LETTERIEICE Electronics Express, Vol.11, No.10, 1–7

Dependence of Brillouinfrequency shift on temperaturein poly(pentaﬂuorostyrene)based polymer optical ﬁbers
estimated by acoustic
velocity measurement
Kazunari Minakawa1a), Kotaro Koike2,3, Neisei Hayashi1,
Yasuhiro Koike2, Yosuke Mizuno1, and Kentaro Nakamura1
1

Precision and Intelligence Laboratory, Tokyo Institute of Technology,

Yokohama 226–8503, Japan2

Keio Photonics Research Institute, Keio University, Kawasaki 212–0032, Japan

3

Polymer Research Institute, New York University,Brooklyn, New York 11201, U.S.A.
a) kminakawa@sonic.pi.titech.ac.jp

Abstract: Poly(pentaﬂuorostyrene) (PPFS), which can be easily synthesized and has a low optical loss window at 850 nm, is a promisingalternative for a costly perﬂuorinated polymer as a base material ofpolymer optical ﬁbers (POFs). To investigate the potential of a PPFSPOF as a Brillouin-based temperature sensing ﬁber, the Brillouinfrequency shift and its temperature dependence of PPFS were measured
using an ultrasonic pulse-echo technique. The temperature coeﬃcient,which determines the sensitivity of the temperature sensing, was approximately !7.1 MHz/K independently of the molecular weight andwas nearly identical to that in perﬂuorinated POFs.
Keywords: polymer optical ﬁber, Brillouin scattering, ﬁber-optic
temperature sensor, poly(pentaﬂuorostyrene)
Classiﬁcation: Fiber optics, Microwave photonics, Optical interconnection, Photonic signal processing, Photonic integration and systems
References© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
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1© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
Copyedited May 25, 2014

Introduction

Compared to conventional temperature sensors (thermocouples, thermistors,etc.), ﬁber-optic temperature sensors have many inherent advantages such as
immunity to electromagnetic interference noise and capability of (quasi-)distributed measurement. In particular, distributed temperature sensors basedon Brillouin scattering in optical ﬁbers have gained a great deal of attention
for the past several decades because of their truly distributed measurement
capability with high stability [1, 2, 3, 4, 5, 6, 7, 8]. In conventional ﬁber-optic
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IEICE Electronics Express, Vol.11, No.10, 1–7

Brillouin temperature sensors, their sensing heads comprise glass optical ﬁbers
such as silica single-mode ﬁbers (SMFs) [1, 2, 3, 4, 5, 6], tellurite glass ﬁbers[7], and photonic crystal ﬁbers [8], which are so fragile that they cannot
withstand strains of over several percent.
To extend the measurable strain region, our research eﬀorts have focusedon the sensor applications of Brillouin scattering in polymer optical ﬁbers(POFs) [9, 10, 11, 12, 13], which are in general suﬃciently ﬂexible to withstand large strains of up to tens of percent. We have studied Brillouinscattering in perﬂuorinated graded-index (PFGI-) POFs and reported theirfundamental properties at 1.55 µm [10, 11, 12, 13], which is a common telecommunication wavelength. The Brillouin frequency shift (BFS) in PFGIPOFs at room temperature was approximately 2.8 GHz (1=4 of that in silicaﬁbers) [10] and was strongly dependent on temperature with a proportionality
constant of !4.1 MHz/K [11]. The absolute value of the temperature dependence constant was 3.5 times as large as that in silica ﬁbers, and a potential application of PFGI-POFs for high-precision temperature sensing has
been discussed [11].
The PFGI-POFs were originally developed as a communication medium
and have been continuing to break records for the lowest propagation loss andhighest data transmission speed of all the POFs [14]. However, despite theirimpressive performance, the PFGI-POFs have not been widely utilized because of the prohibitive costs. The base material is a homopolymer made by
the cyclopolymerization of perﬂuoro(butenyl vinyl ether) prepared by a complex process [15], which makes it one of the most expensive synthetic polymers.
In this study, we investigate a partially ﬂuorinated polymer, poly(pentaﬂuorostyrene) (PPFS) as a possible low-cost alternative. Partially halogenated styrene monomers are easily prepared via dehydration of the corresponding alcohols, which are synthesized via Grignard reactions with acetaldehyde[16]. Because of the carbon-hydrogen bonds at the main chain, PPFS and its
POF absorb infrared light [17] and exhibit a considerably higher propagation
loss at 1.55 µm. However, PPFS-POFs have a low-loss window at 850 nm
instead [16], which is also a common optical transmission wavelength used in
Gigabit Ethernet and compatible with commercial light sources including
vertical cavity surface emitting lasers [18]. Unfortunately, at this moment,
Brillouin scattering in POFs at other than 1.32–1.55 µm cannot be directly
measured because some of the measurement devices such as optical ampliﬁers
and circulators are unavailable. Thus, we estimate the BFS and its temperature dependence in PPFS-POFs at 850 nm using an ultrasonic pulse-echotechnique [19, 20]. In addition, we study the inﬂuence of molecular weight ofthe base polymer on the Brillouin properties for the ﬁrst time.
2© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
Copyedited May 25, 2014

Principle

When propagating in an optical ﬁber, incident pump light interacts withacoustic phonons, generating backscattered Stokes light; this phenomenon is
known as Brillouin scattering. The central frequency of the Stokes light3

IEICE Electronics Express, Vol.11, No.10, 1–7spectrum shifts to lower frequency than the pump frequency by the amountcalled BFS vB , which is given by [21]vB ¼

2nva:
p

ð1Þ

where n is the core refractive index, va is the acoustic velocity in the ﬁber, andp is the pump wavelength. When the refractive index of a ﬁber core material
is known, the BFS at an arbitrary pump wavelength can be calculated bymeasuring the acoustic velocity in the ﬁber sample (as well as in the bulk
sample for POFs with relatively large core diameters [19]). The acoustic
velocity is further related to the Young’s modulus E, density ;, and Poisson’s
ratio < by [22, 23]sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ1
E:
ð2Þ
va ¼
ð1  2Þð1 þ Þ 
Since the Young’s modulus has a relatively strong temperature dependence[24], the acoustic velocity and the BFS shift toward higher or lower frequency
depending on the environmental temperature, which is the principle ofBrillouin-based temperature sensing. To develop high-precision temperature
sensors, ﬁbers with strong BFS dependences on temperature are desirable.
3© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
Copyedited May 25, 2014

Methods

3.1 Preparation of polymer samples
We prepared four PPFS samples with diﬀerent molecular weights. PFS
monomer and all other chemicals were purchased from SynQuest Laboratories
and Sigma Aldrich, respectively. The monomer was freshly distilled using aspinning band column before use, and the others were used as received. For
the polymerization, we used glass ampoules with 10 mm inner diameters. The
monomer mixtures, which contained di-tert-butyl peroxide as the free-radicalinitiator, were transferred into the glass ampoules and subjected to repeatedfreeze–pump–thaw cycles. The ampoules were then ﬂame-sealed in vacuo.
The polymerizations were carried out in an oil bath at 120 °C for 90 h. The
glass ampoules were carefully broken, and the polymer rods were obtained.
The molecular weights and distributions of the polymers were measured
using gel permeation chromatography (GPC). All GPC analyses of thepolymers in tetrahydrofuran were performed at a ﬂow rate of 1.0 mL/min
at 40 °C. The chromatograms were calibrated using polystyrene standards.
The refractive indices were measured using a prism coupler with an accuracyof 0:0005. The probe wavelengths were 409.2, 650.3, 833.7, and 1546 nm.
The measured values were ﬁtted to the three-term Cauchy’s equation [25],and the refractive indices at 850 nm were obtained. The molecular weights(Mw : weight average, Mn : number average) and refractive indices at 850 nm ofPPFS samples are summarized in Table I. We varied the molecular weights
by adjusting the concentration of initiator, and Mw was in the range of 5:96{
8:13  105 . The refractive indices were independent on the molecular weights
as expected.

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IEICE Electronics Express, Vol.11, No.10, 1–7

Table I. The amount of initiator for polymerization, molecular weights, and refractive indices of PPFS
bulk samples
SampleInitiator[mol %]Mw (105 )[g/mol]Mn (105 )[g/mol]Mw =MnRefractive index
at 850 nm

P-1

0.20

8.13

4.13

1.97

1.467

P-2

0.30

7.69

3.99

1.93

1.467

P-3

0.40

6.84

3.57

1.92

1.466

P-40.50

5.96

2.65

2.25

1.467

Fig. 1. Experimental
setupforacoustic
measurement in PPFS bulk samples.

velocity

3.2 Brillouin frequency shift measurements
For BFS measurements, the obtained polymer rods were cut into 4-mm-thick
10-mm-diameter cylinders with ﬂatly-polished bottom and top surfaces. The
sample was placed on a stage with an 8-mm-diameter hole at the center, which
was ﬁxed in degassed water as shown in Fig. 1. A pulsed converging ultrasonicwave with 20-MHz center frequency (note that MHz ultrasonic wave has beenexperimentally shown to be available for BFS estimation [19]), which wasgenerated with a focus-type transducer connected to a pulser/receiver, waslaunched into the sample, and the reﬂected waves from the top and bottom
surfaces were detected with the same transducer. The small hole below thesample avoided the interference with the other wave reﬂected from the stage.
The time delay between the reﬂected waves was observed with an oscilloscope.
The acoustic velocity va in the sample was calculated from the time delay tand the sample thickness l byva ¼ 2l=t:

ð3Þ

By substituting Eq. (3) into Eq. (1), the BFS at 850 nm was obtained. Thesesteps were repeated at diﬀerent temperatures to investigate the temperaturedependence of the BFS.
4
© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
Copyedited May 25, 2014

ResultsFig. 2 shows the temperature dependence of the BFS at 850 nm in sample P-2
as an example. The temperature was increased from room temperature to5IEICE Electronics Express, Vol.11, No.10, 1–7Fig. 2. BFS dependence on temperature in sample P-2 at
850 nm. The solid circles and hollow squares indicate
the measured points with increasing and decreasing
temperature, respectively.

Table II. BFSs at room temperature (RT) and their temperature coeﬃcients of the PPFS samples, silica
SMF, and PFGI-POF at 850 nm. Note that thevalues for the silica SMF and PFGI-POF havebeen recalculated using Eq. (1)© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
Copyedited May 25, 2014

Sample

BFS at RT [GHz]

Temperature coeﬃcient [MHz/K]

P-1

6.16!7.27

P-2

6.17

!7.07

P-3

6.16

!7.18

P-4

6.18!7.12

Silica SMF

19.7

2.2

PFGI-POF

5.1!7.5

75 °C, and then decreased to room temperature. The BFS at room temperature was 96.2 GHz, and its temperature dependence was linear with aproportionality constant of approximately !7.1 MHz/K irrespective of increasing or decreasing temperature. The BFS at room temperature and thetemperature coeﬃcient of the four samples are summarized in Table II. Bothvalues were nearly independent of molecular weight. As the Young’s modulus,
Poisson’s ratio, and density of polymers, which determine the acoustic velocity (Eq. (2)), generally have little dependences on molecular weight when themolecular weight is suﬃciently high [26], no correlation between the BFS andmolecular weight was predictable. However, this relation had not been
experimentally investigated, and we believe that the ﬁnding of this study will
be of great help in designing and preparing POFs for Brillouin-based temperature-sensing applications. The estimated BFS and the temperature coeﬃcient in PPFS-POFs at 850 nm were close to those in PFGI-POFs as listed inTable II. The BFS in PPFS-POFs, which was 1.2 times as high as that in6

IEICE Electronics Express, Vol.11, No.10, 1–7

PFGI-POFs, may contribute to the mitigation of the Brillouin signal deterioration caused by Rayleigh noise [27]. Although the absolute value of thetemperature coeﬃcient in PPFS-POFs was slightly lower than that in PFGIPOFs, it was still approximately 3.3 times as large as that in a standard silica
SMF.
5Conclusion

We synthesized PPFS polymers with diﬀerent molecular weights by freeradical polymerization in bulk and studied their BFS properties. The BFS andits temperature dependence in PPFS-POFs were estimated from the bulk
samples using the ultrasonic pulse-echo technique. The BFS and the temperature dependence were independent of the molecular weight over the range
investigated. The BFSs of PPFS samples at room temperature were 6.2 GHzat 850 nm and linearly changed with temperature at the ratio of !7.1 MHz/K.
The absolute temperature coeﬃcient was nearly identical to those of PFGIPOFs and 3.3 times as large as those of silica SMFs. As mentioned in theintroductory part, since the production cost of PPFS-POFs could be considerably lower than that of PFGI-POFs, PPFS-POFs might be a promisingalternative of PFGI-POFs for high-precision temperature sensing at 850 nm in
the near future.
Acknowledgments
This work was partially supported by the Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST), a Grant-inAid for Young Scientists (A) (no. 25709032) from the Japan Society for thePromotion of Science (JSPS), and by research grants from the General Sekiyu
Foundation, the Iwatani Naoji Foundation, and the SCAT Foundation. N.H.
acknowledges a Grant-in-Aid for JSPS Fellows (no. 25007652).

© IEICE 2014
DOI: 10.1587/elex.11.20140285
Received March 28, 2014
Accepted April 11, 2014
Publicized May 2, 2014
Copyedited May 25, 20147