BRIEF NOTEJapanese Journal of Applied Physics 57, 018001 (2018)https://doi.org/10.7567/JJAP.57.018001

Long-term stability enhancement of Brillouin measurement in polymer optical ﬁbers
using amorphous ﬂuoropolymer
Natsuki Matsutani, Heeyoung Lee, Yosuke Mizuno*, and Kentaro Nakamura
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received October 3, 2017; accepted November 21, 2017; published online December 13, 2017
For Brillouin-sensing applications, we develop a method for mitigating the Fresnel reﬂection at the perﬂuorinated-polymer-optical-ﬁber ends bycovering them with an amorphous ﬂuoropolymer (CYTOP, ﬁber core material) dissolved in a volatile solvent. Unlike the conventional method using
water, even after solvent evaporation, the CYTOP layer remains, resulting in long-term Fresnel reduction. In addition, the high viscosity of the
CYTOP solution is a practical advantage. The effectiveness of this method is experimentally proved by Brillouin measurement.
© 2018 The Japan Society of Applied Physics

Brillouin scattering in optical ﬁbers has drawn signiﬁcantattention in the sensing community as it can be exploited
to measure the strain and temperature distributions.1–18) In
2010, Brillouin scattering in polymer optical ﬁbers (POFs)
was observed for the ﬁrst time;19) since then, several researchgroups have been developing POF-based Brillouin sensors20–23) with large-strain measurability,24) greater ease ofhandling, and the so-called memory functions of strain25)
and temperature.26) Unlike the other groups studying POFbased Brillouin sensing with two-end-access conﬁgurations(Brillouin optical time-domain analysis20) and Brillouin
optical frequency-domain analysis21)), we have been in theprocess of developing POF-based single-end-access distributed sensing systems, using a unique technique called Brillouin
optical correlation-domain reﬂectometry (BOCDR).17,18) In
addition to the single-end accessibility, the advantages of
BOCDR include high spatial resolution, random accessibility, and cost eﬃciency. Recently, several high-speed BOCDR
conﬁgurations were implemented27–29) and successfully
applied to POF-based sensing.23)
Various types of POFs have been developed in addition.30–33) For instance, the most common POF composed ofpolymethyl methacrylate is optimized for visible light transmission; however, its propagation loss at telecom wavelength
is considerably high (100 dB=m), rendering it unsuitablefor Brillouin applications.30) The only type in which Brillouinscattering has been experimentally observed is the perﬂuorinated graded-index (PFGI-) POF,19,31) with relatively low
propagation loss, even at telecom wavelength.
In BOCDR using PFGI-POF (core refractive index, n ’
1:36) as the ﬁber under test (FUT), when the Fresnel reﬂections at both ends of the PFGI-POF, i.e., the boundary witha silica single-mode ﬁber (SMF; n ’ 1:46) and that withair (n ’ 1:00), are signiﬁcant, the Brillouin gain spectrum(BGS) is overlapped with interference noise in general.34) It
is temporally unstable, and the measurement accuracy of
the Brillouin frequency shift (BFS), which provides theinformation on strain and temperature, deteriorates for highspeed conﬁgurations in particular.27–29)
When the FUT is a silica SMF, in addition to the weakFresnel reﬂection at the proximal end, the Fresnel reﬂection
at the distal open end of the FUT can also be suﬃciently
reduced by applying considerable bending loss near the end
or by cutting the end with an angle. However, when the FUT
is PFGI-POF, it is diﬃcult to apply suﬃcient bending loss
near the open end of the FUT, because PFGI-POF is amultimode ﬁber with a core diameter of at least 50 µm and isnearly insusceptible to bending loss.31) To employ an angledcut end is one method of suppressing the Fresnel reﬂection,
but it is not constantly eﬀective for a multimode ﬁber.
Therefore, the Fresnel reﬂection at the open end of the PFGIPOF has conventionally been suppressed by immersing theend in water, which has a refractive index (n ’ 1:33) close
to that of the PFGI-POF core. However, when the tip of the
PFGI-POF is removed from water, a high-power light is radiated at the open end and the water evaporates rapidly; thus, it
is not easy to use this method for long-term measurement.
In this work, we develop a new method for mitigating
the Fresnel reﬂection at the open end of the PFGI-POF bycovering the end with an amorphous ﬂuoropolymer (cyclic
transparent optical polymer, Asahi Glass CYTOP®), whichconstitutes the same material as that of the PFGI-POF
core, dissolved in a volatile solvent. Even after the solvent
evaporates, the CYTOP layer remains attached to the open
end, resulting in long-term Fresnel reduction. In addition, as
the core and CYTOP material are the same, the refractive
index matching can be more ideal than in the case of water.
We experimentally demonstrate the eﬀectiveness of thismethod using Brillouin measurement.
A 2.6-m-long PFGI-POF was employed as the FUT. It
consists of a core (50 µm diameter) with cladding (100 µm
diameter) and overcladding (750 µm diameter). The core andcladding layers are composed of doped and undoped CYTOP,
respectively. The refractive index at the center of the core is1.356 and that of the cladding layer is 1.342;35) these values
are not strongly dependent on the optical wavelength.36) The
propagation loss at 1.55 µm is ∼0.25 dB=m.
The experimental setup for measuring the temporal variations in the Fresnel-reﬂected light power, before and after
covering the open end of the PFGI-POF with water and
CYTOP solution, respectively, is depicted in Fig. 1. The
output of a semiconductor laser at 1.55 µm was injected into
one end of the PFGI-POF via an optical circulator (OC) composed of silica SMFs. The incident power was maintained
at 23.5 dBm by using an erbium-doped ﬁber ampliﬁer. The
polarization state was scrambled to suppress the polarizationdependent signal ﬂuctuations. The SMF, which is the second
port of the OC, was butt-coupled to the PFGI-POF;19) thispart was ﬁxed during all measurements, which resulted in aconstant Fresnel reﬂection. Consequently, the power change
was attributed to the Fresnel reﬂection at the other end of the
PFGI-POF. With a photo detector (PD), the reﬂected light

018001-1© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 018001 (2018)

N. Matsutani et al.

(a)

(b)

Fig. 1. (Color online) Experimental setup. EDFA: erbium-doped ﬁber
ampliﬁer; PFGI-POF: perﬂuorinated graded-index polymer optical ﬁber;
PSCR: polarization scrambler.

was converted into an electrical signal, which was observed
using an oscilloscope in real-time. The tip of the PFGI-POF,
which was perpendicularly cut and polished, was entirely
immersed in water or CYTOP solution [CTX-809SP2; concentration: 9%; solvent: CT-SOLV180 (n ’ 1:29)] and was
subsequently removed. Note that the simply calculated reﬂectivities at the PFGI-POF end covered with air, water, andCYTOP solution (before solvent evaporation) are approximately 2.2, 0.006, and 0.03%, respectively. Moreover, BGS
measurement was performed using the setup shown in Fig. 1
by replacing the PD with an electrical spectrum analyzer.
In this conﬁguration, without using an explicit reference
light path, the Fresnel-reﬂected light operates as the reference
light for BGS detection.37)
The temporal variations in the Fresnel-reﬂected light
power, before immersing the open end of the PFGI-POF inwater, while immersed in water, and shortly (1 min) after
removing from water are depicted in Figs. 2(a), 2(b), and2(c), respectively. The vertical axis was normalized to makethe average power in Fig. 2(a) (−54.2 dBm), 0 dB. In thisﬁgure, the Fresnel reﬂection at the open end was temporally
unstable with a standard deviation (SD) of ±1.67 dB. Then,
when the open end was immersed in water, the temporalsignal ﬂuctuations were suppressed to an SD of ±0.94 dB
owing to refractive index matching. However, 1 min after
removing the PFGI-POF from water, the signal ﬂuctuations
increased to almost the same value as those before immersion, with an SD of ±1.29 dB. This is because the remaining
water on the core evaporated rapidly owing to high-power
light radiation and the index matching was no longer valid.
Thus, covering the PFGI-POF end with water was demonstrated to be unsuitable for reducing Fresnel reﬂection.
Subsequently, similar measurements were performed using
CYTOP solution. The temporal variations in the Fresnelreﬂected light power before immersing the open end of the
PFGI-POF in the CYTOP solution, while submerged in thesolution, and 1 min and 24 h after removal from the solution
are shown in Figs. 3(a), 3(b), 3(c), and 3(d), respectively.
In Fig. 3(a), the signal ﬂuctuations are relatively large withan SD of ±1.64 dB, comparable to that in Fig. 2(a) as themeasurement condition is similar. When the open end was
immersed in the CYTOP solution for refractive index matching, the signal ﬂuctuations were suppressed to an SD of±0.52 dB, as in water. Then, 1 min after removing the PFGI-(c)
Fig. 2. (Color online) Temporal variations in the Fresnel-reﬂected light
power (a) before immersing the open end of the PFGI-POF in water,(b) while immersed in water, and (c) 1 min after removal from water.
The SDs of the signal ﬂuctuations are indicated.

(a)

(b)

(c)

(d)

Fig. 3. (Color online) Temporal variations in the Fresnel-reﬂected light
power (a) before immersing the open end of the PFGI-POF in CYTOP
solution, (b) while immersed, and (c) 1 min and (d) 24 h after removal fromthe CYTOP solution. The SDs of the signal ﬂuctuations are indicated.

POF end from the CYTOP solution, the suppression of thesignal ﬂuctuation continued to an SD of ±0.58 dB, due to thehigh viscosity of the solution. Moreover, 24 h after removalfrom the CYTOP solution, the signal ﬂuctuations were018001-2© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 018001 (2018)

N. Matsutani et al.
Association, the ESPEC Foundation for Global Environment Research andTechnology, the Association for Disaster Prevention Research, the Fujikura
Foundation, and the Japan Association for Chemical Innovation.

Fig. 4. (Color online) BGSs measured before immersing the PFGI-POF
end in CYTOP solution (blue) and 25 h after removal (orange).

suppressed further to an SD of ±0.33 dB. Unlike the case ofwater, after solvent evaporation, the CYTOP solidiﬁed andremained on the core. The enhanced suppression of the signal
ﬂuctuations was probably caused by the more ideal refractive
index matching between the PFGI-POF core (n ’ 1:36) andthe CYTOP (n ’ 1:35) (note that the refractive index of the
CYTOP solution is lower than ∼1.35, because the refractive
index of its solvent is ∼1.29). Thus, covering the PFGI-POF
end with high-viscosity CYTOP solution was demonstrated
to be suitable for long-term Fresnel reduction.
Finally, we measured the BGSs of the same PFGI-POF,
before immersing its open end in CYTOP solution and 25 hafter removal, as shown in Fig. 4. Before immersing, theBGS was overlapped with the interference noise caused by
the multiple Fresnel reﬂections at both the ends of the PFGIPOF. The peak interval of the noise was approximately 40
MHz, which corresponds well with the theoretical value calculated using the PFGI-POF length.34) However, it is evident
that the noise was reduced by CYTOP usage, resulting in an
improved long-term BGS measurement accuracy.
In conclusion, to improve the long-term measurement
accuracy of PFGI-POF-based single-end-access Brillouinsensors, we have developed a new Fresnel suppression
method using CYTOP dissolved in a volatile solvent.
Through our experiments, we are able to conﬁrm that Fresnel
reﬂection can be permanently mitigated by covering the open
end of the PFGI-POF with the CYTOP solution, even aftersolvent evaporation. Moreover, the high viscosity of CYTOP
is advantageous compared to the conventional water-based
method. If combined with angled-cut ends, this method
would enhance the measurement accuracy further. Thus,
we believe that this method can be adopted as a standard
technique for improving the performance of PFGI-POFbased Brillouin sensing systems.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 17H04930 and 17J07226, and by research grants from the Japan Gas

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018001-3© 2018 The Japan Society of Applied Physics