accurate distance measurement
Jeremias Seppä and Tuomas Helojärvi
VTT Technical Research Centre of Finland Ltd, Centre for Metrology MIKES,Espoo
Corresponding author: jeremias.seppa@vtt.fi

Keywords: spectroscopy, air, thermometry
Accurate position and distance measurements over tens of metres are needed in e.g. manufacturing and assembly of large components and structures in e.g. aerospace and automobile industry, civil engineering and power
generation. Optical measurement systems are usually selected for provision of a versatile and feasible position
or distance measurement system - but optical distance measurements suffer from dependency on the effective
refractive index of the ambient medium (air) that relates the optical distance to the true, or “mechanical” distance.
When targeting micrometre accuracies over tens of metres distances and longer, usually the most difficult
parameter to measure that affects the average air refractive index over the optical measurement path is theaverage temperature, or effective temperature. For this reason VTT MIKES has already developed methods
for spectroscopic measurement of air temperature in relation to optical distance measurements [e.g. 1-2].
The methods are based on measuring the absorption of laser signal, which is finely tuned in optical frequency
to match a chosen resonance frequency or frequencies of molecular oxygen. From the absorbance, it is possible
to deduce the effective temperature along the absorption path, which can span up to dozens [1] or even hundreds of meters [2]. Optical distance measurements along this path can then be performed with greater precision, with the acquired knowledge of the average refractive index along the measurement path.
In the recently started LaVA project [3-4] we aim to develop faster and more robust measurement than previously, with more compact and cheaper measurement setups that can be integrated with e.g. distance measurement telemeters [5] using retroreflective targets. The integration of the thermometer with a telemeter laser
distance measurement head is to be tested during the project, along with development, verification and comparison measurements with other air thermometry systems. Figure 1 shows a system schematic.
The presented thermometry work is performed within the project Large Volume Applications, which is jointly
fundedby the EMPIR participating countries within EURAMET and the European Union.
And finally, a heads up, we plan to present also something about a new rather fenomenal application of lasersin ambient air that is being developed for commercialization at VTT. The possibilities of the techonology havebeen already discussed in Japan with some potential partners.

Fig. 1. System schematic for air temperature measurement35References
1. T. Hieta, M. Merimaa, M. Vainio, J. Seppä and A. Lassila, “High-precision diode-laser-based temperature measurement for air refractive index compensation,” Appl. Opt. 50, pp. 5990–5998, 2011
2. T .Tomberg. T. Fordell, J. Jokela. M. Merimaa and T.Hieta, “Spectroscopic thermometry for long-distance surveying”
Appl. Opt. 56, pp. 239–246, 2017
3. Large volume metrology applications (LaVA) project page, available at https://www.euramet.org/research-innovation/research-empir/empir-calls-and-projects/call-2017-industry-fundamental-normative-research-potential-supportfor-impact/, cited on 2019-02-28
4. Large volume metrology applications (LaVA) project page, available at https://www.researchgate.net/project/LaVALarge-Volume-Applications, cited on 2019-02-28
5. J. Guillory. R. Smid, J. Garcia-Marquez. D. Truong, C. Alexande and J.-P. Wallerand, “High resolution kilometricrange optical telemetry in air by radio frequency phase measurement” Rev. Sci. Intrum.. 87, 075105, 201636

Label-free and vibration-free imaging using low-coherent quantitative
phase microscope
Natsuki Suzuki1,2 and Katsuhiro Ishii1
1

The Graduate School for the creation of new photonics industries, Hamamatsu, Japan
2
Hamamatsu Photonics K.K., Iwata, Japan
Corresponding author: natsuki.suzuki@gpi.ac.jp

Keywords: low-coherence interferometry, cell biology, quantitative phase imaging
Cell morphology and its dynamics are widely studied intrinsic properties of biological cells. For example, themotility of cancer cells correlates with their metastatic properties [1], and the membrane fluctuation of red
blood cells correlates with their membrane stiffness [2]. In order to study image cell morphology withoutlabeling, imaging techniques exploiting the interference of the optical field were applied to living cells and arecalled quantitative phase imaging (QPI), quantitative phase microscopy (QPM), or digital holography microscopy (DHM) [3-5].
Regarding the coherence of a light-source, researchers have proven that spectrally and spatially incoherent
illumination provides better image quality [6]. A conventional but powerful tool for providing an interference
microscope with spectrally and spatially incoherent illumination is a two-beam interference microscope withindependent reference and sample arms [7]. However, two-beam interference microscopes used to be a large
and heavy.
In this paper, we introduce a compact and standalone QPM setup with independent reference and sample arms.
Due to an optical path-length stabilization system implemented in the setup, our setup realized sub-nanometer
reproducibility in the optical thickness even without optical tables. In terms of the spatial uniformity, our setup
provided 1-2 nanometers of background flatness owing to the spatial incoherency of the illumination.
Figure 1 shows a schematic illustration of quantitative phase microscope system. The observation light fromthe LED light source is split by the beam splitter, reflected from the sample and reference mirror, and then
combined again to form interference fringes on the imaging surface of the camera. The test cells are cultured
on the half mirror, and the sample light is specularly reflected from the half mirror immediately below the test
cells after passing through the test cells, and interferes with the reference light reflected from the reference
mirror. The interference image is taken with phase shift by λ / 4, and the phase image of the cell can be obtainedby calculation of φ = arctan ((I3-I1) / (I2-I4)). A part of the interference light is guided to a photo detector (PD),and an electrical signal from the PD is input to a control circuit that performs phase shift detection and feedback
control to a desired phase. Our control circuit achieves stabilization of the optical path length by controlling
the reference mirror with a 5 kHz feedback circuit while detecting the phase shift at a sampling rate of 20 kHz.
Figure 2 shows the imaging results of cultured cell line HeLa cells (derived from human cervical cancer).
Figure 2 (a) is a quantitative phase image, and (b) and (c) are emulations of a phase contrast image and adifferential interference image created by image processing of the spatial phase of the quantitative phase image
and spatial frequency filtering, respectively. The use of LED incoherent lighting provides a precise image
without speckle noise. The objective lens used for imaging was a 10X objective lens (NA = 0.45).
We achieved quantitative phase imaging with a two-beam interferometer by using an optical path length stabilization system in an environment without an air table. Our compact system can be applied to the imaging ofmany biological samples and industrial samples because it can be operated with a reduction in the optical path
length even on a normal laboratory desk.

37

Controlcircuit
Camera

L1

PD

OB2
PZT

LED
RM

OB1

L2

SampleX, Y, α, β stage

Fig. 1: Schematic illustration of quantitative phase microscope system. PD: photodetector, PZT: piezoelectric transducer, RM: reference mirror, OB: objective lenses, BS: beam splitter, L: lens.

radians
(a)

5
4

(c)

(b)

3

2
1
0

Fig. 2. Full-field images of HeLa cells. (a): Quantitative phase image. (b): Emulated phase contrast image constructed from (a). (c): Emulated differential interference contrast image constructed from (a).
References[1] A. W. Partin, J. S. Schoeniger, J. L. Mohler et al., “Fourier analysis of cell motility: correlation of motility withmetastatic potential,” Proc Natl Acad Sci USA, 86(4), 1254-8 (1989).
[2] G. Popescu, T. Ikeda, K. Goda et al., “Optical measurement of cell membrane tension,” Phys Rev Lett, 97(21),218101 (2006).
[3] H. Iwai, C. Fang-Yen, G. Popescu et al., “Quantitative phase imaging using actively stabilized phase-shifting lowcoherence interferometry,” Opt Lett, 29(20), 2399-401 (2004).
[4] Z. Wang, L. Millet, M. Mir et al., “Spatial light interference microscopy (SLIM),” Opt Express, 19(2), 1016-26(2011).
[5] P. Marquet, B. Rappaz, P. J. Magistretti et al., “Digital holographic microscopy: a noninvasive contrast imaging
technique allowing quantitative visualization of living cells with subwavelength axial accuracy,” Opt Lett, 30(5),468-70 (2005).
[6] S. Shin, K. Kim, K. Lee et al., “Effects of spatiotemporal coherence on interferometric microscopy,” Optics
Express, 25(7), 8085-8097 (2017).
[7] M. K. Kim, [Digital Holographic Microscopy] Springer, (2011).

38

Setup for characterising the spectral responsivity of
Fabry-Pérot-interferometer-based hyperspectral cameras
Oskari Pekkala1 , Tomi Pulli1 , Alexander Kokka1 and Erkki Ikonen1,2
1

Metrology Research Institute, Aalto University, Espoo, Finland
2
VTT MIKES, Espoo, Finland
Corresponding author: oskari.pekkala@aalto.fi

Keywords: hyperspectral imaging, spectral responsivity, camera calibration, Fabry-Pérot-interferometer
Hyperspectral cameras collect radiance data of the imaged view via dozens of narrow-bandwidth spectral channels, as opposed to the three color channels of common digital cameras. Knowledge of the spectrum, i.e.
spectral power distribution, can be used for example in identification and classification of the materials in theimage. Hyperspectral imaging has applications in many fields, for example in agriculture [1] and medicine [2].
A reliable characterisation of the spectral responsivity of the channels is essential for processing hyperspectral
data and when assessing its reliability. Different camera architectures set different requirements for studied
camera features and characterisation measurements. The camera under study was a Fabry-Pérot-interferometerbased camera, which required the characterisation of its channels both spectrally and spatially.
This study presents the measurement setup (figure 1)
and analysis for characterising the spectral responsivity of hyperspectral cameras. The main part of the
setup was a tuneable, monochromatic radiance source.
The camera under study was mounted on a motorised
turntable, imaging an illuminated diffuser at different
wavelengths and from different viewing angles. The
spectral responsivities of the camera channels were extracted from the pixel values of the images, yielding results both as a function of wavelength and as a function of image plane coordinates. The results were validated by imaging an incandescent lamp-based radiancesource with a known spectrum.

monochromatorbaffleparabolic mirrors

radiation
sourceentrance slit

filter wheel

gratingturret exit slit

diffuserturntable

cameraFigure 1: Measurement setup.

The setup was used in characterising the spectral responsivity of a Fabry-Pérot-interferometer-based hyperspectral camera. The results show that the bandwidth and the centroid wavelengths of the channels change acrossthe image plane, and that some of the channels have regions of sensitivity outside the main channel. Channel
centroid wavelengths shift up to 1.5 nm when moving from the image centre to edges of the image, as shownin figure 2. The bandwidth of the channels changes by up to 10% in the image area and with different patterns
for each channel. The validation of the results yielded a root mean square error of 3.6% between the reference
and the measured spectral power distributions. The expanded uncertainties (k = 2) for the measured camera channel characteristics, namely bandwidths, sensitivities and wavelengths, were 7.9%, 9.5% and 0.64 nm,
respectively. The measured spectral responsivities of the channels are shown in figure 3.
The results confirm the viability of the measurement setup for hyperspectral camera calibrations. A more advanced calibration facility will be implemented at the Metrology Research Institute based on the described
setup.

39

Figure 2: Wavelength shift in the image plane.

Relative spectral responsivity

1.2
1.0
0.8
0.6
0.4
0.2
0.0

450

500

550

600

650

700

750

800

850

900

950

Wavelength / nm

Figure 3: Relative spectral responsivities of the measured camera channels.

References[1] M. Teke, H. S. Deveci, O. Haliloğlu, S. Z. Gürbüz, and U. Sakarya. A short survey of hyperspectral remotesensing applications in agriculture. In 2013 6th International Conference on Recent Advances in SpaceTechnologies (RAST), pages 171–176, June 2013.
[2] Guolan Lu and Baowei Fei. Medical hyperspectral imaging: a review. Journal of Biomedical Optics,
19(1):010901, 2014.

40

Comparison of experimental and the Mie scattering coefficients of bulk
suspensions of ideal and deformed cylindrical particles
Harri J. Juttula1, Matti Törmänen2 and Anssi J. Mäkynen3
1Unit of Measurement Technology, University of Oulu, Kajaani
2
Valmet Automation Inc., Oulu
3
Optoelectronics and Measurement Techniques Unit, University of Oulu, Oulu
Corresponding author: harri.juttula@oulu.fi

Keywords: cellulose, scattering
Paper and paperboard are made of paper machine furnish which consists of water suspended mixture of paper
pulp and various additives. Paper pulp consists of cellulose fibers extracted from trees and additives may
include dyes or inorganic fillers like kaolin, calcium carbonate, talc or titanium dioxide, for example. One of
the most important control parameters of paper making is the total mass fraction, or total consistency, of thefurnish which is defined as a mass percentage of all solid material of the furnish. Today, there are a number ofcommercial online instruments which measure the mass fraction. Measurement principles may include, forexample, attenuation and scattering of optical signals, the attenuation of microwaves or mechanical shear force
detectors [1]. Besides the total mass fraction, modern paper making needs online information about massfractions of fibers and fillers separately. Some experimental optical methods are capable of providing separate
mass fractions but may not be practical for industrial use [2, 3]. Alternatively, the existing optical online
instruments can be tuned to provide separate fiber and filler mass fractions. However, the wide variation infiber and filler materials make this kind of tuning very complex and difficult to conduct reliably. For example
depending on fiber physical dimensions, fibrillation and the amount of fiber fines may lead to widely different
results with optical methods. One possible solution would be to utilize quality information about the fibers andthen resolve their effect on optical mass fraction sensors by using precise optical modelling based on thematerial properties. In this work, we have studied thenecessary preconditions for this kind of approach bycomparing experimental scattering properties of ideal andnatural (deformed) fibers and theoretical values based on theirsize distribution as a function of the mass fraction.
Cylindrical fibers with their diameter in the micrometer size
scale interact with light via elastic scattering and absorption
which can be modelled with the Mie scattering theory [4]. Mie
scattering provides solutions for the scattering and absorption
properties of spherical or infinite cylindrical particles as afunction of the wavelength, particle radius and complex
refractive index. Cellulose fibers used in paper making,however, are far from ideal cylinders and they suffer fromdeformations such as bending, flattening, axial twisting or
kinks when compared with ideal cylinders. Simple cylindricalscattering models cannot explain the scattering properties ofsingle fibers. Furthermore, the orientation angle of the fiber
axis with the incidence light affects the scattering efficiency
and cross section of the fiber. We assume that the effects ofstructural deformations and orientation on the bulk scattering
properties of pulp can be taken into account if we treat thediameter and orientation as probability density functions andintegrate Mie solutions over them. With this approach, we canpredict the bulk scattering properties of fiber suspensions as afunction of the mass fraction.
In order to test our assumption we chose cellulose fibers(bleached Nordic pine) used in paper mill and artificial rayon
fibers to represent deformed and ideal cylindrical particles asshown in Figure 1. We determined their cross sectional

41

Figure 1. a) Natural cellulose fibers
resembling flattened deformed cylinders andb) near ideal cylindrical artificial rayon
fibers.

diameter distribution from microscope images and used alog-normal distribution to fit and represent the fibers in
Mie calculations. We used the log-normal diameter
distributions with an additional assumption of isotropicalorientation of the fibers together with the Mie theory inorder to solve the scattering properties of bulk fiber
suspension as a function of the total mass fraction.
We compared the Mie results based on fiber statisticalmorphology with the experimental results of two
independent measurements of fiber suspensions with avarying mass fraction simulating the real life processenvironments. Closed loop circulation measurement setup
was used to examine the sample fibers in aqueous
solutions between 0.1% and 1% mass fractions. The setup
is shown in Figure 2. Two measurement devices were Figure 2. Measurement setup.
attached to the circulation system to provide independent
results. First device used double integrating spheres and narrow flow through cuvette in conjunction with awell known IAD program [5] to determine the scattering coefficient of the sample suspension from themeasured reflectance and transmittance. Second device measured the scattering coefficient by analyzing thebackscattering of a laser with 45 degree angle of incidence [6] with the sample suspension in a cuvette withlarge turbulent volume. Both devices operated at 660 nm wavelength. Two devices were used in order toeliminate the possible orientation effect on the scattering coefficient in the narrow cuvette between integrating
spheres. Measurement results shown in Table 1. were in agreement with each other and were close to the values
determined from the fiber cross sectional diameter distribution with the Mie theory. Both fiber suspensions
showed a linear response of the scattering coefficient to the total mass fraction.

Our experiments show that simple cylindrical scattering model holds true for the ensemble of fibers if theirdiameter is represented with a probability distribution and that the scattering coefficient has a linear responseto the total mass fraction. This is important as many industrial measurements based on the scattering of lightare non-linear by nature and can behave in unpredictable manner due to changes in fiber morphology. While
our experiments were limited to pure fibers with no fillers or fines and no fibrillation these can be taken into
account as separate scatterers which obey their own size distribution and number density. Results also indicate
that, if available, qualitative morphological data could be used to improve the analysis of optical quantitative
measurements.
References
1. B. G. Liptak, "Instrument Engineers' Handbook, Volume One: Process Measurement and Analysis," 4th ed., CRC
Press, 2003.
2. M. Törmänen, J. Niemi, T. Löfqvist, R. Myllylä, "Pulp consistency determined by a combination of optical andacoustical measurement techniques," Meas. Sci. Technol., Vol. 17, 2006.
3. Z. Zhao, M. Törmänen, R. Myllylä, "Backward-mode photoacoustic transducer for sensing optical scattering andultrasonic attenuation: determining fraction consistencies in pulp suspensions," Meas. Sci. Technol., Vol. 21, 2010.
4. C. F. Bohren, D. R. Huffman, "Absorption and scattering of light by small particles," Wiley, New York, 1983.
5. S. A. Prahl, M. J. C. van Gemert, A. J. Welch, "Determining the optical properties of turbid media byusing the adding doubling method," Appl. Opt. Vol. 32, pp. 559–568, 1993.
6. H. J. Juttula, T. P. Kananen, A. J. Mäkynen, "Instrument for measurement of optical parameters of turbid media byusing diffuse reflectance of laser with oblique incidence angle", IEEE Trans. Instrum. Meas., vol. 63, no. 5, pp. 1301-1309, May 2014.

42

Development of metrological atomic force microscope
Virpi Korpelainen, Johan Nysten, Jeremias Seppä and Antti Lassila
VTT MIKES, Espoo, Finland
Corresponding author: Virpi.korpelainen@vtt.fi(Optional small paragraph describing the additional information concerning the authors may be added here, extending to several lines if needed)

Keywords: metrology, AFM, measurements of nanostructures, uncertainty estimates
Traceability to the SI metre is the basis for dimensional measurements in the nanoscale. Atomic Force Microscopes (AFM) are used to measure surface features with subnanometre resolution. Unidirectional measurements, e.g. of grating pitch and step height, are partly self-compensating and can also reach sub-nanometre
uncertainty [1]. However, fully 3D measurements in nanoscale introduce a number of uncertainty sources,
most notably probe geometry effects, probe-sample interaction and probe positioning control [2]. Thus, theexisting level of 5 nm uncertainty falls short of the requirements of industry and research.
A metrological atomic force microscope (MAFM) has been developed at MIKES [3], featuring direct traceability to the SI metre via laser interferometry. The new setup has large measurement range (950 µm × 950 µm
× 100 µm). The development done under the EMPIR initiative’s 3DNano project [4] also brings us closer totruly 3D measurements of vertical and lateral surface features. To achieve this goal, measurement uncertainty
will be reduced to below 1 nm.
Uncertainty components of the MAFM are characterized experimentally, and the total uncertainty budget isevaluated by Monte Carlo sampling on a virtual AFM (VAFM, a numerical model of the system). The VAFM
is a numerical model of the physical system, implementing the key physics affecting the measurement, e.g.
parasitic rotations in linear movement.
Monte Carlo uncertainty estimation can be extended to include any number of uncertainty components without
added complexity. Initial results validate the method bycomparing existing results against model evaluation on existing data. A conceptual model of the VAFM is produced,
to serve as documentation and design aid.
The new setup allows measurements of large areas, whichmakes it more suitable for measurements of e.g. opticalcomponents and surface roughness. That will improvemeasurement uncertainty and fill the gap between MAFM
measurement and other traceable surface characterization
methods (e.g. white light inrference microscopy, stylus instrument).
This project has received funding from the EMPIR programme co-financed by the Participating States and the European Union’s Horizon 2020 research and innovation programme. The work is part of the Academy of Finland Flagship Programme, Photonics Research and Innovation(PREIN), decision 320168.
Fig. 1: CAD model of the updated MIKIKES MAFM
References
1. V. Korpelainen, J. Seppä and A. Lassila, "Measurement strategies and uncertainty estimations for pitch and step heightcalibrations by metrological atomic force microscope." J. Micro/Nanolith. M., and M., 11, 2012.
2. R. Dixson, N. Orji, I. Misumi and G. Dai, “Spatial dimensions in atomic force microscopy: Instruments, effects, andmeasurements”.Ultramicroscopy, 194, pp. 199-214, 2018
3. V. Korpelainen, J. Seppä and A. Lassila, "Design and characterization of MIKES metrological atomic force microscope." Precision Engineering 34, pp. 735-744, 2010
4. Traceable Three-Dimensional Nanometrology, www.ptb.de/emrp/15sib09-project.html, valid 4/201943

3D printed lens project for bachelor’s education
Eero O. Molkoselkä1, Gleb A. Bulygin1, Justus F. Kleine1 and Anssi J. Mäkynen11Unit of Measurement Technology, University of Oulu, Oulu, Finland
Corresponding author: eero.molkoselka@oulu.fi

Keywords: optical design, digital fabrication, 3D printing, lens polishing, teaching
3D printing technology has made steady progress for a while now and is becoming more and more accessible.
While the printing quality and materials of the commercial models might not be good enough to fabricate precise
optical systems, they can still be used for applications where the accuracy isn’t as important. One of these isteaching.
We present a lens design project for educational purposes that is aimed for bachelor’s level students. In the project
students are given a task of designing, 3D printing and polishing their own biconvex or plano-convex lenses. The
process teaches students the basics of lens design in a practical way and allows them to familiarize with some of
the common software used in the industry. The project has been developed and tested as a part of a technical optics
course with two small groups of students. The students joined the course voluntarily using university’s common
registration methods and had varying experience in digital fabrication and the field of optics.
All the students were given a loose lens specification (refractive index, focal length limits and diameter) which they
then used to design their own lenses first by hand using the lens-maker’s equation [1] and then with OpticStudio by
Zemax [2]. OpticStudio was chosen because it provides built-in functionality for exporting lens designs to 3D
printable CAD files. However, for the future courses a Python library and web based user interface for generating
basic spherical lens objects were developed [3]. This way the design process itself can also be done with othercommon programs such as OSLO [4].
Stereolithography based 3D printer Formlabs Form 2 was used for the printing process [5]. The transparent printing
material of Form 2 is light-activated resin with a measured refractive index of 1.54. Hardened resin is an easymaterial to polish and work with which makes it good for beginners. Three different polishing methods were tested:
1. Polishing with sandpaper, sanding pads and liquids,
2. Polishing by grinding the lens against 3D printed counterpart and3. Dipping the lens in liquid resin and post-curing with UV light.
Generally, the sanding and grinding methods produced decent lenses but required more time and effort to polish.
Using pads and liquids also generates extra material costs. The dipping method can be a better choice if there is alarge number of students or multiple lenses for each participant. The quality was also more consistent because thevariance caused by different polishing times and techniques decreased. Some examples of final lenses can be seenin Fig. 1.

Fig. 1. 3D printed lenses, holders and counterparts (a). An object viewed through a 3D printed lens (b).
44

Lenses were tested by using a modified digital projector. The original optical system of the projector was removed
and replaced with 3D printed holder that could fit the lenses. Performance of the lenses was then compared toindustry made glass lens. The chromatic and spherical aberration of the printed lenses could be easily seen in the
projection which helped teaching the theory to the students. Various optical properties such as MTF (modulationtransfer function) could also be approximated from the projection. In the future, the goal is to run the project with alarger group of students and see whether the whole process can still be executed successfully.
References
1. E. Hecht, Optics, (Addison-Wesley, 1987).
2. Zemax OpticStudio: https://www.zemax.com/products/opticstudio, valid 4/2019
3. Our Python lens library: https://github.com/molkoback/lensmesh, valid 4/2019
4. OSLO: https://www.lambdares.com/oslo/, valid 4/2019
5. Formlabs Form 2: https://formlabs.com/3d-printers/form-2/, valid 4/201945

Three-dimensional flow measurements around micro-pillars in water by
micro-digital holographic particle tracking velocimetry
Yasuhiro Matsuda1 · Hiroshi Kigami1 · Noriyuki Unno2 · Jun Taniguchi1 · Shin-ichi Satake1
1

Department of Applied Electronics, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585 Japan
2
Tokyo University of Science, Yamaguchi, 1-1-1 Daigakudo-ri, Sanyo-onoda, Yamaguchi, 756-0884 Japan
Corresponding author: satake@te.noda.tus.ac.jp

Keywords: Digital holography, Index matching method, micro-pillars
A refractive index matching method in water to measure the velocity field around micro-pillars was investigated. The micro-pillars were fabricated from an optical material (MY-133-V2000) whose refractive index
was 1.333, which is the same as that of water. The fabrication method used was UV imprint lithography(UV-NIL) with photolithography with a replica mold. The moving tracer particles on the pillars were measured by micro-digital holographic particle tracking velocimetry (micro-DHPTV). Because the refractive indices of the pillars and water were the same, the hologram images could be obtained clearly without diffraction around the micro-pillars. As a result, tracking of particles and measurement of velocity fields using reconstructed particles could be achieved without optical distortion. The moving particles could be captured
around the micro-pillars.

Fig. 1 SEM image of the microstructures fabricatedfrom MY-133-V2000 by the vacuum UV imprinting
techniqueFig. 2 Experimental setup for a micro-digitalholographic PTV system.

Techniques for three-dimensional velocity measurements are required to detect flow fields in the microfluidic devices used in bioresearch. In a previous study, an investigation into the mechanism of actin network accumulation was carried out by measuring flow fields in a micro-device with a pattern of PDMS micro-pillars [1]. The flow field was obtained by digital inline holographic microscopy (DIHM). However, particle tracking was difficult because of the optical distortion resulting from the difference between the refractive indices of water and PDMS. Given that water is generally used as the working fluid in biofluidics, therefractive index of MY-133-V2000 must match that of water, so that obstacles caused by optical distortion,such as those observed with PDMS, can be avoided. In past research, fluid measurements for macroflow [2]and nanoflow [3] were successfully achieved via a refractive index matching method [4] using MEXFLON,
an optical material whose refractive index matches that of water. In addition, MY-134 (n = 1.34) a ultraviolet(UV)-curable resin [5], is a potential refractive index matching material. In a previous study, a cell on the
micro-pillar fabricated by the material in the cell culture liquid (n = 1.338) was observed. Additionally, thecalibration plate was fabricated for multilayer nanoparticle image velocimetry using UV nanoimprint lithography (UV-NIL) with MY-133-V2000 [6]. The fabrication process involved the use of photolithography and
UV-NIL with a replica mold. In the present study, we measured three-dimensional flow fields around a micro-pillar prepared by the abovementioned technique. Figure 1 shows a micro-pillar pattern imaged by46

scanning electron microscopy (SEM). Figure 2
shows the experimental setup.The equipment
was the same as that used in our previous study[7]. The plate was covered by glass and had micro-pillars made of MY-133-V2000, whose refractive index is 1.333. A high-resolution digitalcharge-coupled device (CCD) camera (Redlake,
MotionProX-3; resolution 1024 × 1024 pixel,
image 12 μm × 12 μm) with an objective lens
(Nikon, 40×, NA = 0.55) was used to record
fringe images of the particles. A neodymiumdoped yttrium lithium fluoride (Nd:YLF) laser(Photonics Industries DS20-527, λ = 527 nm)
was used as the light source, which produced apair of laser pulses at a repetition rate of 1 kHz
with a pulse length of 58 ns and a pulse delay of
100 μs. The polystyrene spherical particles(Thermo Fisher Scientific 4010A Polystyrene)
had a diameter of 1 μm and specific gravity of
1.05. NaCl (10 mM) was added to the particle
suspensions to enable solid contact between theparticles and the micro-pillar pattern. The reFig. 3 Hologram images of microstructures (a)fractive index of the 10 mM NaCl solution is the
fabricated from MY-133-V2000 (dry), (b) fabricatedsame as that of water. Particle suspensions orfrom PAK-01 (dry), (c) fabricated from MY-13310 mM NaCl particle suspensions were dropped
V2000 (underwater), and (d) fabricated from PAKon the micro-pillar pattern, and the plate was
01 (underwater).
covered by a cover glass. Hologram images wererecorded at a repetition rate of 1 kHz. The particles were tracked by DHPTV [7]. Hologram images of the micro-pillar pattern is showed in Figure 3. Figures
3(a) and 3(b) show the micro-pillar patterns of MY-133-V2000 and PAK-01, respectively. Figure 3(c) shows
the micro-pillar pattern of MY-133-V2000 in the 10 mM NaCl particle suspension, while Fig. 3(d) shows the
micro-pillar pattern of PAK-01 in the particle suspension. On the one hand, the edge of the micro-pillar pattern of MY-133-V2000 became invisible as a result of refractive index matching when the pattern was immersed in the 10 mM NaCl particle suspension. On the other hand, the edge of the micro-pillar pattern of
PAK-01 was visible even when the pattern was immersed in the particle suspension. It is clear, therefore,that the micro-pillar pattern of MY-133-V2000 can be used for refractive index matching with water. In thisstudy, the micro-pillar pattern of MY-133-V2000 was fabricated, and refractive index matching was performed using MY-133-V2000 and water. The particle on the micro-pillar was reconstructed and tracked using micro-DHPTV [7]. Refractive index matching with water will help in enhancing our understanding of the
3D flow around microstructures in water. We believe that our method will provide a deeper insight into the3D flow around complex microstructures and allow for the detection of flow fields in the bioresearch andchemical fields.
References
1. Sebastian, Weiße., Matthias, Heydt., Timo, Maier., Simon, Schulz., Joachim, P. Spatz., Michael, Grunze., Tamás,
Haraszti., and Axel, Rosenhahn., Phys. Chem. Chem. Phys., 2011,13, 13395-13402 (2011)
2. Masataka, Kuniyasu., Yusuke, Aoyagi., Noriyuki, Unno., Shin-ichi, Satake., Kazuhisa, Yuki., Yohji, Seki., Optical
Review, Volume 23, Issue 3, 529-534 (2016)
3. Noriyuki, Unno., Shuichiro, Nakata., Shin-ichi, Satake., Jun, Taniguchi.,, Experiments in Fluids 57: 120. (2016)
4. R.Budwig., Experiments in Fluids 17: 350., 350-355 (1994)
5. Andrea, Ravasio., Sree, Vaishnavi., Benoit, Ladoux., Virgile, Viasnoff., Acta Biomaterialia, Volume 14, 1 March
2015, 5360 (2015)
6. Noriyuki, Unno., Hiroshi, Kigami., Taku, Fujinami., Shuichiro, Nakata., Shin-ichi, Satake., Jun, Taniguchi., Microelectronic Engineering 180, 86–92 (2017)
7. Shin-ichi, Satake., Tomoaki, Kunugi., Kazuho, Sato., Tomoyoshi, Ito., Hiroyuki, Kanamori., and Jun, Taniguchi.,
Measurement Science and Tecshnology, 17, 7, 1647-1651, (2006)47

Evaluation of yield stress from velocity fluctuations of microfibrillated
cellulose suspension flow by optical coherence tomography
Janne Lauri1, Antti Koponen2, Sanna Haavisto3-4 and Tapio Fabritius1
1

Optoelectronics and Measurement Techniques Unit, University of Oulu, Oulu
2
VTT Technical Research Centre of Finland Ltd., Jyväskylä
3
Department of Physics, University of Jyväskylä, Jyväskylä
4
Spinnova Ltd.,Jyväskylä
Corresponding author: janne.lauri@oulu.fi

Keywords: shear viscosity, velocity profile, yield stress, optical coherence tomography
Micro- and nanofibrillated cellulose (MFC) have raised great interest as a sustainable and biodegradable material. It is used in various applications, for instance, in composites, barrier materials, viscosity modifiers andantimicrobial films [1]. Many applications of MFC, such as 3D printing and self-assembly of nanocellulosebased filaments, require prior knowledge of flow dynamics. Despite of extensive research, complex flow properties of MFC are still not comprehensively understood and novel experimental data and analysis methods areneeded. An optical method, called optical coherence tomography (OCT), has recently been demonstrated to
be an excellent tool to monitor fiber suspension flow and flocculation in a rotational rheometer [2]. OCT has
a superior resolution compared to other modalities such as ultrasound velocimetry and magnetic resonancevelocimetry. The high resolution enables accurate measurements for wall layer dynamics. Rheological andflocculation analysis in a pipe rheometer equipped with the OCT device, a more realistic geometry for practicalapplications, has been recently presented [3-5]. In this study, a novel method to estimate yield stress fromoptical coherence tomography (OCT) velocity data is presented for 0.5 % microfibrillated cellulose (MFC)
suspension in a tube flow.
The microfibrillated cellulose sample was prepared from never-dried bleached kraft birch pulp via grinding
three times in a supermasscolloider (Masuko Sangyo Co. Ltd., Kawaguchi, Saitama-pref., Japan). The configuration of the experimental system is shown in Fig 1. Measurements were performed in a glass tube with an
inner diameter of 8.6 mm. The OCT device was in-house built. More details of the measurement setup and thedevice can be found in Refs. [4-5].

Fig. 1. The measurement setup. L1 and L2 are locations of the pressure sensors, θD is Doppler angle and P0 is
overpressure.
Measurements were performed at the A-scan rates of 31 kHz and 123 kHz. The phase variance was calculated
from the bulk region of the suspension (distance from the pipe wall more than 20 µm) to eliminate the effectof reduced MFC concentration due to wall depletion. The data was fitted with two lines (Fig. 2.) The obtained
yield stress, defined as the intersection point of the two fitted lines, was 3.4 Pa which correlates excellently
with the values defined directly with a bob and cup rheometer [2] and with the viscosity data variations of the48

pipe rheometer [3]. Furthermore, the relative floc sizes, calculated from the structural data [4], started to decrease at the same point, which indicated breakage of the fibrous networks structure due to shear stress exceeding the yield stress.
The velocity variance data from OCT can obviously be used to determine the yield stress of complex suspensions. In future, more solid concentrations and different grades of MFC will be measured to validate this novel
method for determining the yield stress.

Fig. 2. Measured variances of the OCT velocity signal with two fitted lines. The intersection of the lines depicts the yield stress point. Rectangular and round points have been measured at the 31 kHz and 123 kHz
scan rates, respectively.
References
1. R.J. Moon, G.T. Schueneman, and J. Simonsen, JOM 68, 2383–2394 (2016).
2. T. Saarinen, S. Haavisto, A. Sorvari, J. Salmela, and J. Seppälä, Cellulose 21, 1261–1275 (2014).
3. S. Haavisto, J. Salmela, A. Jäsberg, T. Saarinen, A. Karppinen, and A. Koponen, TAPPI J. 14, 291–302 (2015).
4 J. Lauri, A. Koponen, S. Haavisto, J. Czajkowski, and T. Fabritius, Cellulose 24, 4715–4728 (2017).
5. A.I. Koponen, J. Lauri, S. Haavisto, and T. Fabritius, Applied Sciences 8, 755 (2018).

49

Measurement setup for differential spectral responsivity of solar cells
Petri Kärhä, Hans Baumgartner, Kasperi Kylmänen, Benjamin Oksanen and Erkki Ikonen
Metrology Research Institute, Aalto University, Espoo
Corresponding author: petri.karha@aalto.fi

Keywords: radiometry, solar cell, spectral responsivity, efficacy, electricity
The spectral responsivity of a photovoltaic device is needed for solar cell development and cell analysis. It canalso be used in testing of solar cells, to correct for the spectral mismatch between the light source used in themeasurement from the standardized reference spectral irradiance data. Measurement setups of spectral responsivity have been standardized in [1]. Solar cells can be nonlinear which poses a challenge in measurements.
Differential spectral intensity in biased conditions needs to be measured. Such measurement setups have beenpresented in [2 – 5]. The cells measured are biased with incandescent or Xe light sources. Monochromatic
measurement beam is chopped and measured with a lock-in amplifier from the cell output. Combining the bias
light with the measurement beam can be made in an integrating sphere, or the beams can be imposed on thecell surface.

Fig. 1. Schematic presentation of the differential spectral responsivity setup.
We have developed a measurement setup for differential spectral responsivity of solar cells using towers of
halogen lights (4 ´ 7 ´ 50 W) for bias light, and 30 narrow-band LEDs for measurement (Fig. 1). The bias
lights are used to set radiation conditions close to AM 1.5 (1000 W/m2) as specified in [6]. The measurement
LEDs are mounted in a carousel, guiding one LED at a time to light the cell. The carousel has been earlier usedin measurement of cameras [7]. The LEDs are temperature controlled (Fig. 2) and operated with a modulated
operating current. A lock-in amplifier is used to detect the weak signal from the induced photocurrent of thesolar cell. The cell to be measured is temperature stabilized to 25 °C with liquid cooling. The photocurrent ismeasured with a shunt resistor (2 W) and a source meter keeping the voltage across the cell negligible.
The setup has been fully characterized and tested for measurements. Figure 3 shows the spectra of the measurement LEDs assembled, and a test measurement of a solar cell. As a reference, a trap detector and a precision
aperture is used to measure irradiances of the individual LEDs with the bias lights switched off.
The uncertainty of the obtained differential spectral responsivities over the visible region is 2.5 % (k = 2). Thisis mainly dominated by the standard uncertainties of the spatial uniformity of the measurement beam (0.6 %),gain of the lock-in amplifier (0.6 %), short term stability of the LED source (0.5 %), and the repeatability of50

the measurements (0.5 %). The standard deviation of the spatial uniformity of the bias lighting is 1.7 % over asolar cell area of 15 ´ 10 cm. The corresponding uniformity for the measurement beam is of the order of 0.6 %depending on the LED used. In the UV and IR regions, the non-uniformities are higher.

Fig. 2. Details of the setup. Left: A carousel containing LEDs. The round circuit board in the middle contains
relays used for connecting the electronics to one LED at a time. Right: Four towers containing the bias lights.

Fig. 3. Left: relative spectra of the measurement LEDs. Right: Test measurement of a solar cell.
References
1. IEC 60904-8:2014, Photovoltaic devices - Part 8: Measurement of spectral response of a photovoltaic (PV) device(The International Electrotechnical Commission IEC, Geneva, 2014).
2. Jürgen Metzdorf, “Calibration of solar cells 1: The differential spectral responsivity method, Appl. Opt. 26, 1701-1708(1987).
3. Ghufron Zaid, Seung-Nam Park, Seongchong Park, and Dong-Hoon Lee, “Differential spectral responsivity measurement of photovoltaic detectors with a light-emitting-diode-based integrating sphere source,” Appl. Opt. 49, 6772-6783(2010).
4. Behrang H. Hamadani, John Roller, Brian Dougherty, Fiona Persaud, and Howard W. Yoon, “Absolute spectral responsivity measurements of solar cells by a hybrid optical technique,” Appl. Opt. 52, 5184-5193 (2013).
5. Behrang H. Hamadani, John Roller, Andrew M. Shore, Brian Dougherty, and Howard W. Yoon, “Large-area irradiance-mode spectral response measurements of solar cells by a light-emitting, diode-based integrating sphere source,”
Appl. Opt. 53, 3565-3573 (2014).
6. IEC 60904-3:2014, Photovoltaic devices - Part 3: Measurement principles for terrestrial photovoltaic (PV) solar
devices with reference spectral irradiance data (The International Electrotechnical Commission IEC, Geneva, 2014).
7. Juha-Matti Hirvonen, Tuomas Poikonen, Anna Vaskuri, Petri Kärhä, and Erkki Ikonen, “Spectrally adjustable quasimonochromatic radiance source based on LEDs and its application for measuring spectral responsivity of a luminancemeter,” Meas. Sci. Technol. 24, 115201 (8 pp), (2013).

51

VTT’s versatile 3 µm silicon photonics platform
Päivi Heimala, Mikko Harjanne, Ari Hokkanen, Matteo Cherchi, Tapani Vehmas,
Srivathsa Bhat, Timo Aalto
VTT, Espoo
Corresponding author: paivi.heimala@vtt.fi
Keywords: silicon photonics, optical waveguides, integrated optics, multiplexers, photodetectors

Silicon photonics (SiPh) represents a scalable path to the deployment of photonic integrated circuits (PICs)[1]. The fundamental idea is to integrate a large number of photonic functions in a small footprint on the surface
of a silicon chip. SiPh applies the proven success of silicon microelectronics into photonics.
VTT's PIC platform uses 3µm thick silicon-on-insulator (SOI) waveguides [2] as illustrated in Fig. 1 andFig. 2. This platform, known also as “Thick-SOI” is offered by VTT as open access via multi-project wafer(MPW) runs, dedicated process runs, prototyping and small-to-medium volume production services. The main
benefits of the Thick-SOI platform are low losses (0.1 dB/cm), dense integration (bending radii down to<2 µm), polarization independent operation, wide wavelength range (1.2–4 µm), ability to tolerate high opticalpowers (>1 W) and smooth transition from R&D and prototyping to volume production. Here we show latest
results of some key devices realized on our Thick-SOI platform. We also present device examples to demonstrate its suitability for sensing and datacom/telecom applications.

Fig. 1. Basic building blocks on Thick-SOI PIC platform:
1) SM rib waveguide, 2) horizontal mirror, 3) rib-strip converter, 4) spot-size converter, and 5) up-reflecting mirror.

Fig. 2. Simulated intensity distributions of the fundamental mode fields in a SM rib waveguide (left) and astrip waveguide (right).

Wavelength (de)multiplexers are versatile components that are crucial for increasing link data-rates in telecommunication applications by increasing the channel counts but can also be used for optical spectrum sampling in spectroscopic sensors. The usual way to do the multiplexing is either with an arrayed waveguidegrating (AWG) or with an echelle grating (EG). While AWG has the advantage in terms of easy scaling up of
the channel count for low channel spacing values, EG is potentially more compact and better suited for larger
channel spacing with a comparatively lower channel count. Fig. 3 A and B show a schematic of a 1 x 11
channel AWG design with 400 GHz channel spacing in the 1550 nm wavelength area (C-band), fabricated inthick-SOI. Measured transmission characteristics confirm low loss (1.2-2.0 dB) and high extinction ratio(35 dB) for the measured TE polarization. Similarly, the fabricated EG design (Fig. 3 C and D) centered around1550 nm show extinction of over 25 dB and low losses in the 1.3-2.5 dB (TE) and 0.9-1.8 dB (TM) range.
T ra n s m is s io n ( d B )

0

- 10

CH1
CH3

B

C

D

CH4

- 30

- 40

A

CH2

- 20

CH5

1530

1540

1550

1560

Wavelength (nm)Fig. 3. Schematic of an 11-channel AWG design (A) with 400 GHz channel spacing and the measured transmission(B) showing low insertion loss and high extinction ratio in TE polarization. Microscope image (C) of the fabricated
5x1 EG with 800 GHz channel spacing and the measured transmission characteristic (D) in TE polarization.

For light input and output coupling purposes we have developed up-reflecting total internal reflection (TIR)
mirrors integrated at the end of the Si waveguides (Fig. 4). The 45° degree vertical coupling mirror surface has
been realized with wet etching. The mirror to fiber coupling loss has been measured to be below 0.5 dB overthe entire C-band, and for both polarizations. These mirrors are ideal for integration of vertical-cavity surface

52emitting lasers (VCSELs) and detectors on the SOI chips, and also for characterizing device performance witha wafer-level testing (WLT) setup.

Fig. 4. SEM image of a cross-section of a 45o TIR vertical coupling mirror.

Fig. 5. SEM image of the fabricated horizontal Ge
PDs integrated with 3 µm SOI waveguide

For light detection, we have also developed Ge-based, monolithically integrated PIN photodetectors (PD) on
our Thick-SOI platform (Fig. 5) using the horizontal PD configuration as reported in [3]. Wafer level measurements have shown PD responsivity of about 0.9 A/W with dark current of about 1 µA, measured at -1V
bias. The PDs also show excellent 3dB bandwidth response (>35 GHz).
In the EU project MIREGAS, the platform was used for creating a switchable and tunable light source in the
2.7 to 3.5 µm wavelength range, with a spectral resolution below 1 nm. The mid-IR light source consisting ofan SLED (provided by TAU/ORC) flip-chipped on 3 µm SOI chip is shown in Fig. 6. The 3 µm integrated
optical circuit on SOI contains arrays of optical switches and a wavelength multiplexer.
We have also realized in the Thick-SOI a LIDAR operating at 1.55 µm, the design is shown in Fig. 7. The
beam can be steered horizontally by changing wavelength. The LIDAR is edge emitting and only requires acylindrical lens to reduce vertical beam divergence.
In the EU project PASSION, the platform is used for creating a 40 channel transmitter module capable of up
to 2 Tb/s transmission rates. The chip incorporates four AWG multiplexers, coupled to arrays of VCSELs via
the up-reflecting mirrors. A picture of the fabricated chip is shown in Fig. 8.
We have demonstrated that the Thick-SOI platform can be used in a wide variaty of applications, ranging fromtelecom and datacom to mid-IR gas sensing. For low-cost, low-barrier access the platform is offered as an
MPW service, with full support for scaling up to volume production.

Fig. 6. Mid-IR tunable light source
consisting of an SLED integrated on
Thick-SOI. Chip size 5×10 mm.

Fig. 7. Wavelength tunable
LIDAR design. Component
size is 1 mm×2mm.

Fig. 8. 40 channel transmitter chip for
2 Tb/s. Chip size 20 × 20 mm.

Acknowledgements
The work is part of the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision
320168.
References
1. D. Thomson et al., Journal of Optics 18, 073003 (2016).
2. T. Aalto et. al., J. Sel. Top. Quantum Electron. (2019), DOI 10.1109/JSTQE.2019.2908551.
3. D. Feng et al., Appl. Phys. Lett. 95, 261105 (2009).

53Effect of nanometrically thin nickel catalyst on the properties of
CVD synthesized graphitic films
M. Baah1, T. Kaplas1, 2 and Y. Svirko1
1

2

Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu Finland.
Department of Optoelectronics, Centre for Physical Sciences and Technology, Sauletekio av. 3, LT-10257
Vilnius, Lithuania
Corresponding author: marian.baah@uef.fi

Keywords: Graphitic thin films, chemical vapor deposition (CVD), direct synthesis, nickel catalysis
Abstract – The use of graphitic thin films in optoelectronic and photonic applications is increasing
due to their unique optical and electrical properties. The recently fine-tuned, metal-catalysis,
CVD synthesis of these films also increases their applicability. Even though such films are being
used now, not much light has been thrown on the properties of these films that makes themappealing. Here we present our findings on the spectroscopic and microscopic investigations,highlighting the effect of the nanometrically thin Ni catalyst used in the synthesis.
Introduction – Graphitic films have now made a huge impact on the opto-electronic andnanofabrication industry as they are now used in such applications [1]. One drawback of these thinfilms, when it comes to photonics application, is the ability to synthesize them on transparent dielectric
substrates. Over the past few years scientists have come up with different techniques to overcome thisbackset and one of these is the use of thin films of metal catalysts such as nickel (Ni) and copper (Cu)[2,3]. The most frequently used metal catalyst is the Ni catalyst. This technique of synthesis mainly
involves the receding of the metal catalyst, the pyrolysis of the carbon precursor, growth of carbon filmon the air-metal interface as well as the metal substrate interface.
Results – Our work focuses on the synthesis technique that allows the use of direct growth of graphitic
films on transparent dielectric substrates, by making use of nanometrically thin nickel catalysis. Usinga process temperature of 800 °C, we grow graphitic thin films using the technique of CVD on
transparent dielectric substrate. We now compare films synthesized with and without the Ni
nanocatalyst by looking at their physical properties. Using spectroscopic measurements, we study the
optical and electrical properties of these films establishing the effect of the Ni nanolayer.

Fig. 1. An illustration of the process
flow for the growth of graphitic thinfilms on transparent dielectric
substrate fused silica (SiO 2) using
CVD synthesis process.

From our study, we observed that removal of the Ni nanolayer from the film does not affect the lattice
structure of the film (from Raman) and increases transmittance in the VIS-IR regions of theelectromagnetic spectrum [4]. Such graphitic thin films have exhibited good potential in applications
such as nonlinear optics and surface-enhanced Raman spectroscopy [5, 6].

54

GRF layer

Fig. 2. Top left corner: image ofsynthesized graphitic film, top rightcorner: Raman spectra of graphitic
film with Ni (GRF + Ni) and with Ni
removed (GRF - Ni), by etching.
Lower left corner: SEM image of
GRF + Ni and lower right corner:
SEM image of GRF – Ni.

Summary – To conclude we present our findings of the effect of the inclusion of a thin film of nickel
metal catalyst in our CVD synthesis of graphitic films, we discuss this effect on the optical and physicalproperties of these thin films.

References
1. F. Bonaccorso, T.H.Z. Sun, T. Hassan, and A.C. Ferrari, Nature Photonics. 4(9), 611-622 (2010).
2. R.S. Edwards and K.S. Coleman, Accounts of Chemical. Research. 46(1), 23-30 (2013).
3. T. Kaplas, D. Sharma, and Y. Svirko, Carbon, 50(4), 1503-1509 (2012)
4. M. Baah, P. Obraztsov, A. Paddubskaya, A. Biciunas, S. Suvanto, G. Niaura, and T. Kaplas. (To be submitted2019).
5. T. Kaplas, L. Karvonen, J. Rönn, M.R. Saleem, S. Kujala, S. Honkanen, and Y. Svirko Optical Materials
Express, 2(12), 1822-1827, (2012).
6. O. Fesenko, G. Dovbeshko, A. Dementjev, R. Karpicz, T. Kaplas, and Y. Svirko, Nanoscale research letters,
10(1), 163, (2015).

55

Qualitative analysis for presence of Sudan IV in edible palm oil
Sampson S. Andoh1*, Kenneth Nyave1, Boniphace Kanyathare1,2, Benjamin Asamoah1, Tarmo Nuutinen1,3,
Cheetham Mingle4, Kai-E. Peiponen1 and Matthieu Roussey1
1

University of Eastern Finland, Institute of Photonics, Joensuu, Finland
Dar es Salaam Institute of Technology, Department of Electronics and Telecommunications Engineering, Dar es
Salaam, Tanzania
3
University of Eastern Finland, Department of Environmental and Biological Sciences, Joensuu, Finland
4
Food and Drugs Authority, Food Physio-Chemical Laboratories, Accra, Ghana*Corresponding author: sampson.andoh@uef.fi

2

Keywords: palm oil, Raman spectroscopy, SERS, adulteration, authentication, PCA
Palm oil, a highly nutritious dietary oil and currently the most used edible oil in the world, is found to contain
Sudan IV dye as hue enhancer. Sudan IV dye is an industrial dye that is used fraudulently in enhancing thecolor of palm oil due to its wide availability, low cost, and intense hue. Due to their proven carcinogenicity
and mutagenicity, it is banned as food additives [1]. Yet this problem is still prevalent in developing countries,
The Ghana Food and Drugs Authority (FDA) through a communique in October 2015, cautioned the public ofthe distribution and sale of Sudan IV dye adulterated palm oil adulterated in the open market. This was after
98 % of the samples collected from sellers in different markets in Accra region in Ghana for testing, showed
positive results to Sudan dyes. The method used in identifying these adulterations are usually laboratory based
and requires high level of expertise in operation and interpretation of results. We propose a method based on
the principle of surface-enhanced Raman spectroscopy in authenticating and detecting Sudan IV dye adulteration of edible palm oil from Ghana. We then apply principal component analyses on classical transmission
spectra to confirm the results. This method provides a quick, less expensive, ready and easy-to-use platform
in its operation.
A substrate functionalized with silver nanoparticles (AgNPs) is used in this work. Its fabrication is detailed in[2]. In this method, a silicon wafer is periodically and systematically dipped in two 5 mM precursor solutionsof silver nitrate (AgNO3) (Sigma-Aldrich, 99 % purity) and sodium chloride (NaCl) (Sigma-Aldrich, 99.8 %purity), see Fig. 1. (a), to produce silver chloride particles. This is an intermediary step to prevent tarnishingof the AgNPs. The growth rate and size of the AgCl particles depends on the dipping cycles, concentrations of
the precursor solutions and the duration of each dip. For this work, we used 1.5 s per dip and submersion cycleof 50. The AgCl particles produced are not SERS active they are made as such by exposing section of the AgCl
to about 5 mW of laser power for about 2 min (see Fig. 1(b)), to reveal fresh AgNPs for measurements. The
FDA provided the samples used in the study.

(a)

(b)

Fig. 1. Schematic flow chart of the substrate production: (a) AgCl fabrication process and (b) photo reduction process to reveal fresh AgNPs for measurement. Adopted from [2].
The SERS profile of analytical grade palm oil (STDPO), Sudan IV dye, and four of the Ghanaian samples(labelled as FDA1-4) is measured with a 514 nm laser, power of 50 µW and acquisition time of 10 s. Fig. 2
shows the spectra of Sudan dye and Ghanaian samples. The Raman spectral peak at 1389 cm-1 was the main
identifier peak used in the authentication to predict adulteration in the Ghanaian samples. We were able todetect traces of Sudan IV dye adulteration in all the palm oil samples from Ghana as indicated by the FDA.
This method provides a quick, ready-to-use, on demand, and less expensive platform in determining adulteration, saving time and money.

56

(a)

(b)

Fig. 2. SERS spectra of: (a) Analytical Sudan IV dye and (b) Ghanaian samples, taken with 514 nm laser of
power 50 µW and acquisition of 10 s.
In addition to SERS characterization of the samples, we performed spectrophotometric measurement with a
Perkin-Elmer Lambda 9 UV/VIS/NIR spectrophotometer. We measured both samples collected on the open
markets of several Ghanaian regions and prepared adulterated samples with different concentration of the Sudan IV dye. We compared the results using principal component analysis, which confirmed our results.
This method can be implemented on the field using handheld devices for rapid and cost efficient screening by
local authorities of the market samples. Suspicious samples would however have to be analyzed carefully byrobust methods in laboratory. Finally, the technique can be applied to many other food or cosmetic product tofight against counterfeiting.
References
1. R. Rebane, I. Leito, S. Yurchenko, and K. Herodes, A review of analytical techniques for determination of Sudan
I–IV dyes in food matrixes, J. Chromatogr. A 1217, 2747–2757, (2010).
2. A. Matikainen, T. Nuutinen, P. Vahimaa and S. Honkanen, A solution to the fabrication and tarnishing problemsof surface-enhanced Raman spectroscopy (SERS) fiber probes, Sci. Rep, 5, 8320, (2015).

57

OPtical Imager for Comets (OPIC) for
ESA’s F-class mission Comet Interceptor
Optics in Engineering OIE19
Andris Slavinskis1,3 , Antti Näsilä2 , Mihkel Pajusalu3 , Jaan Praks1 , Anu Reinart3 , Iaroslav Iakubivskyi1,3 ,Tomas Kohout4 , Colin Snodgrass5 , Geraint Jones6
1

School of Electrical Engineering, Aalto University, Espoo
2
VTT Technical Research Centre of Finland, Espoo
3
Tartu Observatory, University of Tartu, Tartu
4
Department of Geosciences and Geography, University of Helsinki, Helsinki
5
School of Physics and Astronomy, University of Edinburgh, Edinburgh
6
Mullard Space Science Laboratory, University College London, London
Corresponding author: andris.slavinskis@ut.ee

Keywords: comet, Öpik–Oort cloud, visual and near-infrared imaging, tunable Fabry–Pérot interferometer
Comet Interceptor is a recently-selected F-class mission to a dynamically-new Solar System object. Enabled
by recent advances in observational surveys which will cover the sky more deeply, coherently and rapidly, thespacecraft will be parked at the Sun–Earth Lagrange Point L2 waiting for a suitable target to be discovered. It
will be a multi-element spacecraft comprising a primary platform which also acts as the communications hub,and sub-spacecraft, allowing multi-point observations around the target during a flyby. The B1 sub-spacecraft
is proposed by University of Tokyo and JAXA. The B2 sub-spacecraft will carry OPtical Imager for Comets(OPIC) whose goal is mapping of the nucleus and its dust jets at visible and infrared wavelengths. Here we
present the preliminary design and considerations. The instrument is named after an Estonian astronomer Ernst
Öpik who proposed a reservoir of comets which is now called the Öpik–Oort cloud, from where the Comet
Interceptor target would originate.

Fig. 1. Left: Comet Interceptor overview. Right: A simulated view from the OPIC on-board the B2 spacecraft
travelling at 65 km/s with respect to the comet. Full animation: https://vimeo.com/321107967/bca114809a
The visible (VIS) camera will be a monochrome imager with a wavelength range of 400–800 nm, while thenear-infrared (NIR) camera will be a spectral imager capable of imaging the comet at 5–20 different wavelength
bands in the 1000–1600 nm region with the spectral resolution of 20 nm. The spectral imaging is realized witha tunable passband filter. The effective aperture for both cameras is in the range of 1.5–3 cm. The OPIC
instrument will provide images continuously as the B2 spacecraft approaches the comet.
As it is considered too risky to point the cameras directly at the direction of travel, OPIC will be pointed tothe side of the spacecraft (i.e., perpendicular to the flight direction) and a folding mirror is used to point the
instrument towards the flight direction (similar to a periscope). During the closest flyby, the mirror is moved
aside and, if the flyby geometry allows, the instrument will point sideways to the comet as the B2 flies past theclosest approach.

58

Fig. 2. OPIC concept.
The instrument also includes a Data Processing Unit (DPU) to prioritize the images for transfer to the main
spacecraft during the flyby. The DPU will be capable of storing the collected raw data, which can then betransferred onwards if the spacecraft survives the flyby.
The NIR spectral imager is based on VTTs tunable Fabry–Pérot Interferometer (FPI) technology, which is
combined with an InGaAs image sensor (640×512 pixels) to create the spectral imager. The NIR spectral
imager concept has been successfully demonstrated in Low Earth Orbit (LEO) onboard the Reaktor Hello
World CubeSat. The same design is considered as the baseline for Comet Interceptor, but the FPI technology
is compatible with any image sensor, so it is possible to modify the design if necessary. The spectral imager
will include an internal calibration source for wavelength calibration, so it will be possible to re-calibrate theinstrument during the commissioning.
The design of the visual spectrum camera is based on a space-qualified 3D-Plus’ FPA 4Mpx CMOS 3DCM73x.
It features the CMV4000 sensor (2048×2048 pixels) from AMS (formerly CMOSIS). The same sensor has beenused in the VTTs visual range hyperspectral camera on the Aalto-1 nanosatellite with good results. Universityof Tartu is also developing a high-sensitivity Earth observation imager using Gpixels GSENSE series scientific
CMOS sensor. If the sensor and readout electronics will be approved within ESAs Industry Incentive Scheme,it can be considered for OPIC.
In order to maximize the science return of OPIC, a DPU is used to automatically prioritize the images fortransfer during the flyby (e.g., to transfer images with the most useful signal and the least smear). This isneeded because the high probability of dust impacts that could harm the instrument and the spacecraft during
the approach and flyby. Image compression will also be used for the high priority data in order to maximise
the link throughput. The DPU also controls imaging parameters in case all images are too dark, saturated orsmeared. The DPU can be based on the space-qualified Q7 system-on-chip by Xiphos Technologies which
integrates dual ARM CortexTM -A9 MPCore processor together with an FPGA. This board has been tested inspace on-board Reaktor Hello World nanosatellite together with VTT’s hyperspectral imager, and it is also
being implemented and tested for ESA’s PICASSO and APEX cubesats.

59

Direct 3D-printing of optical components at the University of Eastern
Finland
Markku Pekkarinen, Petri Karvinen and Jyrki Saarinen
Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80100 Joensuu, Finland
Corresponding author: markku.pekkarinen@uef.fi

Keywords: 3D-printing, additive manufacturing, optics
A need for rapid prototyping and customized small-series production of polymer optics is increasing. While
the production cycle times for new products are decreasing, the component geometries and functions are getting more and more complex. Additive manufacturing (AM), or 3D printing, offers one way to speed up
product development and decrease costs.
Using AM process, an object is created in a layer-by-layer manner. Most commonly known methods
to create 3D printed optics are two-photon lithography [1,2] and micro-stereolithography [3], which are offering promising results for small objects in micro- and millimeter-scale. At the University of Eastern Finland we are utilizing Printoptical Technology [4], proprietary of Luxexcel. This is an inkjet-based method for
fabrication of centimeter-scale components with smooth surfaces. The process yields consistently prints withsurface roughness in single nanometer scale without any post-processing, which is enough to directly print
optical quality components.
In our recent work [5] we have applied 3D printing to realize freeform optics for industrial based tailored illumination, where the goal was to design and fabricate a freeform lens array for light-emitting diodes(LEDs). We demonstrated an iterative fabrication scheme [6], which improves shape accuracy of the 3D
printed optics even up to imaging quality. Utilizing this scheme, we demonstrate an imaging lens with centimeter-scale diameter featuring optical performance comparable to that of a commercial glass lens. In thispresentation we will discuss the features, limitations and most notably potential and realized applications of
the 3D printed optics.
Future research will concentrate on multi-material printing for support materials and materials which
are offering, e.g., different refractive indices. Support materials, which can be dissolved after the printing
process, would allow, e.g., printing of hollow structures. Printing two materials with different refractive indices would give possibility to research gradient-index (GRIN) optics. These developments will allow 3D
printing of optical components that cannot be manufactured with traditional subtractive methods.
References
1. S. Thiele, K. Arzenbacher, T. Gissibl, H. Giessen, and A. M. Herkommer, “3d-printed eagle eye: compound microlenssystem for foveated imaging,” Sci. Adv. 3, e1602655 (2017).
2. C. Liberale, G. Cojoc, P. Candeloro, G. Das, F. Gentile, F. D. Angelis, and E. D. Fabrizio, “Micro-optics fabrication
on top of optical fibers using two-photon lithography,” IEEE Photon. Technol. Lett. 22, 474–476 (2010).
3. X. Chen, W. Liu, B. Dong, J. Lee, H. O. T. Ware, H. F. Zhang, and C. Sun, “High-speed 3d printing of millimetersize customized aspheric imaging lenses with sub 7 nm surface roughness,” Adv. Mater. 30, 1705683 (2018).
4. K. Blessing and R. van de Vrie, “Print head, upgrade kit for a conventional inkjet printer, printer and method forprinting optical structures,” U.S. Pat. Appl. No.13/924 (March 14, 2012).
5. B. G. Assefa, T. Saastamoinen, M. Pekkarinen, J. Biskop, V. Nissinen, M. Kuittinen, J. Turunen, and J. Saarinen,“Realizing freeform optics using 3D-printer for industrial based tailored irradiance distribution,” OSA Continuum 2,
690–702 (2019).
6. B. G. Assefa, M. Pekkarinen, H. Partanen, J. Biskop, J. Turunen, and J. Saarinen, “Imaging-quality 3D-printed centimeter-scale lens,” Optics Express 27, 12630-12637 (2019).

60

Super-resolution imaging by scanning
subdiffraction-limit optical pattern
Yusuke Ogura1 , Daiki Shinkawa1 , Takahiro Nishimura2 , Yosuke Tamada3 , and Jun Tanida1
1

Graduate School of Information Science and Technology, Osaka University, Suita
2
Graduate School of Engineering, Osaka University, Suita
3
Division of Evolutionary Biology, National Institute for Basic Biology, Okazaki
Corresponding author: ogura@ist.osaka-u.ac.jp

Keywords: super-resolution, subdiffraction limit, imaging, computer generated hologram,
Fluorescence microscopy is a promising tool to visualize the structure and/or dynamics of biological samples
including cells and micro-organelles by labeling specific biomolecules with fluorophores. However, the resolution of the image obtained by wide-field illumination is restricted based on the diffraction limit, and many
types of super-resolution techniques have been proposed[1]. Laser scanning microscopy (LSM) is an option
for fluorescence imaging which is based on scan of a focused laser spot for excitation and reconstruction froma set of point-detected fluorescence signals. Although this microscopy can provide high contrast images thanks
to limited-volume excitation, the resolution and speed are still restricted owing to single diffraction-limit-spot
scanning. This paper presents a method of super-resolution imaging where a subdiffraction-limit optical pattern
is used as illumination. Subdiffraction-limit optical patterns are generated flexibly using computer generated
holograms (CGHs), and our method is expected to improve the performance of the LSM. In this study, weinvestigated capability of the method in bio-imaging.
In LSM, the point spread function (PSF) of a reconstructed image is expressed as the multiplication of thefunction of the illumination-intensity distribution and the PSF of the imaging optics. This equation indicatesthat use of a smaller focused spot is effective to improve the resolution. Thus a subdiffraction-limit optical pattern that is finer than the diffraction-limit spot is employed. We have already demonstrated that well-designed
CGHs can generate an array of subdiffraction-limit spots[2]. It is also possible to generate subdiffraction-limit
spots with multiple wavelengths on multiple planes[3]. Note that the subdiffraction-limit optical patterns areprovided by propagating light, and therefore they are straightforwardly utilized as illumination in microscopy.
In our super-resolution method, a subdiffraction-limit optical pattern is generated and scanned on the sample
plane. During scanning, a sequence of fluorescence images is captured; then a high-resolution image is reconstructed through digital processing. When multiple spots are utilized as illumination, the individual spots cancover different field of views (FOVs), and thus time for image acquisition can be shortened.
Figure 1 shows the experimental setup. A light wave from a laser source (wavelength: 488 nm) hits a spatial
light modulator (SLM; HOLOEYE Photonics AG, LC2012) that works as a CGH, then a subdiffraction-limit
Samplef=20

Laser
λ = 488 nm

SLM

f=400

f=450

Objective lens
f=150

Dichroic
mirrorDichroic
filterPolarizer

Imaging lens
Image sensor

Figure 1: Experimental setup.

61

(a)

1μm
(b)

Diffraction limit

Wide-field, overall

Wide-field, enlarged

1μm

1μm

Intensity (a.u.)

Subiffraction limit

1.0
0.8
0.6
0.4
0.2
0
0

200

400

600

800

Position (nm)Scan, diffraction limit

Scan, subdiffraction limit

Figure 2: (a) Images of GFP-labeled tublin. (b) Intensity profiles along yellow lines shown in (a).
optical pattern is generated on the sample plane through optics including an objective lens (100×, oil immersion, numerical aperture (NA): 1.40). The fluorescence images are captured by an image sensor (Hamamatsu
Photonics K.K., C14440-20UP).
To investigate the ability for resolution improvement in bio-imaging, we observed green fluorescence protein(GFP)-labeled tubulin, which is a configuration factor of cytoskeleton. We designed a CGH that generated anarray of 3×3 subdiffraction-limit spots, using the Gerchberg-Saxton algorithm with our own constraints[2]. The
spot size, defined as the full width at half maximum, of the center spot was reduced from 257 nm (diffraction
limit) to 187 nm (subdiffraction limit). In the image reconstruction, the center spot alone among 3 × 3 spots
was used because the size of the center spot was reduced most effectively. Figure 2 shows an imaging result,where the scan pitch is 65 nm and the number of scan steps is 49 × 49. Although a detailed structure is notobserved by wide-field imaging, some tube-like structures are observable when scanning with a diffraction-limit
spot and with the central subdiffraction-limit spot. A comparison of the intensity profiles (Fig. 2(b)) indicatesthat the width of the structure is reduced from 342 nm to 294 nm. The result demonstrates that the use of a
subdiffraction-limit optical pattern is effective to improve the resolution in LSM. We performed an additional
experiment where 5 × 5 subdiffraction-limit spots among 7 × 7 spots were used for illumination, to shorten
the time for capturing images. Different FOVs were assigned to the individual spots in the reconstruction, anda final large FOV image was synthesized by connecting the reconstructed images. The result confirmed that alarge-FOV super-resolution image was obtained by the method.
In this study, super-resolution imaging by laser scanning microscopy with a subdiffraction-limit optical patternis presented. The experimental results indicate the capability for resolution improvement. We will further
investigate the performance to characterize the method in bio-imaging.
This research was supported by the grant of Joint Research by the National Institutes of Natural Sciences (NINS
program No. 01111702).
References
1. L. Schermelleh, R. Heintzmann, H. Leonhardt, J. Cell Biol. 190, 165–175 (2010).
2. Y. Ogura, M. Aino, and J. Tanida, Opt. Express 22, 25196–25207 (2014).
3. Y. Ogura, M. Aino, and J. Tanida, Appl. Opt. 55, 6371–6380 (2016).

62

The beneficial effects of the sauna exposures measured by PPG
Matti Huotari1, Kari Määttä2 and Juha Röning1
1

Biomimetics and Intelligent Systems Group (BISG), University of Oulu, Finland
2
Circuits and Systems (CAS), University of Oulu, Finland
Corresponding author: matti.huotari@oulu.fi

Keywords: Photoplethysmography, pulse waveform, decomposition, sauna exposure, arterial elasticity
This study concerns sauna and environment heat exposures regarding bathing methods for health enhancement,e.g., sauna bathing is very popular in Finland, and hot water bath is popular in Japan. A systematic review has
shown that a regular hot exposure has the potential to provide beneficial health effects on healthy people, butalso for those with cardiovascular-related and many other diseases, such as rheumatological diseases, and also
very good for sport people [1].
Photoplethysmographic (PPG) signals measurement system consists of ir (infra red) and red LEDs which illuminate the responses recorded by the photodiodes in the PPG probes. In PPG technology, the main difficulty
is its quantitative analysis, calibration can be challenging, and comparison of PPG wave easily obtained fromdifferent body locations, several persons, and with their skin color. In our measurements the light intensities
and wavelengths (red 640 nm & infrared 920 nm) are fixed. In practice, the results are the arterial pulse PPG
waveforms which are based on the propagating pulse wave during each heart cycle the pulse coming from theheart’s left ventricle. The pulse waveforms travel through the arterial circulatory system and arrive the peripheral, parallel capillary arteries to the finger and toe tips under the nails to be measured optically. The elasticity
index was calculated as the relation of the peak time of primary percussion wave to the peak time sum of thefour other primary waves [2].
Young female and male volunteers with good heart health status were included from the University departments in the dry sauna test group of 14 subjects. Finnish saunas involve short exposures (20-30 minutes) attemperature of 80oC with dry air. Before the sauna and the recovery from the sauna exposure were measured
with the subjects in the supine position. The study was approved by the ethical review board of the Oulu
University.
The primary wave components can be used for, e.g., elasticity calculations. However, full optical understanding of PPG waveforms and their physiology are still lacking. After investigation the intrinsic connection between infrared and red PPGs from finger and toe tips, and derived an elasticity index to describe the relationship between biophysical parameters, needs fewer assumptions than other methods. PPG recording before(left) (300 s), in sauna (middle) (1200 s), and after (right) (1300 s)sauna exposure is shown and 5 sec insert.
0,20

PPG finger

0,25
0,05

0,15

0,025

0,04

0,20

PPGfinger

0,000-0,025

PPG [V]

0,025-0,050PPG[V]

0,000

0,15

-0,175

0,01
1

2

3

4

5

Time (s)-0,075-0,100

50

51

52

53

54

55

Time (s)PPG [V]0

PPG [V]-0,200-0,050PPG[V]

0,02

0,10-0,125-0,150-0,025

0,03-0,075-0,100

0,05

0,00

0,10
0,05

0,06
0,05

-0,1250,00

PPG [V]

0,04

-0,05

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0,03
0,02
0,01

-0,05

0,00
-0,01
600

-0,175

601

602

603

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605

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0

50

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150

Time (s)

200

250

300

-0,10

0

100 200 300 400 500 600 700 800 900 1000 1100 1200

Time (s)0100 200 300 400 500 600 700 800 900 1000110012001300

Time (s)Fig1. PPG recordings from a 32 y male subject before (left), in sauna (middle), and after sauna (right)
PPG measurement gives bid data without proper analysis procedures, like decomposition of pulse waveforms.
However, further PPG research from sauna settings relating to the physiological mechanisms of sauna exposure that promote both arterial and cardiac health is needed.
References
1. Kamioka H, et al. (2019) Relationship of daily hot water bathing at home and hot water spa bathing with underlying
diseases in middle-aged and elderly ambulatory patients: A Japanese multicenter cross-sectional study, Complementary Therapies in Medicine, Volume 43, Pages 232-239.
2. Peltokangas, M, et al. (2017). Combining finger and toe photoplethysmograms for the detection of atherosclerosis,
Physiological Measurement, 38, (2).

63Creation and observation of a longitudinally-polarized optical needle
Léo Turquet1, Xiaorun Zang1, Joona-Pekko Kakko2, Harri Lipsanen2, Godofredo Bautista1, and Martti
Kauranen1
1

2

Photonics Laboratory, Physics Unit, Tampere University, P. O. Box 692, FI-33014 Tampere, Finland
Department of Electronics and Nanoengineering, Aalto University, P. O. Box 13500, FI-00076 Aalto, Finland
Corresponding author: leo.turquet@tuni.fi; godofredo.bautista@tuni.fi

Keywords: nonlinear microscopy, nanostructures, optical needle, longitudinal electric field
The term longitudinally-polarized optical needle is used to represent longitudinal electric fields that show
spot-like intensity distribution in the transverse (xy) plane with sub-wavelength confinement over a distanceof several l along the longitudinal axis (z), where l is the excitation wavelength. Such optical fields are expected to be valuable in providing alternative illumination conditions in optical microscopes, for multi-plane
imaging or micromanipulation of nanoparticles. Various methods, based on radially-polarized (RP) beams,
have theoretically predicted the generation of optical needles [1]. To date, however, experimental verifications of the generation a longitudinally-polarized optical needle in three dimensions have been scarce and incomplete.
In this work, we experimentally demonstrate the creation and observation of a longitudinally-polarized optical needle that is produced by a radially-polarized Bessel-Gauss beam (RPBG) generated at the focus of a
microscope objective [2]. This choice was motivated by the well-known large depth of field of vector BG
beams [3], and the longitudinal fields that arise from tight focusing of RP beams [4]. The RPBG beam wascreated by focusing a RP thin ring through a high numerical aperture (NA) objective. The ring itself was
generated by a RP doughnut beam and an axicon.
We managed to probe the longitudinally-polarized optical needle in three-dimensions using spatial mappingof the second-harmonic generation (SHG) from a single vertically-aligned GaAs nanowire, which wereshown to be sensitive to longitudinal fields excitation only [5–7].

Fig. 1. Comparison of experimental focal intensity distribution (top) and theoretical focal electric field distribution (bottom) of the optical needle, under identical focusing conditions, in the xy- and yz-planes.
In conclusion, we have experimentally generated and probed a longitudinally-polarized optical needle in the
focal region of a high NA objective. It was formed by a RPBG beam and characterized using SHG from vertically-aligned nanowires, sensitive to longitudinal electric fields. Our optical needle shows a depth of fieldof over 30 l and subwavelength transverse confinement (0.41 l). It also shows great spatial homogeneity
and almost perfect agreement with numerical simulations. This demonstration confirms the new opportunities opened by optical needles for microscopy, light shaping and optical micromanipulation.

64

References
1.
2.
3.
4.
5.
6.
7.

H. Wang, L. Shi, B. Lukyanchuk, C. Sheppard, and C. T. Chong, Nat. Photonics 2, 501–505 (2008).
L. Turquet, X. Zang, J.-P. Kakko, H. Lipsanen, G. Bautista, and M. Kauranen, Opt. Express 26, 27572–27584(2018).
Z. Bouchal and M. Olivík, J. Mod. Opt. 42, 1555–1566 (1995).
B. Richards and E. Wolf, Proc. R. Soc. London Ser. A 253, 358–379 (1959).
G. Bautista, J. Mäkitalo, Y. Chen, V. Dhaka, M. Grasso, L. Karvonen, H. Jiang, M. J. Huttunen, T. Huhtio, H.
Lipsanen, and M. Kauranen, Nano Lett. 15, 1564–1569 (2015).
L. Turquet, J.-P. Kakko, X. Zang, L. Naskali, L. Karvonen, H. Jiang, T. Huhtio, E. Kauppinen, H. Lipsanen, M.
Kauranen, and G. Bautista, Laser Photon. Rev. 11, 1600175 (2017).
L. Turquet, J.-P. Kakko, L. Karvonen, H. Jiang, E. Kauppinen, H. Lipsanen, M. Kauranen, and G. Bautista, J.
Opt. 19, 084011 (2017).

65

Opto-fluidic biosensors by roll-to-roll fabrication
Jussi Hiltunen, Sanna Uusitalo, Sanna Aikio, Marianne Hiltunen, Olli-Heikki Huttunen, Johanna HiitolaKeinänen, Christina Liedert and Pentti Karioja
VTT-Technical Research Centre of Finland, Kaitoväylä 1, 90570 Oulu, Finland
Corresponding author: Jussi.hiltunen@vtt.fi

Keywords: biosensors, microfluidics, SERS, nanoimprinting
Opto-fluidics is defined as a synergistic integration of photonics and microfluidics [1]. The research and development is driven by the potential for enhanced sensing performance and simplification of microsystems.
However, only few concepts have been transferred successfully into upscaled fabrication of integrated microsystems because of the lack of processing and integration methods. The question is not only about directcommercialization of designs and concepts, but also academic researchers benefit from the high-volume sensor
fabrication when the number of samples is not limiting the assessment of the designs and associated test protocols. Our work deals with the production upscale to manufacture optical sensor elements and microfluidic
sample handling platforms at high volumes.
Nano-scaled structures have been widely studied with several application areas as they exhibit interesting
physical and chemical properties which differ from the properties of the bulk material made of the same composition. The development of different planar plasmonic substrates tries to reduce the repeatability issues of
the colloids through periodicity of the nanostructures on top of the substrates. Fig. 1a) shows roll-to-roll (R2R)
imprinted periodic nanostructures. After coating the surface with a thin layer of gold, they are suitable forsurface-enhanced-Raman-spectroscopy (SERS) applications. Scanning-electron-microscope from the surface
is shown in Fig. 1b). We demonstrated the feasibility of such polymer-based low-cost SERS substrate in analyzing the microbes with characteristic Raman spectrum [2]. Microfluidic channels enable flowing sample
solution on chip in a controlled way and the sample molecules can be also accumulated onto the sensor surface
by analyte capture. Fig. 1c) presents an opto-fluidic sensor configuration where optical sensor surface is integrated with polymeric microfluidic and lid layers.

Fig.1. a) Roll-to-roll nanoimprinted optical surface on plastics, b) SEM-image from the surface, c) opto-fluidic
sensor configuration with integrated microfluidics and a sensors surface.
Elastomer poly(dimethylsiloxane) (PDMS) has become, and remained, one of the most used material in realization of microfluidic devices in academic environment. The wide usage of PDMS material is originated fromnumber of factors with most importantly the relatively cheap and easy prototyping of small numbers of devices
66

using master moulds for structure replication. PDMS surface properties can be also tuned and to bond PDMS
with glass and PDMS itself. These factors have enabled made the realization of prototype devices that test newideas in a shorter time and with lower cost than that which is reachable using silicon technology. Probably thebiggest factor in limiting the usage PDMS outside laboratory relates to the lack of high-volume fabrication
methods to produce microfluidics. In our recent work [3], it was demonstrated that also PDMS–based microfluidics can be roll-to-roll produced. Fig. 2a) shows a photograph from the R2R replication process, where
PDMS material is casted on Al-coated paper and microfluidic features are replicated by thermal curing. A test
device cut from a roll is shown in Fig. 2b). For the illustrative purposes, the fluidic channels were filled withcolored water. In order to validate the replicated molecular diagnostic platforms, on-chip amplification of viralribonucleic acid (RNA) with loop-mediated isothermal amplification (LAMP) was demonstrated. Amplification of negative control (no RNA) and positive control (RNA) were monitored optically in real-time by taking
fluorescence images with interval of 1 min. Test arrangement with positive and negative samples is shown in
Fig. 2c) and corresponding fluorescence signal as a function of time in Fig. 2d). LAMP amplification was
triggered by increasing the chip temperature from room temperature (RT) to 70° C within a time period of
about 3 mins. The onset of amplification occurred at a time period of about 13 – 16 min.

Fig.2. a) Roll-to-roll fabrication of PDMS-microfluidics on paper, b) sensor device where fluidic channels are filled withcolored water, c) test configuration for nucleic acid amplification and d) fluorescence signal from reaction chambers withand without RNA template molecules.

References
1.

X. Fan and I. M. White, Nature Photonics, 5, 591–597 (2011)

2.

S. Uusitalo, M. Kögler, A.-L. Välimaa, A. Popov, Y. Ryabchikov, V. Kontturi, S. Siitonen, J. Petäjä, T. Virtanen, R.
Laitinen, M. Kinnunen, I. Meglinski, A. Kabashin, A. Bunker, T. Viitala, J. Hiltunen, RSC Advances, 6, 62981 62989 (2016)

3.

J. Hiltunen, C. Liedert, M. Hiltunen, O.-H. Huttunen, J. Hiitola- Keinänen, S. Aikio, M. Harjanne, M. Kurkinen, L. Hakalahti, L. P. Lee, Lab on a Chip 18, 1552-1559 (2018)67

Quantitative CARS spectral imaging in studying protein droplets
Yelena Kan1,2, Sapun Parekh2,3, Erik M. Vartiainen1 and Lasse Lensu1
1

2

LUT School of Engineering Science, LUT University, Lappeenranta
Dept. Bonn: Molecular Spectroscopy, Max Planck Institute for Polymer Research, Mainz
3
Dept. of Biomedical Engineering, Univ. of Texas at Austin, Austin
Corresponding author: erik.vartiainen@lut.fi

Keywords: coherent anti-Stokes Raman scattering (CARS), microspectroscopy, hyperspectral imaging, intrinsically disordered proteins, protein dropletsProtein-protein interactions underlie nearly all biological functions. Many proteins that mediate DNA transcription in so-called RNA granules appear to have unique characteristics that lead to protein aggregation undercertain conditions. Many of these proteins are ‘intrinsically disordered’ and undergo a liquid-liquid phase separation (forming droplets) within cells [1] and in vitro [2]. The presence of plaques containing these proteins
is linked with many neurodegenerative disorders, including Parkinson’s, Alzheimer’s and Huntington’s disease [3]. The physical-chemical structure of the protein droplets is a subject of much debate as they are speculated to be precursors to proteinaceous plaques and possibly amyloid fibrils. To that end, we applied thebroadband coherent anti-Stokes Raman scattering (CARS) spectral imaging to study protein droplets for obtaining quantitative and chemically specific information. The protein structure was examined in different statesof the RNA-binding protein fused in sarcoma low complexity domain (FUS LC) in vitro.
For CARS measurements, the samples were "sandwiched" between two glass slides and secured on the translation stage in a broadband CARS/bright-field microscope. The broadband CARS microscope (Fig. 1) included
a pump laser (Spectra Physics Tsunami, 800 nm, 1 ps, 80 MHz), a photonic crystal fiber (PCF), a microscopewith a cooled microscope stage, a monochromator and a cooled CCD camera. Light from the laser was split
into two paths, one for generating a broadband (~ 3430 cm-1) Stokes pulse, and the other to provide narrowband
pump/probe pulses. Pulses were focused through the sample to produce a broadband (~ 500 – 3500 cm-1)
CARS signal. Typically, a 30x30-pixel area was scanned with a focused laser beam and from each pixel, a
CARS spectrum was recorded. For each pixel, a typical integration time of the measurement was from 30 to
60 ms.

Fig. 1. Broadband CARS setup.
The raw CARS spectra as such do not provide useful quantitative information. Such information is obtainedby retrieving the Raman spectra from CARS data [4]. Typically, this includes wavelet analysis to remove the(non-additive) experimental artifacts from CARS spectra [5], and then the MEM analysis to automaticallyextract the Raman line-shape spectra from the CARS line-shapes [4]. The CARS line-shapes, where the experimental artifacts are removed, are also used to make integrated microscopic images as shown in Fig. 2(a).
The Raman line-shapes (Fig. 2(c)) provide the basis for the protein analysis, i.e., chemical mapping inside aprotein droplet and to recognize the possible changes in protein structure within phase transitions from a monomeric solution to liquid droplets and from liquid droplets to solid aggregates. Moreover, statistical
68

multivariate methods, such as the Multivariate Curve Resolution–Alternating Least Squares (MCR-ALS) [6],implemented to the extracted Raman data, enables the separation of spectral and spatial distributions of chemical constituents of a sample (Fig. 3).
(a)

(b)

(c)

Fingerprint region

Pixel number

Fig. 2. Broadband CARS imaging of a protein droplet: (a) hyperspectral (integrated) CARS image, (b)broadband CARS spectra, (c) the retrieved Raman line-shapes at fingerprint region, inside (blue lines) andoutside the protein droplet.
(a)

(b)

Pixel number(c)Pixel number

Fig 3. Statistical multivariate analysis (MCR-ALS) on extracted Raman spectra: Pure component spectral
profiles (blue line: comp. 1) and corresponding distribution maps of liquid-liquid phase separated FUS LC.

References
1. A. A. Hyman, C. A. Weber, and F. Jülicher, Annual Rev. Cell Dev. Biol. 30, 39-58 (2014).
2. K. A. Burke, A. M. Janke, C. L. Rhine, and N. L. Fawzi, Mol. Cell 60, 231-241 (2015).
3. E. H. Koo, P. T. Lansbury and J. W. Kelly, Proc. Natl. Acad. Sci. 96, 9989-9990 (1999).
4. E. M. Vartiainen, H. A. Rinia, M. Bonn, and M. Müller, Optics Express 14, 3622–3630.
5. Y. Kan, L. Lensu, G. Hehl, A. Volkmer, E. M. Vartiainen, Optics Express 24, 11905–11916 (2016).
6. J. Felten, H. Hall, J. Jaumot, R. Tauler, de Juan A, and A. Gorzsás, Nature Protocols 10, 217–240 (2015).

69

Synthetic aperture phase-shifting measurement
for high numerical-aperture spherical surfaces
Kenichi Hibino1, Toshiki Kumagai2 and Yasunari Nagaike2
1
Geological Survey of Japan, AIST, Tsukuba, Ibaraki 305-8567, Japan
2

Olympus Corporation, Tatsuno, Kamiina, Nagano 399-0495, Japan
Corresponding author: k.hibino@aist.go.jp

Keywords: phase measurement, interferometry, Fizeau interferometer, phase shift, spherical test
High numerical-aperture (NA) spherical lens are now common and required for wide-angle view andhigh-magnification in optical products such as endoscopes, microscopes, and compact camera. The measurement repeatability of the spherical lens testing has reached a ~0.1 nm by a phase-shifting Fizeau interferometer, which improves the quality control of the optical products.
In a high NA lens testing, however, spatial nonuniformity of the phase modulation becomes critical in
the Fizeau interferometers with mechanical phase shifting. Fig. 1 illustrates the illuminating geometry of aspherical (convex) testing through the transmission reference sphere with a monochromatic light of  = 633
nm wavelength, as an example. When the reference sphere is translated along the optical axis by distance the phase  of the interference fringes is shifted by 4 / radians on the optical axis. In contrast, the amountof phase shift in the marginal region of the aperture decreases by 4 cos / where the cosine factor is a geometrical effect. This nonuniformity can couple with the multiple-reflection light between the reference andtest surfaces and result in the phase measurement errors. There have been designed many error-compensatingphase-shifting algorithms that can compensate for these phase increment errors. However, it is pointed out[1] that these algorithms no longer show enough reduction of the errors when the NA becomes larger than0.7, or nonuniformity exceeds -30% (sin = NA > 0.7). In industry, we like to test spherical lens whose numerical aperture is up to 0.9, in which the nonuniformity is typically -50%.
We have proposed a synthesis of phase-shifting algorithms in which the observing aperture is divided into seven annular regions and the object phase for each region is estimated by a different algorithm [2]. Fig. 2
illustrates the division of the aperture. In each region, the actual phase increment for each shift is approximately equal and is thus represented by 2/N radians (N = 6, 7, 8, …12). The phase-shift algorithm for eachregion is designed with 2 /N radians step. The divisor N should be integer for the sake of the orthogonality
among the harmonic components of the signals. By selecting these different phase steps, the spatial nonuniformity decreased from -49% to -7.7%. Numerical simulations show that the averaged phase measurement
errors by the present technique decreases to less than 1 nm, while those are 4 ~ 6 nm for conventionally covering whole observing aperture by a single algorithm.

Fig. 1. Illumination geometry of a sphericalFig. 2. Observing aperture division into seven annularconvex test in a Fizeau interferometer.

regions.

70

In the experiment, a glass spherical concave surface of 0.86 NA and 10 mm diameter was compared to areference transmission concave surface of the same numerical aperture and 22 mm diameter. To reduce theinternally scattered light noise, the source beam from a 633 nm wavelength He-Ne laser is transmitted
through a rotating ground glass diffuser and a multi-mode fiber of 200 m diameter to reduce the lateral
coherence [3] (coherence within the normal plane to the beam). The reference surface was translated along
the optical axis by a PZT and 13 interference images were recorded with equal time intervals by a CMOS
detector of 1024 x 1024 pixels. The relative phase step between the frames was designed to be 60° at thecenter of the aperture. The seven different algorithms were generated by the convolutions of the tworectangle sampling windows. Fig. 3 shows the phase distribution of the test surface calculated by the present
method. In order to estimate the magnitude of systematic errors, we intentionally introduced a slightly tilt in
the reference surface relative to the test surface, as was not the case of ordinary industrial measurements. The
raw interference image is shown in the small window of Fig. 3 which has several carrier fringes. Since wecannot observe the similar straight lines in the resultant phase map, we can see that the phase shift error wasvery well reduced.

Fig. 3. Phase distribution of the 0.86 NA sphericalFig. 4. Phase difference between null and tilt alignments

concave surface and its raw interference image.

after removing the Zernike 36 components.

Fig. 4 shows the residual phase difference between the null (not shown) and tilt alignment (Fig. 3) phasemaps after we removed the Zernike 36 components from the phase. We can observe the fine structure of therandom and systematic errors. In the figure, we can observe concentric discontinuous errors of ~0.2 nm
magnitudes. The positions of these concentric errors coincide with the borders between the two neighboring
regions. The sensitivity of the algorithm to the phase-shift error is generally different from algorithm to algorithm. If we denote the error coefficient of the phase shift by i we can denote the linear term of the phasemeasurement error as∆𝜑 = 𝐽𝜀1 𝑠𝑖𝑛2𝜑 + 𝐾0 𝜀2 + 𝐾1 𝜀2 𝑐𝑜𝑠2𝜑 + 𝐹1 𝑅𝜀1 𝑠𝑖𝑛𝜑 + 𝐹2 𝑅𝜀1 𝑠𝑖𝑛3𝜑.
(1)
where R is the reflection index of the surfaces and  is the object phase. The error coefficients J, Ki and Fi
distribute from 0.0 to 0.5 depending on the algorithms. We think that these error discontinuities in Fig. 4 arecaused by the different sensitivity of the algorithms to the errors. Our next goal is how to reduce these discontinuous errors caused from the different sensitivities of the algorithms in the divided regions.
References
1. de Groot, P.J., “Correlated errors in phase-shifting laser Fizeau interferometry,” Appl. Opt. 53, 4334-4342 (2014).
2. T. Kumagai, Y. Nagaike, K. Hibino, “Dual phase-shift schemes for internal-reflection noise reduction in a spherical
Fizeau interferometer,” Interferometry XIX: Techniques and Analysis (San Diego), SPIE 10749, 1074915-1-8(2018).
3. Kumagai, T., Hibino, K., and Nagaike, Y., “Dual-phase-shift spherical Fizeau interferometer for reducing the internally scattered light noise,” Opt. Eng. 56 (3), 034102 (2017).

71

3D measurements of freeform samples using multi sensor optical profilometer
Ville Heikkinen1, Johan Nysten1, Ville Byman1, Björn Hemming1 and Antti Lassila1
1

VTT Technical Research Centre of Finland Ltd, Centre for Metrology MIKES, P.O. Box 1000, 02044VTT Tekniikantie
1, 02150 Espoo, Finland
Corresponding author: ville.heikkinen@vtt.fi

Keywords: Traceability, stitching, scanning white light interferometer, chromatic confocal sensor, interferometry
Scanning white light interferometer (SWLI) allow fast measurements of relatively large areas with nm classvertical precision. Compared to coherent interferometers it has the advantage of measuring across verticalsteps. However, measurement of small details and high slopes requires large magnification, while measurement of larger samples requires smaller magnification and larger pixel size. Thus measurement of sloping
surfaces or small features is often impossible using the same magnification that would allow measurement of
the whole sample. This limitation can be solved by stitching several images together into a large high-resolution image. Traditionally stitching would require adjusting the tilt around three axes and 3-D shifts of theadjacent sub-images due to accuracy of the sample/sensor translation. This makes the method computationallydifficult and decreases the accuracy of the combined 3-D profile.
For more accurate and computationally easier approach, a SWLI was combined with precise sample movementon air bearings and heterodyne laser interferometers tracking sample movement and rotation. The precisely
controlled and tracked sample movement allows accurate stitching of sub-images by only adjusting height
between images. The setup also used a chromatic confocal height sensor that is added to the setup for quickcoarse scans of large areas.
Samples up to 10 x 10 cm wide can be measured with 50 nm nanometer level vertical accuracy and nanometerlevel resolution. For fine details, hundreds of SWLI sub images can be combined into datasets of over 100
million pixels. For fast results, the sample can be quickly scanned using the confocal sensor. Slope limit forsmooth samples for the confocal sensor is 15 degrees and for the SWLI the slope limit varies from 3 to 18
degrees using the currently used magnifications. Rough samples can be measured with higher slopes.
To test the profilometer samples up to 10 cm using were measured using the confocal sensor and samples up
to 5 cm in were measured using the SWLI and stitching. Sample movement straightness was below µm in allaxes and confocal noise below 50 nm. Stitching accuracy in 10 nm range could be achieved for >100 SWLI
images as shown in Fig. 1.

nm

nm

Fig. 1. Height map created by stitching 130 sub-images stitched together (left) and standard deviation of
overlapping pixels (right) illustrating stitching accuracy.

72

Thickness distribution measurement with a full-field pulsed SD-OCT
Takamasa Suzuki1, Bin Liu2, Samuel Choi1, and Osami Sasaki1
1

2Faculty of Engineering, Niigata University, Niigata
Graduate School of Science and Technology, Niigata University, Niigata
Corresponding author: takamasa@eng.niigata-u.ac.jp

Keywords: optical coherence tomography, phase analysis, wavelet transform, pulse irradiation
Optical coherence tomography (OCT) has been used in medical applications such as ophthalmoscopy [1],skin examination [2], and circulatory system examination [3] because it allows for noncontact andnoninvasive measurements with high resolution and sensitivity. A spectral domain OCT (SD-OCT) uses a
low coherence light source and consists of a Michelson interferometer, a diffraction grating, and image
sensor for spectroscopy. The sensor captures the spectrum of the interference signal. However, the phase of
the interference signal varies nonlinearly because the phase is inversely proportional to the wavelength thatspreads linearly in space by the diffraction grating, Thus, the interference signal becomes a nonstationary
signal (e.g., chirp signal) in space in an SD-OCT.

The relationship between phase and wavelength is givenby:L     i  i /  2  .

1000

Wmax(a,b)

100

200

400
600
800
(b) Modulus map

1000

Wmax(a,b)

100
200

400
600
(c) Phase map

800

1000

3.14

α [rad]

(3)

200
400
600
800
(a) Interference signal

200

(2)

The phase i that corresponds to i can be calculated by
CWT, which detects the phase and instantaneous
frequency simultaneously. The phase offset  is given by:  h /     2 2  11  /  1  2  .

200
Scale awhere L, , and i are the optical path difference (OPD),
phase offset, and phase distribution, respectively. The
equation indicates that the phase varies nonlinearlyaccording to the linear change of the wavelength. From(1), L is expressed as:

0
-1
0

(1)

Scale ai  2 L / i    i ,f(x)We propose a signal analytical technique that uses continuous wavelet transform (CWT) [4], where aconventional Fourier transform (FT) is not suitable for processing nonstationary signals. Moreover, we
propose a pulse irradiation technique for capturing fringe. It allows us to capture clear fringes even when an
object, such as a transparent glass plate, may have a low reflection rate. Several experiments indicate that thepulse irradiation technique is effective at improving
measurement accuracy.
1

Figure 1 shows the procedure for the data analysis.
0Specifically, the figure schematically explains the signal-3.14
processing in our technique. The parameters of OPD,
0
200
400
600
800 1000
wavelength range, and data number used in the simulation(d) Detected phase distribution
were L = 40 m, I = 730–930 nm, and 1000, respectively.
Fig. 1 Signal processing based on CWT.
The calculated one-dimensional (1D) fringe is shown in Fig.
1(a). Here, we can see that it becomes a chirp signal. Modulus and phase maps were obtained as shown in
Fig. 1(b) and 1(c), respectively, after CWT for the fringe was calculated.
A schematic of the experimental setup is shown in Fig. 2. We used a super-luminescent diode with a spectral
bandwidth of 60 nm at a center wavelength of 845 nm. The beam is split into two beams using a beam
splitter (BS). One beam passes through a cylindrical lens 1 and a galvanometer and was focused onto thesurface of the object. The other beam passes through to the reference mirror. These beams are recombined at
the BS and interfere with each other. The interference signals are focused linearly on a charge-coupleddevice camera using a cylindrical lens 2.

73

Cover glass
CCD
cameraFringeShutter

x

GM

CL2

Obj
CL1 x
BS

Grating

0=845nm
=60nm

Lens

Pulse
generator

SLD

Oscillator

PC

Fig. 2 Experimental setup.
The 2D thickness distribution of the cover glass was measured. Figure 3 shows the results obtained with
continuous irradiation. The contrast of the captured fringe as illustrated in Fig. 3(a) appears not to be highwith continuous irradiation. The obtained 2D thickness distribution is shown in Fig. 3(b). After several
measurements at the same position, we confirmed the repeated measurement error of 0.16 m at root meansquare. The same measurement was conducted with a pulse irradiation. We found that a fringe is captured
with high contrast, as shown in Fig. 4(a). The 2D thickness distribution is shown in Fig. 4(b). The repeatedmeasurement error was 0.09 nm. We confirmed that the repeatability was improved with the pulse irradiation.
We traced 3D thickness distributions measured with continuous and pulsed irradiations, as shown in Fig. 5.
Mean thickness values were 151.08 and 151.25 nm at continuous and pulsed irradiations, respectively. A
repeated measurement error of 0.32 nm at continuous irradiation was improved to 0.23 nm at pulsed
irradiation.

100

100

200

200

300

300

400

400

y[pixel]y[pixel]In this study, a full-field SD OCT driven by a pulse irradiation was proposed and demonstrated. The
nonstationary signal was successfully processed by using continuous wavelet transform. A pulse irradiation
was effective at obtaining a clear fringe and reducing the measurement error.

500
600

500
600
700

700

800

800
900

900

1000

1000

50

100

150

50

200

150

x[pixel]

(a)

(a)

200

(a) Contimuous

180

149.99μm

Thickness[m]Thickness[m]100

x[pixel]

180

150

120

average value
151.08μm

149.93μm
150

120

0

200

400

600

800

1000

average value
151.25μm

0

200

400

600

800

1000

x[pixel]x[pixel](b)(b)

Fig. 3 2-D thickness distribution

Fig. 4 2-D thickness distribution

with continuous irradiation.

with pulse irradiation.

(b) Pulse

Fig. 5 3-D thickness distributions

with (a) CW and (b) pulse irradiation.

References
1. A.G. Podoleanu, J.A. Rogers, D.A. Jackson, S. Dunne, Opt. Express 7 (9), 292–298 (2000) .
2. E. Dalimier, D. Salomon, Determatology 224 (1), 84–92 (2012) .
3. W. Luo, D.L. Marks, T.S. Ralston, S.A. Boppart, J. Biomed. Opt. 11 (2) (2006) 021014.
4. M. Sifuzzaman, M.R. Islam, M.Z. Ali, J. Phys. Sci. 13, 121–134 (2009) .

74

Low-coherence interferometer with 10MHz repetition rate
Katsuhiro Ishii1, Masaharu Hoshikawa1,2, Takeshi Makino3, Takahiro Hashimoto3,
Hideaki Furukawa3 and Naoya Wada3
1

The Graduate School for the Creation of New Photonics Industries, Hamamatsu
2
Electron Tube Division, Hamamatsu Photonics K. K., Hamamatsu
3
National Institute of Information and Communications Technology, Koganei
Corresponding author: ishii@gpi.ac.jp

Keywords: Highly repetitive low-coherence interferometer, time stretch dispersive Fourier transform technique
We constructed a low-coherence interferometer with 10MHz repetition rate and confirmed its basic characteristics. The low-coherence interferometry consists of a mode-locked laser diode and a fiber-optic interferometer. Based on the time stretch dispersive Fourier transform technique, highly repetitive mesurement isacheved. We demonstrate the 10MHz measurement of interference signals and path-length deference.
1. Introduction
Low coherence interferometry (LCI) and optical coherence tomography (OCT) have attracted attention for
in-situ measurement of laser processing [1-3]. In the spectral domain OCT(SD-OCT) and swept source
OCT(SS-OCT), the repetition rate of measurement is limited to several hundred kHz at present. Since laser
processing is a rapid phenomenon with a time scale faster than several hundred ns, a measurement methodwith higher repetition rate is required to observe such a fast phenomenon.
One of high repetition measurement is a time-stretch technique [4-6]. The time-stretch technique is highlyrepetitive spectroscopy, and its repetition rate is several MHz or more. By combining time-stretch techniqueand low-coherence interferometry, we can construct highly-repetitive low-coherence interferometer with arepetition rate of several MHz or more [7].
In this paper, we construct a measurement system of low coherence interferometry with 10MHz repetition
rate using ultra-short pulse and time-stretch technique and confirm its basic characteristics.

Time-stretcher

Fiber optic Michelson interferometerCL
Ultra-shortpulse laser

DCF

50/50
CP

PD

M1

OBJ

CL

M2

Real-time
oscilloscopeMS

Fig. 1. A Schematic diagram of measurement system of highly-repetitive low-coherence interferometer.
DCF: dispersion compensation fiber, CP: fiber coupler, CL: collimating lens, M1 and M2: reflective mirror,
OBJ: objective lens, MS: manual stage, PD: photo diode.

75

2. Measurement system
A schematic diagram of the measurement system is shown in Fig.1. It was composed of an ultra-short pulse
laser with pulse width less than 1ps, time-stretcher using dispersion compensating fiber (DCF), opticalinterferometer, photodiode (PD) and real-time oscilloscope. The ultra-short pulse from MLLD was passed
through a dispersion flat fiber to generate supercontinuum light with 23.5nm wavelength bandwidth. Repetition frequency of the laser pulse is down-converted to be 10MHz by the LiNbO3 modulator. Then it feeds toa time-stretcher composed of a dispersion compensation fiber (DCF) with a wavelength dispersion of
8959ps2. The pulse width after passing through the time-stretcher is 28.2ns. The fiber-optic Michelson interferometer consists of a 50:50 optical fiber coupler, two collimating lenses, an objective lens and two reflective mirrors. Interference signals are detected by photodiode (32GHz) and recorded by a real-time oscilloscope (16GHz, 20GS/s).
3. Results and discussion

(a)

(b)

0.14

0.14

0.12

0.12

0.10

0.10

Intensity (a.u.)

Intensity (a.u.)

Figure 2(a) shows an obtained signal recorded by the real-time oscilloscope. The optical path length difference between the signal and reference paths is set to ΔL = 2 mm. A pulse train is observed every 100ns. Enlarged view of one pulse is shown in Fig. 2(b). An interference signal is also observed in each pulse. It is
confirmed that interference signal measurement of the low-coherence interferometer can be performed at ahigh repetition rate of 10MHz. We also confirmed that optical path length difference can be estimated accurately from the obtained interference signals.

0.08
0.06

0.04
0.02

0.08
0.06
0.04
0.02

0.00

0.00
0

500

1000

1500

2000

Time (nsec)

0

20

40

60

80

100

Time (nsec)

Fig. 2. (a) Pulse train recorded by the real-time oscilloscope and (b) its enlarged view of one pulse.
4. Summary
We constructed a measurement system of highly repetitive low-coherence interferometry using time-stretch
technique. It was confirmed that interference signals can be measured at high repetition rate of 10MHz. The
estimated optical path length and the measured values showed good agreement.
References
1. P. J. L. Webster et. al., Optics Letters 21, 6217-6220 (2014).
2. T. Bautze et. al., Lasers in Manufacturing Conference 2015, 73 (2015).
3. N. Authier et.al, Proc. ICALEO2016, 904 (2016).
4. D. R. Solli, J. Chou, and B. Jalali, Nature Photonics 2, 45-51 (2008).
5. G. Herink, B. Jalali, C. Ropers and D. R. Solii. Nature Photonics 10, 321-51 (2016).
6. H. Furukawa, et. al., Proc. SPIE 10089, 10089A1-H6 (2017).
7. K. Ishii and M. Hoshikawa, Proc. OIE2017, S9 (2017).

76

Analyzing and applying light with randomness;from speckles to fractals and to spectra
Jun Uozumi
Faculty of Engineering, Hokkai-Gakuen University, Sapporo, Japanuozumi@eli.hokkai-s-u.ac.jp

Keywords: laser speckle, fractal optics, near infrared spectroscopy, fractional derivative
The optical engineering abounds with randomness. Most of objects have random features such as their shapes,
surface conditions and inside structures, and intervening optical systems may also contain randomness arisingfrom irregularities, dusts or refractive index ﬂuctuations of optical elements, atmosphere and so on. The development of optical technology is, therefore, always a battle against such randomness. My researches have beenmostly related with analyzing, applying and sometimes controlling light affected by the randomness. In thisreport, I would like to look back to my researches in this respect.
Most common random phenomenon in optics may be scattering. Light is scattered on encounter with any
object. When the object is random, the scattering becomes also a random phenomenon. As a study on aninteresting random scattering, I started my research life with a computer simulation study of image speckle
patterns in relation to surface roughness proﬁle [1]. During this study, I was captivated by speckles, and analysis
and application of speckle statistics continue to be one of my main research subjects since. Throughout myundergraduate and graduate study, I concentrated on analysis of partially developed Gaussian and non-Gaussian
speckles. Partially developed speckle as shown in Fig. 1(a) is generated from a weak diffuser, and its contrast
deﬁned by the ratio of the standard deviation of the speckle intensity to its mean varies between zero and unity,
depending on the surface roughness. Meanwhile, speckle is referred to as Gaussian or non-Gaussian when its
complex amplitude obeys complex Gaussian statistics or not, respectively.
Among different types of randomness of objects, fractals came to attract me because of their exotic and nevertheless ubiquitous features. Since many natural and artiﬁcial objects have fractal properties as Mandelbrot
showed [2], I thought it would be interesting and important to reveal optical phenomena related to fractals.
Therefore, we started to work for fractal optics as my second main research subject, and examined diffraction
and scattering from fractal objects, different kinds of optical procedures to measure fractal properties such as
fractal dimension, and methods for generating fractal waveﬁeld [3]. Noncontact feature of optical techniques
is particularly favorable for fractal objects, which are mostly fragile.
A random fractal object gives rise to speckles in its diffraction ﬁeld. In the study of speckles produced bya randomized Koch curves, we found two strange phenomena; high contrast and clustered speckles. Most
researchers involved in statistical optics may think that the maximum contrast for Gaussian speckle is unity,
corresponding to zero-mean circular complex Gaussian statistics, and such speckle is said to be fully developed(Fig. 1(b)). However, it was revealed
that contrast can be higher than unity even for Gaussian speckles with√the true maximum contrast of 2 [4]. Such a high contrast Gaussian speckle appears when the scatterer hasa central symmetry or binary randomness and, therefore, can be an indication of the existence of such special(a) Partially developed speckle (b) Fully developed speckle(c) Nondiffracting speckle(d) Fractal speckle

Figure 1: Various types of speckle patterns produced by computer simulations.

77

Figure 2: Anisotropic fractal
speckle.

Figure 3: Fractional (left) and fractional absolute (right) derivatives of
NIR spectra of rice ﬂour.

random features in the scattering object. The clustering feature observed in the above situation has led to thestudy of nondiffracting speckle, which is a random version of nondiffracting beam. Namely, illuminating a
diffuser with a ring-slit aperture gives rise to approximately nondiffracting speckles in the space behind a lensplaced a focal length away from the diffuser [5]. They have a clustered and snake-like appearance as shownin Fig. 1(c) with smaller speckle size than the case with a circular aperture for illumination. This feature wasrecently employed in the enhancement of spatial resolution in digital holographic microscopy [6].
In fractal optics, we also interested in generating speckle ﬁeld with fractal property, namely fractal speckles[7, 8]. Since a fractal structure has a long extending correlation tail, fractal speckle is expected to extend themeasurement range in the speckle metrology using spatial correlation. Fractal speckles as shown in Fig. 1(d)can be produced as speckled speckle using two scatterers of random fractal object and an ordinary phase screen.
Fractal speckles are also observed in bio-speckles produced in some situations, and their fractal dimension was
found to convey important information about the biological tissue [9]. It was also shown that, in the diffraction
geometry, the fractal speckle has different dimensions between in the lateral plane and longitudinal direction[10]. I am currently investigating such anisotropic fractal speckles by computer simulations as shown in Fig. 2.
After I ﬁnished my doctoral course, I joined National Food Research Institute (NFRI), Japan, for ﬁve years,
where I was engaged in the near infrared spectroscopy (NIRS) for nondestructive food analysis. This was adrastic change of my research object from speckles to spectra. However, the research on NIRS was anotherbattle against randomness involved in measured spectra; random shape of object, scattering due to internal andsurface irregularities, and random distribution of constituent concentrations, and became another member of
my main research subjecs. This multifold randomness in spectra is processed with various statistical treatments
including elaborated regression analyses. With these statistical analyses, derivative spectra are sometimes employed to resolve complex spectral features. I paid particular attention to the derivative, trying to provide
additional ﬂexibilities by extending the derivative order from integers to fractional values and by controlling
positions of derivative peaks (Fig. 3) [11]. Application of these treatments is still on the way, and I believe that
it will contribute to improving the measurement accuracy in NIRS.
Random light is annoying but sometimes conveys interesting information, proving useful if properly processed.

References[1] H. Fujii, J. Uozumi and T. Asakura, J. Opt. Soc. Am., 66(11) (1976) 1222–1236.
[2] B. B. Mandelbrot, The Fractal Geometry of Nature, (Freeman, San Francisco, 1982).
[3] J. Uozumi and T. Asakura, Current Trends in Optics, ed. J. C. Dainty (Academic, London, 1994) 83–93.
[4] J. Uozumi, K. Uno and T. Asakura, Opt. Rev., 2(3) (1995) 174–180.
[5] M. Ibrahim, J. Uozumi and T. Asakura, Opt. Rev., 5(3) (1998) 129–137.
[6] H. Funamizu, J. Uozumi and Y. Aizu, OSA Continuum, 2(6) (2019) 1822–1837.
[7] K. Uno, J. Uozumi and T. Asakura, Opt. Commun., 124(1,2) (1996) 16–22.
[8] J. Uozumi, M. Ibrahim and T. Asakura, Opt. Commun., 156(4–6) (1998) 350–358.
[9] N. Yokoi, Y. Aizu and J. Uozumi, J. Biomed. Opt., 24(3) (2018) 031018-(1–7).
[10] K. Tsujino and J. Uozumi, Asian J. Phys., 27(9–12) (2018) 515–528.
[11] J. Uozumi, Asian J. Phys., 27(9–12) (2018) 573-585.

78

Wavelength-tunable common-path interferometerwith a diode source
Yukihiro Ishii1,2, Shunpei Yukita1, Kosuke Kiyohara3, Jun Chen4, and Eiji Tokunaga1
1

Department of Applied Physics, Tokyo University of Science, Tokyo 125-8585, Japan
Photonics Control Technology Team, RIKEN Center for Advanced Photonics, Wako 351-0198, Japan
3
Kiyohara Optics Inc., Tokyo 174-0041, Japan
4
Tokyo Polytechnic University, Atsugi 243-0297, Japane-mail; y.ishii@rs.kagu.tus.ac.jp, yukihiro.ishii@riken.jp

2

Keywords: wavelength-tunable heterodyne interferometer, common-path layout, birefringent beam splitter
The temporal carrier beat frequencies in an optical heterodyne interferometer can be generated by ramping
the wavelength in a laser diode (LD) based on the frequency-modulated continuous-wave (FMCW) techniques [1,2]. The beat frequency is proportional to the optical path difference (OPD) of each pair of interfering beams in an interferometer [3]. It enables us to construct an optical heterodyne interferometer on an unbalanced OPD without auxiliary frequency modulators [4,5]. We demonstrate a new type of optical heterodyne common-path interferometer with a birefringent beam splitter with a wavelength-tunable externalcavity laser-diode (ECLD) source. A two-dimensional (2-D) phase map of a phase object in transmission canbe derived from the numerical Fourier transformation of measured heterodyne interference fringes. Successive measurements are precisely made during tuning because the common-path interferometer [6] is quite
insensitive to external disturbances.
A common-path interferometer is shown in Fig. 1 with a plane-parallel plate with a thickness d giving an
unbalanced OPD. A light source is a wavelength-tunable ECLD (-focus, -6300) those widely tuned wavelength diversity in the experiment is = ~2 nm at a central wavelength of =680 nm. A collimated beamfrom an ECLD has an upper object beam with a phase object in transmission adding with an unbalanced
OPD 2nd/ where n is an index of an optical-polished plane-parallel plate glass. An ITO (indium tin oxide)
thin film to be measured is evaporated on a slide glass of an index n, and is put on an object beam as a phaseobject of the thickness variation (x,y) of an index ni. A lower beam is a reference wave. Both object and reference waves are magnified with a microscope objective plus a convex lens and imaged onto a CMOS camera with a 7 lateral magnification. Both beams are incident on a Wollaston prism through a half-wave plateof a 45-azimuth polarization that splits into O and E rays travelling in different direction. Both waves arelaterally sheared upward (red circle) and downward (blue circle), and are interfered with each other after
passing through a polarizer.
Test ITO film

Plane-parallel plate

ECLD
680 nmReference beam
Object beam

Wollaston prism

O
C
M
O

d

(x,y

S

)

Half-wave plate

Pol

E

Fig. 1. Setup of a heterodyne common-path interferometer.
On the measurement, the square wave from an electronic function generator is applied to an ECLD diodesource and triggered to a high-speed CMOS camera for capturing the heterodyne interference signals [7].
Camera data are sent to a computer that calculates the Fourier transform of the interference signals. By extracting the Fourier spectra of the interference signals at a beat frequency fs= 33 Hz, a phase data of a test
ITO evaporated film can be measured as shown in Fig. 2. Phase gradient of ITO film can be measured withsubracting an addtional phase measurement those experiment is made when the ITO thin film does not put on79

an object arm to remove the unnecessary phase errors. Figs. 2 (a) and (c) show the wrapped phase map (a)and the unwrapped phase map (c) in grey demonstration exhibiting the phase 2(n-1)d/. Figs. 2 (b) and (d)show the wrapped phase map (b) and the unwrapped phase map (d) in grey demonstration in which the ITO
thin film is inserted into an object arm exhibiting the phase 2{(ni-1)+(n-1)(d+s)}/. The subtraction of thephase in Fig. 2 (c) from the phase in Fig. 2 (d) gives the phase 2{(ni-1)+(n-1)s}/ of an ITO thin film
shown in Figs. 2 (e) in grey demonstration and (f) in 3-D phase map. The phase demonstrates a stepped partat an edge of the ITO film evaporated on a slide glass. The small phase step of ~2.2 rad (=~/3) in Fig. 2 (f)can be measured.
A heterodyne common-path interferometer with a Wollaston birefringent beam splitter acts to eliminate theexternal disturbances, assuring that precise phase measurement is performed. The optical heterodyne signals
can be generated by a wavelength-tunable laser-diode source. The phase distribution can be measured fromthe Fourier transform of 2-D heterodyne interference signals.

Fig. 2. Subtraction of unwrapped phase (c) of the object film removed from unwrapped phase (d) of theobject inserted gives the measured phase difference of ITO film in (e) and (f).

References
1.
2.
3.
4.
5.
6.
7.

I. P. Giles, D. Uttam, B . Culshaw, and D. E. N. Davies, Electronics Letters 19, 395-396 (1983).
J. Zheng, Optical frequency-modulated continuous-wave (FMCW) interferometry, (Springer,
New York, 2005).
Y. Ishii, Laser-diode interferometry, Progress in Optics, E. Wolf, Ed., 46, 243-309 (Elsevier, Amsterdam, 2004).
M. Suematsu and M. Takeda, Applied Optics 30, 4046-4055 (1991).
F. Lexer, C. K. Hitzenberger, A. F. Fercher, and M. Kulhavy, Applied Optics 36, 6548-6553 (1997).
J. Endo, J. Chen, D. Kobayashi, Y. Wada, and H. Fujita, Applied Optics 41, 1308-1314 (2002).
R. Onodera and Y. Ishii, Applied Optics 35, 4355-4360 (1996).

80Speckle Shearing Interferometric Vibrometer for Heartbeat Monitoring
Ryohei Hanayama and Katsuhiro Ishii
The Graduate School for the Creation of New Photonics Industries, Hamamatsu
Corresponding author: hanayama@gpi.ac.jp

Keywords: speckle interferometry, vibration measurement, human skin, high-speed camera
We developed a speckle shearing interferometer for measuring micro vibration of human skin such as skinvibration caused by heartbeat. We focused on the volume pulse, or plethysmogram, that is used to diagnosediseases of blood vessel. It can be observed as the motion of skin surface. As with this case, it can be thoughtthat the vibration of skin surface shows the condition of human body. Moreover, we are planning to apply
this method for monitoring emotion of a car driver. We adopted monolithic prism type shearing
interferometer. Because it is very robust to disturbances, such as external vibration or shoch. An anti-shock
type high-speed camera is also adopted for this interferometer.
The photo and schematic diagrams of developped interferometer are shown in figs.1 and 2. It consists of twoparts. One is the illumination part, which irradiate laser beam for the target. Another is interferometer part.
The optical system of the interferometer is based on the twyman-green interferometer. The mirror of one armof the interferometer is slightly tilted from the optical axis, to realize the shearing interferometer. Then, thetwo reflected beams are slightly sheared each other on the imaging plane of the camera. This lateral shearing
interferometer measures the spatial differences. In other words, the inclination of the target can be measured.
When the laser beam illuminates the target surface, which is rough surface made of general materials, thespeckle can be observed. The speckle image has the information of the target surface, nevertheless it looks
random pattern. This interferometer brings the information, especially the inclination, into sight. We acquire
the speckle pattern using high-speed camera. The maximum frame rate of this camera is 30kfps when thespatial resolution is 256 × 256 pixels.
We investigate the sensitivity of developed instrument. In the developed interferometer, there is 0.2deg. of
angle difference between the two of beams, because there is 0.1deg. of inclination in one face of the prism(show Fig.2). The angle difference leads the image shearing. The shear amount depends on the distance fromthe interferometer to the object plane. When the working distance is 0.5m, the shear amount is 1.8mm. The
height difference between two of corresponding point on the object surface causes the interferometric signal.
When λ/4 of height difference exists, the inclination change is:(1)

Fig.1 Developped vibration measuring speckle
shearing interferometer81

Fig.2 Schematic diagram of speckle shearing
interferometerFig.3 experimental setup

Fig.4 inclinational deformation simulaterwith piezo actuator

Fig.5 acquired raw image

Fig.6 ESPI image. The red square shows theinclined area

In this case, wavelengthλis 855nm. s is shear amount. Diameter of blood vessel changes by modulation ofblood pressure. It is said that the diameter change is about 1% of the diameter. When diameter of a bloodvessel is 1mm, the amplitude of diameter modulation is about 10μm. If the baseline of inclination is several
mm, the inclination caused by the diameter change is the order of 10-3rad. Therefore it can be thought that
1.2×10-4rad. of inclination sensitivity is enough for heartbeat monitoring. The experimental results showthat the developed interferometer can detect 3×10-4mrad. of inclination. The experiment setup and the inclination simulator are shown in figs 3,4 and 5. In the inclination simulator, micro inclination vibration can becreated on a paper tape using piezo actuator. Fig 6 shows the acquired ESPI image. As we can see the inclined area was highlighted and can be detected, successfully.
As far as it concerns to heartbeat motion, the main and largest spike in electrocardiogram is known as QRS
complex. The usual width of QRS complex is from 60 to 100msec. The largest slope change occurs in from30 msec. to 50 msec. Then it can be thought that the slope change of human skin caused by heartbeat can beacquired using high-speed camera whose maximum frame rate is more than 50 fps. Therefore, it can bethought that the skin vibration can be measured by the developed interferometer and high-speed camera. In
the future work, we will apply this technique to human emotion monitoring during driving a car.
This work was supported by the Center of Innovation Program from Japan Science and Technology Agency,
JST.
References
1. Hirohide Iwata and Masafumi Hirai, J Jpn Coll Angiol 45, 329-332 (2005). (in Japanese)
2, German Da Costa, Opt. Comm. 117, 395-398 (1995).
3, Sergey Y. Kusmin, Proc. SPIE 3053, 9-17 (1997).

82

Multimode interferometers on strip-loaded waveguide for sensing
M. Roussey1, R. Rao1, and S. Pélisset1
1

University of Eastern Finland, Institute of Photonics, Joensuu, FI-80101, Finland
Corresponding author: matthieu.roussey@uef.fi

Keywords: Integrated optics, waveguides, sensing, micro- and nano-structures

A channel waveguide can be either single mode or multimode. In the second case, the intermodal interference
will result in self-imaging effect of input field at periodic intervals [1]. This effect constitutes the basic operation of multimode interference (MMI) devices. An MMI structure consists of one (or several) single-mode
waveguide, i.e., the input field, a free propagation region, i.e., a section with a wide width, where the intermodal interferences occur, and several output single-mode waveguides. MMI structures possess unique properties such as tolerance to a wide wavelength range, low loss, ease of the fabrication, and small footprint. Thes
properties make them attractive for applications in coupling and splitting technologies in different configurations such as Mach-Zehnder interferometers, modulators, switches, and wavelength division (de)multiplexers.
Different MMI configurations as 1xN and NxM power couplers and splitters can be efficiently produced overa very small area of a few hundred microns. In recent years, MMI structures are greatly used in developing
chemical- and bio-sensors also. The idea behind using MMI structure as sensors is due to their high sensitivity
to refractive index changes and polarization states. For instance, the MMI section can act as the sensing zone
if this one is covered or replaced by an analyte. The optical parameters change of the cover material corresponds to the change in the propagation conditions of the modes within the structure. By observing the field
intensity at the output of structure, we can track the chemical parameters of the analyte. The concept can beeasily adapted to various sensing applications by appropriate selection of the sensor material constituting the
MMI itself. The sensitivity can also be increased by varying the geometrical parameters of the MMI section.
We propose here the concept of MMI on strip-loaded platform to perform sensing. A strip-loaded waveguideconsists of a high refractive index thin film to confine vertically light. A polymer strip is deposited on top of
the thin film in order to induce a local effective index change of the slab waveguide mode. This leads to alateral confinement of the field in the thin film [2]. One condition on the correct operation of this type ofwaveguide is that the refractive index of the loading strip must be lower than the effective index of the guided
mode. Otherwise, light will be preferentially guided in the loading strip.
By realizing an MMI device on such a platform one can easily split the field between several outputs. We
recently proposed a demonstration on a strip-loaded slot waveguide platform, for which the thin film is ahorizontal slot waveguide [3-5]. Figure 1 (a) is a micrograph of the top view of the fabricated device on a
TiO2/SiO2/TiO2 slot waveguide using an electron beam resist (AZ nLof) as a loading strip material. One cansee the realized 1x2 MMI coupler. The free propagation region is 226 µm long.

Fig. 1. Demonstration of a 1x2 MMI device on horizontal slot waveguide. (a) Microscope picture of the topview of the device. (b) Photograph of the output modes at  = 1550 nm. The white lines represent the contour of the device.

Fig. 1(b) is a photograph obtain with a near infrared camera of the output modes, which roves the well operationof the device at  = 1550 nm.
The design of such a device is extremely important. This is an interferometric system, which means that all
the parameters may affect the response of the device. However, the fact of using a loading structure, instead83

of a direct pattern etched on the surface of the waveguide, yields weakly guided modes. This means that theeffect of the MMI may be less strong than if it was fabricated in a high refractive contrast platform such as
silicon, for example.
The polarization for instance is a crucial parameter. The effective index of the mode depends on the polarization and the fluctuation on strip-loaded slot waveguide are important between TE (electric field parallel to theslot plane) and TM (electric field perpendicular to the slot plane). One can thus think that the length of the
MMI will be a sufficient parameter to discriminate between TE and TM modes. Fig. 2 shows the results obtained by simulation of the output power for both TE and TM modes while varying the length of the MMI (freepropagation region). One can clearly see that if TM (fundamental slot mode) present a net peak and obvious
optimized length at 240 µm, this is not the case for TE. For this polarization the field is more confined in thetwo rails that are affected differently by the loading strip leading to not so clear maximum. To fit the curve,
we have considered a Lorentz model, as a first approximation.

Fig. 2. Output power in one of the branches of the MMI as a function of the length of the free propagation
region and the polarization (quasi-TM mode in blue and quasi-TE mode in red). Squares and dots are simulation data and the solid lines are Lorentzian fits.

In addition to its dependence to the geometrical and polarization parameters, an MMI depends also
on the refractive index of the cover medium. In the case of a strip-loaded waveguide the mode isburied far from the surface and is thus not much affect by the surrounding material variation. But if
the loading strip is open into a sensing window, the analyte is in direct contact with the thin film andthus changes the effective index of the guided mode. This will then affect the image position inside
the MMI. By carefully designing the shape of the free propagation region, one can attribute to each
output of the MMI a particular effective index and thus a refractive index of the cladding layer: theanalyte material to be sensed.
References[1] Y. Zhang, Z. Li, and B. Li, "Multimode interference effect and self-imaging principle in two-dimensional silicon
photonic crystal waveguides for terahertz waves," Opt. Express 14, 2679-2689 (2006).
[2] H. Furuta, H. Noda, A. Ihaya, “Novel optical waveguide for integrated optics,” Appl. Opt. 13, 322–326 (1974).
[3] V.R. Almeida, Q. Xu, C.A. Barrios, M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29,
1209–1211 (2004).
[4] M. Roussey, L. Ahmadi, S. Pélisset, M. Häyrinen, A. Bera, V. Kontturi, J. Laukkanen, I. Vartiainen, S. Honkanen, M.
Kuittinen, “Strip-loaded horizontal slot waveguide,” Opt. Lett. 42, 211–214 (2017).
[5] S. Pélisset, J. Laukkanen, M. Kuittinen, S. Honkanen, M. Roussey, “Modal properties of a strip-loaded horizontal slot
waveguide,” J. Eur. Opt. Soc. – Rapid Publ. 13:37 (2017).

84

Finnish Flagship Program on Photonics Research and Innovation
Juha Toivonen
Photonics Laboratory, Tampere University, Tampere, Finland
Corresponding author: juha.toivonen@tuni.fi

Keywords: photonics, education, research and innovation cluster
The Flagship on Photonics Research and Innovation (PREIN) is one of the six Finnish Flagships [1], a northern
fleet of excellent science, successful collaboration and solutions for tomorrow. The Flagship programmemanaged by the Academy of Finland provides a new and unique way for R&D&I in Finland. It supports futureknowledge and sustainable solutions to societal challenges and seeks to advance economic growth bydeveloping new business opportunities. PREIN is a light-based technologies competence cluster formed byfour partners: Tampere University, Aalto University, University of Eastern Finland, and VTT Research Center[2]. Combining the extensive resources and infrastructures of all the partners, PREIN covers the value chain
from fundamental research to applied research, product development and commercialization.
The overall rationale of PREIN is to merge leading-edge Finnish activities in photonics through the wholeinnovation value chain. Its four partners share the vision that future breakthroughs in photonics depend on
deep understanding of light-matter interactions, new materials and nanostructures with enhanced opticalresponses, advanced laser sources with unique output characteristics, and photonic integration. Through rapidtransfer of such advances into potential applications, PREIN will produce disruptive innovations to a variety
of application areas and markets. Based on its current high-quality research and commercialization experience,
the Consortium will leverage the full potential of joint work between its partners to significantly increase theimpact of Finnish Photonics and support the economic growth.

Fig. 1. Photonics Flagship is a cluster of three universities and a research center withdiverse profiles and complementary infrastructure.
PREIN organizes outreach events for industry together with Photonics Finland to disseminate information
about photonics product and technology opportunities. The events are organized all around Finland, underthemes “Photonics in X”, where X stands for an industrial sector, such as Food, Forestry, and Healthcare. We
envision organizing 1-2 such events annually.
Academic research provides a crucial platform for educating skilled workforce. Both highly educatedworkforce with technical skills associated with new technologies and experienced people who understand how
problems in the innovation chain can be circumvented by creative solutions. The M.Sc.’s and Ph.D.’s trainedby PREIN are highly employable, and to increase this further, the programme organizes recruitment events ofearly stage researchers for companies and partners, and expand the volume of summer jobs/Master/PhD theses
85

supported by companies. The full educational path from the second degree to polytechnics and universities
needs to be addressed in order to meet the shortage of skilled workforce for various industrial tasks. Pilot
projects on this are starting in the Joensuu and Tampere regions, making Finland one of the few countries
offering photonics education in polytechnics and vocational schools. Via outreach activities in schools, theprogramme informs schoolchildren about photonics and its applications, encouraging them to opt for scientificdisciplines and consider such disciplines as a career option.
References
1. https://www.aka.fi/en/research-and-science-policy/flagship-programme/
2. https://prein.fi

86Designing random structures for random laser media
Takashi Okamoto1 and Keigo Imamura1
1

Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka,
Fukuoka 820-8502, Japan
Corresponding author: okamoto@ces.kyutech.ac.jp

Keywords: random laser, scattering, nanostructures, optical design and fabrication
1. Introduction
Random lasers have been studied for more than two decades from both physical and technical points of view
[1–3]. From a physical point of view, random lasing is an interesting subject in that both optical nonlinearity,
stemming from absorption and amplification, and multiple light scattering coexist in a random laser medium.
From a technical point of view, random lasers are a promising light source because they have high spectral
radiance owing to stimulated emission, while they are also speckle-free light sources owing to their low spatial coherence [4].
It is, however, difficult to control the spectrum and the direction of random laser emissions owing to the intrinsically random nature of their emission mechanisms. Some investigations have focused on tuning theemission spectrum and direction by designing a two-dimensional (2D) pattern of pump intensity distributions[5–7]. Although a single sharp spectral peak or a directional control of laser emission can be realized withthis method, the pump pattern should be optimized for each random laser medium, making it unsuitable formass production.
In this study, we investigate the possibility of designing the structure of a random laser medium itself. To
simplify the complicated random structures of conventional random lasers, we employ a 2D random medium
having a checkerboard pattern structure. This type of device structure has already been utilized in designing apolarization beam splitter as small as 2.4 × 2.4 μm2 with 20 × 20 pixels [8]. We apply this new design approach to random gain media. Simulations of light scattering and emission are performed using the 2D finitedifference time-domain (FDTD) method. The FDTD method is a time-consuming method; however, limiting
the number of scatterer positions in the lattice structure helps to search for an optimal random structure in anaffordable time.
2. Simulations
We performed electromagnetic simulations of light scattering
for the transverse electric (TE) waves (p-polarization). The
model and parameter values used for the simulations are shownin Fig. 1 and Table 1, respectively. The 2D random medium
had optical gain (orange) in an area of 4 × 4 μm2 and a refractive index of 1.5. Scattering particles of 0.2 × 0.2 μm2 (white)with a refractive index of 1.0 (without gain) were distributed
randomly in the gain region. To eliminate the effect of reflection from the boundary of the medium, the refractive index of
the surrounding medium without gain was set at 1.5. The assumed gain medium was Rhodamine 6G, and it was pumped
by a continuous light source with a wavelength of 532 nm.
The observation points were set outside the gain medium, andthe time variations of the electric field at each point were recorded. The spectrum of emitted light was calculated by Fouriertransform of the time variations. We prepared ten samples foreach configuration of the random gain media for averaging.
3. Results
A typical emission spectrum observed in this model is the one
shown in Fig. 2(e). Several spikes appear in the spectrum. We
first examined which conditions are suitable for random lasing
by changing the filling fraction of scatterers from 5% to 50%.

87

Fig. 1. Schematic of the simulation model.
(PML: perfectly matched layer)
Table 1. Parameter values for the simulation.

The peak spectral intensity is maximal at a filling fraction of 20%, while the average intensity in the wavelength range of 570–590 nm decreases with an increase of the filling fraction due to a decrease in the area of
the gain region. We chose 10%, which showed large values for both the peak and the average intensities.
Next, we optimized the random structure for producing an emission spectrum with a single sharp peak. As asingle-peak index, we defined the peak ratio R as the ratio of the highest peak spectral intensity to the second
highest peak intensity. We employed a simplified version of the direct binary search (DBS) method [8] tolimit the computational time. The algorithm is as follows: (1) prepare five different media; (2) calculate spectra observed from each of the four sides (20 spectra in total); (3) choose the one that shows the largest R value (initial state); (4) choose a scatterer and move it by one pixel (0.2 μm) in a randomly selected direction;(5) if the value R is larger than the previous one, fix the scatterer at the new position, and if it is smaller thanthe previous one, move it back to its original position; and (6) perform steps 4 and 5 for all scatterers (final
state).
A successful example is shown in Fig. 2. As the number of trials increases, the difference between the intensities of the largest peak and the other lower peaks in the emission spectrum gradually increases, and eventually only a single peak survives. However, a small change is observed in the peak wavelength from one trial

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 2. Optimization of scatterer positions to generate a single spectral peak. The configuration of scatterers and thecorresponding spectra for the initial state (a, e), the third scatterer (b, f), the 19th scatterer (c, g), and the final state(d, h). The pixels in blue in (a) – (d) represent moved scatterers.

4. Conclusions
We investigated the possibility of designing the scatterer configurationof random laser media for generating a single peak spectral emission.
We found that control of the emission spectrum could be realized with
the DBS method to some extent. However, further work is needed to
produce a specific spectrum in a precise manner.

25

20

Peak ratio R

to the next, which indicates the unstable nature of this nonlinear system.
Figure 3 illustrates this unstable behavior, showing how the peak ratio
can change dramatically by shifting even a single scatterer.

15

10

5

0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Number of scatterers■ Initial state ■ Increased ■ Decreased ■ Final stateFig. 3. Peak ratio R as a function of
the number of trials (one trial foreach scatterer).

References

1. D. S. Wiersma, Nat. Phys. 4, 359–367 (2008).
2. H. Cao, J. Phys. A: Math. Gen. 38, 10497–10535 (2005).
3. M. A. Noginov, Solid-State Random Lasers, (Springer, New York, 2005).
4. B. Redding, M. A. Choma, and H. Cao, Nat. Photonics 6, 355–359 (2012).
5. M. Leonetti and C. López, Appl. Phys. Lett. 102, 071105 (2013).
6. N. Bachelard, S. Gigan, X. Noblin, and P. Sebbah, Nat. Phys. 10, 426–431 (2014).
7. T. Hisch, M. Liertzer, D. Pogany, F. Mintert, and S. Rotter, Phys. Rev. Lett. 111, 023902 (2013).
8. B. Shen, P. Wang, R. Polson, and R. Menon, Nat. Photonics 9, 378–382 (2015).

88

Broadband Electromagnetic Radiation Detector Based on
Photoacoustic EffectJussi Rossi1, Juho Uotila3, Toni Laurila2, Erkki Ikonen2 and Markku Vainio1,4
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
2 Metrology Research Institute, Aalto University, Espoo, Finland
3 Gasera Ltd, Turku, Finland
4 Department of Chemistry, University of Helsinki, Helsinki, Finland
Corresponding author: jussi.rossi@tuni.fi

Keywords: photoacoustic, power detector, silicon cantilever microphone
Power detector is a central component in nearly all measurement and imaging technologies that use electromagnetic radiation for either scientific or industrial purposes. Currently a large selection of different detectors
is needed to cover all the required wavelengths, power levels and sensitivities, especially in research laboratories. The goal of the present research is to develop a highly sensitive general purpose room-temperature photoacoustic power detector that can be used for traceable detection of electromagnetic radiation covering broad
spectral range from the UV to the far-IR and THz (30 µm – 300 µm) range and wide power range between
nW and W levels. In the far-IR and THz regions, the availability of power detectors that do not require cryogenic operating conditions is rather poor [1].
The basic idea of a photoacoustic power detector is the following: incident electromagnetic radiation is
chopped and absorbed by a black absorber, which is placed in a closed volume filled with acoustic carrier gas
converting the small increase in the heat of the absorber into a pressure signal. This pressure signal, which is
directly proportional to the energy of absorbed radiation, is detected with a microphone. The photoacoustic
effect is highly linear and the measurement devices using novel silicon cantilever microphone solutions arehighly sensitive [2] and compact. Such properties combined with cost-effectiveness and wavelength independence make the photoacoustic principle an ideal candidate for power detection applications. In this contribution,
we demonstrate a very good linear dynamic range, high detection sensitivity and broad wavelength coverageusing a photoacoustic optical power detector based on a low-noise silicon cantilever microphone.
We constructed a power detector prototype, which was tested with different types of absorber materials suchas carbon nanotubes and metal foils blackened by painting or sooting. The power detector prototype was characterized at several wavelengths between the visible (633 nm) and mid-infrared (10 µm) spectral regions. Fig.
1 shows an example demonstrating the large linear dynamic range of the detector. As shown in Fig. 1b, thelinear power range can be shifted by changing the radiation chopping frequency. According to the first measurements at 1532 nm, the limit of detection is in the single nW range.

Fig. 1: a) A linearity measurement made with a 633 nm He-Ne laser. The power range is 40 nW – 8 mW. b) The linear
range was extended to 60 nW – 120 mW by increasing the modulation frequency and using a 1064 nm high-power
fiber laser. Reference power levels were measured with NIST calibrated Thorlabs S120C and S305 detectors.

89

The very good linearity is visible in the graphs. Typical R-squared values for a linear fit are of the order
0.99999 – 0.999999. The demonstrated power detector concept has good market potential. Table 1 contains
specifications for six different types of commercial power detectors on market. The values are collected fromthe datasheets of the manufacturers. The last row represents the target specifications for our detector.
Table. 1: Comparison of commercially available power detectors covering wavelengths from visible
to microwave range.
Detector

Detector type

Wavelength range

Dynamic range

Thorlabs S120C

Si photodiode400 – 1100 nm

50 nW – 50 mW

Ophir RM-9 Pyro

Pyroelectric0.15 - 12 µm

100 nW- 100 mW

Ophir RM-9 THZ

Thermal

10 - 3000 µm

100 nW- 100 mW

TYDEX GC-1P

Golay cell

15 - 8000 µm

0.15 nW – 10 µW

IRLabs General 4.2 K

Bolometer

15 - 2000 µm

0.12 pW -TK instruments

Photoacoustic

100 - 10000 µm

5 µW - 500 mW

Our detector

Photoacoustic

0.4 – 400 µm

1 nW – 1 W

We have measured the limit of detection and the maximum detected power with our current detector prototype
to be c.a. 10 nW and 1 W, respectively, which corresponds to a dynamic range of eight orders of magnitude.
The longest calibration wavelength we have used so far is 10 µm but, according to the theory and prior experimental research on hyperblack blackbody absorbers [3], the detector responsivity should remain flat all theway to the THz range.
In the future, the sensitivity of the power detector will be further improved by more detailed modeling of theproperties of the photoacoustic measurement chamber, light absorber and the silicon cantilever microphone.
Measurements and calibrations with far-IR and THz radiation sources will also be carried out.
References
1. Antoni Rogalski, "Infrared detectors: status and trends", Progress in Quantum Electronics 27 (2), pp. 59-210, 2003.
2. Teemu Tomberg, Markku Vainio, Tuomas Hieta, and Lauri Halonen, "Sub-part-pertrillion level detection of hydrogen
fluoride by cantilever-enhanced photo-acoustic spectroscopy”, arXiv:1707.01312, 2017.
3. John Lehman et al., “Planar hyperblack absolute radiometer”, Optics Express, Vol. 24, No. 23, 14 Nov 2016.

90

Measurement facility for high accuracy multiphoton absorption spectra
Meelis-Mait Sildoja1, Charles W. Stark1, Jüri Pahapill1, Matt Rammo1, Katrin Petritsenko1 and Aleksander
Rebane1,21National Institute of Chemical Physics and Biophysics (NICPB), Tallinn
2Montana State University, Bozeman, Montana
Corresponding author: meelis.sildoja@kbfi.ee

Keywords: nonlinear optics, multiphoton absorption, 2PA, MPA
Multiphoton absorption (MPA) is a nonlinear-optical process, where two or more photons induce a verticalelectronic transition from a lower-energy state to a higher-energy state, such that the energy difference between
the states equals the combined energy of the absorbed photons. Such transitions have very low probability
under ambient illumination. However, their efficiency increases rapidly as a power function under increasing
photon flux.
Fig. 1. Absorption processes. 1PA, 2PA and 3PA indicate how
many photons participate in the absorption process.
Quantitative nonlinear multiphoton detection is driven by theneeds of non-invasive medical diagnostics [1], material science[2] and secure optical communications [3, 4]. We have establisheda world-class facility for the measurement of nonlinear
multiphoton spectra. The femtosecond laser system allows broad wavelength tunability throughout VIS andNIR range to determine absolute 2-photon and 3-photon absorption cross-section spectra of emissive
compounds or materials. Detailed description of the system is shown in Figure 2. Emission from the laser isdirected through beam shaping optics and a variable neutral density filter to adjust the pulse power. The
polariser and waveplate before the sample allow studying the effects of varied polarisations on the absorption.
Emission from sample is collected via focusing optics and spectrometer equipped with a photomultiplier tube.
Pulse power is monitored with pyroelectric detector. Signals are recorded with fast 10-bit digital oscilloscope
synced with the laser pulses. The facility is automated and controlled via LabView.

Fig. 2. Schematic of the automated femtosecond 2PA/3PA spectrometer. POL - polarizer; BS - beam splitter;
LPF - long-pass filter; SPF - short-pass filter; BPF - band-pass filter; L1, L2, L3, L4 - focusing lenses; λ/2 half-wave plate; BBO - barium borate crystal; VNDF - variable neutral density filter; ND - neutral density
filter; PED - pyroelectric detector; λ/4 - quarter wave plate; PMT - photomultiplier tube.
91

The unique properties of the facility include the wide tunability of the wavelengths from 315 nm to 2600 nm,
wavelength adjustment resolution of 1-2 nm, consecutive measurements of up to four samples, four ordermagnitude range of signal detection, beam profiling as well as pulse shape measurement capabilities. In Figure
3. the black-dotted line shows our latest high-resolution absorption cross-section spectrum ofRu(1,10-phenanthroline)3(PF6)2 complex. As a comparison, measurements from other research groups (greendiamond and red-triangle lines) and our previous facility (blue-square line) are given. The green and red lines
are extracted from references [5, 6].

Fig. 3. Absorption cross-section spectra, in Goeppert-Mayer (GM) (10-50cm4·s·photon-1), of Ru(phen)3(PF6)2
reference complex in acetonitrile measured by different workgroups. Black line indicates vastly improved
result with respect of range and resolution achieved with the new setup shown in Fig. 2.
Currently the absolute uncertainty of the measured absorption cross-section is in the order of 10-15%. Afterthe full characterisation of the setup we aim to perform traceable 2PA cross-section measurements with lessthan 5% of absolute and relative uncertainty. One factor that helps to achieve the low uncertainty is to measureexactly the time resolved power of the incident high energy pulses (>1030 photon cm-2 s-1) as current indirect
estimation is prone to errors. Namely, we plan to exploit the inherent two-photon absorption properties of GaP
sensors as the generated photocurrent is directly proportional to the nonlinear photon flux [7].
Our broader goal is to develop a world-class metrological facility for nonlinear multiphoton light absorptionand calibration measurements and provide service for other research labs as well as for industry (medical
imaging, fundamental research, quantum information applications). We have already established a database ofour 2PA (in future also 3PA) measurements which can be found in www.kbfi.ee/mpa.

References
1.
2.
3.
4.
5.
6.
7.

A. Mikhaylov, S. de Reguardati, J. Pahapill, P. R. Callis, B. Kohler and A. Rebane, Biomed. Opt. Expr. 9(2),447 – 452 (2018).
R. S. Price, G. Dubinina, G. Wicks, M. Drobizhev, A Rebane and K. S. Schanze, ACS Appl. Mater. Interfaces 7,10795-10805 (2015).
F. Schlawin, J. Phys. B: At. Mol. Opt. Phys. 50, 203001 (2017).
O. Varnavski, B. Pinsky and T. Goodson III, J. Phys. Chem. Lett. 8, 388−393 (2017).
O. Mongin, M. Four, S. Chevreux, M. Blanchard-Desce, G.Lemercier, CHIMIA International Journal for
Chemistry, 69(11), 666-669 (2015).
C. Girardot, G. Lemercier, J.- C. Mulatier, J. Chauvin, P. L. Baldeck and C. Andraud, Dalton Transactions 0(31)(2007).
E. Z. Chong, T. F. Watson, and F. Festy, Appl. Phys. Lett. 105, 062111 (2014).

92

Printed electronics in pilot scale processing and production of organic
photovoltaics, CIGS solar cells and OLEDs
Marja Vilkman1, Kaisa-Leena Väisänen2, Marja Välimäki2, Mari Ylikunnari2, Thomas Kraft2
1
2

VTT Technical Research Centre of Finland, Tietotie 3, P.O. Box 1000, FI-02150 Espoo, Finland
VTT Technical Research Centre of Finland, Kaitoväylä 1, P.O. Box 1100, FI-90571 Oulu, Finland
Corresponding author: marja.vilkman@vtt.fi

Keywords: Printed electronics, organic solar cell, lifetime, production
Printed Electronics offers tools for large-area processing of novel products, enabling e.g. flexibility and freedom of design. This presentation introduces printed electronics as a processing method and shows how it canbe used in production of optoelectronic components. Specifically, I will focus on showing how to guarantee
optimal lifetime, quality and performance in pilot scale processing of organic solar cells (see image from Fig.
1). In addition, I will give you an outlook on production of organic and CIGS solar cells and OLEDs – a
Factory of the Future.

Fig. 1. A picture of a flexible organic solar cell roll.
Understanding the phenomena, which have an effect on performance and lifetime of flexible solar cells, isessential when transferring the process to pilot scale. Often the promising results in laboratory do not directly
convert into good performance with large-area roll-to-roll (R2R) processed and encapsulated solar modules.
We have faced this challenge while testing new materials for organic solar cells.
For example, replacing PCBM with ICBA as an acceptor should result in increased performance due to higher
Voc. In sheet-to-sheet (S2S) scale this is true, but R2R processing gave different results. ICBA-containing
modules required also long light soaking to recover from an S-shaped IV curve, which appeared after encapsulation. In addition, we use plasma treatment for improving the adhesion of a ZnO nanoparticle layer and thisprocess works in R2R scale. However, we have observed that the plasma-treated modules also require more
light soaking after encapsulation to work well. Motivated by these challenges, we wanted to study the S-shape
related phenomena closer. Naturally, the focus was in energy level alignment. I will show that energy level
matching is crucial especially for R2R processed, encapsulated solar cell modules. In addition, I will also showthat the improved adhesion leads to significantly improved lifetime – even without mechanical stressing. Bothof the observations give guidelines for producing stable solar modules in R2R environment.
Finally, even with carefully selected materials and processes, the R2R processing itself can create challenges.
It is not anymore easy to follow the quality and avoid waste when the processing speed is high and the production volumes are large. There are several characterization methods available but they are not necessarily
suitable for large-area production. To overcome this challenge and to be able to follow and control the qualityof large-scale production of organic and CIGS solar cells and OLEDs, we have launched an EU project,
OLEDSOLAR. Together with 16 partners – VTT, Fraunhofer, TNO, IRIS, Opvius, TWI, CSEM, Solibro,
Meyer Burger, Brunel University London, Leitat, Coatema, DuPont Teijin Films, CTR, Advent and Inuru –93we aim to find the best quality control methods and implement those in the pilot and production lines to allow
high yield and process speed. This presentation will summarize the objectives of the OLEDSOLAR projectand give an outlook for future production of flexible electronics.
References
1.
2.

M. Vilkman, K.-L. Väisänen, P. Apilo, R. Po, M. Välimäki, M. Ylikunnari, A. Bernardi, T. Pernu, G. Corso, J.
Seitsonen, S. Heinilehto, J. Ruokolainen, J. Hast, ACS Applied Energy Materials, 1, 5977-5985 (2018).
OLEDSOLAR project: https://oledsolarproject.eu/

94Predictable Quantum Efficient Detector (PQED) based on n-type silicon
induced junction photodiodes
Timo Dönsberg1, Santeri Porrasmaa2, Farshid Manoocheri2 and Erkki Ikonen1,2
1National Metrology Institute VTT MIKES, VTT Technical Research Centre of Finland Ltd , Espoo, Finland
2
Metrology Research Institute, Aalto University, Espoo, Finland
Corresponding author: timo.donsberg@vtt.fi

Keywords: radiometry, optical power, induced junction photodiode, simulation
The Predictable Quantum Efficient Detector (PQED) [1-2] is a semiconductor based primary standard of optical power that can be operated at room temperature [3-4]. It consists of two induced-junction photodiodes,
which are mounted in a wedged trap configuration for the control of specular reflectance losses. Using onedimensional modelling, the near-zero internal quantum deficiency (IQD) of the custom photodiodes has been
predicted with an estimated standard uncertainty of 70 ppm in the visible wavelength range [5]. The modelledresults have been experimentally confirmed with measurements against cryogenic radiometers [2]. There is an
ongoing work to improve the prediction using three-dimensional modelling [6].
Until recently, all PQEDs were based on induced junction photodiodes, where a SiO2 layer is thermally grown
on top of very lightly doped p-type silicon substrate. This structure inherently contains trapped positive surface
charge close to the Si-SiO2 boundary, which generates an n-type inversion layer in the p-type silicon andproduces a depletion region required for photocurrent generation [1]. This approach, however, has two downsides. First, the availability of suitable substrate material is problematic. Second, the thermal growing process
operating at around 1000 K is time-consuming, expensive and increases the risk of contamination.
Recently, a new type of PQED photodiodes were introduced [7] that utilize induced junction, which is manufactured using n-type silicon substrate. The underlying structure of the new photodiode, shown in Fig. 1.a. issimilar to the previous design, but the n-type substrate requires negative charge to be trapped in the passivation
layer in order to form the depletion region. This was achieved by fabricating an Al2O3 layer on top of thesubstrate. Due to the higher fixed charge density, the Al2O3 layer can be a factor of 10 thinner than the SiO2
layers used with p-type PQED photodiodes.

Fig. 1. (a) Cross section of the structure of the set B photodiode (not to scale). (b) A PQED photodiode ofsize 11 mm x 22 mm attached to the photodiode carrier. The added green square indicates the area of (a).
Two sets of photodiodes, denoted as set A and B, were manufactured using substrates with doping concentrations of 2.5·1011 cm-3 and 4.4·1011 cm-3, respectively [7]. The Al2O3 layers were grown using atomic layer
deposition (ALD) [8], which provides a controlled method to produce uniform oxide layers to an atomicallyspecified thickness. Oxide thicknesses ranging from 25 nm to 30 nm were produced. The responsivity wasmodelled for both sets, and detectors from both sets were characterized for temperature dependence of darkcurrent, spatial uniformity of responsivity, reflectance and absolute responsivity.
In addition to smaller doping concentration, the set A photodiodes also had 16 guard rings in the photodiodelayout as opposed to the one of set B (the outermost p+ implantation of Fig. 1.a.). These factors lead to thedecrease of dark current at room temperature by a factor of five, down to around 1 nA level when biased witha 10 V voltage. When the photodiode is cooled in a cryostat [9], the dark current decreases exponentially;
approximately a decade for every decrease of 15 °C in temperature. Improved dark current properties become
significant at low flux applications of the PQED, such as photometric measurements [10-11], or single or few
photon applications [12]. Preliminary measurements indicate that the absolute power of a laser in the picowatt
scale can be measured with a standard uncertainty of 0.3% within 5-minute measurement period.
Due to different oxide thicknesses of the photodiodes, reflectances measured at the wavelength of 488 nm
varied between 110 ppm and 170 ppm. The reflectance is higher than that of p-type PQEDs of around 27 ppm.
95

Nevertheless, the responsivity of the n-type PQED is similarly uniform as that of the p-type PQED; the responsivity in the central active area is flat within 30 ppm peak-to-peak deviation.
A 3D simulation model of the photodiode structure was built into the semiconductor simulation software Cogenda Genius v. 1.8.0. With the exception of surface recombination velocity (SRV) for electrons and holes
and Al2O3 layer fixed charge (FC), the modelling parameters were obtained from the wafer manufacturer orthey can be directly measured. We developed a method to extract the values for the SRV and FC, where simulated bias voltage dependence of photocurrent is fitted to experimental data (see Fig. 2.a.). SRV values of
3·104 cm/s and 5·105 cm/s, and FC values of 3.9·1012 cm-2 and 4.5·1012 cm-2 were obtained for set A and B,
respectively.

Fig. 2. (a) Relative change of photocurrent as a function of bias voltage at 488 nm. The simulated curves are
fitted to experimental values in order to obtain SRV and FC values. (b) Absolute IQD measured at 488 nm(dots) and 532 nm (square). Horizontal lines show the predicted IQD and cyan boxes indicate the estimateduncertainty of the model at 95% confidence level.
The predicted IQD values for set A and B were 18 – 23 ppm and 26 ppm, respectively. Tentative analysis
indicates estimated standard uncertainty of around 40 ppm for the predicted IQD at room temperature. The
responsivities of the n-type detectors were also measured against a p-type PQED, similar to those characterizedin [2] and [3]. The IQD values obtained from responsivity and reflectance measurements, together with thepredicted values, are shown in Fig. 2.b.
A new type of PQED was developed – first ever to utilize n-type silicon and ALD. Two sets of detectors were
manufactured, modelled and characterized. The agreement between experimental and modelled results indicates that the n-type structure is a very promising alternative to the existing p-type PQEDs, and exhibits betterdark current properties. Consequently, it proves that the manufacturing of PQEDs is no longer dependent on
the availability of a particular silicon process. Finally, this work gives additional credibility to the concept ofa modelled quantum detector serving as a primary standard.
References
1. M. Sildoja, F. Manoocheri, M. Merimaa, E. Ikonen, I. Müller, L. Werner, J. Gran, T. Kübarsepp, M. Smîd and M. L.
Rastello, Metrologia 50, 385–394 (2013).
2. I. Müller, U. Johannsen, U. Linke, L. Socaciu-Siebert, M. Smîd, G. Porrovecchio, M. Sildoja, F. Manoocheri, E.
Ikonen, J. Gran, T. Kübarsepp, G. Brida and L. Werner, Metrologia 50, 395–401 (2013).
3. T. Dönsberg, M. Sildoja, F. Manoocheri, M. Merimaa, L. Petroff and E. Ikonen, Metrologia 51, 197-202 (2014).
4. J. Zwinkels, A. Sperling, T. Goodman, T, J. C. Acosta, Y. Ohno, M. L. Rastello, M. Stock and E. Woolliams, Metrologia 53, G1 (2016).
5. J. Gran, T. Kübarsepp, M. Sildoja, F. Manoocheri, E. Ikonen and I. Müller, Metrologia 49, S130-S134 (2012).
6. M. U. Nordsveen, C. K. Tang and J. Gran, Opt. Express 25, 8459-8469 (2017).
7. T. Dönsberg, F. Manoocheri, M. Sildoja, M. Juntunen, H. Savin, E. Tuovinen, H. Ronkainen, M. Prunnila, M.
Merimaa, C. K. Tang, J. Gran, I. Müller, L. Werner, B. Rougié, A. Pons, M. Smîd, P. Gál, L. Lolli, G. Brida, M. L.
Rastello and E. Ikonen, Metrologia 54, 821-836 (2017).
8. M. Leskelä and M. Ritala, Thin Solid Films 409, 138-146 (2002).
9. F. Manoocheri, T. Dönsberg, M. Sildoja, M. Smîd, G. Porrovecchio and E. Ikonen, J. Phys. Conf. Ser. 972, 012021(2018).
10. T. Dönsberg, T. Pulli, T. Poikonen, H. Baumgartner, A. Vaskuri, M. Sildoja, F. Manoocheri, P. Kärhä and E. Ikonen,
Metrologia 51, S276-S281 (2014).
11. T. Pulli, T. Dönsberg, T. Poikonen, F. Manoocheri, P. Kärhä and E. Ikonen, Light Sci. Appl. 4, e332 (2015).
12. A. Vaigu, G. Porrovecchio, X.-L. Chu, S. Lindner, M. Smid, A. Manninen, C. Becher, V. Sandoghdar, S. Götzinger,
and E. Ikonen, Metrologia 54, 218 (2017).

96TAPERED FIBER AMPLIFIER FOR COHERENT LIDAR
Kim Patokoski1, Joona Rissanen2, Teppo Noronen2, Regina Gumenyuk1,2, Yuri Chamrovskii3, Valery Filippov2, Juha Toivonen1
1

Photonics Laboratory, Tampere University, Tampere, Finland2
Ampliconyx Ltd, Tampere, Finland
3
Kotel’nikov Institute of Radio Engineering and Electronics, Moscow, Russian Federation,
Corresponding author: kim.patokoski@tuni.fi

Keywords: optics, lithography, nanostructures
Coherent lidars in atmospheric sensing rely on narrow linewidth fiber amplifier based light sources. The
achievable peak power with typical single mode fibers is limited to tens of watts [1], still enabling the heterodyne detection of wind speed from particle backscatter [2]. Kilowatt-level peak power could enable detection of spectrally broader molecular backscatter, yielding access to simultaneous visibility and particle typeprofiling alongside the wind speed measurement. Even temperature profiling, considered as a holy grail of
weather forecasting, has been demonstrated with high spectral resolution analysis of molecular backscatter[3].
Stimulated Brillouin scattering (SBS) in the optical fibers is the main effect limiting the peak power. The
strong electric field density induces lattice vibrations (phonons) creating a temporary reflection grating inside
the fiber. The backwards travelling wave is especially harmful inside the amplifier as it experiences highgain causing instability and possible damage to the amplifier. In this work, we present successful light source
development results for coherent lidar using tapered ytterbium doped fiber amplifier at 1.053 µm. By combining several SBS mitigation techniques simultaneously we are able to reach more than two order of magnitude enhancement in output peak power compared to single mode fiber.
The light source design follows a typical seed laser - pre-amplifier - power amplifier scheme and is shown indetail in Fig.1.
Fiber coupledlaser diode1053 nmDFB

Acousto-optic
modulator
AOM

Isolator
Yb3+

Preamplifier 1
Isolator
Arbitrary
waveform
generator

Preamplifier 2

Cladding
mode stripper

Yb3+

CMS

BP
Bandpass
filterTaperedYb3+ doped
fiber

Isolator

OutputP
Stabilizedu976 nmmlaser diode p

Fig. 1. Schematic picture of the fiber amplifier system.
Light from distributed feedback laser diode is chopped to pulses by an acousto-optic modulator and coupledthrough a dual stage pre-amplifier. The single mode ytterbium doped gain fibers are core pumped counter tosignal propagation and the amplified spontaneous emission is filtered between stages.
The pulses are finally coupled to a power amplifier containing an ytterbium doped cladding pumped fiber, which
has a tapered longitudinal profile. A cladding mode stripper is used to remove unabsorbed counter propagating
pump. The core radius grows slowly from 16 µm to 50 µm
along the 1.2-m-long fiber and the profile can be seen in
Fig. 2. All fiber components, including the tapered fiber,
are polarization maintaining.
Fig. 2. Measured core size along the tapered double clad fiber
The main advantage compared to other coherent lidar light sources becomes from the power amplifier design. The seed laser and pre-amplifier are similar to typical coherent lidar sources and the output peak power

97

after pre-amplifier is SBS limited to 10 W. However, with the unique tapered power amplifier we achieved
record high peak power of 2.2 kW with linear polarization and single mode output. The polarization extinction ratio is 20 dB and the measured M2=1.05/1.08 indicates excellent beam quality, unreachable by othercompeting technologies such as large mode area fibers.
Coherent lidar signal-to-noise ratio is highly dependent on good beam quality and linear polarization [4].
Combined with high peak power, we expect unparalleled performance with coherent lidar. We continue toassemble a prototype system to demonstrate lidar performance in simultaneous wind speed, visibility andparticle profiling. The tapered fiber amplifier is definitely a key technology enabler for coherent lidar reaching record high multi kilowatt peak power, while maintaining excellent beam quality and linear polarization
state.
References
1. Canat, C.,Jaouën, Y., Mollier, J.C, C. R. Physique, 7, 177-186 (2006).
2. Hirsikko, A. et al, Atmos. Meas. Tech., 7, 1351–1375 (2014)
3. Witschas et al., Optics letters, 39, 1972-1975 (2014)
4. J.-P. Cariou et al., C. R. Physique 7, 213-223 (2006)98

Fourier Transform Photoacoustic Spectroscopy
with Supercontinuum Light Source
Tommi Mikkonen1, Caroline Amiot1, 2, Antti Aalto1, Kim Patokoski1, Goëry Genty1, Juha Toivonen1
1

2

Photonics Laboratory, Physics Unit, Tampere University, PO Box 692, FI-33014 Tampere, Finland
Institut FEMTO-ST, UMR 6174 CNRS - Université Bourgogne Franche-Comté, 15B av. des Montboucons,
F-25030 Besançon, France
Corresponding author: juha.toivonen@tuni.fi

Keywords: broadband photoacoustic sensing, supercontinuum light source, Fourier transform spectroscopy
In contrast to conventional absorption spectroscopy methods, photoacoustic spectroscopy (PAS) measures theabsorbed light power directly, which makes the technique highly sensitive and also enables small sample
volumes [1]. These properties are important in multicomponent trace-gas analysis, which is possible in PAS
using a broadband light source together with a Fourier transform spectrometer (FTS) as a light modulator.
Scanning FTS creates a distinct modulation frequency for each wavelength of the source, which leads to theformation of several pressure waves in the photoacoustic cell due to periodic gas absorption and thermal
relaxation. Pressure variations are then detected using a cantilever microphone with optical readout.
Cantilever-enhanced photoacoustic spectroscopy (CEPAS) is a non-resonant photoacoustic technique enabling
sensitive detection over a wide range of frequencies [2]. The CEPAS technique together with FTS has so far
been used only with conventional infrared radiators, which suffer from poor spectral power density [3].
ComputerIn this work, we demonstrate broadband Fourier transform photoacoustic spectroscopy using two
spectrum
Interferogramrange [4]. ThePA
supercontinuum light sources at 1–3.5 µm wavelength
supercontinua
were generated using thesame optical fibers and a pump laser, whose repetition rate was varied between 70 kHz and 400 kHz. The
experimental setup is illustrated in Figure 1. Compared to conventional infrared radiators, the supercontinua
provide larger spectral power density and therefore largerFourier
photoacoustic signal. We measured rovibrationaltransform
photoacoustic spectra of water vapor in air and methane (400ppm) in nitrogen at atmospheric pressure using
an infrared radiator and the supercontinua as light sources. The sample volume was smaller than 8 mL. The
measured spectra shown in Figure 2 were recorded in 50 s (10 scans) with 120 GHz resolution and they are ingood agreement with a model based on HITRAN database.

Supercontinuum source

FTS
M
BS
M

Gain switched laser,ns pulses, 70–400 kHz
repetition rate

M

W
DSF

ZBLAN

PM

PM

Optical readout of
cantilever position

C

GM

Photoacoustic cellFig. 1. Experimental setup. PM, off-axis parabolic mirror; M, mirror; BS, beam splitter; W,window; GM, gold mirror; C, cantilever. Note that the supercontinuum generated using lower
repetition rate (red spectrum) was long-pass filtered at 2.3 µm.

BS – beam splitter
PM – parabolic mirror
W – window (BaF2)
C – cantilever99

GM – gold mirror
DL – diode laser
PD – photodiodes
PA – photoacoustic

(a)

(b)

Fig. 2. Photoacoustic spectra of (a) water vapor and (b) methane measured using two
supercontinua (blue and red) and a broadband infrared source (black, magnified). Reference
spectra from HITRAN database are also shown (grey, inverted).
We achieved a 20- and 70-fold increase in photoacoustic signal with the supercontinua compared to theinfrared radiator, demonstrating the advantage of larger spectral power density of the supercontinuum sources.
In addition to the improvement in sensitivity, the spatially coherent and collimated supercontinuum beam
enables miniaturization of the FTS and better resolution as compared to the infrared radiator. Thus, thecombination of supercontinuum light source, FTS and CEPAS detection enables broadband multi-gas analysis
using small sample volumes.
References
1. J. Li, W. Chen, B. Yu, Recent Progress on Infrared Photoacoustic Spectroscopy Techniques, Appl. Spectrosc. Rev.
46, 440-471 (2011).
2. T. Kuusela, J. Kauppinen, Photoacoustic gas analysis using interferometric cantilever microphone, Appl. Spectrosc.
Rev. 42, 443-474 (2007).
3. C. B. Hirschmann, J. Uotila, S. Ojala, J. Tenhunen, R. L. Keiski, Fourier transform infrared photoacoustic
multicomponent gas spectroscopy with optical cantilever detection, Appl. Spectrosc. 64, 293-297 (2010).
4. T. Mikkonen, C. Amiot, A. Aalto, K. Patokoski, G. Genty, J. Toivonen, Broadband cantilever-enhanced
photoacoustic spectroscopy in the mid-IR using a supercontinuum, Opt. Lett. 43, 5094-5097 (2018).

100

Novel hyperspectral imager based on angle-tuned multi pass band filter,
Leds and RGB image sensor
Heikki Saari1, Harri Ojanen1, and Ingmar Stuns1
1

VTT Microspectrometers, VTT Technical Research Centre of Finland Ltd., Espoo, Finland
Corresponding author: heikki.saari@vtt.fi

Keywords: optics, tunable filter, hyperspectral imaging
The multi-passband interference filters can be manufactured using the known this film technologies based on
multi-cavity technology as explained in references 1 (Denton et.al.) and 2 (Abdullah et.al.). The transmission
spectrum of the interference filter can tuned by changing the angle of the filter and this kind of filters arecommercially available from Semrock (3). We have created a hyperspectral imager concept (4, 5) which isbased on an angle-tuned multi-passband interference filter, multi wavelength Led illumination and RGB color
image sensor. This construction can be used to form hyperspectral image of the target at narrow wavelength
bands each related to the dedicated pass bands of multiple pass band filter, rotation angle of the filter andillumination of multi wavelength Leds. For example with 12 - 15 different Leds, an angle-tuned TiO2-SiO2
thin film stack based filter and RGB image sensor it is possible to perform hyperspectral imaging in the wavelength range 450 – 850 nm. In case of ambient light illumination or broadband lamp illumination the number
spectral bands is limited by the number spectrally different pixels in the image sensor.
The design of TiO2-SiO2 angle-tuned multi-passband filter was made based implementing the short wavelength range (450 - 625 nm) on the front surface of fused silica substrate and the long wavelength range (625
- 850 nm) on the back surface. The depositions of the thin film stacks on ShinEtsu Synthetic Quartz wafer (d=675 µm, diameter = 150 mm) were made Oplatek Oy with the process detailed design made by Aki Syväluoto
from Oplatek. The short wavelength range filter was based on seven layer Bragg mirrors optimized for 550
nm and operating at FPI order 10 @ 550 nm. The long wavelength range filter has seven layer Bragg mirrors
optimized for 780 nm and it has optical depth between Bragg mirrors corresponding the FPI order 8 @ 780
nm. Measured transmission spectra of the filter are shown in Fig. 1 for the rotation angles 0, 20, 30, 35
and 40.

1
0.9
0.8

Transmission

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
400

425

450

475

500

525

550

575

600

625

650

675

700

725

750

775

800

825

850

875

900

Wavelength/[nm]Rotation angle:
Rotation angle
Rotation angle
Rotation angle
Rotation angle

 0.0  20.0 
Rot_angles1   30.0  35.0  40.0 

Fig. 1. Measured spectral transmissions of the TiO2-SiO2 angle-tuned multi-passband filter for the rotation
angles 0, 20, 30, 35 and 40.

101

We have built a laboratory prototype of the hyperspectral imager
based on the TiO2-SiO2 angle-tuned multi-passband filter designed
for 450 - 850 nm, on 12 different Leds with wavelenghs 450, 470,
525, 540, 595, 625, 660, 690, 729, 780, 810 and 850 nm (three of
each Led type) and on Sony IMX174 CMOS image sensor based
machine vision camera. The camera is the Point Grey USB3 GS3U3-23S6C-C which has 1200 x 1900 pixels and 5.86 µm x 5.86 µm
pixel size.
The results of the hyperspectral imager characterization and specifications are presented in Table 1
Table 1 Specifications and characterization results of the hyperspectral imager laboratory prototype based on angle-tuned multi passband filter, Leds and RGB image sensor.
Parameter

Typical/measured value

System focal length

100 mmF-number

4.0

Spectral range

450 - 850 nm

Spectral
FWHM

resolution@ 2.5…12.5 nm (pending on
the wavelength)

Maximum image/object sizes 1920 x 1200 pixels
7.02 mm/14.0 mm
11.23 mm/22.5 mm
13.3 mm/26.6 mm
Dimensions

80 mm x 80 mm x 300 mm

Fig 2. Hyperspectral imager laboratory
prototype based on angle-tuned multipass band filter, Leds and RGB image
sensor.
We have shown that it is possible to build a hyperspectral imager using low cost parts for the wavelength range
450 - 850 nm. The concept can be used in a wide wavelength range from 250 to 5000 nm by designing themulti pass band filter high (HfO2, ZrO2, Poly-Silicon, TiO2 etc.) and low (SiO2, MgF2 etc.) refractive index
materials accordingly.
References
1. D. Morton, Sociecty of Vacuum Coating Technology, conf. proc, (2003), pp. 1-6.
2. H. A., Gaillan, R. M. Bushra, A. Nahla and Al. Jabar, Design Thin Film Narrow Band-pass Filters For Dense Wavelength Division Multiplexing, International Journal of Advances in Applied Sciences (IJAAS), Vol. 1, No. 2, June
2012, pp. 65-70.
3. Semrock angle-tuned filter information can be found in: https://www.semrock.com/Data/Sites/1/semrockpdfs/opticalengineering_anderson.pdf
4. US9835847 B2, 5.12.2017, Hyperspectral imaging arrangement.
5. FLIR (Point Grey) camera information can be found in: https://www.flir.eu/products/grasshopper3-usb3?model=GS3U3-23S6C-C

102

Miniaturized spectral imaging technologies at VTT
Antti Näsilä, Heikki Saari, Kai Viherkanto, Christer Holmlund, Rami Mannila, Roberts Trops, Endija Briede,
Akseli Miranto, Altti Akujärvi, Harri Ojanen, Ismo Näkki, Ingmar Stuns, Hans Toivanen, Tahvo Havia, Martti
Blomberg, Sari Elomaa and Tapani Antila
VTT Technical Research Centre of Finland Ltd, Espoo, Finland
Corresponding author: antti.nasila@vtt.fi

Keywords: optics, hyperspectral imaging, fabry-perot interferometerIn the recent years, VTT Technical Research Centre of Finland Ltd has developed several miniaturized spectral
imaging instruments, that are suitable for applications ranging from nanosatellites to skin cancer detection andcell phone integrated gas sensors. The common topic in all these applications is that they require small androbust instrumentation. VTT’s spectral imagers are based on a tunable spectral filter, a Fabry-Perot Interferometer. This filter allows the acquisition of images at a single, freely selectable, wavelength, i.e. it is possible
to record a complete 2D image at any single wavelength with a single snapshot. In the past similar functionality
has been usually achieved by filter wheels, which are not very suitable for small instruments to their bulky
construction. Also the tuning speed of VTT’s spectral filters is very fast, which enables the recording of thecomplete spectrum in a short time. The filter structure is very simple, compact and robust, and thus makes thistechnology usable in small airborne platforms or handheld devices, which has not been possible in the past.
After a decade of research and development, spectral filter solutions have been realized for almost any wavelength from ultraviolet to thermal infrared. This means the technology can be tailored for almost all applications which utilize optical spectroscopy in some form. This has already been demonstrated in several cases,
where the same basic instrument principle has been used for skin-cancer detection, ship emission monitoring,gas sensing and precision agriculture, only to list a few.
In addition to ground based applications, VTT has pioneered spectral imaging in space with nanosatellite platforms. The first space demonstration of the technology was on-board the Aalto-1 nanosatellite, which waslaunched in June 2017. The Aalto-1 Spectral Imager operates in the visible and near-infrared wavelengths
between 500 and 900 nm. Aalto-1 Spectral Imager was also the first nanosatellite compatible camera capableof hyperspectral imaging ever to be flown in space. The second demonstration mission was for the shortwave
infrared wavelengths (900 - 1400 nm) on-board the Reaktor Hello World nanosatellite, which was successfullylaunched in November 2018. The Hello World Spectral Imager was the first demonstration of shortwave infrared spectral imager on a nanosatellite. Both of these missions have been successful, and all the technicalobjectives have been completed. Future space applications include planetary exploration and missions focusedon atmospheric science.
To summarize, VTT’s spectral imaging technology has already been successfully demonstrated in many different applications. This has been made possible by advances in the development of the tunable Fabry-Perot
Interferometers, and now VTT can provide solutions to almost any wavelength range, from UV to thermal
infrared. We believe that the development will continue, as further miniaturization of the instruments will
create more applications in the future.

103

Author Index
Lastname
Aalto
Aalto
Aikio
Aizu
Akujärvi
Al-Sharabi
Amiot
Amoani
Andoh
AntilaAnttonen
Aoi
Aoki
AsamoahBaah
Baumgartner
Bautista
BawuahBhat
Blomberg
BriedeBulygin
Byman
Chamrovskii
Chen
Cherchi
Choi
Costache
Das
DeguchiDumitru
Dönsberg
ElomaaFabritius
Fazeli
Filippov
Funamizu
Furukawa
Gane
Genty
Gumenyuk
HaavistoFirstname
Timo
Antti
Sanna
Yoshihisa
Altti
Mohammed
Caroline
James
Sampson S.
TapaniVuokkoToshi
Yoshinobu
Benjamin O.
M.
Hans
Godofredo
PrinceSrivathsa
MarttiEndija
Gleb
Ville
Yuri
Jun
MatteoSamuelMariana
Kaustav
Takahiro
Adrian
Timo
Sari
Tapio
Elnaz
ValeryHideki
Hidekai
PatrickGoëryReginaSanna

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HagenHanayama
HaradaHariyadiHarjanne
Hasegawa
Hasegawa
Hashimato
Havia
HayasakiHeikkilä
Heikkinen
HeimalaHelojärvi
Hemming
Hibino
Hiitola-Keinänen
Hiltunen
Hiltunen
Hiroshi
HokkanenHolmlundHoshikawa
HristuHuotari
Huttunen
Huttunen
Hänninen
Iakubivskyi
IkonenImamura
Ishii
Ishii
Ishikawa
Ito
Jones
JuttulaKaikkonen
Kaikkonen
Kakko
Kan
Kanyathare
KaplasKarioja
Karttunen

NathanRyohei
Kenji
Soetedho
Mikko
Shuma
HiroshiTakahiro
Tahvo
Yoshio
Rauno
Ville
Päivi
TuomasBjörn
KenichiJohanna
Jussi
Marianne
OohashiAri
Christer
Masaharu
Radu
Matti
Mikko J.
Olli-Heikki
Pekka
Iaroslav
Erkki
Keigo
Katsuhiro
Yukihiro
TatsuyaYujiroGeraint
Harri J.
Ville A.
Ville
Joona-Pekko
YelenaBoniphaceT.
Pentti
Anssi-Pekka

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Karvinen
KasedaKauranen
Ketolainen
KigamiKiyohara
KleineKobori
Koho
KohoutKokka
Koponen
Korhonen
Korpelainen
Kraft
KukkolaKumagaiKübarsepp
Kylmänen
Kämppi
Kärhä
Lassila
Lauri
LaurilaLensu
LiedertLipsanen
Liu
MakinoMannilaManoocheriMarklMatsudaMatsudaMikkonen
MingleMirantoMiyataMolkoselkä
Mäkynen
Määttä
NagaikeNielikäinen
NishimuraNiskanen

Petri
Kento
MarttiJarkkoHiroshi
KosukeJustus F.
Yuta
Sami
Tomas
Alexander
Antti
Ossi
Virpi
ThomasKatri
ToshikiT.
Kasperi
Antti
Petri
Antti
Janne
Toni
Lasse
Christina
Harri
Bin
Takeshi
Rami
FarshidDanielKiyofumi
Yasuhiro
Tommi
CheethamAkseli
Yuki
Eero O.
Anssi J.
Kari
Yasunari
Nanni
Takahiro
Ilpo

60
3
64
33
46
79
44
24
22
58
39
48
33
43
93
20
70
28
50
20
50
43
48
89
68
66
64
73
75
103
95
33
29
46
99
56
103
13
1
1
63
70
20
13
29

72

20
20

44
41

61

xi

44

Noronen
Nuutinen
Nyave
NystenNäkki
Näreoja
Näsilä
Odoi
Ogura
OjanenOkamotoOksanen
OshimaOtani
Pahapill
Pajusalo
ParekhPatokoski
Peiponen
Pekkala
Pekkarinen
Pélisset
Peltonen
Petritsenko
Pikkarinen
Pokatilov
Porrasmaa
Porrovecchio
Praks
Pulli
Rammo
Rantanen
Rao
RebaneReinartRidgway
Rissanen
Rossi
Roussey
Räty
Röning
Saari
Saarinen
Saarinen
Sakurai

Teppo
Tarmo
KennethJohan
Ismo
Tuomas
Antti
Keisuke
YusukeHarri
TakashiBenjaminJuro
Yukitoshi
Jüri
MihkelSapun
Kim
Kai-Erik
Oskari
MarkkuS.
Juha
KatrinNiklasA.
Santeri
G.
Jaan
Tomi
Matt
AleksiR.
Aleksander
Anu
Cathy
Joona
Jussi
Matthieu
Jukka
Juha
HeikkiJarkko J.
Jyrki
Sho

97
56
56
43
103
22
58
3
13
101
87
50
3
17
91
58
68
97
26
39
60
83
22
91
20
28
95
28
58
39
91
20
83
91
58
33
97
89
26
29
63
101
5
60
31

72

103
61
103

29

99
33

56

56

83

103

xii

SasakiSatake
Sato
Seppä
Shibata
Shibata
Shimomura
Shinkawa
Sildoja
Slavinskis
Snodgrass
Stanciu
Stark
Stuns
SuzukiSuzukiSuzuki
SvirkoTamadaTanidaTaniguchi
TannerTate
Taue
Temerov
Toivanen
Toivonen
Tokunaga
TomitaTrops
TurquetTörmänen
Unno
UotilaUozumi
Uusitalo
Wada
VainioVartiainen
VehmasViherkanto
Vilkman
Väisänen
Välimäki
Ylikunnari

Osami
Shin-ichi
Ryo
Jeremias
ShuheiYutaroSuguruDaiki
Meelis-Mait
AndrisColin
Stefan C.
Charles W.
IngmarMasayukiNatsukiTakamasa
Y.
Yosuke
Jun
Jun
Tarja
Naoya
Shuji
Filipp
Hans
Juha
Eiji
Yasuo
Roberts
Léo
Matti
Noriyuki
Juho
Jun
Sanna
Naoya
MarkkuErik M.
TapaniKai
Marja
Kaisa-Leena
Marja
Mari

73
46
9
35
17
31
13
61
91
58
58
18
91
101
17
37
73
54
61
13
46
20
13
15
5
103
85
79
3
103
64
41
46
89
77
66
75
89
68
52
103
93
93
93
93

43

103

61

97

xiii

99

Yokota
Yuasa
YukitaZang
Zeitler

MasayukiTomonori
Shunpei
XiaorunJ. Axel

7
24
79
64
33

xiv