Spiral Propagation of Polymer Optical Fiber
Fuse Accompanied by Spontaneous Burstand Its Real-Time Monitoring Using
Brillouin Scattering
Volume 6, Number 3, June 2014
Yosuke Mizuno, Member, IEEE
Neisei Hayashi
Hiroki Tanaka
Kentaro Nakamura, Member, IEEE

DOI: 10.1109/JPHOT.2014.2323301
1943-0655 Ó 2014 IEEEIEEE Photonics Journal

Propagation of POF Fuse and Its Monitoring

Spiral Propagation of Polymer Optical Fiber
Fuse Accompanied by Spontaneous Burstand Its Real-Time Monitoring Using
Brillouin Scattering
Yosuke Mizuno, Member, IEEE, Neisei Hayashi, Hiroki Tanaka, andKentaro Nakamura, Member, IEEE
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
DOI: 10.1109/JPHOT.2014.2323301
1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received April 14, 2014; revised May 8, 2014; accepted May 8, 2014. Date of publication
May 13, 2014; date of current version May 21, 2014. This work was supported in part by a Grant-in-Aid
for Young Scientists (A) under Grant 25709032 from the Japan Society for the Promotion of Science(JSPS) and in part by research grants from the General Sekiyu Foundation, the Iwatani Naoji Foundation,and the SCAT Foundation. The work of N. Hayashi was supported by a Grant-in-Aid for JSPS Fellows
under Grant 25007652. Corresponding author: Y. Mizuno (e-mail: ymizuno@sonic.pi.titech.ac.jp).

Abstract: We study the propagation behavior of the polymer optical fiber (POF) fuse at a
power density up to several tens of kW/cm2 (corresponding to subwatt power). The propagation velocity is raised in proportion to the power density, reaching 41 mm/s at 67 kW/cm2 .
We also observe spiral oscillation and spontaneous termination of the fuse propagation, withthe latter accompanied by a burst. We then develop a new method of detecting the location of
the propagating POF fuse remotely and nonvisually in real time using Brillouin scattering,which can be clearly observed at such a high power density. This method requires neither
additional light injection nor signal integration, and it could be used to monitor the propagating
fuse in glass fibers.
Index Terms: Polymer optical fibers, fiber fuse, Brillouin scattering, remote sensing, realtime monitoring, nonlinear optics.

1. Introduction
Despite their higher loss than that of silica single-mode fibers (SMFs), polymer (or plastic) opticalfibers (POFs) [1], [2] provide various advantages such as easy and cost-efficient connection, safety,
and extremely high flexibility, leading to medium-range applications [3] and large-strain monitoring
applications [4]. Since the first observation of Brillouin scattering [5]Vone of the most importantnonlinear effectsVin POFs in 2010 [6], its properties have been extensively studied, especially forsensing applications [7]–[11]; and recently, distributed Brillouin sensing of strain and temperature in
POFs has been successfully demonstrated [12], [13], proving its high-precision temperature sensing
capability [7] as well as large-strain sensing capability [8]. The signal-to-noise ratio (SNR) of thesystem is, however, not sufficiently high when the spatial resolution is set to centimeter-order [13],because of the low Brillouin-scattered power in POFs resulting from their relatively large core
diameters and multimode nature [6]. One solution is, as a previous study predicts [9], simply to boost
the incident power; but recent reports have clarified that such high-power light injection into POFs

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Propagation of POF Fuse and Its Monitoringcauses not merely burning or damage at the POF-to-SMF interfaces [10] but also a continuous selfdestruction process of the POFs, i.e., fiber fuse.
A fiber fuse [14]–[18] occurs when high-power light propagating along the fiber locally heats thefiber material and initiates an optical discharge, which is then trapped in the fiber core andpropagates back toward the light source, while consuming the optical energy and leaving damage.
As the fiber is no longer usable after the fuse passage, this phenomenon is generally considered to
be one of the critical factors that limit the maximal optical power to be delivered [19]. Thus, it is ofgreat importance to thoroughly investigate the fuse properties in order that all possible measures aretaken to avoid this effect.
Recently, we have reported on the first observation of the fiber fuse in graded-index (GI-) POFs at
1.55 m [20], and its unique properties have been investigated from various angles [20], [21].
Although its propagation behavior is similar to that of the fuse in silica glass fibers [14]–[18], [22]–[25]from a macroscopic perspective, its velocity is 20 mm/s [20], which is one to two orders slower. The
threshold power density is 6.6 kW/cm2 [20], which is over 150 times lower than that of silica SMFs.
Spectral measurement has clarified that the POF fuse is not a plasma but an optical discharge at atemperature of approximately 3600 K [21]. In addition, microscopic studies have revealed not onlythat the damage left after the passage of the bright spot looks like a black curve that is slightly
oscillatory [20], which can be explained by taking the multimode nature into consideration [21], butalso that gas bubbles are sometimes formed simultaneously with the damage [21].
However, these properties have been reported only in the case of a relatively low incident power
density (G 13 kW/cm2 ; corresponding to a power of 130 mW in the POFs used in Refs. [20], [21].
See Eq. (1) in Ref. [20] for the calculation method), and thus, to clarify the propagation behavior of
the POF fuse under a higher power density is an important issue. Another significant task for study
is to remotely and nonvisually detect the location of the propagating POF fuse on a real-time basis
to minimize the damage extension. As POF-based sensors are often assumed to be used after
being embedded in various materials and structures (the POFs can no longer be visuallymonitored), even though the measurement range is relatively short, a remote-detection method isrequired. Abedin et al. [24] have developed a method of detecting the fuse location in silica SMFs
based on optical time-domain reflectometry, but the injection of high-peak-power optical pulses may
cause damage at the POF-to-SMF interface; needs for additional light at a different wavelength andfor the time-consuming process of signal integration for SNR enhancement are also problematic.
Note that Abedin et al. [25] have developed another fuse-detecting method based on opticalcoherence-domain reflectometry, which can rapidly detect the fuse initiation but does not provide its
location information.
In this work, we investigate the propagation behavior of the POF fuse at a power density up to
several tens of kW/cm2 (corresponding to a sub-Watt power). The propagation velocity is raised inproportion to the incident power density, reaching 41 mm/s at 67 kW/cm2 . Spiral propagation ofdamage along with a burst induced by a considerable amount of generated gas is newly observed.
We then develop and demonstrate a new method of detecting the location of the propagating POF
fuse in real time using Brillouin scattering, in which a continuous wave (CW) is used and neither
signal integration nor additional light injection is required.

2. Principle
When propagating in an optical fiber, light is partially returned via spontaneous Brillouin scattering.
The backscattered light spectrum is called Brillouin gain spectrum (BGS) [5], and the central
frequency of the BGS is downshifted from the incident frequency by the amount called Brillouinfrequency shift (BFS). The BFS is known to be 10.8 GHz for a silica SMF [5] and 2.8 GHz for aperfluorinated GI-POF [6] at 1.55 m. According to the theory [5], the Brillouin-scattered power isroughly in proportion to the effective length Leff defined as
Leff ¼Vol. 6, No. 3, June 20141  expðLÞ

(1)

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Propagation of POF Fuse and Its Monitoring

Fig. 1. Schematic setup of Brillouin-based POF-fuse monitoring system. EDFA, erbium-doped fiber
amplifier; ESA, electrical spectrum analyzer; PC, polarization controller; PD, photo detector; POF,
polymer optical fiber.

where  is the propagation loss and L is the fiber length. The fuse propagation can be regarded as
the shortening of L, leading to the reduction in the Brillouin signal. Though light can still propagate
through a POF after the fuse passage [20], the Brillouin signal from this part is negligibly small.
One may think of an idea of exploiting the Rayleigh-scattered light in the same way, but it isdifficult because of its spectral overlap with the Fresnel-reflected light, which is extremely unstable
in power. We have also confirmed that the fuse emission is so weak that its direct detection isdifficult.

3. Methods
POFs employed in the experiment were perfluorinated GI-POFs [2] with a numerical aperture of
0.185, a core diameter of 55 m (different from that in Refs. [20], [21]), a cladding diameter of100 m, an overcladding diameter of 750 m, a core refractive index of 1.35, and a propagation
loss of 250 dB/km (i.e.,  ¼ 0:056 =m) at 1.55 m. The core/cladding layers and the overcladding
layer were composed of amorphous perfluorinated polymer and polycarbonate, respectively.
The experimental setup is depicted in Fig. 1. The POF fuse was initiated in the same way as inRefs. [20], [21]. The Brillouin detecting system based on self-heterodyne was essentially the sameas that previously reported in Ref. [6]. The high-power light at 1.55 m, which was boosted with a
1-W-output erbium-doped fiber amplifier (EDFA; HPA-200, Alnair Labs), not only provided energy forthe initiation and propagation of the fuse but also served as Brillouin pump light. The polarization
state was optimized using polarization controllers (PCs). The fuse propagation was recorded with avideo camera to obtain its velocity and location.

4. Experimental Results on Fuse Characterization
The measured dependence of the POF-fuse propagation velocity on the power density is shown in
Fig. 2. The power (density) was calculated by taking into consideration the coupling loss at the
POF-to-SMF interface, the propagation loss of the POF, and other losses caused in the opticalcirculator, etc. With increasing power density, the propagation velocity was linearly raised, andreached 41 mm/s at 67 kW/cm2 (corresponding to 800 mW). The threshold power density was
8.4 kW/cm2 , which is close to the previous report (6.6 kW/cm2 ) [20]. The proportionality constantwas 440 mm  s1 MW1 cm2 in this range, which is 3.5 times smaller than the previous report(1560 mm  s1 MW1 cm2 ) [20]. This discrepancy may be elucidated by Todoroki’s [26] theory on
fuse propagation modes proposed for silica SMFs; further study is needed on this point.
Microscopic analyses also revealed some unique features. Fig. 3(a) shows a micrograph of thedamage left spirally after the fuse passage at 42 kW/cm2 . The period of the spiral oscillation wasapproximately 1400 m, which moderately agrees with the theoretical period of the ray propagating
in a GI fiber (1204 m in this POF) [27]. The diameter of the spiral oscillation was  200 m(measured by observing the cross section of the damaged POF with a microscope; Fig. 3(a) does
not provide correct information on its diameter because of the lens effect of the POF side surface),
which is larger than the core/cladding diameter ð100 mÞ, indicating that the overcladding layer
was thermally damaged at this high power density. The conversion of the spiral direction was alsoVol. 6, No. 3, June 2014

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Fig. 2. POF-fuse propagation velocity vs. power density. The red circles are measured points, and thegreen line is a linear fit.

Fig. 3. (a) and (b) Two examples of the paths of the POF fuse at a power density of 42 kW/cm2 .

Fig. 4. (a) Path of the POF fuse at a power density of 67 kW/cm2 , (b) that with a burst, and (c) its
magnified view.

observed as shown in Fig. 3(b), which suggests the possible change in the fuse propagation mode,
supporting the mechanism of the POF fuse propagation reported in Ref. [21], i.e., the bright spot
travels only along the optical path of a particular propagating mode (that with the highest energy)
that provides it with energy directly.
When the power density was as high as 67 kW/cm2 , not only did the spiral oscillation becomeless clear (Fig. 4(a)) but the fuse propagation was also sometimes spontaneously ceased

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Fig. 5. (a) BGSs in POF measured every 2 s after the fuse initiation. (b) Peak power dependence on
the remaining POF length. The blue circles are points measured every 1 s, and the red curve is atheoretical fit.

accompanying a burst, as shown in Fig. 4(b) and (c). This burst appears to have been induced by aconsiderable amount of generated gas; and the higher the incident power was, the more frequently
it was observed. Thus, it is difficult to make the POF fuse propagate for a relatively long distance(longer than several tens of centimeters) at a power density higher than 100 kW/cm2 (or at a
power higher than 1 W).

5. Experimental Results on Fuse Monitoring
The incident light power was 450 mW, corresponding to a power density of 38 kW/cm2 . The room
temperature was 19  C. A fuse was initiated at around the open end of a 62-cm-long POF. Fig. 5(a)shows the BGSs measured every 2 s after the fuse initiation, the vertical axis of which is in linear
scale. The noise floor was 0.6 nW. The BFS was 2850 MHz, which agrees with the previously
reported value [6]–[11]; a slight discrepancy was caused by the different room temperature andvideo bandwidth of an electrical spectrum analyzer (ESA). As the fuse propagated, the peak powerof the BGS was continuously reduced. In the weakest BGS presented in Fig. 5(a), an additional
peak was observed at approximately 2960 MHz, which originated not from the heated POF but fromthe noise floor of the ESA, because the BFS in a POF is lowered with increasing temperature [7].
Fig. 5(b) shows the peak power, measured every 1 s (this interval can be even shorter; if BGSs areunnecessary, we need not use an ESA with a relatively low sampling rate to measure the peak
power), plotted as a function of the remaining POF length. The red line indicates a fitted curve
calculated using the effective length given by Eq. (1), which can be regarded as almost linear with aslope of 0.032 nW/cm in this range (When the POF length is shorter than 10 m, linear
approximation is valid [9]). Thus, the fuse location can be identified using the peak power. The
measurement error seems to originate from polarization- and multimode-dependent signal fluctuations. The fuse propagation velocity calculated from Fig. 5(b) was 29.4 mm/s, which well agrees
with the result in Fig. 2. As long as a POF fuse is initiated, the maximum length of the remotedetection is, in principle, not limited, because the optical power required for Brillouin observation ismuch lower than that for fuse initiation [20]. Although demonstrated using the POF fuse, we expect
this real-time monitoring method is applicable also to the glass optical fiber fuse.

6. Conclusion
The propagation behavior of the POF fuse at a power density up to several tens of kW/cm2 (or at a
sub-Watt power) was investigated. The propagation velocity was proportional to the power density,and reached 41 mm/s at 67 kW/cm2 . The spiral propagation mode of damage as well as

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Propagation of POF Fuse and Its Monitoring

spontaneous termination of the fuse propagation accompanied by a burst was newly observed. A
novel method of detecting the location of the propagating POF fuse in real time using Brillouinscattering was also developed. This CW-based method is free from the necessity of injecting
additional light and/or integrating signal, and could be implemented also for glass-fiber fusemonitoring. We believe that this work will be useful in developing POF-based systems for highcapacity transmission and distributed Brillouin sensing [28]–[32] in the near future.

Acknowledgment
The authors are grateful to R. Nedelcov (Department of Language Arts, Tokyo University of theArts) for his English editing.

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