Applied Physics Express 8, 062501 (2015)
http://dx.doi.org/10.7567/APEX.8.062501

Polarization scrambling in Brillouin optical correlation-domain reﬂectometry
using polymer ﬁbers
Neisei Hayashi, Kazunari Minakawa, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received March 26, 2015; accepted April 19, 2015; published online May 13, 2015
We evaluate whether the measurement stability of Brillouin optical correlation-domain reﬂectometry (BOCDR) using polymer optical ﬁbers (POFs)
can be enhanced by polarization scrambling. In this study, two major factors that affect the signal-to-noise ratio in BOCDR, speciﬁcally, the spatialresolution and incident power, are varied, and their inﬂuences on distributed measurements with polarization scrambling are experimentallyinvestigated. We thus conﬁrm that in POF-based BOCDR, unlike in BOCDR using standard silica glass ﬁbers, polarization scrambling is aneffective means of enhancing the measurement stability only when the spatial resolution is sufﬁciently low or when the incident power is sufﬁciently
high. © 2015 The Japan Society of Applied Physics

or the past several decades, increasing attention
has been directed toward Brillouin scattering in
optical ﬁbers,1) which can be used to develop lasers,1)
microwave signal processors,2) optical memories,3) phaseconjugators,4) and slow-light generators.5) Strain and temperature sensors6–10) are also promising applications of Brillouinscattering due to its ability to measure the strain and temperature distributions along optical ﬁbers. To date, the headsof Brillouin sensors have been composed mainly of glass
optical ﬁbers including silica single-mode ﬁbers (SMFs),
which, however, lack ﬂexibility and cannot withstand strainsof over ∼3%. One promising solution to this drawback is toemploy polymer optical ﬁbers (POFs),11,12) which are highly
ﬂexible and can withstand large strains of over several tens ofpercent. Another advantage of POF-based sensors is a uniquefunction called a “memory eﬀect”,13) with which the information on the applied large strain can be stored by theirplastic deformation.
Previous studies on Brillouin scattering in POFs14–17) haverevealed that this phenomenon can potentially be used as amechanism of large-strain sensing16) and of highly sensitive
temperature sensing with less sensitivity to strain.17) Actual
distributed Brillouin measurements using POFs were obtained by three research groups almost simultaneously in2014. Minardo et al.18) employed Brillouin optical frequency-domain analysis (BOFDA)8,19) and examined a 4.0m-long heated section (located at the edge) of a 20-m-long
POF. The spatial resolution and signal-to-noise ratio (SNR)
were, however, not suﬃciently high for practical use. Donget al.20) employed Brillouin optical time-domain analysis(BOTDA)7) and investigated the evolution of mode couplingwith 3.3 m spatial resolution along a 40-m-long POF. Theyobserved signiﬁcant ﬂuctuations of the Brillouin frequency
shift (BFS) along the POF, a characteristic that is uniqueto pump–probe-based systems, including BOTDA, indicating
the diﬃculty in using the current BOTDA system to measure
the strain=temperature distributions along POFs. On the otherhand, our group21) employed Brillouin optical correlationdomain reﬂectometry (BOCDR)10) and investigated a 10-cmlong heated section of a 1.3-m-long POF with a theoreticalspatial resolution of 6.0 cm.
In BOCDR using a glass optical ﬁber, polarization
scrambling22,23) is often employed to suppress the polarization-dependent signal ﬂuctuations and to enhance thestability of long-term monitoring and of distributed measurements along the entire length of the ﬁber under test (FUT).24)

FIn contrast, all distributed measurements using POF-based
BOCDR have been performed without polarization scrambling, in other words, with an optimized polarization state(strictly speaking, a relative polarization state between
Brillouin Stokes and reference light beams).21) The measurements have been conducted in this manner because, in a
POF with a BFS ∼4 times lower than that in a silica SMF,
the Rayleigh wing is raised and overlaps the Brillouin gain
spectrum (BGS) by averaging the polarization state,25)
leading to signiﬁcant deterioration of the SNR of theBrillouin measurement. The lowered SNR is likely to induce
fatal measurement errors in POF-based BOCDR, because theBrillouin signal in a POF is much weaker than that in a silica
SMF due to the large core diameter and multimode nature.14)
However, in long-term monitoring using POF-based
BOCDR, polarization-dependent signal ﬂuctuations need tobe mitigated (an entire-length but fast measurement along a
short POF does not suﬀer from the signal ﬂuctuations).
In this work, we experimentally evaluated whether themeasurement stability of POF-based BOCDR can be enhanced by polarization scrambling. In general, major factorsthat aﬀect the SNR in BOCDR are the spatial resolution,incident pump power, and propagation loss (i.e., sensingposition). Among these three, the inﬂuences of the ﬁrst twofactors were investigated; a 1.0-m-long section (positionﬁxed) of a 3.0-m-long POF was heated. When the incident
power was ﬁxed at 31 dBm, distributed measurement with aspatial resolution of 0.67 m was feasible even with polarization scrambling, whereas that with a resolution of 0.45 m
failed unless the polarization state was optimized. Furthermore, when the spatial resolution was ﬁxed at 0.67 m,distributed measurement with an incident power of 27 dBm
was feasible even with polarization scrambling, whereas thatwith an incident power of 23 dBm needed to be performed
with an optimized polarization state. These results indicatethat polarization scrambling can be exploited to enhance themeasurement stability of POF-based BOCDR only when thespatial resolution is suﬃciently low or when the incident
power is suﬃciently high.
BOCDR10,26) is known to be a Brillouin-based distributed
sensing technique with intrinsic one-end accessibility andhigh spatial resolution. Its operating principle is based on
the correlation control of continuous light waves in a selfheterodyne scheme. In other words, the pump and reference
lights [instead of the probe light used in Brillouin opticalcorrelation-domain analysis (BOCDA)9)] are sinusoidally

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Appl. Phys. Express 8, 062501 (2015)

N. Hayashi et al.

Fig. 2. Structure of tested POF.
Fig. 1. Schematic setup of BOCDR containing PSCR in the reference
path. AC, alternating current; DC, direct current; EDFA, erbium-doped ﬁber
ampliﬁer; ESA, electrical spectrum analyzer; PC, polarization controller; PD,photo detector.

frequency-modulated at f m , with “correlation peaks” 27) generated periodically along the FUT. The measurement range
dm is determined by their interval, which is inversely
proportional to f m as26)
dm ¼

c;
2nfm

ð1Þ

where c is the light velocity in a vacuum and n is therefractive index of the ﬁber core. By sweeping f m , thecorrelation peak (namely, the sensing position) can bescanned along the FUT to acquire BGS and BFS distributions. The spatial resolution Δz is given by 26)
z ¼cB;2n fm  f

ð2Þ

where Δ f is the modulation amplitude of the opticalfrequency and ΔνB is the Brillouin bandwidth. In a basic
conﬁguration, the upper limit of Δ f is half the BFS of theFUT due to the inevitable noise caused by Rayleighscattering.10,26)
As an FUT, we employed a 3.0-m-long perﬂuorinatedgraded-index POF 27) with a numerical aperture of 0.185, core
diameter of 50 µm, cladding diameter of 70 µm, overcladding
diameter of 490 µm, core refractive index of ∼1.35, and propagation loss of ∼250 dB=km at 1.55 µm. The core=cladding
layers and overcladding layer were composed of amorphous
perﬂuorinated polymer and polycarbonate,28) respectively.
A schematic setup of POF-based BOCDR is depicted inFig. 1, which is essentially identical to that previously
reported,10) except for some devices used to control thepolarization state.24) All the optical paths excluding the FUT
were silica SMFs. One end of the FUT was butt-coupledto the end of the second port of an optical circulator.14) The
output from a distributed-feedback laser diode at 1.55 µm
with 1 MHz linewidth was sinusoidally frequency-modulatedby direct modulation of the driving current. It was divided
into two light beams with an optical coupler; one was guided
through a 1-km-long delay ﬁber to adjust the correlation peakorder and a polarization scrambler (PSCR) to switch on=oﬀ
the average function of the polarization state. It was then usedas the reference light of heterodyne detection (here, the“polarization state” means, in a precise sense, the relative
polarization state between the Brillouin Stokes and reference
light beams). The other beam was ampliﬁed with an erbiumdoped ﬁber ampliﬁer (EDFA), polarization-adjusted with apolarization controller (PC), and then injected into the POF
as the pump light. Subsequently, the optical beat signal of theStokes and reference lights was converted to an electricalsignal with a photo detector (PD) and was ﬁnally monitored
as BGS with an electrical spectrum analyzer (ESA) with a300 kHz frequency resolution.
As stated above, polarization scrambling in POF-basedBOCDR drastically deteriorates the signal-to-noise ratio(SNR) of the measurement. To evaluate its inﬂuence, weﬁrst show that, when polarization scrambling is employed
and the incident pump power is ﬁxed, there exists a thresholdspatial resolution, above which distributed measurements
cannot be properly performed. The modulation frequency f m
was swept from 35.002 to 35.198 MHz, corresponding to themeasurement range dm of 3.2 m [see Eq. (1)]. The modulation amplitude Δ f was set to 0.227 and 0.151 GHz, corresponding to nominal spatial resolutions Δz of 0.45 and0.67 m [see Eq. (2); the Brillouin bandwidth ΔνB is reported
to be ∼100 MHz14) in POFs]. The incident power was ﬁxed at
31 dBm, and the polarization state was scrambled. The orderof the correlation peak used for the measurement was 167,
and the overall sampling rate of single-location BGS acquisition was 3.3 Hz. The room temperature was 18 °C. Figure 2
shows the structure of the FUT, where a 1.0-m-long section(position ﬁxed) of a 3.0-m-long POF was heated to 45 °C.
Figures 3(a) and 3(b) show the normalized BGS distributions measured with 0.67 and 0.45 m spatial resolutions, respectively, and their BFS distributions are plotted in Fig. 3(c).
The BFS in the unheated section was 2.83 GHz, which agreed
well with the previously reported value.14) With 0.67 mresolution, the 1.0-m-long heated section was clearly detected
and was determined to have an average BFS value of ∼2.73
GHz, which is reasonable considering that the temperaturedependence coeﬃcient of the BFS is −3.2 MHz=K.29) The
measurement error (standard deviation) was calculated to be∼±1.0 °C. In contrast, with 0.45 m resolution, the heatedsection was not properly detected because the raised Rayleighwing overlapped with the BGS from lower frequencies,
unlike in the case of silica-SMF-based BOCDR. This result
indicates that there is a threshold spatial resolution between
0.45 and 0.67 m (precise determination is diﬃcult due to thesignal ﬂuctuations) and that measurements with resolutions
higher than the threshold cannot be performed with polarization scrambling.
Next, we demonstrated that, when polarization scramblingis employed and the spatial resolution is ﬁxed, there exists
a threshold incident pump power, below which distributed
measurements fail. Figures 4(a), 4(b), and 4(c) show the BGS
distributions with incident powers of 27, 23, and 20 dBm,respectively, and their BFS distributions are summarized inFig. 4(d). The nominal spatial resolution was ﬁxed at 0.67 m.
The heated section was clearly detected with 27 dBm incident
power, while the overlap of the raised Rayleigh wing
disturbed proper detection of the heated section with 23062501-2© 2015 The Japan Society of Applied Physics

Appl. Phys. Express 8, 062501 (2015)(a)N. Hayashi et al.
(b)

(c)

Fig. 3. Normalized BGS distributions measured with nominal spatial resolutions of (a) 0.67 and (b) 0.45 m. (c) BFS distributions with resolutions of (i) 0.67
and (ii) 0.45 m. Polarization state was scrambled.

(a)

(b)

(c)

(d)

Fig. 4. Normalized BGS distributions with incident powers of (a) 27, (b) 23, and (c) 20 dBm. (d) BFS distributions with incident powers of (i) 27, (ii) 23,and (iii) 20 dBm. Polarization state was scrambled.

and 27 dBm incident powers. Thus, the threshold incident
power was veriﬁed to lie between 23 and 27 dBm under thesemeasurement conditions.
Finally, we conﬁrmed that, when the polarization stateis scrambled, the SNR of the measurement is deteriorated
compared to that when the polarization state is optimized.
Figure 5(a) shows the BFS distribution measured when theincident power was ﬁxed at 31 dBm (same as in Fig. 3) andthe spatial resolution was set to 0.38 m (<0.45 m; Δ f was set
to 0.269 GHz). The heated section was successfully detected.
In addition, Fig. 5(b) shows the BFS distributions measured
when the spatial resolution was ﬁxed at 0.67 m (same as inFig. 4) and the incident power was set to 23, 20, and 19 dBm.
The heated section was properly detected with 23 dBm
power, but detection failed at 20 and 19 dBm powers,indicating that the threshold power is lower than 23 dBm.
These results clearly suggest that polarization scrambling
lowers the SNR of the measurement compared to that when
the polarization state is optimized; in other words, polarization scrambling enhances the long-term measurement
stability at the cost of SNR deterioration and should be usedwith suﬃciently low spatial resolution and suﬃciently highincident pump power.
In conclusion, we experimentally investigated the inﬂuence of polarization scrambling on measurements by POFbased BOCDR. The spatial resolution and incident pump

(a)

(b)

Fig. 5. (a) BFS distribution measured with 0.38 m spatial resolution.
(b) BFS distributions with incident powers of (i) 23, (ii) 20, and (iii) 19 dBm.
Polarization state was optimized.

power, which signiﬁcantly aﬀect the SNR, were varied, andtheir inﬂuences on the distributed measurements with polarization scrambling were evaluated. A 1.0-m-long section(position ﬁxed) of a 3.0-m-long POF was heated. When theincident power was ﬁxed at 31 dBm, distributed measurementwith a spatial resolution of 0.67 m was successfully performed even with polarization scrambling, while that with aresolution of 0.45 m proved unsuccessful unless the polarization state was optimized. Meanwhile, when the spatialresolution was ﬁxed at 0.67 m, distributed measurement withan incident power of 27 dBm was successfully conductedeven with polarization scrambling, but that with an incident

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Appl. Phys. Express 8, 062501 (2015)

N. Hayashi et al.

power of 23 dBm had to be performed with an optimized
polarization state. These results indicate that, unlike the caseof silica-SMF-based BOCDR, polarization scrambling can beused to improve the measurement stability of POF-based
BOCDR only when the spatial resolution is suﬃciently lowor when the incident power is suﬃciently high. The threshold
resolution and incident power should be dependent on otherparameters, such as the sensing position and optical wavelength; therefore, further study is required on this point. We
believe that these results will provide guidelines useful forthe implementation of POF-based BOCDR systems with highmeasurement stabilities in long-term structural monitoring in
the near future.
Acknowledgments This work was partially supported by Grants-in-Aidfor Young Scientists (A) (No. 25709032) and for Challenging Exploratory
Research (No. 26630180) from the Japan Society for the Promotion of Science(JSPS) and by research grants from the Iwatani Naoji Foundation, the SCAT
Foundation, and the Konica Minolta Science and Technology Foundation. N. H.
acknowledges a Grant-in-Aid for JSPS Fellows (No. 25007652).

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