工学部SSSVプログラム
日中共同セミナー(２０１２年３月５日―３月６日)

Research on self-mixing interference in fiber lasers and its
applications
Ming Wang
School of Physical Science and Technology, Nanjing Normal University, No. 1
Wenyuan Road, Nanjing, Jiangsu 210046, China

ABSTRACTIn this paper, theory of self-mixing interference (SMI) in fiber ring lasers (FRLs) is analyzed. The
theoretical model of vibration measurement based on SMI is designed. The relationship between
vibration parameters and SMI signal is deduced. Experiment system is demonstrated and experimental
results are compared with theoretical results. Theoretical model of self-mixing speckle effect (SMSE)
in FRLs is analyzed. According to this theory, a model for velocity measurement is designed. The
theory shows that the faster the velocity is, the more similar to Gaussian form the probability density
distribution is. A method based on the fractal boxes of a section of speckle waveform is adopted for
demodulation. The relationship between the numbers of boxes and velocities of the object is linear.
Dual-channel SMI is presented demonstrating the potential for multiplexing in FRLs. The dual
wavelengths of the FRL are selected by two fiber Bragg gratings (FBGs). The output power ofdual-channel with optical feedback is theoretically deduced and simulated. The output variation andmodes competition between the two channels in a fiber laser is also analyzed. The experimental results
show a good agreement with the simulation, and indicate that SMI with parallel dual-channel is anefficient approach for multiple displacement measurement.

Self-Mixing Interference with sinusoidal phase modulation technique and its application for
displacement sensing
Wei Xia and Ming Wang
Department of Physics Science and Technology, Nanjing Normal University
Wenyuan Road 1, Nanjing, 210046, China

1. Introduction
Self-mixing interference(SMI) phenomenon has been widely
researched for the past decades, notably for displacement sensing[1]. SMI in a He-Ne laser has already been theoretically andexperimentally demonstrated[2], and was used to measure displacement with an accuracy of λ/2 by counting the number ofinterference signal peaks. In order to improve the resolution of
displacement measurement, a novel self-mixing interferometry
based on sinusoidal phase modulating technique is proposed inthis report. An electro-optic modulator(EOM) is situated in theexternal cavity of the laser to yield sinusoidal phase modulationof the interference signal. Moreover, the self-mixing interference
signal is processed in frequency-domain to retrieve the vibration
waveform of an object with an accuracy of tens of nanometer.

by Agilent 5529A. And the measurement result obtained by SMI
is in good accordance with reference vibration. Simultaneous
error between the two vibration waveforms is shown in Fig. 3(b)and the maximum displacement error is better than 20nm.

2. Experimental setup and results
The experimental setup is shown in Fig. 1, the light source is alinear polarized He-Ne laser. The beam emitted from the front
mirror of the laser is sent through a variable attenuator, an
EOM(New Focus, 4002), and then reflected back to the laser
cavity by the external object(Corner Cube 2). The variable attenuator helps to assure that the self-mixing interferometry is running in the weak feedback regime. Therefore, the SMI signal isvery close to a standard sinusoidal form. The EOM can provide
pure phase modulation with extremely low amplitude modulation,
when the polarization direction of the laser is parallel to the principal axis of the electro-optic crystal. The external object is fixed
on a high-precision commercial PZT(PI P762.2l) which can obtain displacement accuracy of 1nm. The interference signal ismonitored by a photodiode(PD) situated behind the back mirrorof the laser. The electric output of PD is fed through low-passfilter, amplifier and data acquisition card. Hence, the digital signal is processed by a computer for reconstruction the vibration
waveform of the object.
A calibration system is included in the experimental setup toevaluate the performance of the self-mixing interferometry. Orthogonal polarized dual-frequency laser emitted from the calibrator head(Agilent 5529A) is projected on the polarization beam
splitter(PBS). One part of the laser with polarization direction
perpendicular to the paper is reflected by PBS and corner-cube 1
and then collected by the detector of the calibrator head. The
other part of the laser with polarization direction parallel to thepaper is sent through the PBS, reflected by corner-cube 2 andthen collected by the detector of the calibrator head as well.
Combining the collected light, the calibrator provides a reference
displacement for evaluating the performance of the proposedinterferometry.
Firstly, PZT is controlled to vibrate at a sinusoidal form with
fixed frequency of 10Hz. A set of reconstructed vibration waveforms at different amplitudes from 200 nm to 1000 nm are shownin the Fig. 2. In displacement measurement, only the relativevalue of two positions is important. So, only peak-to-peak amplitude is examined and the maximum error between the measured
peak-to-peak amplitude of the vibration and reference peak-topeak amplitude is less than 20nm.
Secondly, a instant start-up of a 2D-PZT movement has beentested. In Fig. 3(a), the dotted curve is the reconstructed vibration
waveform by SMI and the solid curve is the reference vibration

Fig.1 Experimental setup of self-mixing interferometry for displacement measurement.

Fig.2 Reconstructed vibration waveforms of commercial PZT at different amplitudes.

Fig.3 A measurement result of PZT from an instant
start-up to steady state vibration. (a) Result comparison between SMI and calibrator; (b) Error.

3. Conclusion
An entire evaluation of the performance of self-mixing interferometry with sinusoidal phase modulation is firstly demonstrated.
Experimental vibration measurement results of a commercial
PZT are compared with simultaneous measurement results provided by Agilent 5529A and an accuracy of 20nm is achieved. It
is shown that the phase-modulated laser self-mixing interferometer is suitable for real-time continuous vibration measurement on
an accuracy of tens of nanometer.

References[1]U. Zabit, et al., Opt. Lett., 35, 8 (2010).
[2]S. Merlo, et al., IEEE J. Quantum Electron., 33, 4 (1997).

Sol-Gel co-assembly of hollow cylindrical inverse opals
Ni Haibin, Wang Ming and Chen Wei
Jiangsu Key Laboratory on Opto-Electronic Technology,
Department of Physics and Technology, Nanjing Normal University, Nanjing 210046, China

1. Introduction
Photonic band gap fibers (PBGFs) with hollow core guiding
structures are a kind of microstructure optical fibers (MOFs) to
overcome the limitations of traditional silica optical fibers [1].
Two dimensional photonic crystal fibers have been commerciallyavailable and widely used as sensing detectors, high power optical waveguides and nonlinear optical devices. Three dimensional(3D) PBGFs were proposed subsequently as its unique structure
enables various applications in optical sensing and signal processing. It is challenging to fabricate silica air core cylindricalinverse opal fibers or waveguides both for template fabricating
and infiltration materials to realize inverse opal structure. Recently Hatton et al. developed a sol-gel co-assembly method to
overcome these difficulties [2]. We applied this method to producing inverse opals on capillaries’ inner wall.

ter to capillary diameter is very large. The average pore size ofthe fabricated inverse opals are 615nm, 500nm and 430nm,
which made from PS spheres in 490nm, 580nm and 690nm diameters, with a contraction of 12.2%, 13.8% and10.8%, respectively.

2. Experimental setup and results
Glass capillaries in 1.8mm, 850μm, 670μm diameters and in
10cm length were selected as an ideal cylindrical substrate.
Aqueous plain PS microspheres solution with solids content ~10
wt. % was bought from Bangs Laboratories, Inc, and was only
diluted to a certain concentration without any other treatment
before use. The surface of the PS microspheres was covered withnegatively-charged sulfate groups. Monodispersity of the PS
microspheres is less than 10%. Capillaries were cleaned in an
ultrasonic bath first using acetone and then deionized (DI) waterfor 10 minutes respectively. Then cleaned capillaries were dried
with nitrogen before use. To fabricate HCIOs on the internalsurface of capillaries, PS colloidal/TEOS solution dispersed in
DI water with concentration of ~1vol % (PS spheres) and~2vol % (TEOS solution [2]) was injected into a capillary. The
height of the solution inside the capillary is 10-95mm and can be
precisely controlled by a micro-pump. Length of the HCIOs wasaffected by the height of the inside solution. One end of the capillary was sealed to control the evaporation direction of solvent.
Silica inverse opals fabricated on a capillary’s internal wall arewith good quality, as seen in Fig.1. Fig.1 (a), (b), (c) exhibit
optical microscope images observed in reflection mode: (a) abare capillary, (b) a capillary with PS-silica gel CCC coated on
its internal wall and (c) the capillary with silica inverse opals
preserved on the internal wall after sintering. The axial uniformity together with the symmetric color distribution on the capillaryinternal edges attests to the high quality of the silica HCIO,
which spectrally shifts the stop band with the viewing angle
around the capillary curved surface. The color on the internaledge of the capillary has a blue shift after removing the PS colloidal crystal template as the effective refraction index decreased,
as seen in Fig.1 (b) and (c). Fig.1 (d) ~ (g) are SEM images ofthe longitudinal-section of the capillary in (c). Image (d) is a top
view at low magnification and (e) is longitudinal cross section of
the silica inverse opals on the edge of the capillary internal wallwith 7 layers. The robust inverse opals on the capillary internal
wall are with good connectivity with the capillary substrate.
Image (f) and (g) are curved surface of the HCIO with increased
magnification. High uniform and ordered structure and small
pores connected the big pores which reveal good connectivity ofthe structure can be observed. Fig.1 (h) and (i) show the top viewof the cross section of a capillary with silica inverse opals coatedon its internal wall. The internal diameter of capillaries could
range from ~100μm to several millimeters as the ratio PS diame-Fig.1. (a) ~ (c) are optical microscope images of capillaries in 350μm internal diameter. (a) bare capillary, (b) internal wall coated with composite colloidal crystal, (c) internal wall preserved with silica inverse opals after sintering. (d) ~ (i) are SEM imagesof inverse opals on the internal wall of a capillary,(d) longitudinal section of the capillary whose internal wall was coated with silica inverse opals, (e)longitudinal cross section of inverse opals, (f)surfaceof the inverse opals on the internal curved wall, (g)high magnification of the surface in (f), (h) and (i)are cross section of a capillary with silica inverse
opals on its internal wall.

3. Conclusion
An approach of fabricating hollow cylindrical inverse opals by
sol-gel co-assembly method was proposed. PS colloidal suspension added with hydrolyzed silicate precursor solution was used
to vertical self-assemble composite colloidal crystals which consist of PS colloidal crystal template and infiltrated silica gel.
Continuous silica hollow cylindrical inverse opal films have been
produced on capillaries’ inner surface.

References[1]J. Li, et al., Opt. Express, 13 17 (2005)[2]B. Hatton, et al., P. Natl. Acad. Sci. USA, 107 10354 (2010).

The Simulation of a Novel SU-8 Fiber Fabry-Perot Pressure Sensor
Lihua Dai and Ming Wang
Jiangsu Key Lab of Opto-electronic Technology , School of Physical Science and Technology,
Nanjing Normal University, Nanjing 210046, China

1. Introduction
With the fast evolving MicroElectroMechanical Systems(MEMS) technology, researchers are in more and more urgent
need of improvement in the structure, fabrication or packaging ofthese Microsystems. The introduction of optics provides an opportunity to extend the use of these sensors to harsh environments. SU-8 photoresist is a kind of near UV photosensitive
epoxy-based polymer. The molecular composition of this epoxy
negative photoresist enables to produce uniform coatings. Thishigh contrast photoresist has been used extensively in micromachining and other microelectronic applications, where a thick,chemically, and thermally stable image is desired, with an aspectratio larger than 1:10.

2. Simulation and optimizations
The major difference between the proposed micro sensor and a
traditional one is that the F-P sensor is designed for sensing lateral displacement instead of vertical one. The direction of F-P
cavity change and the direction of exerting pressure are always
the same in most reported sensors. Because of the F-P cavity
length is microscale, the same direction could limit the range ofthe sensor’s linear measurement. If it is overloaded, the membrane’s deformation would get large enough to reach the othersurface of F-P cavity, the sensor will out of order or, even worse,
cause irreparable damage to itself. However, to make sure of thesingle-valued relation between deformation and the phase ofinterferential beam within the bandwidth of the light source, thecavity length should be small enough to keep the free spectrum
range inside a little bit bigger than the working range. In thisdesign, the directions of sensing membrane deformation frompressure and the change of cavity length are separated into two
relative parts to meet requirements above perfectly.

Fig.1. Schematic description of the micro Fabry-Perot sensor.
On account of the structure involving a connected movement,
the deformation under load cannot be easily figured out throughthe existing formula. The behaviors of the sensor proposed can
only be calculated by numerical simulation. The whole simulation is carried out by using commercial software CoventorWare.
Considering the stress concentration in all the corners, the circular membrane is selected as the fundamental shape of silicondiaphragm.
Fig.3 is demonstrated that there is a certain position where thesquare column be placed to acquire a maximum value of lateraldisplacement under a constant pressure. A conclusion can be
drawn from Fig.4 that the dimension of the square column
doesn’t impact the lateral displacement a lot. The shift of peakvalue is only because of the size of contact area, which affects
the deformation of the diaphragm and the angle of tilt of thecolumn.

Fig.2. The distribution of the deformation.

Fig.3. Relationship between the position of thesquare column and the attainable maximal lateraldisplacement.

Fig.4. The Comparison of different design dimensions.

3. Conclusion
The novel design broadedens the range of linear measurement ofpressure sensors. The sensitivity is 2.606μm/Mpa by simulation.
The characteristic result here verifies the feasibility of the design.
Through numerical simulations, we further confirm and optimize
the parameters of the structure. The accurate micro-pressure
detection is widely needed in industrial field. The excellent characters make the perspective more wide.

References[1]Y.Ge, et al., Sensors and Actuators A: Physical, 143, 224 (2007).
[2] I. Pardon, et al., Sensors Journal, 11, 343 (2011).
[3] C. J. Lin, et al., Sensors and Actuators A: Physical, 113, 12 (2004).

High channel-count ultra-narrow comb-filter based on asampled fiber Bragg grating
Hongpu Li and Xuxing Chen
Department of Electrical and Electronic Engineering, Shizuoka University,
3-5-1, Johoku, Hamamatsu, 432-8561, Japan
dhli@ipc.shizuoka.ac.jp

Abstractaobtained by utilizing two phase-only sampled 51-channelpiezoelectric transducer (PZT)-induced phase-shift in a

gratings which have identical characteristics. The gratings arephase-only sampled FBG is proposed and experimentally

particularly designed with channel spacing of 0.8 nm exactly

demonstrated. Moreover, the high channel-count scheme based

fitted with the ITU channel grids. The grating strength is about

on a triply sampled FBG is firstly proposed.

10 dB with a length of about 12 cm and the flat-top bandwidthAnultra-narrowcomb-filter

formedbyfor each channel is about 0.6 nm. One of the gratings is1.

working in the transmission with π phase-shift introduced byIntroduction

The high channel-count comb filters with ultra-narrow

a PZT which is wholly glued to the grating. The other (without

bandwidth have attracted lots of interesting in the fields such as

the glued PZT) is placed in a temperature chamber and working

the wavelength selection, multiwavelength fiber laser and RF

in the reflection. Fig. 1(b) shows the measured 5-channel

single generation[1-2]. The multi-channel ultra-narrow filtertransmission spectrum for these two cascaded FBGs (thecan easily be realized by introducing a phase shift into a

voltage applied on the PZT is 150 V) where the inset shows the

sampled fiber Bragg grating (FBG). To date, several kinds ofwhole spectrum coving 51-channel. It can be seen thatMulti-channel FBG have been proposed and demonstrated,

multichannel band-pass filter has been obtained and each of thesuch as the sinc-sampled FBG, the superimposed FBG, thechannel has almost the same characteristics with a bandwidth

Talbot-effect based FBG and the amplitude-phase sampled FBG.

of about 0.027 nm.

In particular, the phase-only sampled FBG, a special case of theamplitude-phase sampled FBG, has the advantages of highchannel counts, minimum index-modulation, high uniformityof inter-channel, and compatibility with the standard ofinternational telecommunications union (ITU) grids. In thisstudy, a PZT-induced phase shift in a linearly chirped
51-channel FBG is firstly demonstrated and a comb filter with
51-channel is thus successfully obtained. Moreover, based on asampledFBG

withsimultaneous

utilization

of

two

amplitude-assisted phase sampling (AAPS) functions and aphase-only sampling (POS) function, a comb filter withconsecutive 1377 channels enabling to cover whole band of(O+E+S+C+L+U) is numerically demonstrated, which has achannel-bandwidth of 1.5 GHz and channel spacing of 25 GHz.

2. Realization of comb filter based on a phase-shift
51-chanel phase-only sampled FBG
Principle of the proposed comb-filter is schematically shown in
Fig. 1(a). The comb filter with ultra-narrow bandwidth can be

Fig. 1. (a) Setup for the realization of the proposed
comb-filter and (b) transmission spectrum for the two
cascaded multi-channel FBGs.

3. High channel-count Comb-filter based on a
triply sampled FBG

P3=P2/(2N2+1) are satisfied.

Most recently, we have reported a doubly sampling approach enabling
to create an ultrahigh-channel-count FBG with both high channel
uniformity and high in-band energy efficiency [2]. In this study, a triple
sampling method is firstly proposed for the design of a FBG-based
comb-filter. Unlike the previous doubly-sampling scheme, here thesampling function adopted is a typical one consisting of three sampling
functions which can be expressed as

S( z ) = S a 1 (z ) ⋅ S a 2 (z ) ⋅ S p 3 (z ) ,(1)
Fig. 2. Fourier analysis of the proposed triple sampling method
for the FBG based comb filter.

where Sa1(z) and Sa2(z) both are the pre-optimized
amplitude-assisted phase-only sampling (AAPS) functions.
Each of them has small in-band channels of (2N1+1) and(2N2+1), respectively. Meanwhile Sp3(z) is a pre-optimized
phase-only sampling (POS) function with a relative larger
number of in-band channels (2N3+1). Figure 2 shows theprinciple of the triply sampled FBG based on the Fourier
analysis. The detailed processes can be summarized as follows.
Step 1: As a seed grating, a single channel FBG withultra-narrow bandwidth is firstly designed by usingthetransfer matrix method (TMM); Step 2: By utilizing thesimulated annealing method and the Gerchberg-Saxton
algorithm, optimizations for the three sampling functions are

To verify the above proposals, design for a FBG with 1377
channels is implemented. Firstly, two AAPS functions with 3-,9-channel and a POS function with 51-channel are optimallydesigned. Secondly, a seed grating is designed by using the TMM
method. The seed grating is designed to have a narrow bandwidth of
1.5 GHz. By multiplying the seed grating with the 3-, 9- and51-channel sampling functions simultaneously in spatial
domain, an ultrahigh channel-count FBG with 51 sets of 3x9
channels can be obtained. Figure 3 shows the designed results forthe spectrum of the comb filter, where high uniform 1377-channel witha narrow bandwidth of 1.5 GHz is successfully obtained.

accomplished. A (2N1+1) channel FBG can be generated in thefrequency domain by multiplying the function Sa1 with the seedgrating in spatial domain, which could be seen in Fig. 2(b).
Step 3: In the spatial domain, the second amplitude-assisted
phase sampling function Sa2 is then multiplied by the originated(2N1+1) channel FBG, then in the frequency domain, an FBG
with (2N1+1)( 2N2+1) channels could be obtained as is shownin Fig. 2(c). Note that, to avoid overlapping between thegenerated channels, the condition P2 ≤ P1 / (2 N 1 + 1) should

Fig. 3. Design results for a 1377-channel comb filter withspacing of 25 GHz. The inset show the whole spectra one.

be satisfied. Step 4: The phase-only sampling function Sp3(z)with a higher channel-count of 2N3+1 is then multiplied by the4.

two amplitude-assisted phase-only sampled FBG as is shown

In this study, a comb filter with 51-channel is experimentally

Conclusions

Fig. 2(d). To avoid overlapping between the generated channels,

demonstrated. Moreover, based on the triple sampling method,

the condition P3 ≤ P2 / (2 N 2 + 1) should be satisfied. Figurea FBG based filter having a bandwidth@3-dB of 1.5 GHz,2(e) shows the reflection spectrum of the realized FBG using

channel spacing of 25 GHz, and a consecutive 1377 channel isthe triple sampling method, it can be seen that one can easily

numerically demonstrated.

obtain consecutive (2N1+1)(2N2+1)(2N3+1) channels with

5.

channel spacing ofΔν 1 = c / 2 n eff P1 in the frequency

domain as long as the conditions P2=P1/(2N1+1) and

References[1] M. Ibsen et al, IEEE Photon. Tech. Lett. 10, 842 (1998).
[2] H. Li et al, Opt. Lett. 34, 938 (2009).

Fabrication of a phase-shifted long-period fiber grating by using CO2 laser
CO2 レーザによる 位相シフト長周期ファイバグレーティングの作製
Keisuke Hishiki Taki Yosuke
日紫喜 圭介
瀧 洋丞

Goto Yuji
後藤 祐志

Lunlun Xian線 倫々

Hongpu Li李 洪譜

Faculty of Engineering, Shizuoka University
静岡大学 工学部

In the past decades, long-period fiber gratings (LFG) havebeen widely studied and found many applications in the fields of
optical communications and optical sensing system, such as the
band-rejection filter, gain-spectrum flattening filter, temperatureand refractometric sensors, etc. Recently, the phase-shifted LFG
has attracted a progressive interest due to its potential applications being able used as an optical differentiator and a subpicosecond pulse shaper [1-2]. In this study, a simple approach towrite LFG and create a typical phase-shift in the LFG have been
proposed and firstly demonstrated, which is based on the thermal
shock effect realized by using a high frequency CO2 laser pulses.

forward core-mode and one of cladding-modes (LP15 mode in
our case). It is also seen that the resonant attention is larger than
25dB, giving that the coupling efficient κ combined with thegrating length L is given to be κL ≈ 0.482π . Furthermore, wemeasure the transversal profile of the written LFG by using an
atomic force microscopy (AFM), no any damages and defects
have been found, which in return means that the large indexchange in the core is caused by the change of the densificationand the residual stress of the fiber core through the thermal shock
effect [2].
0Transmission (dB)

1. Introduction

2. Fabrication setup

-10
-20
-30
-40
1520

1540

1560

1580

1600

1620

Wavelength (nm)Fig. 2. Transmission spectrum of LPG without phase shift.

Secondly, we also wrote several phase-shifted LFGs by using
the same setup as shown in Fig. 1, where the inserted phase-shift
are realized by purposely change the local period of the moving
stage. In our case, a π phase shift is assumed to be inserted at
middle of the grating. In order to realize it, we simply change themoving period of motored-stage from 650 μm to 325 μm just forthe period at middle of the grating (i.e., the 23th period in ourcase). Figure 3 shows the measurement result for a typical phaseshifted grating, from which it can be seen that the LP15 peakshown in Fig. 2 is split into two, which agrees well with the theoretical one what the Ref [3] expects.
0

Transmission (dB)Figure 1 shows the experimental setup, which consists of a
CO2 laser controlling system, a beam moving and a focusing
system, a fiber alignment stage, and a in-line system to measure
the transmission spectrum of the fabricated LFG, where the utilized fiber is single-mode one provided by Fujikura Inc(FutureGuide®-SR15E), the CO2 laser (SYNRAD-20, emission
at the central wavelength of 10.2 μm) works at a typical frequency of 5 kHz with a duty-cycle changeable form %1 to 99%.
For the grating fabrication, the aligned CO2 laser beam is focused through a ZnSe lens with a focus length of 70 cm. The
minimum spot size (CO2 laser) on the fiber is 〜100 μm, whichmakes the focusing laser mainly absorbed by the fiber in a very
short time, and thus makes a permanent index-change within
the exposure region of the fiber core based on the thermal
shock effect. Period of the fabricated LFG is precisely controlled by periodically moving the motored stage (including thereflection mirror) and turning on/off the laser shutter throughthe LabView controlling software. For the measurement, anamplified spontaneous emission (ASE) source and an optical
spectral analyzer (OSA) are utilized to measure the transmission spectrum of the LFG in the real time.

-10

-20

-30
1520 1540 1560 1580 1600 1620

Wavelength (nm)Fig. 3. Transmission spectrum of LPG with a phase shift.

4. Conclusions
Fig. 1. Fabrication setup for the long-period fiber grating.

3. Experimental results and discussions
At first, we fabricated a couple of long-period gratings, while
the average power of the CO2 laser is kept in 5 W. The gratings
are continuously written with number of 46 period and each period adopted is 〜650 μm. Figure 2 shows the transmission spectrum of a typical grating, from which it can be seen that the typical characteristic of LFG is really realized. There exist a resonant
attention peak located at a wavelength between 1540 and 1560
nm, which can be attributed to the strong coupling between the

A simple approach based on the utilization of a highfrequency CO2 laser has been proposed and successfully demonstrated for the fabrication of both a LFG and a phase-shifted LFG.
This kind of devices may provide potential applications to fiber
optical sensor and all-optical signal processing.
References[1] Y. J. Rao, et al., Opt. Communications, 229, 209 (2004).
[2] R. Slavik, et al., Opt. Express, 14 , 10699 (2006).
[3] H. Ke et al., IEEE Photon. Techn. Lett. 19, 1596 (1998).

Enhanced Slow Light in a Phase-Shifted Multichannel Fiber Bragg Grating Assisted by Stimulated Brillouin Scattering
1Lunlun Xian1 and Hongpu Li2
Graduate School of Science and Technology, Department of Electrical and Electronic Engineering, Faculty of Engineering,
Shizuoka University, Johoku 3-5-1, Hamamatsu, 432-8561, Japan2

1. Introduction
Slow light has found many applications such as optical buffering, high-resolution interferometers etc. Slow light in single
channel fiber Bragg grating (FBG) has already been theoreticallyand experimentally demonstrated [1]. However, to date there isno demonstration of slow light in a multichannel FBG. On theother hands, stimulated Brillouin scattering (SBS) is anotherimportant method to realize a tunable slow light in optical fibers[2]. In this report, a multichannel slow-light is firstly demonstrated in a piezoelectric transducer (PZT)-induced phase-shifted
multichannel FBG. The group delay is controlled by applying
different voltages on the PZT. Moreover, the group delay in eachchannel can be further enhanced by incorporating SBS.

V. By carefully tuning the wavelength of LD2, the amplification
due to the SBS can be clearly verified by the RFSA. Figure 3
shows the recorded output waveforms. Compared with above
results, it is obviously seen that the group delay is further enhanced from 0.06 to 1.473 ns at Brillouin pump power of 50 mW.

2. Experimental setup and results
The experimental setup is shown in Fig. 1, where the input
signal is generated by externally modulating a tunable distributed-feedback (DFB) laser (LD1: with a linewidth of 20 MHz)through a LiNbO3 amplitude modulator (LM), which is driven bya sinusoid signal with a frequency of 450 MHz. The resulted
pulses are amplified by an Erbium doped fiber amplifier (EDFA
1) and transmit through a phase-shifted multichannel FBG. The
FBG is a phase-only sampled 51-channel FBG [3] possessing achannel spacing of 0.8 nm, a grating strength of about 10 dB, alength of about 12 cm and a flat-top bandwidth for each channelof about 0.6 nm. The phase shift is induced by a PZT attached to
the grating at the central position [3]. To monitor the notch
wavelength of the phase-shifted FBG which determines the
wavelength selection of LD1, an amplified spontaneous emission(ASE) source incorporated with a circulator CL2, a 3-dB coupler(CPLR), and an optical spectrum analyzer (OSA) are utilized.
Moreover, an optical isolator (OI1) is used to isolate the lightreflected by the grating and the isolator OI2 is used to isolate theBrillouin pump light. Note that the output signal from the phaseshifted FBG is used as Brillouin probe signal. To realize SBS
amplification, another tunable DFB laser (LD2: with a linewidthof 100 kHz) is used as the SBS pumping source amplified by anamplifier (EDFA 2). In addition, a 500-m high-nonlinear fiber(HNLF) is used as Brillouin gain medium. The signal after thecirculator CL1 is detected by a fast photodiode (PD) which isconnected to a sampling oscilloscope (OSC) or a RF spectrum
analyzer (RFSA).
First, only the phase-shift multichannel FBG is used for slow
light while the SBS is not employed. The wavelength of LD1 istuned to 1550.4 nm as reference (Ref) as shown in Fig. 2. The
signal transmitted through the FBG is recorded by OSC. Thenthe wavelength of LD1 is tuned to notch wavelength of 1549.932
nm in channel #1. Two voltages of 40 V and 90 V are in turnapplied on PZT to control the phase shift. The transmitted signals
for the two cases are recorded by OSC. Comparing with Ref, thedelay times are 0.041 ns and 0.06 ns for 40 V and 90 V, respectively. To verify multichannel operation, the wavelength of LD1
is then tuned to 1550.739 nm in #2. The measurement of thetransmitted signal is repeated. Comparing with Ref, the delay
times with #2 are 0.031 ns and 0.076 ns for 40 V and 90 V, respectively. Next, SBS is employed to further increase the groupdelay within any individual channel. For convenient, channel #1
is only selected for the test. The voltage applied on the PZT is 90

Fig.1. Experimental setup for the slow-light measurement.

Fig.2. Measured reflection spectrum with different
voltages on the PZT. Inset shows notches in #1.

Fig.3. Measurement results for the output waveforms
assisted with SBS.

3. Conclusion
Slow light utilizing a PZT-induced phase-shift multichannel FBG
is firstly demonstrated in experiment. The group-delay can betuned by adjusting the voltage on the PZT. The delayed time ineach channel can be further enhanced by incorporating SBS.

References[1]S. Longhi, et al., Electron. Lett., 41, 1075 (2005).
[2]K. Song, et al., Opt. Express, 13, 82 (2004).
[3]X. Chen, et al., Photon. Technol. Lett.. 23, 498 (2011).