Fairy Lights in Femtoseconds: Aerial and Volumetric Graphics
Rendered by Focused Femtosecond Laser Combinedwith Computational Holographic Fields
YOICHI OCHIAI
University of Tsukuba
KOTA KUMAGAI
Utsunomiya University
TAKAYUKI HOSHI
Nagoya Institute of Technology
JUN REKIMOTOThe University of Tokyo
andSATOSHI HASEGAWA and YOSHIO HAYASAKI
Utsunomiya University
We present a method of rendering aerial and volumetric graphics using
femtosecond lasers. A high-intensity laser excites physical matter to emit
light at an arbitrary three-dimensional position. Popular applications canthus be explored, especially because plasma induced by a femtosecond laser
is less harmful than that generated by a nanosecond laser. There are two
methods of rendering graphics with a femtosecond laser in air: producing
holograms using spatial light modulation technology and scanning of a laser
beam by a galvano mirror. The holograms and workspace of the system
proposed here occupy a volume of up to 1 cm3 ; however, this size is scalable
depending on the optical devices and their setup. This article provides detailsof the principles, system setup, and experimental evaluation, and discusses
the scalability, design space, and applications of this system. We tested
two laser sources: an adjustable (30–100fs) laser that projects up to 1,000
pulses/s at an energy of up to 7mJ/pulse and a 269fs laser that projects up
to 200,000 pulses/s at an energy of up to 50μJ/pulse. We confirmed that the
spatiotemporal resolution of volumetric displays implemented using theseY. Ochiai and K. Kumagai are joint first authors.
Authors’ addresses: Y. Ochiai, 1-2 Kasuga, Tsukuba-shi, Ibaraki, 3058550 Japan; email: wizard@slis.tsukuba.ac.jp; K. Kumagai, S. Hasegawa,and Y. Hayasaki, 7-1-2 Yoto, Utsunomiya-shi, Tochigi, 321-8585 Japan;
emails: {kumagai_k, hasegawa_s, hayasaki}@opt.utsunomiya-u.ac.jp; T.
Hoshi, Gokisocho, Showa-ku, Nagoyashi, Aichi, 466-855 Japan; email:star@nitech.ac.jp; J. Rekimoto, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033
Japan; email: rekimoto@acm.org.
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DOI: http://dx.doi.org/10.1145/2850414

laser sources is 4,000 and 200,000 dots/s, respectively. Although we focus onlaser-induced plasma in air, the discussion presented here is also applicableto other rendering principles such as fluorescence and microbubbles in solid
or liquid materials.
CCS Concepts:
nologiesr Hardware → Emerging optical and photonic tech-

Additional Key Words and Phrases: Volumetric display, laser plasma, femtosecond laser, aerial interaction, touchable aerial images
ACM Reference Format:
Yoichi Ochiai, Kota Kumagai, Takayuki Hoshi, Jun Rekimoto, Satoshi
Hasegawa, and Yoshio Hayasaki. 2016. Fairy lights in femtoseconds: Aerial
and volumetric graphics rendered by focused femtosecond laser combinedwith computational holographic fields. ACM Trans. Graph. 35, 2, Article 17(February 2016), 14 pages.
DOI: http://dx.doi.org/10.1145/2850414

1.

INTRODUCTION

Three-dimensional (3D) displays have attracted great attention overthe past 5 decades. Virtual 3D objects were originally displayed witha head-mounted display in Sutherland [1968]. Since then, continuous efforts have been made to explore 3D displays that have planar
surfaces, and several methods have been developed to provide stereopsis for binocular vision [Benzie et al. 2007]. The technologies that
employ glasses to achieve this are based on anaglyphs, time division, and polarization. On the other hand, those technologies that
do not rely on glasses are based on a parallax barrier and lenticularlens array [Masia et al. 2013]. Although these methods can offer
effective 3D images, they require calculation and generation of precise images for multiple viewpoints, and users have to stay within
a limited view angle.
A different approach to realizing advanced 3D displays uses aphysical 3D space instead of a planar surface to render graphics
and forms a visual representation of an object in three physicaldimensions, as opposed to the planar image of traditional screens
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

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Fig. 1. A map of related work divided into four categories regarding to
non–position-control/position control and reflection/emission. This study
falls into the position-control and emission category.

that simulate depth through various visual effects [Masia et al.
2013]. These 3D displays, which are called volumetric displays,
allow users to view the displayed images from any angle.
Volumetric displays arrange “voxels” in a 3D space. They are
divided into four categories: nonposition-control/position-control
and reflection/emission (Figure 1). Note that some of them are
not 3D but 2.5D, and position control in 2.5D produces surface
deformation. In this study, we focus on laser-induced plasma,
which is in the position-control emission category.
Laser-induced plasma has the following advantages. First, it does
not require special materials arranged and suspended in air to emit
light. Second, it does not require wires and structures that possibly
obstruct the line of sight because power is transmitted wirelessly.
Third, the laser can be precisely controlled owing to progress in
optical technologies.
We envision a laser-induced plasma technology with general applications for public use. If laser-induced plasma aerial images weremade available, many useful applications such as augmented reality(AR), aerial user interfaces, and volumetric images could be produced (Figure 2). This would be a highly effective display for the
expression of 3D information.
Volumetric expression has considerable merit because the content
scale corresponds to that of the human body; therefore, this technology could be usefully applied to wearable materials and spatial
user interactions. Further, laser focusing technology adds an additional dimension to conventional projection technologies, which
offer surface mapping, whereas laser focusing technology is capable of volumetric mapping. Thus, this technology can be effectively
used in real-world-oriented user interfaces.
Plasma-based 3D displays were previously developed using ananosecond laser [Kimura et al. 2006] and femtosecond (100fs)
laser [Saito et al. 2008]. Although these studies of laser-plasma
graphics were innovative, they did not provide the details of theprinciples and technologies, and users were not allowed to touch
the rendered graphics. Our motivation is to provide further discussion of this laser-plasma graphics technology and to expand theachievements of earlier work by enabling users to touch the rendered graphics.
In this study, we use femtosecond lasers with pulse durationsof 30–100fs and 269fs. The plasma thus generated is less harmful
than that obtained using nanosecond lasers, so the system can be
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

incorporated into everyday life. The design space and possible application scenarios of the plasma-based 3D display are discussed.
In addition, we use an optical device called a spatial light modulator(SLM) to modify the phase of light rays and produce various spatial
distributions of light based on interference.
The primary contribution of this article is the production by an
ultrashort pulse duration laser of in-air SLM-based laser-plasma
graphics that enable physical contact and interaction (Figure 3).
In addition, the principles are theoretically described, the characteristics of this technology are experimentally examined, and theapplications and scalability are discussed.
The rest of this article is organized as follows. First, we describe
the principles and design parameters of femtosecond-laser-based
volumetric displays. We explore a less harmful, high-resolution,
adaptable laser-based volumetric display using a femtosecond laser
and an SLM. Second, we introduce the setup we designed. Third, wegive examples of applications. Finally, we conduct experiments on
the generation and control of lasers and their damage to the skin. We
also discuss the system’s limitations and estimate its scalability. We
believe that this study fills gaps in the design space of plasma-based
3D displays that were left unresolved by previous studies.

2.

RELATED WORK

2.1

Volumetric Displays

2.1.1 Non–Position-Control Types. Reflection: In this category, the work space is filled with small objects of a material
that can passively reflect projected or environmental light. Threedimensional displays based on mechanical motion of a mirror or
screen are discussed in Parker [1948]. A spinning mirror is usedwith a high-speed projector in Jones et al. [2007], where different
images are projected onto the mirror according to its azimuthal angle to express the 360◦ light field of an object. Similarly, images
are projected onto a rotating screen [Favalora et al. 2002] and a rotating diffuser plate [Karnik et al. 2011]. The systems proposed in
Rakkolainen et al. [2005a] and Lee et al. [2009] use fog as thereflecting material. A thin layer of fog is generated, and images areprojected onto it. In Eitoku et al. [2006], falling water drops are
utilized as a screen. Because of their lens-like properties, the water
drops deliver projected images to users’ eyes. Subsequently, multilayer water-drop screens were implemented [Barnum et al. 2010],
and different images were projected onto different layers by synchronizing the projector with the water valves. In DepthCube [Sullivan 2004], a multilayered liquid crystal shutter is illuminated by ahigh-speed projector. Photochromic materials are used in Hashida
et al. [2011] to form a volumetric and multicolor display controlled
by an ultraviolet projector. In Holodust [Perlin and HAN 2006],
small floating particles are illuminated by lasers. Small particles arelaunched into the air and illuminated by a projector in Matoba et al.
[2012].
Emission: In this category, objects occupying the work space
actively emit light to show images. Clar [2008] created a 3D cubic array of light-emitting diodes (LEDs). In this setup, the LEDs
are supported by a framework, and their relative positions (i.e.,
voxel locations) are fixed. Fabrication-type 3D volume displays are
currently being explored [Willis et al. 2012]. Three-dimensional
printed objects with embedded light paths can display information
when the objects are placed on a flat display. As the objects become
more complex, the light paths also become complex, which makes
it difficult to design the object to be printed. Pereira et al. [2014]
solved this problem by algorithmically computing the arrangement

Fairy Lights in Femtoseconds

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Fig. 2. These figures show the example applications of proposed laser-based graphics technology. (a) Images superposed on a hand and a box. (b) Floating
button with haptic feedback. (c and d) Volumetric images rendered in open and closed areas.

Fig. 3. Application images of Fairy Lights in Femtoseconds, aerial and volumetric graphics in air rendered by femtosecond lasers. (a) A “fairy” flying infront of a finger. (b) A “sprout” coming out from a seed. (c) Interference between a point cloud and a finger. (d) The SIGGRAPH logo.

of the light paths so that their endings form a desired surface shape,such as that of a face.
2.1.2 Position-Control Types. Reflection: In this category, thepositions of reflection objects are controlled to render graphics.
Studies focusing on controlling the surface shape of a screen or
display have also been pursued. For example, the surface shapeof the deformable screen in Project FEELEX [Iwata et al. 2001]
is changed by linear actuators. The deformable screen in Form
[Follmer et al. 2013] not only displays images but also interacts with
objects. Ochiai et al. [2013] used focused ultrasound to deform asoap film, without making contact, to show a bump on it. Pixie Dust
[Ochiai et al. 2014] is a floating display consisting of small particles
that are suspended and moved by means of acoustic levitation.
Emission: Light sources are moved to realize 3D displays in thiscategory. This type of volumetric display was originally reported in
Jansson and Berlin [1979]. Many types of volumetric displays havebeen explored for 35 years. Grossman and Balakrishnan [2006]
surveyed this area well. Macfarlane [1994] proposed a voxel-based
spatial display. LUMEN [Poupyrev et al. 2004] consists of LEDs
attached to linear actuators and shows information in the form of
red-green-blue and height variations. Laser plasma, which is freefrom physical support and connections, is used as a light source inKimura et al. [2006]. We also work with this technology to use thisadvantage.

2.2

the pulse duration (e.g., to nanoseconds or shorter) under a limited
time-averaged power.
The basic concept was demonstrated using a nanosecond laser inKimura et al. [2006], where the rendering speed was 100 dots/s. The
color of the voxels was bluish white because of plasma emission.
Later, 1,000 dots/s was achieved [Saito et al. 2008], and rendering
algorithms for a point cloud were discussed in Ishikawa and Saito[2008a, 2008b]. A femtosecond (100fs) laser was used there, but
this information on the pulse duration appeared only on a Japanesewebsite.1 Although these studies of laser-plasma graphics were innovative, the published papers did not provide detailed discussionsof the light emission, design space, scalability, and so on. We discussthese issues in this article and demonstrate complete laser-basedgraphics in air from principles to applications.
Laser-based 3D displays in materials other than air were also
demonstrated. An in-water type2 of laser-based volumetric display
was developed in Kimura et al. [2011] and achieved 50,000 dots/s.
Although no detailed principle was provided, we infer that thisin-water type is based not on laser plasma but on laser-induced microbubbles. The dots of green light generated by a green laser canbe explained by diffusion of the incident laser by the microbubbles.
Fluorescent materials were used in Soltan et al. [1992], Downing
et al. [1996], and Hasegawa and Hayasaki [2013]. The pulse peakintensity required for microbubble- and fluorescence-based rendering is experimentally confirmed in Section 5.3. This method offers

Laser-Based Volumetric Displays

As mentioned above, laser-plasma 3D displays are categorized as
position-control emission-type 3D displays. Voxels in air are generated by high-intensity laser pulses, which are realized by shortening

1 http://www.aist.go.jp/aist_j/press_release/pr2007/pr20070710/pr2007071

0.html (in Japanese) (last accessed June 17, 2015).
2 http://www.diginfo.tv/v/11-0231-r-en.php (last accessed June 17, 2015).

ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

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Table I. Comparison between the Previous and This Study
MediumLaser

Kimura 2006
Air
Nanosecond

Saito 2008
Air
Femtosecond
100fs

Dot/sec

100

1,000

Hologram

No

No

Interaction
Touch

No
No

No
No

This study
Air
Femtosecond
A: 30-100fs, 2mJ
B: 269fs, 50μJ
A: 1,000
B: 200,000
Yes
LCOS-SLM or LCSLM
Yes
Yesa higher rendering speed than plasma-based rendering so that not a
set of lines but a surface can be represented [Ishikawa et al. 2011].
Fig. 4. Laser plasma induced by focused femtosecond laser.

2.3

Aerial Interaction

Volumetric, aerial, and/or 3D displays can usually interact withusers’ hands. For example, users can directly interact with graphics
rendered on a thin layer of fog [Rakkolainen et al. 2005b]. Touchable
Holography [Hoshi et al. 2009] and RePro3D [Yoshida et al. 2010]
show 2D and 3D images in air, respectively, and also provide haptic
feedback. Small particles are acoustically levitated in Ochiai et al.
[2014], and users can touch them. ZeroN [Lee et al. 2011], although
it is a tangible system rather than a graphic system, magneticallylevitates a sphere, and users can touch and also handle it.
There are two necessary conditions for aerial interaction withvolumetric displays: They should be safe and accessible. The previous works [Kimura et al. 2006; Saito et al. 2008] did not satisfy
these conditions. The in-air type [Kimura et al. 2006] is harmful tousers’ hands because of the high laser power, and the in-water type
[Kimura et al. 2011] renders images in a transparent container filledwith water. In this article, we attempt to demonstrate a less harmful
and touchable laser-based volumetric display.

2.4

Purpose of this Study

Conventional studies have encountered two problems in the development of a volumetric display: how to suspend and emit voxels.
The application of laser plasma technology to a volumetric display overcomes these two issues because laser plasma generates anemission point at an arbitrary position in a 3D space. In addition,
studies of previous laser volumetric displays have not sufficiently
discussed the theoretical principles and scalability.
This study focuses on a system for rendering volumetric graphics in air using a femtosecond laser. Because our system uses anultrashort-pulse laser and an SLM, we can explore touch interaction and computer-generated holograms (Table I). These explorations and evaluations are useful for discussing the scalability andapplication space of a plasma-based volumetric display using ahigh-intensity laser for general, widespread application.

Plasma is produced by tunnel ionization, which predominantly
occurs when the laser intensity is greater than 1014 W/cm2 [Keldysh
1965]. The potential well of a molecule or atom is deformed by theelectric field of the high-intensity laser to have a potential barrier;then, an electron has the opportunity to leave the atom (i.e., ionization of the atom is possible) as a result of the tunnel effect. It
is known that a higher laser intensity leads to a higher tunnel ionization probability; that is, more electrons are ionized [Ammosov
et al. 1986]. The ionized electron is recombined with the atom after
a half-cycle, and a photon is emitted. This effect is called laser
breakdown. The emitted light looks bluish white.

3.2

Laser Filamentation

An emission dot generated by a high-intensity laser has a tail along
the propagation direction. This tail is generated as the self-focusing
behavior due to the optical Kerr effect competes with the natural
diffraction of the laser beam; however, this effect is undesirable
when rendering 3D graphics in air. Practically, this effect is invisible
to the human eye because the light from the focal point is muchbrighter, but it might be taken into consideration in some special
cases.

3.3

Voxel Sizes

We assume that the size of an emission dot (i.e., a voxel) is equal to
the size of the focal point of the laser. The focal point is usually an
oval that has two diameters. One is the diameter perpendicular tothe laser beam, wf , which is the diffraction limit and is determinedby the original beam width a, focal length r, and wavelength λ such
that
r
(1)
wf = 2λ .
aIn this section, we show how to generate light spots in air using
lasers.

The other is the diameter along the laser beam, wd , which is geometrically obtained from the relationship a : wf = r : w2d , such
that
 2
r
wd = 4λ
.
(2)
a

3.1

3.4

3.

PRINCIPLES

Laser-Induced Light Spot

There are three types of laser-induced effects that produce lightspots: Fluorescence in special materials, cavitation in fluids, andplasma in air. The required instantaneous power for these effects isquite different. In this study, we focus on the third effect (Figure 4),
which requires the most power.
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

Computational Phase Modulation

The use of SLMs is one method of rendering holograms. An SLM
generally has an array of computer-controlled pixels that modulatea laser beam’s intensities, phases, or both. This optical device isused in, for example, laser processing to generate an arbitrary laser
pattern [Hayasaki et al. 2005].

Fairy Lights in Femtoseconds

Fig. 5. Example of a computer-generated hologram (CGH). (a) An original
image, (b) a converted spot-array image of the original image, and (c) a CGH
to be displayed on the SLM.

A liquid crystal SLM (LCSLM), which contains a nematic liquid
crystal layer, is used in this study. The molecule directions within
this layer are controlled by electrodes, i.e., pixels, and the phase of
the light ray reflected by each pixel is modulated according to thedirection of the liquid crystal molecule. In other words, this device
acts as an optical phased array.
The spatial phase control of light enables control of the focusing
position along both the lateral (XY ) and axial (Z) directions. The
complex amplitude (CA) of the reconstruction from the computergenerated hologram (CGH) Ur is given by the Fourier transform ofthat of a designed CGH pattern Uh :Uh (x, y) exp[−i2π (xνx + yνy )]dxdy
Ur (νx , νy ) == ar (νx , νy ) exp[iφr (νx , νy )](3)

Uh (x, y) = ah (x, y) exp[iφh (x, y)] ,(4)

where ah and φh are the amplitude and phase of the hologram plane
displayed on the SLM, respectively. In the experiment, ah is constant because the light irradiating the CGH is considered to be aplane wave with a uniform intensity distribution. φh is designed using the optimal rotation angle (ORA) algorithm [Bengtsson 1994].
On the other hand, ar and φr are the amplitude and phase of thereconstruction plane, respectively. The spatial intensity distributionof the reconstruction is actually observed as |Ur |2 = ar2 .
To control the focusing position along the lateral (XY ) direction,
the CGH is designed on the basis of a superposition of the CAs of
blazed gratings with various azimuth angles. If the reconstruction
has N -multiple focusing spots, the CGH includes N -blazed gratings. To control the focusing position along the axial (Z) direction,
2 +y 2a phase Fresnel lens pattern φp (x, y) = k x 2f
with a focal length
f is simply added to φh , where k = 2π
is
a
wave
number. In thisλcase, the spatial resolution of the SLM determines the minimum
focal length, according to the theory discussed in Section 3.3.
The ORA method is an optimization algorithm for obtaining thereconstruction of a CGH composed of a spot array with a uniform
intensity (Figure 5). It is based on the addition of an adequate
phase variation calculated by an iterative optimization process to
the CGH. In the i-th iterative process, the amplitude ah and phaseφh(i) at a pixel h on the CGH plane and the CA Ur(i) at a pixel r
corresponding to the focusing position on the reconstruction plane
are described computationally as follows: (i)Ur(i) = ωr(i)
uhr
h

=

ωr(i)


h

ah exp[i(φhr + φh(i) )],

(5)

•

17:5

where uhr is the CA contributed from pixel h on the CGH plane to
pixel r on the reconstruction plane, φhr is the phase contributed bylight propagation from pixel h to pixel r, and ωr(i) is a weight coefficient that controls the light intensity at pixel r. To maximize thesum of the light intensity r |Ur(i) |2 at each pixel r, the phase variation φh(i) added to φh(i) at pixel h is calculated using the following
equations.
 S2
φh(i) = tan−1
,
(6)
S1
S1 =



ωr(i) ah cos(φr − φhr − φh(i) ),(7)

ωr(i) ah sin(φr − φhr − φh(i) ),

(8)

r

S2 =


r

where ωr is the phase at pixel r on the reconstruction plane. The
phase of the CGH, φh(i) , is updated by the calculated φh(i) as
follows.
φh(i) = φh(i−1) + φh(i) .

(9)Furthermore, ωr(i) is also updated according to the light intensity of
the reconstruction obtained by the Fourier transform of Equation (9)in order to control the light intensity at pixel r on the reconstruction
plane.
 (d) αIωr(i) = ωr(i−1) r(i) ,(10)
Ir
where Ir(i) = |Ur(i) |2 is the light intensity at pixel r on the reconstruction plane in the i-th iterative process, Ir(d) is the desired lightintensity, and α is a constant. The phase variation φh(i) is optimized by the above iterative process (Equations (6)–(10)) until Ir(i)is nearly equal to Ir(d) . Consequently, the ORA method allows us todesign a high-quality CGH.

3.5

Positioning of Graphics

The galvano mirror used in this study covers an area of 10 × 10 mm2 .
Further, the SLM also renders graphics within approximately thesame area. This means that we have two options for placing a point at
an intended position: leading a laser there using the galvano mirrors
or modifying the spatial distribution of the laser using the SLM. The
conditions and/or response times of these devices determine which
is suitable.
The theoretical rendering limit is 33 dots/s for 30 frames/s (fps)because the femtosecond laser is pulsed at a frequency of 1kHz. The
SLM is used to render additional dots in a single frame, whereas
the galvano mirror is used primarily for positioning the rendered
holograms.

3.6

Spatiotemporal Resolution

The number of dots per frame (dpf) must be evaluated forlaser-based volumetric displays. We now assume that the dots are
displayed in darkness; therefore, the minimum required power foreach dot is equal to the laser breakdown threshold Elbd . The total
output power Etot is divided among the dots by the SLM. The
number of dots per laser pulse, Ndot , is expressed as
Ndot =

Etot
.
Elbd

(11)

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Fig. 6. Setup of our light circuit. The host computer controls (2) the SLM for hologram generation, (5) the galvano scanner for XY control, and (9) the
varifocal lens for Z control.

The number of dots per frame is determined by Ndot , the repeat
frequency Frep of the laser pulses, and the frame time Tf , which isdetermined on the basis of the persistence of human vision. Hence,
dpf = Ndot × Frep × Tf .

(12)

For example, if Ndot = 100, Frep = 1kHz, and Tf = 100ms, ananimation of 10,000dpf is played in 10fps. Note that, in practice,
the number of dots per frame is determined by the bottleneckproduced by the time response of the galvano mirrors and/or the
SLM, instead of by Frep .

4.

Table II. Specifications of Laser
Sources
SystemMaker
Pulse duration
Repeat cycle
Energy/pulse
Dots/sec
Average powerA
Coherent
30–100fs
1kHz
2mJ
1,000
2W

B
IMRA
269fs
200kHz50μJ
200,000
10W

IMPLEMENTATIONIn this section, we describe our system implementation. First, we
present an overview of our system. Next, we describe our lightsource, optical circuit (i.e., arrangement of optical devices), 3D
scanning system, SLM, and control system.

4.1

Overview

Figure 6 shows the system configuration of our basic setup. Thissystem aims to produce a simultaneous, multipoint volumetric display. It consists of a femtosecond laser source, an XYZ scanner(galvano scanner plus varifocal lens), and a liquid crystal on silicon
SLM (LCOS-SLM) displaying a CGH for simultaneously addressedvoxels. Our system was tested and investigated at 20.5◦ C. The atmosphere was ordinary air (80% N2 and 20% O2 ).
The setup was tested using two light sources (A and B), thespecifications of which are given below. We primarily used a femtosecond laser source developed by Coherent Co., Ltd., which hasa center wavelength of 800nm, repetition frequency of 1 kHz, andpulse energy in the 1mJ to 2mJ range. The specifications of the laser
sources are shown in Table II. Figure 7 shows examples of resultsfor our system in air.
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

Fig. 7. Relationship between the XYZ-coordinate and the focused laser
beam. Voxels are rendered above the objective lens.

The galvano mirror scans the emission dot along the lateral directions (X and Y scanning), whereas the varifocal lens can vary its
focal point in the axial direction (Z scanning). The Fourier CGH
is used for simultaneously addressed voxels [Hayasaki et al. 2005].
The CGH, which is designed using the ORA method, is displayedon the LCOS-SLM, which has 768 × 768 pixels, a pixel size of
20 × 20μm2 , and a response time of 100ms. The specificationsof each component are shown in Tables III and IV. In addition toFairy Lights in Femtoseconds
Table III. Specifications ofGalvano Mirrors
SystemMaker
Scan angle
Error
Resolution

A
Canon±0.17 rad<5 μrad20 bitB
Intelliscan±0.35 rad< 5mrad20 bitTable IV. Specifications of
Varifocal Lens
SystemMaker
Aperture
Response time
Focal length

A and B
Optotune
10mm<2.5ms+45 to +120mm

these components, we use a microscope, which is connected to thecomputer via USB, for monitoring and recording.

4.2

Light Source

4.4

3D Scanning System

In this subsection, we describe our scanning system in detail. Figure 6 shows the galvano and varifocal lenses. We employ galvano
mirrors to scan the lateral directions (X and Y scanning), while thevarifocal lens can change its focal point in the beam axial direction(Z scanning). For system A, we utilize a Canon GH-315 driven
by GB-501 as the galvano mirror, and for System B we employ
an Intelliscan 20i to scan the beams. Both are connected by PCI
boards. Table III shows the specifications of each of the galvano
mirrors. As the varifocal lens for both Systems A and B, we employ
an Optotune EL-10-30, which is connected via serial USB to a personal computer. The specifications of the varifocal lens are shownin Table IV. These devices are operated by original applications
coded in C++.

4.5

4.3

4.6

Here, we describe our optical circuit in terms of the path of the laser.
Figure 6 shows the optical setup of System A. The laser is generated
by the femtosecond light source and then phase-modulated by the
SLM. The SLM energy conversion rate is 65% to 95%. Then, thebeam spot is varied by two lenses (F = 450 and 150mm). This
two-lens unit reduces the beam spot by a factor of 1/3. The spotis then reflected by the galvano mirror, which determines the XY
position of the light. The galvano mirror and SLM are connectedin an object–image correspondence. Subsequently, the beam spot
is adjusted by two lenses (F = 100 and 150mm); this two-lensunit magnifies the beam spot 1.5-fold. Then, the light enters thevarifocal lens. The varifocal lens and galvano mirror are connectedin an object–image correspondence, and the former adjusts the zaxis focal points. The light enters the objective lens (F = 40mm).
Once it exits this lens, it excites the display medium (air). Theenergy conversion rate of System A is 53%.
System B has the same structure but lacks an SLM. In addition, the lens sets are slightly different from those of System A.
Specifically, System B has no lens before the galvano mirror, as the

17:7

varifocal lens is positioned after the galvano mirror. Then, the beam
spot is adjusted by the two-lens unit (F = 50 and 80mm). An F20
objective lens is employed, and System B’s total energy conversion
rate is 80%.

We use two light sources. The primary light source used for evaluation and application was developed by Coherent Co., Ltd., andhas a center wavelength of 800nm, repetition frequency of 1kHz,
and pulse energy of up to 2mJ; the pulse width is adjustable from30 to 100fs. Figure 8 shows the spectra and pulse intensities of
the 30fs and 100fs settings of this light source. Ultrashort pulses
are generated by converting low-intensity and long-duration pulses
to high-intensity and short-duration ones. If the time-averaged laser
power is unchanged, the peak intensity differs according to the pulse
width. In fact, the 30fs pulse width has a threefold greater peak intensity than the 100fs pulse width at the same time-averaged power.
We refer to the system using this light source as System A.
The other light source we use is the FCPA μJewel DE1050 from
IMRA America, Inc. The laser has a center wavelength of 1,045nm,
repetition frequency of 200kHz, pulse energy of up to 50μJ, andpulse width of 269fs. We refer to the system with this light source
as System B. Note that the peak intensity of the laser, rather thanthe pulse width, is important for producing aerial plasma. Systems
A and B both have sufficient peak intensity to excite the air andgenerate an emission dot.

Optical Circuit

•

LCSLM

The LCSLM (Hamamatsu, PPM) is a parallel-aligned nematic liquid crystal SLM (PAL-SLM) coupled with a liquid crystal display(LCD) and a 680nm laser diode (LD). This device, which can perform phase-only modulation of more than 2rad, is frequently used
to display real-time CGHs. The PAL-SLM is composed of a liquid
crystal (LC) layer, a dielectric mirror, and an optically addressed
photoconductive (PC) layer containing amorphous silicon, which
are sandwiched between two transparent indium tin oxide electrodes. The LC molecules are aligned in parallel. When incident
light illuminates the PC layer, the impedance of this layer decreases,
and the electric field across the LC layer increases accordingly. With
this increased field, the LC molecules become tilted in the propagation direction of the readout light, and the effective refractive index
of the LC layer decreases. Pure phase modulation occurs only when
the polarization direction of the femtosecond laser is parallel to thealigned direction of the LC molecules. The CGH pattern on the LCD
illuminated by the LD is applied to the PC layer through imaging
optics.

Control System

Figure 6 shows our system diagram. The system is controlled using
a personal computer running the Windows operating system, and all
programs are coded in C++. The control system operates the SLM,
galvano mirror, and varifocal lenses. To monitor the interaction,a USB microscope is connected to the system. The galvano andvarifocal lenses run along different threads and are synchronizedwhen new draw patterns are input. The user input is captured at
20Hz, and the SLM is connected to the computer as an external
display.

5.

EXPERIMENTS AND EVALUATIONSIn this section, we describe our experiments and system evaluation
procedures. First, we present an overview of our experimental planand results. Then, we report the results of the following tests: energy
versus ionized plasma brightness, brightness versus pulse peak, simultaneously addressed voxels for aerial images, and skin damage.
In the experiments, the brightness is measured as a summation of
all the pixel values within a close-up image of the plasma taken by
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

17:8

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Y. Ochiai et al.

Fig. 8. Spectra of 100fs, 30fs, and 269fs lasers (from left to right). The rightmost illustrates the peak intensity and the pulse width of each femtosecond lasers.

a digital camera, which is a common definition in the field of laser
optics.
We tested not only gas-ionized plasma but also photon absorption
and cavitation in order to compare the various energy consumption
performances and the means of applying the femtosecond laser
system to display technology. All experiments were conducted using
System A, which is described in Section 4.

5.1

Overview of Experiments

In this study, we propose a femtosecond-laser-based display systemdesign. In previous works [Kimura et al. 2006; Saito et al. 2008], therequirements, scalability, and risks of such laser-based systems are
not thoroughly discussed. There are several factors that we should
explore. In Section 5.2, we examine the voxel brightness, which
is important in relation to the energy and display spatiotemporal
resolution, as discussed in Section 3. In Section 5.3, we explore
the relationship between the pulse duration and brightness. Thisis important for scalability, particularly when a faster laser source
is developed. In Section 5.4, we explore simultaneously addressedvoxels using an SLM. This is important for scalability with regard
to increasing the spatiotemporal resolution. Then, in Section 5.5, we
examine risk issues and the effect of the plasma on the skin. Thisis important because this technology is intended for widespread
general use. Finally, in Section 5.6, we examine audible soundfrom the plasma generated by femtosecond lasers. This is importantbecause this technology is intended for use in everyday life.

5.2

Energy versus Brightness

We conducted this experiment to evaluate the relationship between
the plasma-production energy level and the resultant brightness of
the image. This experiment aimed to confirm the feasibility of oursystem and to investigate how it can be applied to display voxels;
thus, the minimum peak intensity value was determined.
We conducted the experiments using System A (30fs) and employed a microscope to capture the resultant image. With our setup,the laser source can provide a time-averaged power of up to 7W;
however, unwanted breakdown occurs in the light path before theobjective lens when the power is too high. Hence, the full power ofthe laser source cannot be used. Moreover, the capacity of our SLM
is not guaranteed above 2W. The experiments were conducted for atime-averaged power range of 0.05 to 1.00W.
Figure 9 (30fs) shows the experimental setup and results. The
experiments were conducted under energies per pulse of 0.16 to
0.55mJ. The 30fs laser can produce plasma at a pulse energyof 0.2mJ. The cross-sectional area of the focal point is theoretically calculated to be 2 × 10−7 cm2 . Then, the peak intensity is
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

Fig. 9. Experimental setup and results on brightness of light emission in
air induced by 30fs and 100fs lasers.

36PW/cm2 , which is clearly higher than the ionized plasma threshold (>1PW/cm2 ).

5.3

Brightness versus Pulse PeakThe relationship between the pulse peak and the resultant image brightness was also examined, as the peak intensity plays
an important role in plasma generation. This experiment aimed toclassify systems of different pulse width in terms of the display voxel
brightness.
As in the previous experiment, we conducted experiments using
System A (30 and 100fs). Pulses of 30 and 100fs yield different
spectra and instantaneous power for the same time-averaged power.
In addition, the 30fs setting yields a threefold higher peak pulse. We
employed the same microscope to capture the image as that used in
Section 5.1, and the results are shown in Figure 9. The experiments
were conducted for a power range of 0.05 to 1.00W.

Fairy Lights in Femtoseconds

Fig. 10. Experimental results on brightness of light emission in air, water,
and fluorescence solution induced by 30fs laser.

It was found found that a 100fs laser can generate plasma ata pulse energy of 0.45mJ. Then, the peak intensity is 24PW/cm2 ,which is clearly higher than the ionized plasma threshold(>1PW/cm2 ). Further, it is confirmed that the 30fs pulse requires
less energy than the 100fs pulse to produce plasma under the sametime-averaged power.
Additionally, we conducted other experiments comparing media
materials (air, water, and fluorescent solution). The results are shownin Figure 10, which shows that the required pulse energy differs
dramatically depending on the medium.

5.4

Simultaneously Addressed Voxels

One of the main contributions of this article is the application of
an SLM to in-air laser plasma graphics. This enables simultaneously addressed voxels using CGHs. (Note that in previous works[Kimura et al. 2006; Saito et al. 2008], multiple voxels were notgenerated simultaneously.) Simultaneous addressing is importantfor increasing the spatiotemporal resolution, although the simultaneously addressed voxels are darker than a single point because theenergy is distributed among them. This experiment was designed to
explore the resolution scalability by using an SLM with a single lightsource. Simultaneous addressing can be obtained for both the lateral(X, Y ) and beam (Z) axes by displaying appropriate holograms on asingle SLM. Here, we investigated simultaneous addressing for thelateral axis. Again, the experiments were conducted using System A(30fs), and Figure 11 shows the results and the holographic images
used in the SLM. We employed the microscope shown in Figure 9.
We conducted the experiments using time-averaged laser powers
from 0.05 to 1.84W. We had one to four simultaneously addressedvoxels, and five or more voxels were not visible.

5.5

Skin Damage

Another main contribution of this article is an estimation of the riskof femtosecond laser systems. Plasma can be harmful to humans.
However, a femtosecond pulse is an ultrashort pulse and is used forbreaking without the use of heat for industrial purposes. It is also
used for ultrashort-scale fabrication on the submicrometer scale.
Thus, we supposed that such pulses may not damage human skinseriously. In addition, our display scans a 3D space very rapidly;
therefore, the laser spot does not remain at a specific point fora long period. On the other hand, this plasma still poses dangers

•

17:9

to the retina. However, we believe that the potential for generalapplication still exists with appropriate installation.
Therefore, we conducted this experiment to explore the damage
to skin structure caused by femtosecond plasma exposure. In theseexperiments, we employed leather as a substitute for human skin.
The experiments were conducted using System A (30fs and 1W,
100fs and 1W), and the plasma exposure duration was varied between 50 and 6,000ms. Figure 12 shows the results. It was foundthat the 30fs and 100fs pulses have almost the same effect on theskin. As we described above, the 30fs pulse has a threefold greater
instantaneous power and can generate brighter voxels. However,
50 shots occur in 50ms, and there is almost no difference between
the 30fs and 100fs results. In this experiment, the time-averaged
power is the factor determining the result. For exposures of lessthan 2,000ms (2,000 shots), only 100μm-diameter holes appeared,
and there was no heat damage to the leather. For a period of longer
than 2,000ms, heat effects were observed around the holes.
We conducted a test with a nanosecond laser for comparison withthis result. With the nanosecond laser, the leather burned within
100ms. This means that the pulse duration, repetition time, andtime-averaged power are important factors affecting the level ofdamage caused by the laser.
Hence, this laser is sufficiently less harmful for use. Further, there
are two ways to reduce the damage to the skin: using an ultrashortpulse laser, which is bright and has a time-averaged power that is
not intense, or increasing the scanning speed.

5.6

Noise Level

Laser plasma in air radiates not only visible light but also audible
sound. We conducted an experiment to evaluate the radiated sound.
The position of the laser plasma was fixed. The time-averaged laser
power was set to 1.0 or 1.2W. The pulse width was set to 40, 60, 80,
or 100fs. The noise level was measured by a noise level meter (NL52, Rion Co., Ltd.), which was placed 20mm from the laser plasma.
The background noise level was 55.7dB SPL. The brightness of thelaser plasma was also recorded.
The results are shown in Figure 13. The maximum noise level
was 77.2dB SPL with 40-fs pulses, which also yielded the brightest
radiation. This noise level was not very subjectively annoying, and
it is acceptable in everyday life. Brighter plasma emission tends tobe accompanied by louder sound; 40fs pulses radiate louder soundand brighter light.

6.

APPLICATIONSIn this section, we describe potential applications of our system. We
introduce a 3D aerial display system and describe the interaction
between the system and users (Figure 2).

6.1

Aerial Displays

In this subsection, we describe our aerial display using laser plasma.
We developed our application for both Systems A and B, and theresults are shown in Figures 14(a), 14(b), and 14(d). For Systems
A and B, the workspaces are 10 × 10 × 10 and 8 × 8 × 8mm3 ,respectively. These workspaces are smaller than those in previous
works [Kimura et al. 2006; Saito et al. 2008], but the resolutions
are 10 to 200 times higher than those in previous works. The maximum spatiotemporal resolution is 4,000 points/s (with simultaneous addressing of four voxels) for System A and 200,000 point/s
for System B. The image frame rate is determined by the numberof vertices used in the image.
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17:10•Y. Ochiai et al.

Fig. 11. Experimental results on simultaneous addressing. One to four addressing were tested. The intensity is the normalized value of the summation of all
the pixel values of the photos of the voxels, which is taken a evaluative value of brightness.

Fig. 12. Experimental results on skin damage. Leather sheets were exposed to the 30fs and 100fs lasers and the irradiation time was controlled. The exposure
longer than 2,000ms burns the leather surface.

6.1.1 Spatial AR to Real-World Object. This aerial display canbe used with real-world objects, as shown in Figures 14(e) and 14(f).
One of the merits of the spatial AR to real-world object techniqueis that the AR content is on the same scale as the object, whichis overlapped. In addition, this system was developed to include a
microscope, which can detect an object in the workspace, overlap
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

it with the contents, and modify the contents when contact between
the object and the plasma occurs.
This has an advantage over conventional AR approaches in termsof correspondence to the 3D spatial position. Digital content andinformation are directly provided in a 3D space instead of on a 2D
computer display.

Fairy Lights in Femtoseconds

•

17:11

using multiple fluorescent materials. This is acceptable because
the invisible ultraviolet source leaves only the emission visible.
On the other hand, when cavitation in water is applied, a visible
wavelength should be used, because the incoming wavelength
is diffused by microbubbles and observed. This feature leads tofull-color rendering with multiple lasers of different colors.
The softness of the medium determines the possible forms ofinteraction. With aerial plasma in air, users can insert their hands
into the workspace and touch the plasma. This is also possible
with nonfluorescent or fluorescent liquid media. However, for a
fluorescent solid medium, the voxels cannot be touched directly.

7.2

Fig. 13. Experimental results on noise level vs. brightness of light emission.
The background noise level was 55.7dB SPL.

6.1.2 Aerial Interaction with Aerial Content. Our system has
the unique characteristic that the plasma is touchable. It was foundthat contact between the plasma and a finger causes a brighter light.
This effect can be used as a cue indicating contact.
Figures 14(c) and 14(g) show examples of this interaction. One
possible control is touch interaction in which floating images change
when touched by a user. The other is damage reduction. For safety,
the plasma voxels are shut off within a single frame (17ms = 1/60s)
when users touch the voxels. This is sufficiently less than the harmful exposure time (2,000ms) determined in Section 5.4.
6.1.3 Haptic Feedback on Aerial Images. Shock waves are
generated by plasma when a user touches the plasma voxels. Thenthe user feels an impulse on the finger as if the light has physical substance. A detailed investigation of the characteristics of thisplasma-generated haptic sensation with sophisticated spatiotemporal control is beyond the scope of this article.
However, example applications such as an aerial “check box” areat least expected. Figure 14 shows the interaction between the user
and the aerial image.

7.

DISCUSSION

7.1

Laser-Induced Emission Phenomena

There are two laser-induced emission phenomena other than plasma
emission: fluorescence and diffusion by cavitation, as mentioned in
Section 3.1. Both can be applied to displays using the laser-andSLM system. In this section, the differences between the emission
phenomena are explained.
The display medium is the key factor determining the potential
interactions. Whtereas plasma is generated in air, fluorescence requires fluorescent materials (e.g., ink or pigment), and cavitation
requires a fluid medium. The medium also determines the powerthat is required for light emission. The order of the required power
decreases from air (petawatts per centimeter squared) to water tofluorescent materials (megawatts per centimeter squared).
The available wavelengths also differ in these cases. The plasmacolor is wavelength-independent; hence, it is reasonable to use invisible wavelengths (e.g., infrared or ultraviolet). For fluorescence,
multielectron fluorescence is reasonable; in this case, multiple
photons are absorbed by molecules, and a single photon with ashorter wavelength is emitted. Full-color rendering is possible byDrawbacks and Limitations

An SLM is not resistant to an intense laser because its reflectanceefficiency is not 100%. We cannot use the maximum time-averaged
laser power, and the reflected light is also decreased. However,
we are optimistic that improvement of the reflectance efficiency of
SLMs will relax this limitation in future. Then greater numbers ofsimultaneously addressed voxels can be generated.
In addition, the optical circuit should be developed and treatedcarefully. Because our system utilizes high-intensity lasers, ionization may occur along the optical circuit. This also limits the availabl
laser power so that damage to optical components is avoided. Further, plasma generation is a nonlinear phenomenon, and carefulhandling is required. These issues should be considered well toensure safety.
In addition, focusing and aberration are limitations of our systems. We have to focus the light to make focal points to generate
aerial plasma. Thus, the aperture of the objective lens determines the
maximum workspace, which limits the angle range of the galvano
mirror. In addition, high-speed variation of the varifocal lens would
cause an aberration problem. The characteristics of these lenses areimportant in development of the optical circuit.

7.3

Scalability

7.3.1 Size of Workspace. A scalable workspace size is a mainconcern of our project. Aerial plasma is limited mainly by the objective lens after the varifocal lens. Laser plasma generation requires
an instantaneous laser power of petawatts per centimeter squared,
and an objective lens is required for this purpose. An objective lenswith a larger aperture allows a larger angle range of the galvano
mirror (i.e., XY scanning).
The instantaneous laser power required to excite fluorescenceand microbubbles in water is small compared to that required forlaser plasma, and an objective lens is not required. Thus, theseworkspaces are limited by the angle range of the galvano mirror
and the depth range of the varifocal lens.
7.3.2 Voxels Per Second. The spatiotemporal rendering ability(voxels per second) is determined by the number of voxels simultaneously addressed by the SLM, the refresh rate of the SLM, thescanning speed of the galvano mirror, and the response time of thevarifocal lens. The galvano mirror is the fastest (more than 1kHz),and the other components work at less than 100Hz. It is hence reasonable to use mainly the galvano mirror. In addition, the SLM canrender up to four points simultaneously. Then the four voxels are
moved together by galvano scanning, and 4,000 dots/s is achieved.
Although the use of the SLM increases the cost and complexityof the optical circuit, the multiplication of voxels is a considerable
benefit.
We have to develop three factors to scale up our system foreveryday applications: increasing the time-averaged power of the
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17:12•

Y. Ochiai et al.

Fig. 14. Results of aerial rendering. (Uppermost) The setups of Systems A and B. (a) The SIGGRAPH logo, (b) a cylinder, (c) a “heart” that is broken bytouch, (d) a “fairy,” (e) “sprouts” coming out from seeds, (f) a light point that changes into a “jewel” in contact with a ring, and (g) direct interaction between
a light point and a finger.

laser source, shortening the pulse width to increase the instantaneous
power, and increasing the scanning speed. These factors enable us
to have a number of simultaneously addressed and scanned voxels
within a single frame, maintaining the features of visibility andtouchability.
A higher time-averaged laser power yields more simultaneously
addressed voxels. The laser power is limited by the damage tothe skin, unwanted ionization along the optical circuit, and thereflection/transmission characteristics of the optical devices.
Shortening the pulse width has two benefits. One is a higher
repetition frequency (i.e., more dots per second), which maintains
the high instantaneous power required for plasma generation. The
other is reduced harmfulness to human skin because of the lower
amount of pulse energy with a fixed instantaneous power.
ACM Transactions on Graphics, Vol. 35, No. 2, Article 17, Publication date: February 2016.

There is little room to improve the scanning speed of the galvano
mirror and varifocal lens. Employing multiple laser systems is oneof the solutions for generating multiple voxels.

7.4

Risk of Laser

Class 4 laser sources are used in this article. The proposed display
system was carefully designed and operated on the basis of International Electrotechnical Commission (IEC) 60825-1:2014. There
are two concerns regarding the risk of lasers: damage to the eyes
and to the skin.
Direct viewing of the laser beam by users should be avoided.
Because the laser plasma emits visible light in all directions at the
focal point, users can see it from the side of the laser beam. It is still

Fairy Lights in Femtoseconds
recommended that users wear glasses with infrared filters until thisdisplay technology is mature.
There are a few reports on damage to the skin by femtosecond lasers. The minimum visible lesion thresholds for porcine skinfor pulsed lasers were evaluated in Cain et al. [2007]. The ED50
for a femtosecond laser (44fs, 810nm, and 12mm spot size) was
determined to be 21mJ from the observation that the lesions produced by lasers with less than that energy value disappeared at 24hours after exposure. The energy (2mJ and 50μJ for lasers A andB, respectively) and spot size (<10 μm) are much smaller, andwe expect that damage by these femtosecond lasers is negligible.
We also investigated the exposure time in this article. The resultshows that there was a discontinuous expansion of the damaged
area when the exposure time reached 2,000ms. We can minimize
the damage by keeping the exposure time below 2,000ms by, forexample, feedback control based on the detection of brighter plasma
emission at the surface of the finger in contact with the aerial laser
plasma.

8.

CONCLUSION

In this article, we introduced a system for rendering volumetricgraphics in air using a femtosecond laser. Aerial laser-induced
plasma emits light without interaction with any special materials,
and one advantage of the femtosecond-laser display system is that
it is less harmful to the skin than a system using a nanosecond
laser.
There are two methods of rendering graphics in air with a femtosecond laser: producing holograms by SLM technology and scanning of a laser beam by a galvano mirror. The hologram size andworkspace of the current system have maximum values of 1cm2 and5cm3 , respectively. Although these demonstrated sizes are currently
too small for use in the applications shown in Figure 2, this study
is the first step in discussing and designing laser-based aerial volumetric displays. These sizes are scalable depending on the opticaldevices and setup.
This paper reports the details of the theoretical principles, system setup, and experimental evaluations, and also discusses thesystem’s scalability, limitations, and applications. Although we
focus on laser-induced plasma, the same considerations can beapplied to other emission techniques such as fluorescence andcavitation.
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Received June 2015; revised November 2015; accepted November 2015