Research Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum

874

Enhanced stability and sensitivity ofslope-assisted Brillouin opticalcorrelation-domain reflectometry using
polarization-maintaining fibers
H EEYOUNG L EE , *N AKAMURAYOSUKE M IZUNO,AND K ENTARO

Institute of Innovative Research, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku, Yokohama
226-8503, Japan* hylee@sonic.pi.titech.ac.jp

Abstract: We show that, even when a polarization scrambler is switched off, PMF-based
SA-BOCDR can operate with higher stability than that of standard silica-fiber-based systems.
This leads to reduced cost and enables the use of the optimized state of polarization for higher
sensitivity. After investigation of the strain/temperature dependencies of the Brillouin frequency
shift and the Brillouin spectral power in the PMF, we show that the strain/temperature sensitivity of
the PMF-based SA-BOCDR is 1.4 times the value of the standard silica-fiber-based configuration;
we then demonstrate distributed temperature measurement with higher stability and sensitivity.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1.

Introduction

Distributed strain and temperature sensing is a key technology for monitoring a variety ofcivil infrastructures and enhancing human safety. In particular, fiber-optic distributed sensing
techniques based on Brillouin scattering [1] are immensely promising for their ability to detect
sub-centimeter or even sub-millimeter strained or heated sections along long optical fibers[2,3]. From the aspect of light injection to sensing fibers, Brillouin sensors can be categorized
into two-end-access “analysis” and single-end-access “reflectometry”. The former includes
Brillouin optical time-, frequency-, and correlation-domain analysis (BOTDA [4–6], BOFDA[7–9], and BOCDA [10–14]), while the latter includes Brillouin optical time-, frequency-,
and correlation-domain reflectometry (BOTDR [15–17], BOFDR [18], and BOCDR [19–26]).
Although analysis systems generally have higher signal-to-noise ratios (SNRs) owing to the useof stimulated Brillouin scattering, reflectometers based on spontaneous Brillouin scattering havehigher degrees of freedom in embedding the sensing fibers into structures and thus are convenient
for end-users. Here, we focus on BOCDR, which is the only scheme that can simultaneously
achieve single-end accessibility, high spatial resolution, and cost efficiency.
Since its first proposal [19], BOCDR has been vigorously studied, and many configurations
have been developed to improve its performance [21–26]. Recently, two types of high-speed
BOCDR systems have been implemented. One is phase-detected (PD-) BOCDR [26], whichenables a >100 kHz sampling rate, but the strain dynamic range is severely limited. The otheris slope-assisted (SA-) BOCDR [27], which exploits not the Brillouin frequency shift (BFS)
itself but the power on the slope of the Brillouin gain spectrum (BGS). The sampling rate ofSA-BOCDR is lower than that of PD-BOCDR, but SA-BOCDR has a wider strain dynamic
range. In addition, SA-BOCDR has been shown to possess a unique “beyond-nominal-resolution”
effect [28], with which a 2-mm strained section has been detected (sub-millimeter detection
has also theoretically shown to be feasible) [3]. In addition, very recently, an example of
SA-BOCDR-based composite structural diagnosis has been reported, showing its practical use in#357479
Journal © 2019https://doi.org/10.1364/OSAC.2.000874
Received 11 Jan 2019; revised 8 Feb 2019; accepted 12 Feb 2019; published 13 Mar 2019

Research Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum875

the future [29]. Thus, there has been considerable progress on the SA-BOCDR technology, but
one of its major drawbacks is the significant polarization-dependent power fluctuations, whichlead to unstable operation of the system. To date, to mitigate this noise, a polarization scrambler(PSCR) has been employed in the system [3,27–29], but the averaged state of polarization (SOP)
leads to non-ideal sensitivity (as experimentally shown below), and in addition, the use of the
PSCR increases the cost and encumbers the downsizing of the system. Polarization diversity has
also been employed in BOCDR [30], but the negative situation cannot be remedied because of
the use of a polarization switch, and what is worse, the sampling rate inevitably becomes lessthan half for data processing.
In this paper, to resolve these shortcomings, we develop polarization-scrambling-free SABOCDR using a polarization-maintaining fiber (PMF) as a fiber under test (FUT). A PSCR is
not employed in this configuration, leading to the cost reduction (when the FUT is relatively
short). In addition, what is more important, the SOP can be optimized so that the sensitivity
can be higher. First, we experimentally show that, unlike the case where the FUT is a silica
single-mode fiber (SMF), PMF-based SA-BOCDR can operate correctly even when the PSCR
is switched off. Subsequently, we characterize the fundamental sensing properties, such as thestrain/temperature dependencies of the BFS and the Brillouin spectral power in the PMF, andshow that the strain sensitivity of the PMF-based SA-BOCDR is 1.4 times higher than that of theconventional configuration. Finally, we demonstrate distributed temperature sensing with higher
stability and sensitivity.

Fig. 1. Experimental setup of SA-BOCDR, in which the polarization scrambler (PSCR)
can be switched on and off. EDFA, erbium-doped fiber amplifier; ESA, electrical spectrum analyzer; OSC, oscilloscope; PC, polarization controller; PD, photodetector; PMF,
polarization-maintaining fiber.

2.

Experimental setup

As the FUTs for distributed measurements, we used a silica SMF (LPSGK-S-SM, Sigma Koki)
and a polarization-maintaining and absorption-reducing (PANDA) fiber (most commonly used
PMF; P3-1550PM-FC, Thorlabs) [31]. The PANDA fiber had a core diameter of 8.5 µm and a
relatively low propagation loss of ∼0.4 dB/km. The BFSs of the silica SMF and PANDA fiber
were 10.855 and 10.863 GHz, respectively, at 1.55 µm at room temperature (20 °C). The lengthsof the two FUTs were both 9.15 m. Figure 1 schematically shows the experimental setup of
SA-BOCDR, in which a PSCR can be switched on and off. Compared to the previous report[27], a polarization controller (PC) was newly employed to control the SOP when the PSCR
was switched off. The 1.55-µm light from a distributed-feedback laser diode (NLK1C5GAAA,NTT Electronics) was divided into two: pump and reference lights. After amplified to 26 dBm
using a high-power erbium-doped fiber amplifier (EDFA), the pump light was injected into theResearch Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum

876

FUT. In this measurement, no special attempt was made to align the polarization state of thepump light to one of the eigenaxes of the PMF. To suppress the Fresnel reflection, a bending loss
was applied near the distal end of the FUT. The backscattered Brillouin Stokes light from the
FUT was amplified to ∼1 dBm using another EDFA and coupled with the reference light. The
reference light was amplified to ∼2 dBm in advance after passing through a delay fiber (which
was employed to avoid the appearance of the zero-optical-path-difference point, i.e., the 0th
correlation peak, in the FUT; the length can be shortened to <<1 km, if necessary). Then, usinga photodetector (PD), the self-heterodyned optical signal was converted into an electrical signal,
which was observed as a BGS using an electrical spectrum analyzer (ESA). The spectral powerchange at a fixed frequency (10.86 GHz for the PMF (see below) and 10.85 GHz for the SMF)
[27] was obtained using a zero-span mode of the ESA and observed using an oscilloscope (OSC)with 256 times averaging. The resolution and video bandwidths of the ESA were 10 MHz and 10
kHz, respectively. The room temperature was 20 °C.
By sinusoidally modulating the optical frequency of the laser output, a correlation peak
was generated and scanned along the FUT, which enabled distributed measurement. Based on
Eqs. (15) and (16) in [20], the spatial resolution and the measurement range were ∼72.5 mm and∼11.4 m by setting the modulation frequency and amplitude to 8.990–9.036 MHz and 1.5 GHz,
respectively. Only when we performed single-point measurements to investigate the system
performance, the modulation frequency was maintained at 9.023 MHz to obtain the Brillouinsignal only from the correlation peak (located 7.15 m away from the end of the second-port
pigtail of an optical circulator).
3.

Experimental results

First, we performed distributed measurements of Brillouin spectral powers using the partiallystrained SMF and PMF under three SOPs: 1) optimized using the PC so that the power change
at the strained section was maximized, 2) scrambled by switching the PSCR on so that thepolarization-dependent power change along the FUTs was suppressed, and 3) anti-optimized using
the PC so that the power change at the strained section was minimized. Strains of 1000 µ wereapplied to 0.3-m-long sections (7.0–7.3 m away from the circulator) in both the FUTs. Figure 2(a)and (b) show the power distributions along the SMF and PMF, respectively, under three SOPs. In
the case of the SMF, only when the SOP was scrambled, the power distribution was relatively flatand the strained section was correctly detected. However, when the SOP was optimized, although
the strained section was detected with higher strain sensitivity, the baseline was significantly
distorted and the distributed measurement was clearly erroneous. In addition, when the SOP was
anti-optimized, not only was the distributed measurement erroneous but also even the signal at
the strained section was completely buried under the distorted baseline. Thus, when the SMF
was used, polarization scrambling was found to be of paramount necessity for correct distributed
measurements. In contrast, in the case of the PMF, regardless of the SOPs, the baselines wererelatively flat and the strained sections were correctly detected. The strain sensitivity at thestrained section was highest at the optimized SOP and lowest at the anti-optimized SOP (notethat, when the SOP was scrambled, the results were almost identical between the SMF and PMF).
Thus, when the PMF was used, polarization scrambling was not necessary any longer, leading to
the cost reduction of the system (when the FUT length is relatively short; considering the highcost of the FUT, the PMF-based configuration is not suitable for long-distance measurements).
Besides, from the viewpoint of strain sensitivity, the SOP should be optimized instead of being
scrambled.
In the experiments below, we characterized the fundamental operations of PMF-based SABOCDR and, in both experiment and theory, quantitatively evaluated how much the strain
sensitivity can be improved by optimizing the SOP. First, we investigated the BFS dependencies
on strain and temperature in a 0.57-m-long PMF. Without applying frequency modulation, theResearch Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum

877

Fig. 2. Spectral power distributions along (a) the SMF and (b) the PMF measured at threedifferent states of polarization, when 1000 µ strain was applied to the FUTs (7.0–7.3 m).

BGS was observed when strains from 0 to 4800 µ were applied to the whole length of the FUT.
The BFS was then plotted as a function of the strain (refer to Appendix). With increasing strain,
the BGS linearly shifted to higher frequency with a coefficient of 425 MHz/%, which is ∼0.82
times the value in a typical SMF [32]. In the same manner, the BFS dependence on temperature
was also measured (refer to Appendix). With increasing temperature from 20 to 80 °C, the BFS
also increased with a coefficient of 1.1 MHz/°C, which is almost the same as that in an SMF [33].
Subsequently, after frequency modulation was applied, we measured the BGSs under threedifferent SOPs (Fig. 3(a)). The Brillouin spectral power was highest at the optimized SOP
and lowest at the anti-optimized SOP, and using the spectral slope at 10.86 GHz (derived from
Fig. 3(a)) and the strain coefficient of the BFS, we can calculate the theoretical strain sensitivitiesof the spectral power to be 0.96×10−4 , 0.76×10−4 , and 0.66×10−4 dB/µε (corresponding to
the temperature sensitivities of 2.49×10−3 , 1.97×10−3 , and 1.72×10−3 dB/°C) at optimized,
scrambled, and anti-optimized SOPs, respectively. Finally, at three different SOPs, we measured
the spectral power dependencies on strain (Fig. 3(b)). The dependencies were almost linear
and the coefficients were 1.09×10−4 , 0.78×10−4 , and 0.69×10−4 dB/µε at optimized, scrambled,
and anti-optimized SOPs, respectively. These values are in moderate agreement with theaforementioned values calculated from the BGS. In this measurement, the standard deviationof the power fluctuations along the non-strained section was approximately +/–0.01 dB, whichcorresponds to a strain uncertainty of ∼190 µ. The most important finding here is that the strain
sensitivity at the optimized SOP is 1.4 times higher than that at the scrambled SOP. Note that
it is difficult to derive this value theoretically based on the theory developed by Deventer et al.
[34], because the BGS observed in correlation-domain techniques is inherently influenced bybell-shaped background noise [35]. It is also not easy to compare the increased sensitivity withthe previously reported sensitivities [3,27–29,36] because SA-BOCDR has the measurement

Fig. 3. (a) Measured BGS in the PANDA-type PMF at three different SOPs. (b) Spectral
power plotted as a function of the applied strain at three different SOPs. The dotted lines arelinear fits.

Research Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum

878

sensitivity dependencies on various experimental conditions including incident light power, spatial
resolution, and resolution and video bandwidths of an ESA [36]. Nevertheless, our results clearly
indicate that, compared to conventional SMF-based SA-BOCDR, the PMF-based SA-BOCDR
can offer polarization-scrambling-free operation with high strain/temperature sensitivity alongwith the high stability (Refer to Fig. 4(a) (Visualization 1) and Fig. 4(b) (Visualization 2) for
demonstrations of distributed temperature measurement using the PMF, in which the spatial
resolutions were approximately ∼0.2 m and ∼0.1 m, respectively).

Fig. 4. Demonstration movies of distributed temperature measurement using PMF-based
SA-BOCDR. (a) Lower spatial resolution (Visualization 1) and (b) higher spatial resolution(Visualization 2).

4.

Conclusions

We developed and characterized the PMF-based SA-BOCDR with no need of polarization
scrambling. First, we showed that, unlike silica-SMF-based systems, PMF-based SA-BOCDR
can operate with higher stability even when the PSCR is switched off. This enables the useof the optimized SOP for higher sensitivity. After investigation of the strain/temperaturedependencies of the BFS and the Brillouin spectral power in the PMF, we finally showed thatthe strain sensitivity of the PMF-based SA-BOCDR is 1.4 times the value of the standard
silica-SMF-based configuration. We also presented two demonstration movies of distributed
temperature measurement. Considering the cost of PMFs, this configuration is suitable forhigh-resolution sensing with a relatively short measurement range. We anticipate that thisPMF-based SA-BOCDR will be of significant use for practical structural monitoring in the futureowing to its single-end accessibility, higher stability, and enhanced sensitivity.
Appendix
The BFS dependencies on strain and temperature in the PMF are shown in Figs. 5(a) and 5(b),respectively.

Fig. 5. BFS dependencies on (a) strain and (b) temperature in the PANDA-type PMF. The
dotted lines are linear fits.

Research Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum

879

Funding
Japan Society for the Promotion of Science (JSPS) (17H04930, 17J07226); Japan Gas Association;
Association for Disaster Prevention Research; Fujikura Foundation; Japan Association for
Chemical Innovation (JACI).
References
1. G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, 1995).
2. T. D. Vo, J. He, E. Magi, M. J. Collins, A. S. Clark, B. G. Ferguson, C. Xiong, and B. J. Eggleton, “Chalcogenide
fiber-based distributed temperature sensor with sub-centimeter spatial resolution and enhanced accuracy,” Opt.
Express 22(2), 1560–1568 (2014).
3. H. Lee, Y. Mizuno, and K. Nakamura, “Detection of 2-mm-long strained section in silica fiber using slope-assisted
Brillouin optical correlation-domain reflectometry,” Jpn. J. Appl. Phys. 57(2), 020303 (2018).
4. T. Horiguchi and M. Tateda, “BOTDA–nondestructive measurement of single-mode optical fiber attenuation
characteristics using Brillouin interaction: theory,” J. Lightwave Technol. 7(8), 1170–1176 (1989).
5. Y. Dong, L. Chen, and X. Bao, “Time-division multiplexing-based BOTDA over 100 km sensing length,” Opt. Lett.
36(2), 277–279 (2011).
6. Y. Peled, A. Motil, L. Yaron, and M. Tur, “Slope-assisted fast distributed sensing in optical fibers with arbitrary
Brillouin profile,” Opt. Express 19(21), 19845–19854 (2011).
7. D. Garus, K. Krebber, and F. Schliep, “Distributed sensing technique based on Brillouin optical-fiber frequency-domain
analysis,” Opt. Lett. 21(17), 1402–1404 (1996).
8. R. Bernini, A. Minardo, and L. Zeni, “Distributed sensing at centimeter-scale spatial resolution by BOFDA:
Measurements and signal processing,” IEEE Photon. J. 4(1), 48–56 (2012).
9. A. Minardo, R. Bernini, and L. Zeni, “Distributed temperature sensing in polymer optical fiber by BOFDA,” IEEE
Photon. Technol. Lett. 26(4), 387–390 (2014).
10. K. Hotate and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiber using acorrelation-based technique–Proposal, experiment and simulation–,” IEICE Trans. Electron. E83-C(3), 405–412(2000).
11. K. Y. Song, M. Kishi, Z. He, and K. Hotate, “High-repetition-rate distributed Brillouin sensor based on opticalcorrelation-domain analysis with differential frequency modulation,” Opt. Lett. 36(11), 2062–2064 (2011).
12. A. López-Gil, S. Martin-Lopez, and M. Gonzalez-Herraez, “Phase-measuring time-gated BOCDA,” Opt. Lett. 42(19),3924–3927 (2017).
13. C. Zhang, M. Kishi, and K. Hotate, “5,000 points/s high-speed random accessibility for dynamic strain measurement
at arbitrary multiple points along a fiber by Brillouin optical correlation domain analysis,” Appl. Phys. Express 8(4),042501 (2015).
14. W. Zou, C. Jin, and J. Chen, “Distributed strain sensing based on combination of Brillouin gain and loss effects inBrillouin optical correlation domain analysis,” Appl. Phys. Express 5(8), 082503 (2012).
15. T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa, and Y. Koyamada, “Brillouin optical-fiber time domainreflectometry,” IEICE Trans. Commun. E76-B(4), 382–390 (1993).
16. A. Masoudi, M. Belal, and T. P. Newson, “Distributed dynamic large strain optical fibre sensor based on the detectionof spontaneous Brillouin scattering,” Opt. Lett. 38(17), 3312–3315 (2013).
17. G. Tu, X. Zhang, Y. Zhang, Z. Ying, and L. Lv, “Strain variation measurement with short time Fourier transform-based
Brillouin optical time-domain reflectometry sensing system,” Electron. Lett. 50(22), 1624–1626 (2014).
18. A. Minardo, R. Bernini, R. Ruiz-Lombera, J. Mirapeix, J. M. Lopez-Higuera, and L. Zeni, “Proposal of Brillouin
optical frequency-domain reflectometry (BOFDR),” Opt. Express 24(26), 29994–20001 (2016).
19. Y. Mizuno, W. Zou, Z. He, and K. Hotate, “Proposal of Brillouin optical correlation-domain reflectometry (BOCDR),”
Opt. Express 16(16), 12148–12153 (2008).
20. Y. Mizuno, W. Zou, Z. He, and K. Hotate, “Operation of Brillouin optical correlation-domain reflectometry:
theoretical analysis and experimental validation,” J. Lightwave Technol. 28(22), 3300–3306 (2010).
21. N. Hayashi, Y. Mizuno, and K. Nakamura, “Distributed Brillouin sensing with centimeter-order spatial resolution inpolymer optical fibers,” J. Lightwave Technol. 32(21), 3999–4003 (2014).
22. R. Shimizu, M. Kishi, and K. Hotate, “Enlargement of measurement function in Brillouin optical correlation-domainreflectometry with combining four performance improvement schemes,” Proc. SPIE 10323, 1032390 (2017).
23. Y. Mizuno, N. Hayashi, H. Fukuda, and K. Nakamura, “Single-end-access distributed strain sensing with wide
dynamic range using higher-speed Brillouin optical correlation-domain reflectometry,” Jpn. J. Appl. Phys. 56(7),072501 (2017).
24. Y. Yao, M. Kishi, and K. Hotate, “Brillouin optical correlation domain reflectometry with lock-in detection scheme,”
Appl. Phys. Express 9(7), 072501 (2016).
25. N. Hayashi, Y. Mizuno, and K. Nakamura, “Characterization of stimulated Brillouin scattering in polymer opticalfibers based on lock-in-free pump-probe technique,” J. Lightwave Technol. 31(19), 3162–3166 (2013).
26. Y. Mizuno, N. Hayashi, H. Fukuda, K. Y. Song, and K. Nakamura, “Ultrahigh-speed distributed Brillouinreflectometry,” Light: Sci. Appl. 5(12), e16184 (2016).

Research Article

Vol. 2, No. 3 / 15 March 2019 / OSA Continuum

880

27. H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, “Slope-assisted Brillouin optical correlation-domain reflectometry:
proof of concept,” IEEE Photon. J. 8(3), 1–7 (2016).
28. H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, “Operation of slope-assisted Brillouin optical correlationdomain reflectometry: comparison of system output with actual frequency shift distribution,” Opt. Express 24(25),29190–29197 (2016).
29. H. Lee, Y. Ochi, T. Matsui, Y. Matsumoto, Y. Tanaka, H. Nakamura, Y. Mizuno, and K. Nakamura, “Distributed strain
measurement and possible breakage detection of optical-fiber-embedded composite structure using slope-assisted
Brillouin optical correlation-domain reflectometry,” Appl. Phys. Express 11(7), 072501 (2018).
30. O. Furukawa, S. Tezuka, M. Tsukamoto, S. Matsuura, M. Kishi, and K. Hotate, “Beyond 21 km distributed strain
measurement with Brillouin optical correlation-domain reflectometry using polarization diversity method andtemporal gating scheme,” IEEJ Trans. Fund. Mater. 137(1), 52–57 (2017). [In Japanese]
31. Y. Sasaki, K. Okamoto, T. Hosaka, and N. Shibata, “Polarization-maintaining and absorption-reducing fibers,” inOptical Fiber Communication Conference, 1982 OSA Technical Digest Series (Optical Society of America, 1982),
paper ThCC6.
32. T. Horiguchi, T. Kurashima, and M. Tateda, “Tensile strain dependence of Brillouin frequency shift in silica opticalfibers,” IEEE Photon. Technol. Lett. 1(5), 107–108 (1989).
33. T. Kurashima, T. Horiguchi, and M. Tateda, “Thermal effects on the Brillouin frequency shift in jacketed opticalsilica fibers,” Appl. Opt. 29(15), 2219–2222 (1990).
34. M. O. van Deventer and A. J. Boot, “Polarization properties of stimulated Brillouin scattering in single-mode fibers,”
J. Lightwave Technol. 12(4), 585–590 (1994).
35. K. Y. Song, Z. He, and K. Hotate, “Optimization of Brillouin optical correlation domain analysis system based on
intensity modulation scheme,” Opt. Express 14(10), 4256–4263 (2006).
36. H. Lee, Y. Mizuno, and K. Nakamura, “Measurement sensitivity dependencies on incident power and spatial resolution
in slope-assisted Brillouin optical correlation-domain reflectometry,” Sens. Actuators A 268, 68–71 (2017).