Brillouin gain spectrum dependence on large
strain in perfluorinated graded-index polymer
optical fiber
Neisei Hayashi,* Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama
226-8503, Japan*hayashi@sonic.pi.titech.ac.jp

Abstract: We investigate the dependence of Brillouin gain spectra on large
strain of > 20% in a perfluorinated graded-index polymer optical fiber, andprove, for the first time, that the dependence of Brillouin frequency shift(BFS) is highly non-monotonic. We predict that temperature sensors evenwith zero strain sensitivity can be implemented by use of this nonmonotonic nature. Meanwhile, the Stokes power decreases rapidly when theapplied strain is > ~10%. This behavior seems to originate from thepropagation loss dependence on large strain. By exploiting the Stokes
power dependence, we can probably solve the problem of how to identify
the applied strain, when the identification is difficult only by BFS becauseof its non-monotonic nature.
©2012 Optical Society of America
OCIS codes: (160.5470) Polymers; (280.4788) Optical sensing and sensors; (290.5830)
Scattering, Brillouin.

References and links
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4. K. Hotate and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiber using acorrelation-based technique – Proposal, experiment and simulation,” IEICE Trans. Electron. E83-C, 405–412(2000).
5. Y. Mizuno, W. Zou, Z. He, and K. Hotate, “Proposal of Brillouin optical correlation-domain reflectometry(BOCDR),” Opt. Express 16(16), 12148–12153 (2008).
6. M. G. Kuzyk, Polymer Fiber Optics: Materials, Physics, and Applications (CRC Press, 2006).
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OTDR,” Proc. SPIE 5855, 807–810 (2005).
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measurement,” Appl. Phys. Express 5(3), 032502 (2012), doi:10.1143/APEX.5.032502.
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at telecommunication wavelength,” Appl. Phys. Lett. 97(2), 021103 (2010), doi:10.1063/1.3463038.
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high-accuracy temperature sensing,” Opt. Lett. 35(23), 3985–3987 (2010).
14. G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, 1995).

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15. T. Horiguchi, T. Kurashima, and M. Tateda, “Tensile strain dependence of Brillouin frequency shift in silica
optical fibers,” IEEE Photon. Technol. Lett. 1(5), 107–108 (1989).
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1. Introduction
Due to their light weight, small diameter, immunity against electro-magnetic noise, etc.,
optical fiber sensors have been increasingly required for monitoring diverse civil structures,such as buildings, dams, levees, bridges, pipelines, tunnels, and aircraft wings. Above all,
Brillouin scattering-based fiber-optic sensors have been extensively studied because they canmeasure strain/temperature distribution along the fibers [1–5]. Up to now, only glass opticalfibers (GOFs) have been used for their sensor heads, but they are quite fragile and cannot
withstand strains of over several %. As one way to solve this problem, employing polymer
optical fibers (POFs) in such Brillouin sensors has attracted considerable attention, which has
extremely high flexibility and can withstand > 50% strain [6]. Besides, POFs have a uniquefeature called the “memory effect” [7], with which the information on the applied large strain
can be stored due to their plastic deformation.
Commercially-available POFs are classified into two types: poly(methyl methacrylate)based (PMMA-) POFs and perfluorinated graded-index (PFGI-) POFs. The former mainlytransmit visible light at around 650 nm, whereas the latter transmit not only visible light but
also telecom wavelength light at up to 1.55 μm. Brillouin scattering in PMMA-POFs has notbeen experimentally observed yet, because some of the optical devices required for themeasurement are extremely difficult to prepare at visible wavelength. Therefore, we havedeveloped a method to characterize the Brillouin properties in POFs using a so-called
ultrasonic pulse-echo technique [8], and predicted that the strain dependence on Brillouinfrequency shift (BFS) in PMMA-POFs is non-monotonic when applied strain is larger than
~10% [9]. Meanwhile, Brillouin scattering in PFGI-POFs has already been experimentallyobserved, since various optical devices are available at telecom wavelength [10–13]. The BFS
dependence on strain in PFGI-POFs has been measured to be linear with a coefficient of−121.8 MHz/%, but, in our previous experiment, the applied strain ranged merely from 0% to0.8% [13]. Putting their memory effect in perspective, the BFS dependence on large strain inPFGI-POFs needs to be clarified.
In this study, we experimentally investigate the dependence of Brillouin gain spectra(BGS) on large strain of up to 20% in a PFGI-POF. The BFS dependence is found to be nonmonotonic, which agrees with our prediction concerning PMMA-POFs [8]. By exploiting thisnon-monotonic nature, temperature sensing even with zero strain sensitivity will be feasible.
As for the Stokes power, it is found to decrease drastically when the applied strain is larger
than 10%. We show that this behavior is well explained by the propagation loss dependence
on large strain. Utilizing the Stokes power dependence is one way to solve the problem of
how we should identify the applied strain, when it is difficult only by BFS due to its nonmonotonic nature.
2. Principle
When a light beam is propagating in an optical fiber, it interacts with acoustic phonons andgenerates a backscattered light beam called the Stokes light [14]. This phenomenon is known
as Brillouin scattering, and the Stokes light spectrum is called the BGS. The center frequency#171301 - $15.00 USD(C) 2012 OSA

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of the BGS is known to be down-shifted from that of the incident light; the amount of thisfrequency shift νB, called the BFS, is given asυΒ =2n υ A,λ(1)In Eq. (1), n is the refractive index, vA is the acoustic velocity in the fiber, and λ is thewavelength of the incident light.
If strain (or temperature change) is applied to the fiber, the BFS shifts toward higher orlower frequency depending on the fiber core material, which is the basic principle of fiberoptic Brillouin sensors. The BFS dependence on strain has been investigated for a variety ofoptical fibers. They include silica single-mode fibers (SMFs) [15], tellurite glass fibers [16],germanium-doped photonic crystal fibers (PCFs) [17], and PFGI-POFs [13], the strain
coefficients of which are reported to be + 580, −230, + 409 (main peak), and −122 MHz/%,
respectively. Here, we should note that the value for the PFGI-POF is valid only for strain of< ~1%.
As well as the BFS, the Stokes power is also strain-dependent; for instance, the Stokes
power in silica SMFs is known to decrease with the increasing strain [18]. The Stokes power(or the Brillouin gain coefficient) is a function of many structural quantities [14], one ofwhich is the effective length Leff defined as
Leff =

1- e -α Lα,(2)where α is propagation loss and L is the fiber length.
3. Experimental setup
We employed a 1.27-m-long PFGI-POF as a fiber under test (FUT), which had numericalaperture (NA) of 0.185, core diameter of 50 μm, cladding diameter of 750 μm, core refractive
index of ~1.35, and propagation loss of ~250 dB/km at 1.55 μm. The experimental setup forinvestigating the BFS dependence on large strain in the PFGI-POF was basically the same asthat previously reported in [10], where the BGS can be observed with a high resolution (3MHz in this experiment) by self-heterodyne detection. One end of the PFGI-POF was buttcoupled to a silica SMF via an SC connector, and the other end was guided to an opticalpower-meter. Polarization state was adjusted for each measurement with polarization
controllers so that the Stokes power may be maximal. Different strains of up to 20% wereapplied to the whole length of the PFGI-POF fixed on two translation stages. The temperature
was kept at 27 °C for all the measurements.
4. Experimental results
Figure 1 shows a stress-strain curve of a 0.1-m PFGI-POF of the identical type, which wasobtained with the same method (strain-applying speed: 100 mm/min) as in Ref [9]. The crosssectional area was assumed to be constant during the measurement. Fracture strain of thePFGI-POF was 71% and its elastic-plastic transition was apparently induced at several %strain [19]. The initial peak up to ~10% indicates the elastic-to-plastic transition [20].

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Fig. 1. Measured stress-strain curve of the PFGI-POF.

The measured BGS dependence on large strain of up to 18.3% in the PFGI-POF is shownin Fig. 2(a). It took tens of seconds to manually apply a specific strain to the PFGI-POF; then,a few minutes later, the BGS measurement was performed. When the strain was 2.6%, a small
peak was clearly observed at approximately 2.8 GHz. This peak was caused by a 6-cm
portion of the PFGI-POF end connected to the silica SMF, to which proper strain was notapplicable. From this measurement, the dependences of the BFS and the Stokes power onlarge strain can be plotted as shown in Figs. 2(b) and (c), respectively. In Fig. 2(b), the BFS
dependence on strain was non-monotonic; with the increasing strain, the BFS shifted at first
toward lower frequency (0-2.6%) (this shift agrees well with the result under small strain[13]), then toward higher frequency (2.6-8.1%), and finally became almost constant (8.118.3%). This behavior may be caused by the Young’s modulus dependence on large strain[13]. In Fig. 2(c), with the increasing strain, the Stokes power decreased, and its reduction
grew drastic when the strain was over ~10%. At ~20% strain, the Stokes power became solow that the target BGS was buried by the noise, i.e., by the BGS of the small portion of thePFGI-POF without strain applied. The fluctuations in the Stokes power were caused by theunstable polarization state. The dependence of the Brillouin linewidth on large strain is also asignificant property, but we did not evaluate it because the Stokes power was so small that its
fair measurement was not feasible. The repeatability of the results above has been confirmed
by performing the same measurements for other samples.

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Fig. 2. Measured large-strain dependences of (a) the BGS, (b) the BFS, and (c) the Stokes
power in the PFGI-POF.

To clarify the origin of the Stokes power dependence on strain given in Fig. 2(c), thepropagation loss was also measured as a function of strain as shown in Fig. 3(a). The loss
drastically increased when the strain was over ~10%. Then, based on this figure, the strain
dependence of the effective length was calculated using Eq. (2), which is given in Fig. 3(b).
With the increasing strain, the effective length started to decrease drastically when the strain
was over ~10%, which is in good agreement with the Stokes power dependence. In Fig. 3(b),when the strain was smaller than 5%, the effective length was slightly increased with thestrain, because the actual fiber length was elongated owing to the strain and it compensatedfor the increase in the loss. This behavior is different from that in the Stokes power
dependence in Fig. 2(c), but it is valid if we consider that the small peak at ~2.8 GHz causedby the small portion of the PFGI-POF was overlapped with the target BGS at such small
strains. Thus, we presume that the reduction of the Stokes power in the PFGI-POF with strain
is attributed to that of the propagation loss.

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Fig. 3. Measured large-strain dependences of (a) the propagation loss and (b) the effective
length in the PFGI-POF.

By exploiting these unique Brillouin features of PFGI-POFs, some useful devices andsystems will be developed. For instance, the BFS in a PFGI-POF to which 10-15% strain isapplied has no strain dependence, and consequently it may be used for temperature sensing
with almost zero strain sensitivity. We must note that, due to the non-monotonic nature ofBFS, the identification of the applied large strain is sometimes difficult only by BFS. We may
solve this problem by applying ~2.6% strain beforehand and/or by using the Stokes power
dependence on strain as well.
5. Conclusion
By applying large strain of up to 20%, the strain dependence of BGS in the PFGI-POF wasmeasured in detail. The BFS exhibited a non-monotonic nature, with which temperaturesensing even with zero strain sensitivity will be feasible. The Stokes power drastically
dropped when the applied strain was larger than ~10%. This behavior is probably caused by
the large-strain dependence of the propagation loss. The identification of the applied strain issometimes difficult only by BFS due to its non-monotonic nature; in that case, using theStokes power dependence will be one of the solutions. Since this nature may vary dependingon the time of applying strain or the types of the POFs (PMMA-POFs, partially-chlorinated
POF, etc.), further investigation is required on this point. We believe that these results are ofgreat significance in developing large-strain sensors based on Brillouin scattering in POFs.
Acknowledgments
We are indebted to Professor Hosoda and Ms. Naruse of Tokyo Institute of Technology,Japan, for instructing us in the use of the tension tester. This work was partially supported bythe Research Fellowships for Young Scientists from the Japan Society for the Promotion ofScience (JSPS), and by research grants from the Murata Science Foundation and theFoundation of Ando Laboratory.

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