Applied Physics Express 5 (2012) 032501
DOI: 10.1143/APEX.5.032501

Enhancement of Brillouin Scattering Signal in Optical Fibers by Use of Pulsed Pump Light
Yosuke Mizuno and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received January 16, 2012; accepted February 23, 2012; published online March 6, 2012
We propose a new method of enhancing the Brillouin scattering signal with a relatively low-cost low-power erbium-doped fiber amplifier by using
pulsed pump light. When a pulsed pump light with an average optical power of 20 mW, a duty ratio of 20%, and a pulse period of 2 s was injected
into a 1-km-long silica single-mode fiber, the Brillouin signal was enhanced by 25 dB relative to that with a continuous-wave pump light having thesame average power. We believe that this method can also be used to enhance the weak Brillouin scattering signal in polymer optical fibers for its
detailed characterization. # 2012 The Japan Society of Applied Physics

rillouin scattering in optical ﬁbers1,2) is one of themost signiﬁcant nonlinear eﬀects, and has been
extensively studied. It has many useful applications,such as lasing,3) microwave signal processing,4) slow lightgeneration,5) phase conjugation,6) tunable delay,7) corealignment,8) optical storage,9) and strain/temperature sensing.10–15) Up to now, Brillouin scattering has been studied
not only for glass optical ﬁbers (GOFs) but also for polymer
optical ﬁbers (POFs), including poly(methyl methacrylate)(PMMA)-based POFs16) and perﬂuorinated graded-index(PFGI-) POFs.17–20) Brillouin scattering in PFGI-POFswas experimentally observed at 1.55 m,17) but the signal
was quite small due to their large core diameter and highpropagation loss compared with those of GOFs. In orderto enhance the Brillouin signal in POFs for more accuratecharacterization, a high-power pump light needs to beemployed. However, commercially available high-power
optical ﬁber ampliﬁers such as erbium-doped ﬁber ampliﬁers(EDFAs) are expensive.
In this paper, we propose a new method of enhancing
the Brillouin scattering signal only with a relatively lowcost low-power EDFA by use of a pulsed pump light,and demonstrate its eﬀectiveness using silica single-mode
ﬁbers (SMFs). [Note that, in Brillouin optical time-domain
reﬂectometry (BOTDR),10,11) the pulsed pump light has
already been used to spatially resolve the measurement
spot, but no detailed study on its signal enhancementeﬀect for accurate Brillouin characterization has beenreported.] We show both theoretically and experimentally
that, even when the pulsed pump light has low average
power, if its peak power is higher than the Brillouinthreshold power, the temporally averaged Brillouin scattering signal can be enhanced. When a pulsed pump lightwith an average optical power of 20 mW, a duty ratio of20%, and a pulse period of 2 s was used, the Brillouinsignal was enhanced by 25 dB relative to that with
continuous-wave (CW) pump light having the same averagepower.
The dependence of Brillouin Stokes power on CW pump
power is known to be nonlinear;2) the Stokes power begins togrow exponentially when the pump power is higher than theBrillouin threshold power Pth , and then reaches saturation.
This behavior indicates the transition from spontaneous to
stimulated Brillouin scattering (SBS). Pth of an SMF is givenby2)

BPth ¼

21Aeﬀ;
KgB Leﬀ

Fig. 1. Two-state model of the Stokes power dependence on pump power.
Both axes are in linear scale.

where Aeﬀ is the eﬀective cross-sectional area, gB is theBrillouin gain coeﬃcient, and Leﬀ is the eﬀective lengthdeﬁned asLeﬀ ¼

1  expðLÞ;ð2Þ

where  is the propagation loss and L the ﬁber length. K is apolarization-dependent constant, which is 0.667 when thepolarization is not maintained.
When a pulsed pump light with its peak power higher thanPth is employed, the temporally averaged Stokes power canbe enhanced compared with that with a CW pump lighthaving the same average power. Such pulsed pump light witha high peak power can be generated with a low-power EDFA,
which typically has a small-signal gain of about 20 to 40 dB
and a saturation output power of about 10 to 20 dBm. In thepulsed regime, as long as the average output power does notexceed the saturation output power, the input pulsed lighteven with several mW peak power experiences the smallsignal gain. This condition can be satisﬁed by suﬃciently
decreasing the pulse duty ratio, and thus, pulsed pump lightwith (sub-)watt-range peak power can be obtained.
Prior to experiments, we theoretically analyzed the eﬀectiveness of this method. The Stokes power dependence on
pump power was approximated using a simple two-statemodel as shown in Fig. 1, where  and  represent the slopesof the lines (  ). The average input power Pin , corresponding to the output power of a low-power EDFA, wasassumed to be lower than the Brillouin threshold power Pth .
When the pump light is CW, the Stokes power is given byð1Þ
032501-1

PStokes ¼   Pin :ð3Þ# 2012 The Japan Society of Applied Physics

Appl. Phys. Express 5 (2012) 032501

Y. Mizuno and K. Nakamura

Fig. 2. Experimental setup. DC, direct current; EDFA, erbium-doped ﬁber
ampliﬁer; ESA, electrical spectrum analyzer; FUT, ﬁber under test; IM,intensity modulator; ISO, isolator; OSC, oscilloscope; PC, polarization
controller; PD, photodiode; VOA, variable optical attenuator.

Fig. 3.

Measured Stokes power dependence on CW pump power.

When the pump light is ideally pulsed, its peak power isgiven byPpeak ¼ Pin 100;x

ð4Þ

where x is the duty ratio (in %). When Ppeak is lower than or
the same as Pth , the temporally averaged Stokes power isgiven byx¼   Pin ;PStokes ¼   Ppeak ð5Þ100
which is the same as eq. (3). In contrast, when Ppeak is higher
than Pth , the Stokes power is given byxPStokes ¼ ½  Pth þ   ðPpeak  Pth Þ 100x:
ð6Þ¼   Pin  ð  Þ  Pth 
100
Here, the pulse width dependence of Pth 21) is not considered
for simplicity. Using   , eq. (6) can be further simpliﬁedasx PStokes ¼   Pin  Pth ;
ð7Þ
100
which indicates that, as the duty ratio decreases, theStokes power increases, ﬁnally reaching Pin . Thus, it wastheoretically shown that, when Pin is lower than Pth , theStokes signal can be enhanced by reducing the duty ratio of
the pulsed pump light so that its peak power is higher thanPth .
The experimental setup is depicted in Fig. 2. In order
to measure the Brillouin gain spectrum (BGS) with highresolution, self-heterodyne detection17) was used. All theoptical paths were composed of silica SMFs. A distributedfeedback laser diode (DFB-LD) at 1547 nm was used as alight source, and its output was divided into two light beams
with a coupler. One of the beams was directly used as thereference light of heterodyne detection, after passing apolarization controller (PC) and a variable optical attenuator(VOA). The other beam was converted to optical pulses witha LiNbO3 intensity modulator (IM) having an extinction
ratio of over 20 dB, ampliﬁed with a low-power EDFA
having a small-signal gain of 35 dB, and injected into a ﬁber
under test (FUT) as pump light. Part of the pump light wasdirectly guided to a photodiode (PD) after passing a VOA,and its waveform was monitored with an oscilloscope (OSC)

Fig. 4. Measured BGS with the duty ratio varied from 100 (CW) to 5%.

to optimize the input voltages to the IM. Then, the opticalbeat signal between the Stokes light and the reference light
was converted to an electrical signal with a PD. Finally, thesignal was ampliﬁed by 23 dB with an electrical preampliﬁer, and monitored with an electrical spectrum analyzer(ESA).
First, a 1-km-long silica SMF was used as an FUT.
The Brillouin threshold Pth of this FUT was approximately
50 mW, as shown in Fig. 3. This is in good agreement withthe theoretical value of 53 mW, which was obtained bysubstituting in eqs. (1) and (2) the following typical values
for silica SMFs:2) Aeﬀ ¼ 50 m2 , gB ¼ 3  1011 m/W, and ¼ 0:2 dB/km.
Figure 4 shows the measured BGS when the output powerof the EDFA, i.e., the average power of the pump light, wasset to 13 dBm (¼ 20 mW < Pth ). The pulse period was ﬁxed
at 2 s, and the duty ratio was varied from 100 (CW) to 5%.
When the duty ratio was 40%, the calculated peak power
was almost the same as Pth . As the duty ratio decreased, theStokes power was enhanced.
Figure 5 is the Stokes power dependence on the dutyratio. When the duty ratio became lower than 40 mW (i.e.,
when the peak power became lower than Pth ), the Stokes
power began to grow drastically, and it gradually reachedsaturation at approximately 10 dBm. The former behavior

032501-2# 2012 The Japan Society of Applied Physics

Appl. Phys. Express 5 (2012) 032501

Fig. 5.

Y. Mizuno and K. Nakamura

CW. This probably originates from the nonlinearity of theStokes power dependence on CW pump power even within
the range below Pth .
In conclusion, a new scheme for enhancing Brillouin
Stokes power was developed, which exploits pulsed pumplight and a low-power EDFA. First, it was theoreticallyshown that, even when the pulsed pump light has low
average power, the temporally averaged Stokes power canbe enhanced if its peak power is higher than the Brillouinthreshold power. Then, in the experiment, when a pulsed
pump light with an average power of 20 mW, a duty ratio of20%, and a pulse period of 2 s was injected into a silica
SMF, the Stokes power was enhanced by 25 dB relative tothat with CW pump light having the same average power.
We believe that this method will provide a useful andcost-eﬀective way of characterizing the Brillouin scattering
properties in POFs, such as Brillouin frequency shift (BFS)
and its dependence on strain, temperature, moisture, andcore materials.

Measured Stokes power dependence on duty ratio.

Acknowledgment This work was partially supported by Research Fellowships for Young Scientists from the Japan Society for the Promotion of Science(JSPS).

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032501-3# 2012 The Japan Society of Applied Physics

Fig. 6. Measured BGS when the pump light was CW (red) and pulsed(blue) with 10% duty ratio.

can be well explained by eq. (7), while the latter behavior
can be understood by the saturation of exponential growthin Stokes power with the increasing pump power.2) Thus,
drastic enhancement (>25 dB) of the Stokes power wasachieved when the duty ratio was lower than 20%.
Then, a 2-m-long silica SMF was also used as an FUT, thetheoretical Pth of which was 26.2 W. Figure 6 shows themeasured BGS with CW pump light and with pulsed pumplight with 10% duty ratio. The average power of the pumplight was set to 19 dBm (¼ 80 mW), and the calculated peak
power of the pulsed pump light was 800 mW. Although
this is much lower than Pth , with the pulsed pump light,
the Stokes power was slightly enhanced relative to that with

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