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Characterization of Stimulated Brillouin Scatteringin Polymer Optical Fibers Based on Lock-in-Free
Pump–Probe Technique
Neisei Hayashi, Yosuke Mizuno, and Kentaro Nakamura, Member, IEEE

Abstract—We observe stimulated Brillouin scattering (SBS) in
polymer optical fibers (POFs) using a pump–probe technique without lock-in detection, and fully investigate the dependences of theBrillouin gain spectrum on the POF length, pump power, probe
power, and temperature. Since the POF has relatively high propagation loss of 250 dB/km at 1.55 μm, an optimal POF length
exists for SBS observation, which is found to be approximately
3.8 m with 21.1-dBm pump and 22.2-dBm probe waves. As theprobe and pump powers are increased, the Stokes power is also
raised but nonlinearly. The temperature dependence of the Brillouin frequency shift is –4.02 MHz/K, which agrees well with theprevious report. These results indicate that the Brillouin signal in
POFs observed with this technique can be directly applied to thedevelopment of POF-based Brillouin optical time-domain analysis
systems for high-precision temperature sensing.
Index Terms—Brillouin optical time-domain analysis (BOTDA),
distributed measurement, optical fiber sensing, polymer opticalfibers, stimulated Brillouin scattering.

I. INTRODUCTION
OR the past several decades, increasing attention has been
directed toward Brillouin scattering in optical fibers [1],which can be applied to a variety of devices and systems, suchas lasers [1], microwave signal processors [2], optical memories [3], phase conjugators [4], slow light generators [5], anddistributed strain/temperature sensors [6]–[10]. Brillouin properties in many specialty fibers have been investigated to improvetheir performances, including tellurite glass fibers, chalcogenide
glass fibers, bismuth-oxide glass fibers, photonic crystal fibers,
and rare-earth-doped glass fibers [11]. All of these glass optical fibers (GOFs) are, however, fragile and require carefulhandling; in addition, in sensing applications, they cannot be
used to measure large strain of over several tens of percentage.

F

Manuscript received February 21, 2013; revised July 26, 2013; accepted
August 19, 2013. Date of publication August 20, 2013; date of current version
September 13, 2013. This work was supported in part by a Grant-in-Aid forYoung Scientists (A) (no. 25709032) and a Grant-in-Aid for the Japan Society
for the Promotion of Science (JSPS) Fellows (no. 25007652) from the Ministryof Education, Culture, Sport, Science and Technology (MEXT), Japan, and byresearch grants from the Mazda Foundation, the JFE 21st Century Foundation,and the General Sekiyu Foundation.
The authors are with the Precision and Intelligence Laboratory, Tokyo
Institute of Technology, Yokohama 226-8503, Japan (e-mail: hayashi@sonic.
pi.titech.ac.jp; ymizuno@sonic.pi.titech.ac.jp; knakamur@sonic.pi.titech.
ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2013.2279252

Recently, we have put forward a new approach for resolving
these problems by exploiting Brillouin scattering in polymer
optical fibers (POFs) with extremely high flexibility, which
can withstand >50% strain. Especially for sensing applications,some spontaneous Brillouin properties of perfluorinated gradedindex (PFGI-) POFs have been investigated at 1.55 μm, and their
Brillouin gain coefficient [12], Brillouin threshold power [12],Brillouin frequency shift (BFS) [12], and BFS dependences on
strain and temperature [13], [14] have been clarified. These
results show that Brillouin scattering in POFs is potentially applicable both to high-precision temperature sensing [13] and tolarge-strain sensing [14].
Candidate techniques for adding distributed measurement
capability to strain/temperature sensors based on Brillouin scattering in POFs include Brillouin optical time-domain reflectometry (BOTDR) [6], Brillouin optical correlation-domainreflectometry (BOCDR) [7], Brillouin optical time-domain
analysis (BOTDA) [8], Brillouin optical frequency-domain
analysis (BOFDA) [9], and Brillouin optical correlation-domain
analysis (BOCDA) [10]. Considering the relatively small Brillouin signal in POFs because of their large core diameter andmultimode nature, stimulated Brillouin scattering (SBS)-based
analysis systems are preferable to spontaneous Brillouin scattering (SpBS)-based reflectometers. Although each analysis system has its own advantages and disadvantages, here we focus
on BOTDA, which is most widely used in the world. SBS in
POFs was already observed with so-called pump–probe technique [15], but, since the experimental setup included a lock-in
detector, it was not directly applicable to standard BOTDA systems. As is well known, lock-in detection enables us to measurevery small ac signals based on a technique known as phasesensitive detection to single out the component of the signal ata specific reference frequency and phase. The reason for thedifficulty in observing SBS in POFs without lock-in detection
has been clarified to originate not from their structural problems (large core diameter, etc.) but from their physical problems(high propagation loss, etc.) [16]. Here, we should note that researchers have tried to detect SBS signals using a 100-m-long
PFGI-POF, but to no effect [16].
In this paper, we observe SBS in POFs, for the first time,using the pump–probe technique without lock-in detection, andfully investigate the dependences of the Brillouin gain spectrum(BGS) on POF length, pump power, probe power, and temperature. The optimal POF length for SBS observation is found tobe ∼3.8 m when the probe and pump powers are 21.1 dBm
and 22.2 dBm, respectively. The Stokes power is raised

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nonlinearly with increasing probe and pump powers. The temperature dependence of the BGS is also measured. These results will be significant information in developing POF-based
BOTDA systems.
II. PRINCIPLE
When a pump wave is injected into an optical fiber, backscattered Stokes light is generated due to SpBS. The Stokes wave
spectrum is called the BGS [1]. The center frequency of the
Stokes wave is known to be lower than that of the pump wave.
The amount of this frequency shift is called the BFS, which is,
at 1.55 μm, typically ∼10.8 GHz for silica single-mode fibers(SMFs) [1] and ∼2.8 GHz for PFGI-POFs [12]. In general, theStokes power is raised with increasing pump power. When thepump power becomes higher than a certain power called Brillouin threshold power Pth , the Stokes power begins to drasticallyincrease due to the transition from SpBS to SBS. For sensing
applications, the Brillouin Stokes power should be as high as
possible, because it leads to high spatial resolution, long measurement range, short measurement time, etc. However, since
Pth of POFs is extremely high (∼53.8 dBm for a 10-m-long
PFGI-POF [17]), observing large SBS signal is not easy.
One effective method to generate SBS irrespective of thehigh Pth is a so-called pump–probe technique. The main idea
is to provide the SBS seed artificially by using probe wave at
the same frequency as the Stokes wave, which is additionallyinjected into the other end of the fiber. Thus, SBS can be induced
at much lower pump power. This scheme has been exploited indeveloping silica SMF-based BOTDA [6], [18] and BOCDA
systems [9], [19]. Although many researchers have attempted toinduce SBS in POFs, hardly any experimental results have beenreported thus far. In the only successful report, a complicatedsetup based on lock-in detection was utilized [15], which cannot
be directly applied to standard BOTDA systems. The difficultyin observing SBS in POFs with this technique has been shown toconsist in the physical problems including their high propagation
loss [16]. One solution is to optimize the POF length and thepump/probe powers; when the length is too long, the pumpand the probe waves are highly attenuated and do not interact
efficiently with each other.
III. EXPERIMENTAL SETUP
We employed nine kinds of PFGI-POFs with the lengths ranging from 0.3 to 50 m as fiber under tests (FUTs), all of which
had numerical aperture (NA) of 0.185, core diameter of 50 μm,
cladding diameter of 750 μm, core refractive index of ∼1.35,
and propagation loss of ∼250 dB/km at 1.55 μm. Both endsof the FUTs were connected to silica SMFs with a multimodefiber (MMF)-assisted coupling technique [20], which can mitigate the damage or burning at the POF/SMF interfaces. Two
1-m-long silica MMFs with BFS of ∼10.4 GHz [16] at room
temperature were inserted, which have practically no influenceon the observation of the BGS in the POFs.
The experimental setup for observing SBS with the pump–probe technique is schematically shown in Fig. 1. Two laser
diodes (LDs) at 1.55 μm were used as light sources; one (Santec,

Fig. 1. Experimental setup for observing SBS in POF with pump–probe technique. EDFA, erbium-doped fiber amplifier; ESA, electrical spectrum analyzer;FG, function generator; FUT, fiber under test; LD, laser diode; OSA, optical spectrum analyzer; OSC, oscilloscope; PC, polarization controller; PD,
photodetector.

TSL-210) was for providing a pump wave, and the other (NEC,NX8562LB) was for a probe wave. The center frequency of thepump wave was swept from ν0 – 1.3 GHz to ν0 + 1.3 GHz bymodulating the LD driving current with a sawtooth waveform
at 80 Hz (ν0 is the center frequency with no modulation appied)
provided by a function generator (NF Electronic Instruments,
1930A). The center frequency of the probe wave was fixedat ν0 – BFS. Small fractions (10%) of the pump and probe waves
were coupled, and their beat signal was divided equally andmonitored with an optical spectrum analyzer (OSA) (Anritsu,
MS9710A) and after the signal was converted into an electricalsignal with a photodetector (PD) (Discovery Semiconductors,
DSCR 402; bandwidth, 10 GHz; saturation power, 7 dBm),
an electrical spectrum analyzer (ESA) (Anritsu, MS2663C) forrough and precise adjustment of their relative frequency, respectively. The pump wave was amplified with an erbium-doped fiber
amplifier (EDFA) (Luxpert, LXI 2000), polarization-adjustedwith a polarization controllor (PC), and injected into one endof the FUT. The probe wave was also amplified, polarizationadjusted, and injected into the other end of the FUT. The Stokes
wave was directed through an optical circulator to a PD (New
Focus, 2053; bandwidth, 10 MHz; saturation power, 10 mW),
converted into an electrical signal, and monitored as a BGS withan oscilloscope (OSC) (Tektronix, TDS 3014B) triggered by the
LD current modulation.
IV. EXPERIMENTAL RESULTS
A. BGS Dependence on POF Length
Fig. 2 shows the measured BGS in a 3.8-m-long POF. The
pump and probe powers were 21.1 and 22.2 dBm, respectively.
The vertical axis was normalized so that the peak Stokes power
was 1.0. Despite the relatively short POF length, a clear BGS
was observed even without temporal averaging (sampling rate
was 80 Hz in this measurement), which is desirable for futureapplications to averaging-free BOTDA systems. The BFS was

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Fig. 2.

Measured BGS in 3.8-m-long POF without averaging.

Fig. 5.

Measured BGS dependences on probe power.

Fig. 3.

Measured BGS dependence on POF length.

Fig. 6.

Stokes power dependence on probe power.

because, as the POF length is longer, the Brillouin effective
length also becomes longer (leading to higher Stokes power)
but the pump and probe waves suffer from more significant
attenuation. We predict that the optimal POF length can beelongated by employing the pump and probe waves with muchhigher powers.
B. BGS Dependences on Probe and Pump Powers

Fig. 4.

Stokes power dependence on POF length.

∼2.80 GHz, which is slightly lower than the previously reported
value of 2.83 GHz [12], probably due to the difference in room
temperature [13]. The ripples on the noise floor are probably
caused by the interference of the waves Fresnel-reflected at the
POF/MMF and MMF/SMF interfaces.
Since SBS in POFs has been shown to be influenced bytheir physical properties, including high propagation loss [16],the POF length is one of the most important parameters to be
optimized to obtain a large SBS signal. Fig. 3 shows the BGS
dependence on the POF length with 32-times averaging. The
Stokes power was drastically changed according to the POF
length. The Stokes power dependence on the POF length isshown in Fig. 4. The maximum Stokes power was observed at
the POF length of 3.8 m. Here, the optimal POF length exists

The BGS dependence on probe power in the 10-m-long POF
was measured when the pump power was fixed at 21.1 dBm,
as shown in Fig. 5. Averaging was performed 32 times. With
increasing probe power, the Stokes power was raised, verifyingthat this BGS originates from the pump–probe interaction, i.e.,
SBS. The Stokes power dependence on probe power is shownin Fig. 6. The Stokes power began to increase rapidly at theprobe power of approximately 20 dBm; this behavior seems tobe due to the pump depletion [1]. The Stokes power was alsomeasured as a function of pump power as shown in Fig. 7, wherethe vertical axis was normalized so that the Stokes power with
24.8-dBm pump was 1.0. The probe power was fixed at
20.5 dBm. With increasing pump power from 0 dBm, the Stokes
power was gradually enhanced; and it also began to increase
rapidly when the pump power was higher than ∼20 dBm, which
is much lower than the theoretical Brillouin threshold without
the pump–probe technique (∼53.8 dBm [15]). These results suggest that the pump and probe waves with the powers of higher
than ∼20 dBm should be used to efficiently induce SBS in POFs
with this technique.

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POFs observed with this technique can be directly applied tohigh-precison temperature sensing.
V. CONCLUSION

Fig. 7.

Stokes power dependence on pump power.

SBS in POFs was observed, for the first time to the best of
our knowledge, using the pump–probe technique without lockin detection, and the dependences of the BGS on POF length,
pump/probe powers, and temperature were fully investigated.
We clarified that the optimal POF length for SBS observationis 3.8 m when the probe and pump powers are 21.1 dBm and22.2 dBm, respectively. This length can be, we predict, elongated by the pump and probe waves with much higher powers.
We also showed that the Stokes power was raised nonlinearlywith increasing probe and pump powers, and that >20-dBm
power is required for both pump and probe waves to efficiently
induce SBS in POFs. In addition, the BFS dependence on temperature was measured to be –4.02 MHz/K, which was in good
agreement with previous experiments. We believe that theseresults will offer a crucial basis for implementing distributed
Brillouin sensors (especially, BOTDA systems) based on POFs
with extremely high flexibility.
ACKNOWLEDGMENT
We express our gratitude to the two anonymous reviewers,
whose comments helped us to improve the article significantly.

Fig. 8.

Fig. 9.

Measured BGS dependence on temperature.

BFS dependence on temperature.

C. BGS Dependence on Temperature
We also measured the temperature dependence of the BGS
in the 10-m-long POF, as shown in Fig. 8. The temperature
was set to 25 ◦ C, 55 ◦ C, and 70 ◦ C. Averaging was performed
32 times. With increasing temperature, the BGS shifted toward
lower frequency. The Stokes power was decreased probably due
to the polarization fluctuations and the nonuniform temperature
distribution along the POF. Fig. 9 shows the temperature dependence of the BFS. Note that the dependence is well known
to be linear in the range from 20 ◦ C to 80 ◦ C [13], [15]. The
slope of –4.02 MHz/K is in good agreement with the previously reported values [13], [15], which means that the BGS in

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Yosuke Mizuno was born in Hyogo, Japan, on October 13, 1982. He received the
B.E., M.E., and Dr.Eng. degrees in electronic engineering from the Universityof Tokyo, Tokyo, Japan, in 2005, 2007, and 2010, respectively.
In 2004, he worked on optical information processing at the University ofTokyo. From 2005 to 2007, he was involved in spintronics and studied spin hotcarrier transistors for his master degree. From 2007 to 2010, he was involvedin Brillouin optical correlation-domain reflectometry for his Dr.Eng. degree atthe University of Tokyo. From 2007 to 2010, he was a Research Fellow (DC1)of the Japan Society for the Promotion of Science (JSPS). From 2010 to 2012,as a Research Fellow (PD) of JSPS, he worked on polymer optics at the Tokyo
Institute of Technology, Yokohama, Japan. In 2011, he stayed at BAM Federal
Institute for Materials Research and Testing, Germany, as a Visiting Research
Associate. Since 2012, he has been an Assistant Professor at the Precision andIntelligence Laboratory, Tokyo Institute of Technology, where he is active infiber-optic sensing, polymer optics, and ultrasonics.
Dr. Mizuno received the Funai Research Award 2010, the Ando Incentive
Prize for the Study of Electronics 2011, the NF Foundation R&D Encouragement
Award 2012, and the Kenjiro Nakamura Prize 2012. He is a Member of theJapanese Society of Applied Physics (JSAP), and the Institute of Electronics,
Information, and Communication Engineers (IEICE) of Japan.

Neisei Hayashi was born in Gunma, Japan, on April 13, 1988. He received the
B.E. degree in advanced production from the Gunma National College Technology (GNCT), Maebashi, Japan, in 2011, and the M.E. degree in electronicengineering from Tokyo Institute of Technology, Yokohama, Japan, in 2013.
From 2008 to 2010, he studied a ring-wandering phenomenon for his bachelor degree at the GNCT. From 2011 to 2013, he worked on nonlinear optics
in polymers for his master degree at Tokyo Institute of Technology. In 2013,
he became a Research Fellow (DC1) of the Japan Society for the Promotion of
Science (JSPS), and since then, he has been continuing to study polymer optics
for his Dr.Eng degree at Tokyo Institute of Technology. His research interests
include fiber-optic sensors, polymer optics, and ultrasonics.
Mr. Hayashi received the TELECOM System Technology Award for Student
2012. He is a Member of the Japanese Society of Applied Physics (JSAP),
and the Institute of Electronics, Information, and Communication Engineers(IEICE) of Japan.

Kentaro Nakamura (M’00) was born in Tokyo, Japan, on July 3, 1963. He received the B.E., M.E., and Dr.Eng. degrees from Tokyo Institute of Technology,
Yokohama, Japan, in 1987, 1989, and 1992, respectively.
Since 2010, he has been a Professor at the Precision and Intelligence Laboratory, Tokyo Institute of Technology. His research interests include the applications of ultrasonic waves, measurement of vibration and sound using opticalmethods, and fiber-optic sensing.
Prof. Nakamura received the Awaya Kiyoshi Award for encouragement ofresearch from the Acoustical Society of Japan (ASJ) in 1996, and the Best Paper Awards from the Institute of Electronics, Information, and Communication
Engineers (IEICE) in 1998 and from the Symposium on Ultrasonic Electronics(USE) in 2007 and 2011, respectively. He also received the Japanese Journal ofApplied Physics (JJAP) Editorial Contribution Award from the Japan Societyof Applied Physics (JSAP) in 2007. He is a Member of the IEEE, the ASJ, theJSAP, the IEICE, and the Institute of Electrical Engineers of Japan (IEEJ).