LETTER

Applied Physics Express 13, 052001 (2020)
https://doi.org/10.35848/1882-0786/ab827b

Effect of laser temperature control on Brillouin optical correlation-domainreflectometry
Yosuke Mizuno1,2* , Naoki Motoishi2 , Kohei Noda2 , Heeyoung Lee3 , and Kentaro Nakamura2
1

Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
3
College of Engineering, Shibaura Institute of Technology, Tokyo 135-8548, Japan2*

E-mail: mizuno-yosuke-rg@ynu.ac.jp

Received February 9, 2020; revised March 12, 2020; accepted March 22, 2020; published online April 3, 2020
We show that 1 °C drift of laser temperature yields a strain error of ∼8 με in Brillouin optical correlation-domain reflectometry. Under theassumption that the strain measurement accuracy is +/–20 με (severe requirement), +/–2.5 °C drift of the laser temperature can be permitted forproper system operation. We prove this prediction by performing distributed strain measurements at different laser temperatures. Our results
indicate that, if we allow a strain measurement accuracy of approximately +/–100 με (or more), we can even remove the thermal control of thelaser, leading to cost reduction and downsizing of the system. © 2020 The Japan Society of Applied Physics

Distributed strain and temperature sensors are key
tools for monitoring the conditions of various civil
infrastructures. In particular, distributed sensing
techniques based on Brillouin scattering in optical ﬁbers
have been the focus of attention because of their ability todetect sub-centimeter strained or heated sections along long
optical ﬁbers.1) Based on the conﬁgurations of light injection
to sensing ﬁbers, Brillouin sensors can be divided into twoend-access “analysis” and one-end-access “reﬂectometry.”
The former includes Brillouin optical time-, frequency-, andcorrelation-domain analysis (BOTDA,2–5) BOFDA,6–8) and
BOCDA),9–13) whereas the latter includes Brillouin optical
time-, frequency-, and correlation-domain reﬂectometry(BOTDR,14–16) BOFDR,17) and BOCDR).18–25) In general,
analysis systems exhibit higher signal-to-noise ratios (SNRs)
due to the use of stimulated Brillouin scattering. However,despite their lower SNRs, reﬂectometers based on spontaneous Brillouin scattering have higher degrees of freedom in
embedding the sensing ﬁbers into structures; in addition,measurements can be continued even when the sensing ﬁbers
are broken. Needless to say, each reﬂectometry has its own
merits and demerits, and here, we focus on BOCDR.
BOCDR was ﬁrst proposed in 2008 and has been
extensively studied since then.18) Its performance has beensigniﬁcantly improved;19–25) for instance, the trade-off relation between the spatial resolution and the measurement
range has been mitigated by temporal-gate scheme anddouble-modulation scheme.20,21) Plastic-ﬁber-based operation has also been demonstrated.22) In addition, one of themost recent achievements is real-time operation of the systemby phase-detected BOCDR and slope-assisted BOCDR.23,24)
However, implementation cost of BOCDR is not yet sufﬁciently low, though some trials have been made.25) Thus, it isextremely important to reduce the cost of the system forpractical use in future.
In most of the laser-based sensing systems with highaccuracy, the optical wavelength is stabilized by precise
thermal control of the laser, which is often performed using
thermoelectric cooling (TEC) based on the Peltier effect.26,27)
If the thermal control is absent in the laser, the output
wavelength signiﬁcantly ﬂuctuates, leading to measurement
error. As Brillouin frequency shift (BFS) is in inverse
proportion to wavelength,1) Brillouin sensors are also subject
to error. However, as all the conﬁgurations of BOCDR arebased on interferometry,18–25) the inﬂuence of the wavelength
ﬂuctuations on the measurement accuracy can be suppressed
to a certain range. If thermal control of the laser turns out
unnecessary (at least for some applications that do not require
high accuracy), the implementation cost of BOCDR will be
reduced.
In this work, ﬁrst, we investigate the thermal behavior of asemiconductor laser and its inﬂuence on the BFS measured
by heterodyne detection. Then, we discuss the relation
between the accuracy of the thermal control of the laser
and the measurement accuracy of strain and temperature.
Finally, we experimentally show that, if ambient temperatureof the sensing ﬁber is constant and a moderate accuracy is
acceptable, a TEC-free laser can be used for distributed strain
measurement based on BOCDR for cost reduction.
BOCDR is a distributed sensing technique with operation
by single-end light injection, relatively high spatial resolution, random accessibility to sensing points, and a highsampling rate.18–25) It operates based on the synthesis of an
optical coherence function.28) The optical output from a laser
is divided into pump and reference lights. The pump light isinjected into a sensing ﬁber, and the Stokes light is directed
into a photodetector. The reference light is used as an optical
local oscillator. The electrical beat signal of the two lightbeams is monitored using an electrical spectrum analyzer(ESA). Because of the frequency difference of ∼11 GHz
between the Stokes and reference lights,1) this conﬁguration
is called self-heterodyne.
To resolve the position along the sensing ﬁber, the opticalfrequency of the laser output is often modulated in asinusoidal waveform by directly modulating the injection
current of the laser. From the viewpoint of temporal averaging, the coherence function is synthesized into a series of
periodical peaks, whose interval is inversely proportional to
the frequency of the sinusoidal modulation. We control themodulation frequency to generate only one correlation peakwithin the range of the sensing ﬁber, so that only the
Brillouin-scattered light at that peak has high correlation
with the reference light, giving high heterodyne output. The
peak frequency observed using the ESA gives the BFS
caused at the speciﬁc position. By sweeping the modulation
frequency, the position of the correlation peak is scannedalong the sensing ﬁber, and thus the distribution of theBrillouin gain spectrum (BGS) or the BFS can be obtained.

052001-1© 2020 The Japan Society of Applied Physics

Appl. Phys. Express 13, 052001 (2020)

Y. Mizuno et al.

(a)

(b)

(c)

Fig. 1. (Color online) (a) Output frequency versus laser temperature at three driving currents. The solid lines are linear ﬁts. (b) Temporal variations of thelaser temperatures at three driving currents. (c) Saturated laser temperature versus driving current. The dotted curve is a quadratic ﬁt.

The spatial resolution and the measurement range of standard
BOCDR are expressed as Eqs. (15) and (16) in Ref. 19,
respectively.
In general, if TEC is not used in a laser, its output
wavelength ﬂuctuates. Therefore, if a TEC-free laser is
employed in BOCDR, a certain amount of error in strain
and temperature measurements is anticipated, because the
BFS is inversely proportional to wavelength.1) It is thus ofconsiderable interest to quantitatively evaluate the inﬂuenceof the wavelength ﬂuctuations on the measurement accuracyof BOCDR and to discuss under what conditions such a
TEC-free laser can be used in BOCDR for cost reduction.
As preliminary experiments, we investigated the basic
characteristics of a typical commercially-available semiconductor laser (NX8563LB, NEC; rated maximum
temperature = 65 °C; spectral linewidth =∼1 MHz) connected to a temperature controller (LDC-3900, ILX
Lightwave; displayed temperature stability <0.01 °C). First,
when TEC was active, the output frequency was measured asa function of a laser temperature at three different driving
currents [Fig. 1(a)]. Note that BOCDR has been driven
around these driving currents. The frequency decreased withincreasing temperature and current. Next, at three different
driving currents, the temporal variations of the laser temperature were measured [Fig. 1(b); at time t = 0, TEC was
switched off; the initial temperature was set to room
temperature (26.1 °C)]. Regardless of the current, the laser
temperature increased with time and became almost constantat t = ∼60 s. Then, the saturated laser temperature (measured
at t = 120 s) was plotted as a function of the driving current[Fig. 1(c)]. The saturated temperature increased with increasing driving current, following a quadratic ﬁt reﬂecting
Joule’s ﬁrst law. Seeing that the saturated temperature even at
100 mA is much lower than the rated maximum temperature,
it appears that the laser is not destroyed by excessive heat
under normal operation of BOCDR even when TEC is not
employed.
The experimental setup of BOCDR used in this work is
depicted in Fig. 2. The pump power was 20 dBm, and thesensing ﬁber was a 3.6 m long silica single-mode ﬁber. A
2.0 km delay line and a polarization scrambler were inserted in
the reference path. For distributed measurements, the modulation frequency was 1.1730–1.1744 MHz and the modulation

Fig. 2. (Color online) Experimental setup of BOCDR. EDFA: erbiumdoped ﬁber ampliﬁer.

amplitude was 2.84 GHz, corresponding to the spatial resolution of 0.29 m and the measurement range of 87.6 m. The BGS
observed at a single measurement position was averaged 10
times on the ESA. The BFS was simply calculated as thefrequency at which the BGS became maximal. The video
bandwidth and the resolution bandwidth of the ESA were
1 kHz and 3 MHz, respectively. The total measurement timewas 10 s. The room temperature was 25.9 °C.
First, we experimentally investigated the dependence of thelaser temperature on the BFS. Here, to observe the light
Brillouin-scattered in the whole length of the sensing ﬁber toenhance the SNR, the optical frequency was not modulated.
The normalized BGS dependence on the laser temperature isshown in Fig. 3(a). With increasing laser temperature, the BGS
shifted to lower frequency. The BFS is then plotted as afunction of the laser temperature [Fig. 3(b)]. The BFS linearlydecreased with increasing laser temperature with a coefﬁcient of–0.41 MHz °C−1. Considering the strain and temperature dependence coefﬁcients of the BFS at 1.55 μm are +480 MHz/%and +1.18 MHz °C−1,29,30) respectively, we can estimate that
1 °C drift of the laser temperature corresponds to strain andtemperature measurement errors of 0.0008% (=8 με) and
0.34 °C. Let us suppose that the ambient temperature of thesensing ﬁber is constant. Then, even if we assume that the strain
measurement accuracy of BOCDR is +/–20 με (applicable todetection of local yielding or cracking in structure; note that theactual accuracy, which depends on various experimental
conditions such as spatial resolution, pump power, signal
processing (averaging, ﬁtting, etc), and many others, is muchworse than this), +/–2.5 °C drift of the laser temperature ispermitted in the BOCDR operation.

052001-2© 2020 The Japan Society of Applied Physics

Appl. Phys. Express 13, 052001 (2020)

Y. Mizuno et al.

(a)

(b)

Fig. 3. (Color online) (a) Normalized BGS observed at different laser temperatures. (b) BFS dependence on the laser temperature. The dotted curve is a linear
ﬁt.

(a)

(b)

Fig. 4. (Color online) Normalized BGS distributions at laser temperatures of (a) 20 °C and (b) 25 °C; 0.4% strain was applied.

Subsequently, to verify this result, we performed distributed strain measurements when the laser temperature was
20 °C and 25 °C (imitating a drift of +/–2.5 °C). Strains of
0.4% and 0.6% were applied to a 1.0 m long section (1.0–
2.0 m from the proximal end) of the 3.6 m sensing ﬁber. The
normalized BGS distributions measured when 0.4% strain
was applied at the laser temperatures of 20 °C and 25 °C areshown in Figs. 4(a) and 4(b), respectively. In both results, thelocal shift of the BGS at the strained section was clearly
detected. Finally, Figs. 5(a) and 5(b) show the comparisonsof the BFS distributions at the laser temperatures of 20 °C
and 25 °C, measured when 0.4% and 0.6% strains wereapplied, respectively. It is clear that, regardless of the amountof strain, the discrepancies caused by the two different laser
temperatures (in average, –0.10 MHz at 0.4% and+0.48 MHz at 0.6%) were negligibly small compared tothe inherent ﬂuctuations of the BFS (the standard deviations
from the theoretical trends at 20 °C were +/–11.3 MHz at
0.4% and +/–11.8 MHz at 0.6%). This result indicates that,for proper operation of BOCDR, the laser temperature does
not need to be precisely controlled (e.g. < +/–0.01 °C) andcan be roughly adjusted with an accuracy of +/–2.5 °C.

Considering the result of the preliminary experiments[Fig. 1(c)], if we allow the strain measurement accuracy ofapproximately +/–100 με or more, even removal of thethermal control of the laser will be feasible.
In conclusion, the thermal behavior of a commercial laser
and its inﬂuence on the BFS were investigated, and 1 °C
drift of the laser temperature was estimated to correspond tostrain measurement error of 8 με. This means that, assumingthat the strain measurement accuracy of BOCDR is +/–20
με (severe requirement), +/–2.5 °C drift of the laser
temperature is permitted in the BOCDR operation. Then,we experimentally proved this prediction by performing
distributed strain measurements at the laser temperatures of
20 °C and 25 °C. Our results indicate that, allowing thestrain measurement accuracy of approximately +/–100 με(or more), we can even remove the thermal control of thelaser. We believe that employment of a TEC-free laser will
enable cost reduction and downsizing of the BOCDR
system (note that our conclusion will hold true not onlyfor BOCDR but also for all types of Brillouin analysis/
reﬂectometry based on heterodyne detection), promoting its
practical use in the future.

052001-3© 2020 The Japan Society of Applied Physics

Appl. Phys. Express 13, 052001 (2020)

Y. Mizuno et al.

(a)

(b)

Fig. 5. (Color online) BFS distributions at laser temperatures of 20 °C and 25 °C; strains of (a) 0.4% and (b) 0.6% were applied.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 17H04930 and 17J07226.
ORCID iDs Yosuke Mizuno
https://orcid.org/0000-0002-33624720 Naoki Motoishi
https://orcid.org/0000-0001-6830-2265 Kohei Noda
https://orcid.org/0000-0003-4976-9657 Heeyoung Lee
https://orcid.org/
0000-0003-3179-0386 Kentaro Nakamura https://orcid.org/0000-0003-28994484

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