Applied Physics Express 4 (2011) 102501
DOI: 10.1143/APEX.4.102501

Measurement of Acoustic Velocity in Poly(methyl methacrylate)-Based
Polymer Optical Fiber for Brillouin Frequency Shift Estimation
Neisei Hayashi, Yosuke Mizuno, Daisuke Koyama, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received August 1, 2011; accepted August 27, 2011; published online September 14, 2011
The acoustic velocity in a poly(methyl methacrylate) (PMMA) polymer optical fiber (POF) was measured to be 2:8  103 m/s using ultrasonicpulse-echo technique, from which its Brillouin frequency shift (BFS) was estimated to be 13 GHz at 650 nm. The BFS was also predicted to show
a negative dependence on temperature with a coefficient of approximately 17:1 MHz/K at 650 nm, 14:5 times larger than that of a silica fiber at
1.55 m. # 2011 The Japan Society of Applied Physics

rillouin scattering in optical ﬁbers has been widely
studied,1,2) and has various applications, such as
lasing,2) slow light generation,3) core alignment ofbutt coupling,4) strain/temperature sensing,5–7) and manyothers. To improve their performance, Brillouin scattering
not only in standard silica-based optical ﬁbers but also insome specialty ﬁbers, including tellurite glass ﬁbers,8,9)
bismuth-oxide ﬁbers,10,11) and photonic crystal ﬁbers,12,13)
has been vigorously studied so far. Recently, we havesucceeded in observing Brillouin scattering in perﬂuorinatedgraded-index polymer optical ﬁbers (PFGI-POFs)14,15) at
1.55 m, which oﬀer extremely high ﬂexibility, easy andlow-cost connection, and high safety compared with otherstandard glass ﬁbers.16) The Brillouin frequency shift (BFS)
was 2.83 GHz, and the Brillouin gain coeﬃcient wasestimated to be 3:09  1011 m/W, which was almost thesame as that of a silica single-mode ﬁber (SMF). This result
indicates that Brillouin scattering in PFGI-POFs can beapplied to a variety of practical devices and systems in thesame way as that in silica SMFs. We have also investigatedthe dependences of the BFS on strain and temperature in the
PFGI-POF at 1.55 m, and found that Brillouin scatteringin PFGI-POFs has a big potential for use in the developmentof high-precision temperature sensors with reduced strain
sensitivity.17) However, Brillouin scattering in standard
poly(methyl methacrylate) (PMMA)-based POFs has notbeen experimentally observed yet for the following two
reasons: (1) it is diﬃcult to develop a Brillouin monitoring
system including an optical ampliﬁer, optical circulator, etc.,
at 650 nm, where the propagation loss is minimal, and (2)their core diameter is so large (typically 980 m) that theBrillouin threshold power is extremely high. We believe thatclariﬁcation of the Brillouin scattering properties in PMMAPOFs is as signiﬁcant as that in PFGI-POFs, since PMMAPOFs are much more cost-eﬀective and more widely used incommunication applications.
In this paper, we measure the acoustic velocity in
POFs using ultrasonic pulse-echo technique, and estimate
their BFSs. First, in order to conﬁrm the validity of thismethod, we show that the estimated BFS of a PFGI-POF
of 2:9 GHz at 1.55 m moderately agrees with the actual
value previously reported. Then, we measure the acoustic
velocity in a PMMA-POF with a 980 m core diameter tobe 2:8  103 m/s, from which its BFS is estimated to be5:4 GHz at 1.55 m and 13 GHz at 650 nm. Finally,
we estimate the coeﬃcient of the temperature dependenceof the BFS in a PMMA-POF to be 7:2 MHz/K at 1.55 mand 17 MHz/K at 650 nm, which are 6:1 and 14:5

B

times larger than that of a silica ﬁber at 1.55 m,respectively.
When pump light is injected into an optical ﬁber,backscattered light, also called Stokes light, is generated
due to the interaction with acoustic phonons, and it propagates in the direction opposite to the pump light. Thisphenomenon is called spontaneous Brillouin scattering. The
Brillouin-scattered light spectrum, also known as Brillouingain spectrum (BGS), takes the shape of a Lorentzianfunction.2) The frequency at which the peak power isobtained in the spectrum is downshifted by several GHz
from the incident light frequency, and the amount of thisfrequency shift is known as BFS. When the pump lightwavelength is 1.55 m, the amounts of BFS are reported
to be 10:8 GHz for silica-based SMFs2) and 2:83 GHz
for PFGI-POFs.14) In optical ﬁbers, the BFS B is given as2)
B ¼

2nvA
;
p

ð1Þ

where n is the refractive index, vA is the acoustic velocity inthe ﬁber, and  p is the wavelength of the incident pumplight. Thus, we can estimate the BFS using the value of vA . Ifeither tensile strain or temperature changes in standard silica
SMFs, the BFS moves to higher frequency in proportion to
the applied strain (+580 MHz/%)18) or temperature change(+1.18 MHz/K).19) In PFGI-POFs, the BFS is known tomove to lower frequency with increasing applied strain(121:8 MHz/%)17) or temperature (4:09 MHz/K).17) In
both cases, by measuring the BFS in the ﬁbers, the strain
amplitude or temperature change can be derived.
We employed two kinds of POF samples. One was a
PFGI-POF with numerical aperture (NA) of 0.185, core
diameter of 120 m, cladding diameter of 490 m, corerefractive index of 1:35, and propagation losses of 150
dB/km at 1.55 m and 100 dB/km at 650 nm. The othersample was a PMMA-POF with NA of 0.5, core diameter of980 m, cladding diameter of 1000 m, core refractive indexof 1:49, and propagation loss of 150 dB/km at 650 nm(extremely high loss at 1.55 m). The lengths of the PFGIPOF and PMMA-POF were 4.2 and 3.9 mm as shown in
Figs. 1(a) and 1(b), respectively. The entire measurement
setup is depicted in Fig. 2. The ultrasonic pulse generated
with an unfocused transducer (Panametrics M316) connected to a pulser-receiver (Panametrics 5900PR) was
directed to the POF samples ﬁxed in degassed water, andthe reﬂected wave was received with the same transducer.
The center frequency of the pulse was 10.0 MHz, and thediameter of the ultrasonic beam was 3.2 mm. The magniﬁed

102501-1# 2011 The Japan Society of Applied Physics

Appl. Phys. Express 4 (2011) 102501(a)

N. Hayashi et al.

(b)
(a)

Microscope images of (a) the PFGI-POF sample and (b) the
PFGI-POF sample.
Fig. 1.

(b)
Fig. 4. Schematic setups (a) with the PFGI-POF sample and (b) with the
PMMA-POF sample.
Fig. 2.

Entire experimental setup for measuring the acoustic velocity in

POFs.

(a)
Fig. 5.
Fig. 3.

(b)

Measured echo waveforms (a) without and (b) with the PFGI-POF

sample.

Schematic setup without the POF samples.

view around the transducer when the POF samples were not
employed is schematically shown in Fig. 3. The aluminum(Al) plate had a hole of 1 mm diameter to ﬁx the PMMAPOF. The magniﬁed views with the PFGI- and PMMA-POF
samples employed are depicted in Figs. 4(a) and 4(b),respectively. The PFGI-POF was perpendicularly placed on
the Al plate, while the PMMA-POF was ﬁxed perpendicularly to the Al plate by making use of the hole.
Figure 5(a) shows the measured echo waveform without
the POF sample. The ﬁrst signal, where the time delay t wasdeﬁned as 0 s, represents the launched wave. Several large
peaks observed at t > 11:8 s are due to the reﬂection fromthe Al plate. The ﬁrst peak (A) at t  11:8 s and the second
peak (B) at t  12:4 s correspond to the reﬂected waves
at the surface and bottom of the Al plate, respectively. Other
peaks (C) following them at t > 13:0 s are due to themultiple reﬂection in the Al plate. Figure 5(b) shows themeasured echo waveform with the 4.2 mm PFGI-POF
sample employed. By comparing Figs. 5(a) and 5(b),we know that the two peaks at t  6:4 s and at t  11:4 scorrespond to the reﬂected waves at the surface and bottomof the PFGI-POF, respectively. The peak at t  11:4 s wasmuch smaller than the peak (A) at t  11:8 s because of thesmall core area and low acoustic reﬂectivity of the PFGIPOF/water boundary compared with those of the water/Alboundary. From the magniﬁed view around these two peaks,
the exact time delay was found to be 4.98 s, and theacoustic velocity vA was calculated to be 1:7  103 m/s.
By substituting this value in eq. (1), with the pump wavelength of 1.55 m, its BFS was estimated to be approximately 2.9 GHz, which is in moderate agreement with thepreviously reported value of 2.83 GHz.14) Though thisdiscrepancy seems to be caused by the insuﬃcient resolution
in calculating vA , it was shown that this method can be used
to roughly estimate the BFS. When the pump wavelength
was 650 nm, where the propagation loss in PMMA-POFs
is minimal, the BFS in the PFGI-POF was estimated to beapproximately 7.0 GHz.

102501-2# 2011 The Japan Society of Applied Physics

Appl. Phys. Express 4 (2011) 102501(a)

N. Hayashi et al.

(4:09 MHz/K17)) at 1.55 m, respectively. On the other
hand, when the pump wavelength is 650 nm, the coeﬃcientis calculated to be as large as 17:1 MHz/K. The larger
temperature coeﬃcient leads to precision enhancementof the temperature measurement, and consequently, theBrillouin scattering in the PMMA-POF can be potentially
utilized to develop high-precision temperature sensors.
In conclusion, the acoustic velocities in POFs were
measured based on the ultrasonic pulse-echo technique
for the estimation of their BFS. First, the BFS of a PFGIPOF at 1.55 m was estimated to be 2:9 GHz. This value
moderately agreed with the actual value previously reported,
indicating that this method can be used to roughly estimate
the BFS. Then, the acoustic velocity in a PMMA-POF with
980 m core diameter was measured to be 2:8  103 m/s,
from which its BFS was estimated to be 13 GHz at 650 nm
and 5:4 GHz at 1.55 m. In addition, the coeﬃcient of thetemperature dependence of the BFS in a PMMA-POF wasestimated to be 7:2 MHz/K at 1.55 m and 17:1 MHz/K
at 650 nm, which are 6:1 times and 14:5 times larger thanthat of a silica ﬁber at 1.55 m, respectively. We expect that
this information will be a useful guideline for observing
and exploiting Brillouin scattering in PMMA-POFs forvarious applications in the future including distributed
Brillouin sensors with low cost and robustness. The experimental observation of Brillouin scattering in PMMAPOFs might be feasible by employing high-power visible
light, at the wavelength of which their propagation loss
is minimum.

(b)

Fig. 6. Measured echo waveforms (a) without and (b) with the
PMMA-POF sample.

Fig. 7. BFS dependence on temperature in PMMA-POF at 1.55 m,estimated using the data in ref. 20.

Next, we measured the acoustic velocity in the PMMAPOF and estimated its BFS. Figure 6(a) shows the measured
echo waveform without the PMMA-POF sample, which canbe interpreted in the same way as Fig. 5(a). The positions of
the peaks (A, B, and C) diﬀered from those in Fig. 5(a),because the vertical position of the transducer was slightly
lower. The measured echo waveform with the 3.9 mm
PMMA-POF sample employed is shown in Fig. 6(b). By
comparing Figs. 6(a) and 6(b), it is clear that the two peaks
at t  8 s and at t  11 s correspond to the reﬂected waves
at the surface and bottom of the PMMA-POF, respectively.
The exact time delay was measured to be 2.77 s, and theacoustic velocity vA in the PMMA-POF was calculated to be
2:8  103 m/s, which is nearly comparable to the acoustic
velocity of 2644 (25) m/s in bulk PMMA.20) Its BFS at
650 nm was also estimated to be 13 GHz, which is close to11 GHz of silica ﬁbers at 1.55 m.2) In contrast, its BFS at
1.55 m was estimated to be 5:4 GHz, which is about twiceof 2.83 GHz in PFGI-POFs at 1.55 m.14)
Since the measured acoustic velocity in the PMMA-POF
was close to that in bulk PMMA, we can estimate the temperature dependence of the BFS in the PMMA-POF using
the data in ref. 20, where acoustic velocities in bulk PMMAat each temperature from 0 to 90  C are provided. The
estimated BFS dependence on temperature in PMMA-POF
at 1.55 m is shown in Fig. 7. We can see that the temperature coeﬃcient is approximately 7:2 MHz/K, theabsolute value of which is 6.1 and 1.8 times larger thanthose of a silica ﬁber (+1.18 MHz/K19)) and a PFGI-POF

Acknowledgments The authors are indebted to the staﬀ of the Technical
Department, Tokyo Institute of Technology, for providing the Al plate used in
the experiment. The work of Y. Mizuno was supported by the Research
Fellowships for Young Scientists from the Japan Society for the Promotion of
Science (JSPS).

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# 2011 The Japan Society of Applied Physics