LetterVol. 44, No. 8 / 15 April 2019 / Optics Letters

2097

Noise suppression technique for distributed
Brillouin sensing with polymer optical fibers
YOSUKE MIZUNO,1,*

HEEYOUNG LEE,1

NEISEI HAYASHI,2

1

AND KENTARO NAKAMURA

1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan*Corresponding author: ymizuno@sonic.pi.titech.ac.jp

2

Received 15 January 2019; revised 18 March 2019; accepted 19 March 2019; posted 22 March 2019 (Doc. ID 357498); published 15 April 2019

We develop a new noise suppression technique to perform
distributed strain and temperature sensing based on ahigher-speed configuration of Brillouin optical correlationdomain reflectometry even when a polymer optical fiber(POF) is used as a sensing fiber. We acquire the spectral difference between with and without reference light, leading
to selective observation of the beat signal of Brillouinscattered light and reference light, which is effective for
distributed sensing. After experimentally showing theusefulness of this technique, we demonstrate POF-based
distributed temperature sensing and dynamic strain
sensing. © 2019 Optical Society of America
https://doi.org/10.1364/OL.44.002097

Brillouin scattering in optical fibers has been exploited todevelop distributed strain and temperature sensors [1]. Suchsensors are categorized into two configurations: one is twoend-access “analysis,” which includes Brillouin optical time[2,3], frequency- [4], and correlation-domain [5,6] analysis(BOTDA, BOFDA, and BOCDA); the other is single-endaccess “reflectometry,” which includes Brillouin optical time[7], frequency- [8], and correlation-domain [9] reflectometry(BOTDR, BOFDR, and BOCDR). Combination of someof these configurations has also been developed [10,11].
Different configurations have different advantages and disadvantages, and here we focus on BOCDR [9], which has advantages such as operation by single-end light injection, relatively
high spatial resolution, random accessibility, and high cost efficiency. One of the major disadvantages of a standard BOCDR
configuration is its long measurement time caused by frequency
sweeping required to obtain a Brillouin gain spectrum (BGS) at
each measurement position. The sampling rate of the standard
configuration was limited to 19 Hz when a silica single-mode
fiber (SMF) was used as a fiber under test (FUT) [12]. To enhance the flexibility of the FUT, some researchers [13–16] havemade attempts to use polymer optical fibers (POFs) for
Brillouin sensing. To date, a 0.1 m heated section of a 1.3 m
POF has been detected using a standard BOCDR configuration, but the sampling rate was limited to 3.3 Hz becauseof the low signal-to-noise ratio (SNR) [16].
0146-9592/19/082097-04 Journal © 2019 Optical Society of America

To boost the sampling rate of BOCDR and demonstratereal-time distributed sensing, we recently developed afrequency-sweeping-free configuration named phase-detected
BOCDR and achieved a sampling rate of >100 kHz [17].
However, the maximal measurable strain was limited to 0.2%.
Another frequency-sweeping-free configuration named slopeassisted BOCDR has also been developed, but its strain dynamic range was also limited to <0.3% [18]. To extend thestrain dynamic range to >2.0% (close to fracture strain of silica
fibers), still another BOCDR configuration with higher-speed
frequency sweeping has been developed, and its usefulness has
been proved using a silica SMF as an FUT [19].
In this higher-speed configuration, the noise floor was suppressed by acquiring the spectral difference between with andwithout pump light. When the FUT was a silica SMF, the electrical noise was then mitigated, and distributed measurement
was correctly performed [19]. However, when the FUT was a
POF, the Fresnel reflection at the SMF-to-POF boundary was
so strong that, even though the electrical noise was mitigated as
expected, the final signal involved not only the beat signal of
Brillouin-scattered light and reference light (effective for distributed measurement) but also the beat signal of Brillouinscattered light and the Fresnel-reflected light (not effective).
As the latter beat signal is sometimes extremely strong [20],it was difficult to directly employ this higher-speed configuration to perform POF-based distributed sensing.
In this study, we develop a new noise suppression techniqueto appropriately perform distributed sensing based on higherspeed BOCDR even when the FUT is a POF. By acquiring
the spectral difference between with and without reference light(not pump light), we selectively observe the beat signal of
Brillouin-scattered light and reference light, removing the influence of the beat signal of Brillouin-scattered light andFresnel-reflected light. We experimentally show the effectiveness of this technique and demonstrate POF-based distributed
temperature sensing and dynamic strain sensing.
BOCDR is a fiber-optic distributed Brillouin sensing technique based on synthesis of optical coherence functions[9,21,22]. By sinusoidally modulating the driving current of
the laser, its output frequency is also modulated, generatinga so-called correlation peak (corresponding to a measurement
position) in an FUT. By sweeping the modulation frequency,

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Vol. 44, No. 8 / 15 April 2019 / Optics Letters

Letterthe location of the correlation peak can be scanned along the
FUT, enabling distributed measurement. The spatial resolution, Δz, and the measurement range, d m , are given by [9,21]
Δz 

cΔvB
,
2πnf m Δf

(1)

c
,
2nf m

(2)

dm 

where c is the velocity of light in a vacuum, ΔνB is the Brillouinbandwidth, n is the refractive index of the fiber core, f m is themodulation frequency, and Δf is the modulation amplitude of
the optical frequency. The upper limit of Δf is known to behalf the Brillouin frequency shift (BFS) [9,21]. In a standard
BOCDR configuration, the sampling rate was limited to 19 Hz
by the frequency-sweeping speed of an electrical spectrum analyser (ESA). To enhance the sampling rate while maintaining
the strain dynamic range, we have developed a new configuration of BOCDR with higher-speed frequency sweeping [19].
In the higher-speed BOCDR, the BGS is converted fromthe frequency domain to the time domain for higher-speed
processing. First, the Brillouin-scattered signal is heterodynedwith the reference light and experiences optical-to-electricalconversion using a photo detector (PD). The Brillouin signal
is then mixed with the output of a microwave frequency
sweeper (MFS), the frequency of which is repeatedly swept
for several hundred megahertz to scan the BGS in the frequency
domain. Subsequently, the power at a fixed frequency component of the mixed signal is output from the ESA using a zerospan mode, and the BGS is repeatedly reproduced in the time
domain. The BGS acquisition must be slower than the calculation time required for the BFS derivation from the BGS,
which limits the maximal sampling rate of this configuration.
Finally, the BGS signal in the time domain is input to a computer via a sound board and monitored using a virtual oscilloscope triggered at the repetition frequency of the MFS.
Conventionally, to suppress the noise floor, we acquired thespectral difference between with and without the pump light.
When the pump light is injected into the FUT, the heterodyned signal at the PD is composed of three factors: (i) thebeat signal of the Brillouin-scattered light and reference light,(ii) the beat signal of the Brillouin-scattered light and Fresnelreflected light, and (iii) the electrical noise. The spectral shapeof (i) varies according to the location of the correlation peak andis effective for distributed measurement. In contrast, (ii) does
not vary according to the location of the correlation peak andserves as noise in distributed measurement. (iii) is the electricalnoise mainly caused by the output-power fluctuations of the
MFS accompanied by frequency sweeping. In the meantime,
when the pump light is not injected in the FUT, only (iii)
is included in the observed spectrum. Therefore, by calculating
the spectral difference, the final spectrum includes (i) and (ii).
When the FUT is a silica SMF, the Fresnel-reflected light isextremely weak (note that, in previous experiments, the distal
end of the FUT was cut with an angle [19]) and thus (ii) is
negligibly small. Therefore, by using the spectral difference
between with and without the pump light, silica-SMF-based
distributed measurements have been successfully demonstrated
in the higher-speed BOCDR configuration [19].
However, when the FUT is a POF, the Fresnel reflection
at the boundary of a silica SMF (second port of an opticalcirculator) and the POF is so strong that (ii) is not negligible(this can be interpreted as a pseudo-correlation peak [23],which is universally located at the proximal POF end).
Considering that the pump power is generally high (e.g.,>20 dBm), (ii) is sometimes larger than (i). Therefore, the final
spectrum includes both (i), which is effective, and (ii), which is
not effective, leading to difficulty in performing POF-based
distributed sensing with this conventional technique. Note that
insertion of a tapered fiber between the SMF and the POF cannot completely suppress the Fresnel reflection.
To tackle this problem, we develop a new noise suppression
technique for POF-based distributed sensing. Here we acquire
the spectral difference with and without the reference light, notthe pump light. When the reference light is present, the final
spectrum includes (i), (ii), and (iii); while the reference light is
not present, it includes (ii) and (iii). Therefore, their difference
gives only (i), which directly contributes to distributed sensing.
Note that this technique is applicable not only to POFs but also
standard silica SMFs.
Figure 1 schematically shows the experimental setup of
POF-based BOCDR for verifying the new noise suppression
technique. The basic structure of the setup is the same as previously reported [9,21]. A 3.0 m perfluorinated graded-index
POF [24] with a core diameter of 50 μm was employed as the
FUT. It had a propagation loss of ∼0.25 dB∕m at 1550 nm, a
BFS of 2.85 GHz at 1550 nm [25], and its strain and temperature dependence coefficients were −121.8 MHz∕% [26] and−3.2 MHz∕°C [27], respectively. The SMF of the second portof the optical circulator was 0.8 m in length and was connectedto the POF by butt-coupling [25]. The other end of the POF
was cut with an 8° angle. The pump light was amplified using
an erbium-doped fiber amplifier (EDFA), and its power injected into the FUT was 22 dBm. The power of the reference
light was 4 dBm (before being injected into the final optical
coupler). Variable optical attenuators (VOAs) were newly inserted in the pump and the reference path following EDFAs
so that they were used to switch on/off the pump light andthe reference light for the noise suppression.
First, we evaluated the conventional noise suppression technique by acquiring the spectral difference between with andwithout the pump light. The correlation peak was located at
the midpoint of the FUT ( f m  13.552 MHz), and thenominal spatial resolution was set to approximately 0.65 m(Δf  0.4 GHz). The spectra measured with and without
the pump light are shown in Figs. 2 and 3(a), respectively.
Figure 2 exhibited a clear BGS peak, but the spectrum was distorted by the electrical noise. Figure 3(a) shows the electricalFig. 1. Experimental setup including a polymer optical fiber (POF)
for verifying the new noise suppression technique. EDFA, erbiumdoped fiber amplifier; ESA, electrical spectrum analyzer; FG, function
generator; PC, polarization controller; PD, photo detector; VOA,
variable optical attenuator.

LetterVol. 44, No. 8 / 15 April 2019 / Optics Letters2099

Fig. 2. Measured raw BGS.

noise spectrum, which did not include a clear BGS peak. The
spectral difference of these two, obtained using a calculating
function of the ESA, is shown as a solid curve in Fig. 3(b) [notethat the three spectra were not simultaneously obtained andthat the direct subtraction of Fig. 3(a) from Fig. 2 slightly differs from Fig. 3(b)]. As the electrical noise was suppressed, thespectral shape seemed to be ideal, but this spectrum cannot bedirectly used for distributed measurement. To clarify this issue,
a 0.6% strain was applied to a 1.0 m section around the midpoint of the FUT where the correlation peak was located. The
spectrum then shifted to lower frequency, as indicated by a dotted curve in Fig. 3(b). However, the downshift of the peak frequency was ∼33 MHz (calculated after Lorentzian fitting),
which was lower than half the theoretical value (∼73 MHz;calculated using the aforementioned strain dependence coefficient of the BFS in the POF). This is because, as strain wasapplied, part of the spectrum [corresponding to (i)] shifted tolower frequency, but the remaining part of the spectrum [corresponding to (ii)] did not shift. Consequently, the overlap ofthese two spectra yielded a slightly shifted spectrum. Thus, it isdifficult to use the conventional noise suppression technique toderive the correct value of the BFS change caused by strain.
Subsequently, under the same measurement conditions, weevaluated the new noise suppression technique by acquiring thespectral difference between with and without the reference
light. The spectrum measured with the reference light is basically the same as Fig. 2. The spectrum measured without thereference light is shown in Fig. 4(a), which included not only
the electrical noise spectrum but also a clear BGS peak. This
peak corresponds to (ii) and does not contribute to distributed
measurement. The spectral difference of these two is shown asa solid curve in Fig. 4(b). It is natural that the height ofthe spectrum is lower than that in Fig. 3(b), because thewhole spectrum is effective for distributed sensing in Fig. 4(b).
To prove this, a 0.6% strain was applied in the same manner,

Fig. 3. Spectra measured with the conventional noise suppression
technique. (a) Noise floor. (b) Noise-suppressed BGS. The dottedcurve is the spectrum when a 0.6% strain was applied.

Fig. 4. Spectra measured with the new noise suppression technique.
(a) Noise floor. (b) Noise-suppressed BGS. The dotted curve is the
spectrum when a 0.6% strain was applied.

and the shifted spectrum is shown as a dotted curve in Fig. 4(b).
Compared to Fig. 3(b), as strain was applied, the whole spectrum shifted to lower frequency, and the shift of the peak frequency was ∼73 MHz (calculated after Lorentzian fitting),
which was in good agreement with the theoretical value.
Thus, the new noise suppression technique was proven to givea correct value of the BFS change caused by applied strain. This
indicates that the new technique is inherently superior to theconventional one for distributed sensing. Note that, if the BGSof the whole length of the POF is to be analyzed, the conventional technique is more suitable considering its seemingly
higher SNR.
Finally, we performed two demonstrations of POF-based
higher-speed BOCDR using the new noise suppression technique. The setup was very similar to that which was previously
reported [19]. The length of the POF was 3.0 m. The modulation frequency f m was swept from 13.515 to 13.590 MHz(corresponding to the measurement range of ∼8.2 m ), andthe modulation amplitude Δf was ∼0.4 GHz (corresponding
to the nominal spatial resolution of ∼0.65 m ). The sampling
rate was 42 Hz, which cannot be achieved using a standard
BOCDR configuration. The room temperature was 25°C.
In the first demonstration, a 1.0-m-long section of the POF
was heated to 55°C [Fig. 5(a)]. The measured BGS (normalized) and BFS distributions are shown in Figs. 5(b) and 5(c),Fig. 5. Demonstration of distributed temperature sensing.
(a) Structure of the FUT. Measured distributions of (b) BGS and(c) BFS.

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Vol. 44, No. 8 / 15 April 2019 / Optics Letters

Letterthe FUT is a silica SMF because non-zero optical reflection iscaused also at an SMF-to-SMF boundary. Thus, we anticipate
that the new technique will be one of the most fundamental
tools for BOCDR-based distributed sensing, especially whena POF is employed as an FUT, in the near future.
Funding. Japan Society for the Promotion of Science(JSPS) (17H04930, 17J07226); Fujikura Foundation.
REFERENCES

Fig. 6. Demonstration of dynamic strain sensing. (a) Structure of
the FUT. Measured temporal variations of (b) BGS and (c) BFS.

respectively. The number of the measured points were 84 andno averaging was performed, resulting in the measurement timeof 2 s. The error of the BFS measurement (calculated as astandard deviation of the BFS values along the non-strained
sections) was approximately 4 MHz in this demonstration.
The frequency downshift at the heated section was roughly
100 MHz, which well agreed with the theoretical value(∼96 MHz). Thus, distributed measurement was correctly performed. The BFS fluctuations can be mitigated by averaging at
the cost of the measurement time. In the second demonstration, by setting the modulation frequency f m to 13.552 MHz,the correlation peak was located at the midpoint of the POF.
A 0.3% static strain was applied in advance to a 1.0-m-longsection around it, and then a dynamic strain of 0.3% wassinusoidally applied at 2.0 Hz to the pre-strained section[Fig. 6(a)]. The measured temporal variations of the BGS (normalized) and the BFS are shown in Figs. 6(b) and 6(c), respectively. The amplitude of the BFS variation was roughly
35 MHz, which agreed with the theoretical value (∼37 MHz).
Thus, a dynamic strain at 2.0 Hz was correctly measured with
the new noise suppression technique. The sampling rate can beenhanced to several hundreds of hertz simply by replacing the
MFS with a voltage-controlled oscillator (or another MFS witha higher frequency-sweeping rate).
In conclusion, we developed a new noise suppression technique for POF-based distributed strain/temperature sensing
based on higher-speed BOCDR. This technique is not suitable
for non-distributed measurement; namely, the conventional
technique is more suitable for the BGS analysis of the whole
length of the POF because of its higher SNR. However, the
new technique has an effect of SNR enhancement even when

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