BRIEF NOTEJapanese Journal of Applied Physics 52 (2013) 058003
http://dx.doi.org/10.7567/JJAP.52.058003

Broad and Flat Brillouin Gain Spectrum in Optical Fiber
Obtained by Modulating Driving Current of Laser Diode
Yosuke Mizuno, Neisei Hayashi, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received February 21, 2013; accepted March 31, 2013; published online April 19, 2013
A simple method for obtaining a broad and flat Brillouin gain spectrum (BGS) is presented. By modulating the driving current of a pump laser diodewith triangular waveform, a BGS with lower than 0.3 dB gain variation over >200 MHz is achieved. With this method, the bandwidth of the flattened
BGS can be controlled by adjusting the amplitude and/or frequency of the current modulation. # 2013 The Japan Society of Applied Physics

Brillouin scattering is one of the most signiﬁcant nonlinear
eﬀects in optical ﬁbers, and has been applied to various
devices and systems.1) In some of these applications,
including gain-ﬂattened Brillouin ampliﬁers,2) slow-light
generators,3) and narrow-linewidth Brillouin lasers,4) a broadand ﬂat Brillouin gain spectrum (BGS) can be of great
beneﬁt. Conventional methods that can be used to obtain atemporally-averaged ﬂat BGS in an optical ﬁber are basedon the overlapping of multiple narrow BGSs with slightly
diﬀerent Brillouin frequency shifts (BFSs), which include
the employment of longitudinal variations in strain/temperature,5) core diameter,6) and/or dopant concentration.7,8)
Recently, Dragic has succeeded in obtaining a BGS with again variation of <0:5 dB and a bandwidth of >200 MHz by
combining two acoustically anti-guiding optical ﬁbers,8) but
the BGS in each ﬁber needs to be precisely controlled byoptimizing the radial distribution of the doping concentration of Al, P, and Ge, which is somewhat complicated. A
much simpler way to achieve a time-averaged ﬂat BGS isrequired.
In this Note, we demonstrate a simple method for
obtaining a broad and ﬂat BGS. By modulating the driving
current of a pump laser with triangular waveform, its output
spectrum can be broadened and ﬂattened, resulting in abroad and ﬂat BGS. We experimentally achieve a BGS
with <0:3 dB gain variation over >200 MHz, which is evenﬂatter than Dragic’s result.8) The bandwidth of the ﬂattened
BGS can be controlled by adjusting the amplitude and/or
frequency of the current modulation. The increase in gain
variation is observed when the bandwidth is broader than200 MHz, the reason of which is also clariﬁed.
Since the output optical frequency as well as the output
optical power of a laser diode (LD) depends on the driving
current, the optical frequency can be modulated by directly
modulating the LD current. When high-frequency amplitude
modulation is applied to the LD current, the time-averaged
optical spectrum becomes broad. If we neglect the induced
modulation of the output power, to obtain a ﬂat spectrum,
the LD current modulation should be performed with notsinusoidal but triangular waveform, because, ideally, theoutput power is uniformly distributed over the covered
frequency range with the triangular modulation. Since theelectrical circuit to drive the LD has an intrinsic response time
(much longer than that of the LD itself ), the bandwidth of theoutput optical spectrum depends not only on the modulation
amplitude but also on the modulation frequency. The BGS
measured by heterodyne detection is known to be broadened,
when broadband light source is employed as a pump LD.9)

Fig. 1. (Color online) Experimental setup. DFB-LD, distributed-feedback
laser diode; EDFA, erbium-doped ﬁber ampliﬁer; ESA, electrical spectrum
analyzer; FG, function generator; FUT, ﬁber under test; ISO, isolator; MOD,external modulation port; PC, polarization controller; PD, photo diode;
T-LD, tunable laser diode; VOA, variable optical attenuator.

The ﬁber under test (FUT) employed in the experiment
was a 1-km-long silica SMF with core diameter of 9 m,core refractive index of 1:47, numerical aperture (NA) of0.13, propagation loss at 1.55 m of 0:5 dB/km, and BFSof 10:8 GHz at 1.55 m at room temperature. The experimental setup is depicted in Fig. 1. A distributed-feedback(DFB-) LD (DenseLight DL-CLS101B) at 1.55 m wasused as a pump light source. The driving current of the LD
was modulated with triangular waveform using a function
generator connected to the external modulation port of an
LD driver (Thorlabs LDC-500). After being ampliﬁed to
16 dBm with an erbium-doped ﬁber ampliﬁer (EDFA), thepump light was injected into the FUT. The open end of theFUT was bent to suppress the Fresnel reﬂection. As thefrequency-modulated pump light cannot be directly used as areference light for heterodyne detection in this experiment(for details, refer to Refs. 10 and 11), another tunable LD(T-LD; Santec TSL-210) at 1.55 m was employed, theoutput of which was coupled with the Brillouin Stokes light,and then, after power adjustment with a variable opticalattenuator (VOA), converted into an electrical signal with aphoto diode (PD). The polarization states of the pump andthe reference light beams were optimized with polarization
controllers (PCs). The electrical signal was, after being preampliﬁed, observed as a BGS with an electrical spectrum
analyzer (ESA).
Figure 2(a) shows the measured BGS dependence on
the peak-to-peak modulation amplitude applied to the LDcurrent. The modulation frequency was ﬁxed at 500 kHz.
With no modulation (amplitude: 0 mV), the BGS was058003-1# 2013 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 52 (2013) 058003(a)Y. Mizuno et al.

(b)

Fig. 2. (Color online) (a) Measured BGS dependence on modulation
amplitude of LD current. (b) Magniﬁed view when modulation amplitude
was 290 mV.

(a)

(b)

Fig. 3. (Color online) (a) BGS bandwidth vs modulation amplitude.
(b) Brillouin gain variation vs BGS bandwidth.

observed not at 10:8 GHz but at 9:7 GHz, because thecenter frequency of the pump light was 1:1 GHz higher
than that of the reference light. By tuning the output frequency of the T-LD, the optical frequency at which the BGS
is observed can be arbitrarily controlled. As the modulation
amplitude was raised, the BGS bandwidth was broadened
and the maximum Stokes power was decreased, which isreasonable if we consider that this broadening eﬀect is theconsequence of temporal averaging. With increasing modulation amplitude, the center frequency of the whole BGS
was slightly shifted to the lower frequency, because theoutput frequency dependence of the LD on driving current
was not completely linear. Figure 2(b) shows a magniﬁedview of the ﬂattened region when the modulation amplitude
was 290 mV. Over the range of >200 MHz, the gain variation was lower than 0.3 dB, which is ﬂatter than Dragic’s
report.8)
The BGS bandwidth was measured as a function ofmodulation amplitude, as shown in Fig. 3(a). Here the BGSbandwidth was deﬁned as the 3-dB bandwidth calculatedwith reference to the maximum Stokes power in the whole

BGS. As the modulation amplitude was raised, the bandwidth was also almost linearly increased in this range. TheBrillouin gain variation was also measured as a functionof BGS bandwidth, as shown in Fig. 3(b). With increasing
bandwidth, the gain variation was also increased, especiallywhen the bandwidth was broader than 200 MHz. The increase in gain variation according to the bandwidth broadening is due to the distortion of the ﬂattened BGS, as shownin Fig. 2(a), which seems to originate partly from the lowpass characteristics of the electronic circuit (note that, whenthe modulation amplitude was 790 mV, and the modulation
was switched from triangular to sinusoidal waveform, thebroadened BGS scarcely changed), and partly from theoutput power dependence of the LD on driving current.
In conclusion, we demonstrated a simple method for
obtaining a broad and ﬂat BGS by modulating the driving
current of a pump LD. The BGS with <0:3 dB gain variation
over >200 MHz was experimentally achieved. We also
showed that the bandwidth of the ﬂattened BGS can becontrolled by adjusting the amplitude of the current modulation. When the bandwidth was broader than 200 MHz,
gain variation was increased, probably due to the the lowpass characteristics of the LD driving circuit as well as theoutput power dependence of the LD on driving current. We
believe that, though the broad and ﬂat BGS obtained withthis method can be applied only to a limited range of devices
in which temporally-averaged spectra are exploited, it willbe of great use in developing gain-ﬂattened continuous-wave
Brillouin ampliﬁers, etc. We also predict that the BGS canbe further broadened and ﬂattened by modulating the LDdriving current with optimized arbitrary waveform.
Acknowledgements This work was partially supported by a Grant-in-Aidfor Research Activity Start-up (24860029) from the Japan Society for thePromotion of Science (JSPS), and by research grants from the Hattori-Hokokai
Foundation, the Mazda Foundation, and the JFE 21st Century Foundation.

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058003-2# 2013 The Japan Society of Applied Physics