SUBJECT AREAS:

Propagation mechanism of polymer
optical fiber fuseNONLINEAR OPTICS

Yosuke Mizuno1, Neisei Hayashi1, Hiroki Tanaka1, Kentaro Nakamura1 & Shin-ichi Todoroki2

OPEN

POLYMERS
1

Received
3 February 2014
Accepted
9 April 2014
Published
25 April 2014

Correspondence andrequests for materials
should be addressed to
Y.M. (ymizuno@sonic.
pi.titech.ac.jp)

Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503,
Japan, 2Photonic Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.

A fiber fuse phenomenon in polymer optical fibers (POFs) has recently been observed, and its uniqueproperties such as slow propagation, low threshold power density, and the formation of a black oscillatory
damage curve, have been reported. However, its characterization is still insufficient to well understand themechanism and to avoid the destruction of POFs. Here, we present detailed experimental and theoreticalanalyses of the POF fuse propagation. First, we clarify that the bright spot is not a plasma but an opticaldischarge, the temperature of which is ,3600 K. We then elucidate the reasons for the oscillation of thedamage curve along with the formation of newly-observed gas bubbles as well as for the low threshold power
density. We also present the idea that the POF fuse can potentially be exploited to offer a long photoelectricinteraction length.

Compared with other glass fibers, polymer optical fibers (POFs)1,2 offer easy and cost-efficient connection,high safety, and extremely high flexibility. Consequently, despite their higher loss than that of silica singlemode fibers (SMFs), POFs have been used in medium-range applications such as home networks andautomobiles3 and in large-strain monitoring applications4. Several years ago, Brillouin scattering5—one of themost important nonlinear effects—in POFs was observed for the first time6, and since then its properties havebeen investigated, especially for distributed strain and temperature sensing applications7–11, suggesting its potential applicability to high-precision temperature sensing7 and to large-strain sensing8. The Brillouin-scattered
power in POFs is, however, quite low on account of their relatively large core diameters and multimode nature6,
which needs to be enhanced to improve the signal-to-noise ratio of Brillouin distributed sensing systems12–17. One
solution is simply to raise the incident power, the validity of which has been experimentally confirmed9. Up tonow, it has been reported that such high-power light injection into POFs causes not only burning or damage at the
POF-to-SMF interfaces10 but also a so-called fiber fuse phenomenon.
Fiber fuse18–22 is generally known as the continuous self-destruction of a fiber by propagating high-power light,
which leads to local heating and the initiation of an optical discharge. It is then captured in the fiber core andtravels back toward the light source, consuming the light energy and leaving damage. Since the fiber can no longer
be used after the passage of the fuse, this effect is now regarded as one of the critical factors limiting the maximal
optical power that can be delivered23. Therefore, the fuse properties need to be well investigated so that all possible
measures are taken to avoid this phenomenon. According to previous studies18–22,24–27, the fiber fuse in silica glass
fibers is typically induced at an input optical power of one to several watts (one to several megawatts per squarecentimeter) and to have a propagation velocity of one to several meters per second.
Very recently, we have observed for the first time the fiber fuse in POFs with a graded-index profile using
continuous light at telecom wavelength28. Its macroscopic appearance is in perfect analogy to the fuse in silica
SMFs, but its propagation velocity is as slow as ,0.02 m/s. The threshold power density is 6.6 kW/cm2, which is
180 times lower than that of silica SMFs. The microscopic behavior of the POF fuse is also striking: the damage left
after the passage of the bright spot looks like a black oscillatory curve, the period of which is moderately consistent
with the theoretical oscillation period of the ray. In addition, fuse propagation can be terminated, by exploiting thehigh elasticity of the polymer, at a point pressed with an outer metal attachment. Thus, the fuse propagation
mechanism in POFs appears distinctly different from that in silica glass fibers, and there are still many points to befurther studied and better understood, such as (i) whether the POF fuse is a plasma or not, (2) the temperature of
the bright spot, (3) the reason for the oscillation of the damage curve, and (4) the reason for the low threshold
power density.
In this paper, we report detailed experimental and theoretical analyses of the POF fuse propagation, offeringanswers to the aforementioned questions. The spectral measurement suggests that the POF fuse is not a plasma
but an optical discharge at a temperature of approximately 3600 K. The oscillatory damage curve is explained by

SCIENTIFIC REPORTS | 4 : 4800 | DOI: 10.1038/srep04800

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taking the multimode nature into consideration, while the low
threshold power density is explained by the unique temperaturedependence of the optical absorption of the POF. We also show thatgas bubbles are partially periodically formed after the passage of thebright spot, and that, by using the electrical conductivity of thedamage curve, the POF fuse can be a candidate for providing a longphotoelectric interaction length.

Results
Polymer carbonization. The measured temperature dependence ofthe optical absorption of a perfluorinated graded-index POF2 (itsdetailed properties are provided in Methods) is shown in Fig. 1(a).
The absorption increased drastically at around 120uC, which is much
lower than the corresponding increase of a silica SMF that occurs at
1050uC (also shown in Fig. 1(a)). Then, using a thermal analyzer(STA 449 Jupiter; Netzsch), a thermogravimetry measurement was
performed in an argon atmosphere with a sample prepared by
selectively etching the overcladding layer of the POF using chloroform29; the 10-mg sample consisted of only the core and cladding
materials. As indicated in Fig. 1(b), the sample carbonized at,500uC, which is probably a result of heat degradation and gas
generation. Carbonization was confirmed by comparing two polymer samples, each held in an argon-filled quartz tube with an
18.5 mm outer diameter (Fig. 1(c)) and kept at ,350 or ,600uC
for 10 min. Transparent polymer, solidified from the molten state,
was observed when the sample was heated to ,350uC (Fig. 1(d)),
while carbide was formed at ,600uC (Fig. 1(e)).
Spectral analysis. The emission spectrum of the bright spot propagating along the POF (,300 mW incident power) was measured
using a spectrophotometer (Maya2000 Pro; Ocean Optics; 200–
1100-nm wavelength), as shown in Fig. 2(a). The slow fuse propagation had hardly any influence on the measured spectral shape. Its
comparison with the blackbody-like spectra of an incandescent lightbulb (Fig. 2(b)) and sunlight (Fig. 2(c)) shows that, although the POF
fuse spectrum has some characteristic peaks, all three spectra aresimilar (the sharp lines at ,280 nm are the instrumental noise).
This would indicate that the bright spot of the POF fuse originates

not so much from plasma emission as from thermal radiation,
because if the bright spot mainly consists of plasma, the emission
spectrum will generally contain some line-shaped components30,31(blackbody-like plasma emission can be induced by laser-driven
implosion at a temperature of the order of keV (,107 K)32,33,
which is not the case here). Spectra theoretically calculated using
Planck’s law (Fig. 2(a)) indicate that the temperature of the brightspot is ,3600 K (also verified using Wien’s displacement law); thedifference in the spectral shape is due to the limited wavelength rangeof the spectrophotometer. Note that the temperature of the light bulb
seems higher than that of the POF fuse by several hundreds ofKelvins, and that the surface temperature of the sun is .5000 K.
Propagation mechanism. The fiber fuse has also been observed inglass multimode fibers (MMFs), in which the shape of the molten
area corresponds to a summation of the optical paths of all the
propagating modes34,35. In contrast, the entire cross section of thecore and cladding of a POF seems to melt (See Fig. 3(b) in Ref. 28),
partly because their glass-transition temperature is as low as,108uC, and partly because the fuse propagation velocity isrelatively slow; and the boundary between the molten and solid
areas of the fiber cannot be observed. In the molten area of the
POF, the bright spot travels only along the optical path of aparticular propagating mode (that with the highest energy) thatprovides the bright spot with energy directly. The graded-index
profile may be destroyed by high temperature, but the observed
oscillating damage curve indicates that its time scale is longcompared with the fuse propagation velocity. Here, the bright spot
is probably attributed to the carbide; once generated at hightemperature (.500uC), it absorbs the light and heats the adjacentpolymer above the decomposition temperature, leading to its growthalong the optical path. This behavior is analogous to metal particle
manipulation by laser irradiation in glass36,37, i.e., carbide plays thesame role as the metal particle that moves, emitting bright visible
light and melting the surrounding glass by photothermal conversion,
which supports the fact that the POF fuse is not a plasma but an
optical discharge. The result is the oscillatory carbonized curve
indicating the passage of the POF fuse. Interestingly, unlike the

Figure 1 | (a) Optical absorption of a POF (orange) and silica SMF (blue) measured as a function of the temperature. Light of 20 dBm at
1546 nm was injected into an 8-m POF, and the data for a 1-m-long germanium-doped silica SMF is taken from Ref. 19 (wavelength of 1064 nm).
(b) Thermogravimetry curve, i.e. relative mass as a function of the temperature, of the polymer. (c–e) Photographs of the polymer samples in argon-filledquartz tubes before the temperature increase, after heating at ,350uC for 10 min, and after heating at ,600uC for 10 min, respectively.
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Figure 3 | (a,b) Two example micrographs of the POF fuse pathcontaining voids.

the POF, which will be useful in developing various optical/electricaldevices.
Void formation. Figures 3(a) and (b) show the microscopic imagesof example POF fuse paths, obtained using a digital microscope(VHX-600; Keyence), along which gas bubbles are observed. The
bubbles form randomly at places where the curvature of the fusepassage is maximal, but the formation is so irregular that sometimes bubbles are not observed, as in Fig. 3(b) in Ref. 28. As thesurrounding polymer cools slowly, the gas generated by thepassage of the fuse can move to energetically stable positions, as far
away as possible from the hot solidified carbonized areas of the fiber.
Despite their lateral shift from the optical path, the resulting bubbles
are partially periodic, and somewhat similar to the void train
observed in glass fibers. This experimental observation provides
reasonable explanations for the slow propagation and the low
threshold power density of the POF fuse, as discussed below.

Discussion
Irrespective of the fiber material, the fiber fuse phenomenon is an
example of a dissipative soliton38,39, which is a useful concept toanalyze the fuse properties including the fuse propagating velocity.
According to the literature39, by compressing the material contributions into one parameter, i.e. without using the physical properties
particular to a silica SMF, the velocity of the fuse propagation can bemathematically expressed using a somewhat complicated equation(Eq. (9) in Ref. [39]), which can be simplified as Eq. (10) in Ref. [39].
Simple mathematical analysis of this equation qualitatively showsthat, with decreasing thermal conductivity, the maximal velocity
achievable is reduced. Here, the molten polymer after the fuse passage is cooled slowly enough for the generated gas to move, whichindicates that the thermal conductivity in the POF is much lower
than that in a silica SMF. Consequently, the velocity of the fusepropagation in the POF is slower than that in a silica SMF.
To explore the reason for the low threshold power density of the
POF fuse, let us consider the threshold power Pth given as40
Pth ~K| Tp {To =Ap ,ð1Þ

Figure 2 | Emission spectra of (a) the bright spot of the POF fuse, (b) an
incandescent light bulb, and (c) sunlight. In (a), the aqua, green, and red
dashed curves represent the normalized blackbody radiation spectra
calculated theoretically at 2600, 3600, and 4600 K.

case for a silica MMF, light and electric current can simultaneously
propagate through the POF after the passage of the fuse, because thegenerated continuous carbonized curve is electrically conductive
while the optical propagation loss is approximately 1.4 dB/cm28,
which is too high for telecom use but sufficiently low for centimeterorder light propagation. This characteristic provides a possible
scheme for a long photoelectric interaction length, and the opticalabsorption (or electric current/resistance) might be controlled byadjusting the electric current (or optical power) propagating along
SCIENTIFIC REPORTS | 4 : 4800 | DOI: 10.1038/srep04800

where K is a proportionality constant, Tp is the temperature at which
the optical absorption is maximal, To is room temperature (,300 K),
and Ap is the absorption at Tp. This equation is valid under theassumption that the absorbed power is equal to the effective heatsink40, i.e. that the velocity of the fuse propagation is sufficiently highcompared to the thermal conduction so that it can be assumed that,from the viewpoint of the bright spot, a cold medium is constantly
flowing by. This assumption is valid for the POF judging from theexperimental results presented here. In the POF, Pth is 0.065 W,
which corresponds to a power density of 6.6 kW/cm2 and is much
lower than the ,1.3-W threshold in a silica SMF40. Consequently, theTp and Ap values of the POF are much lower and higher, respectively,
than those of the silica SMF. The lower threshold power density isthus a result of the lower temperature—at which the absorption is
maximal—and the higher absorption at that temperature (as seen in
Fig. 1(a)).
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In summary, detailed analyses of the POF fuse propagation were
presented. By spectral measurement, the bright spot was clarified tooriginate not from plasma emission but from thermal radiation, thetemperature of which reaches approximately 3600 K. Furthermore,
the reasons for the oscillation of the damage curve and for the low
threshold power density were theoretically elucidated. The formation of gas bubbles was also newly reported. Moreover, potential useof the POF fuse as a scheme for a long photoelectric interaction
length was presented. We hope that these findings will be useful inthe design and implementation of future devices and systems exploiting high-power polymer fiber optics, including Brillouin-based distributed strain and temperature sensors.

Methods
POF samples. The POF used in the experiment is the same as in Ref. 28, i.e., aperfluorinated graded-index POF2 (ID050; Sekisui Chemical) with a core (50 mm
diameter), cladding (100 mm diameter), and overcladding (750 mm diameter)
encased in polyvinyl chloride. The core and cladding layers are composed of doped
and undoped polyperfluorobutenylvinyl ether, respectively. The refractive index at
the center of the core is 1.356, whereas that of the cladding layer is 1.342. The
propagation loss is relatively low (,250 dB/km) even at telecommunication
wavelengths. The fiber fuse was initiated using a light source at 1546 nm in the same
way as in Ref. 28.
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Acknowledgments
The authors wish to acknowledge Richard Nedelcov (Department of Language Arts, Tokyo
University of the Arts) for his English editing, and Akitoshi Okino, Takahiro Iwai, Ken
Kakegawa, Yosuke Watanabe, Yukiko Ishihara, Yoshitaka Utsunomiya (Department ofEnergy Sciences, Tokyo Institute of Technology), Hideki Hosoda, Tomoya Inamura,
Masaki Tahara, Taku Aoki, Hyunbo Shim, Masatoshi Ii, Wei Qiu, Kazunari Minakawa, andMingjie Ding (Precision and Intelligence Laboratory, Tokyo Institute of Technology) for
their experimental assistance. This work was partially supported by a Grant-in-Aid forYoung Scientists (A) (no. 25709032) from the Japan Society for the Promotion 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).

Author contributions
Y.M. and N.H. designed and performed the experiments. Y.M., H.T. and K.N. performed
the experimental data analysis. Y.M. and S.T. performed the theoretical analysis. Y.M. wrote
the manuscript with input from all co-authors.

Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Mizuno, Y., Hayashi, N., Tanaka, H., Nakamura, K. & Todoroki, S.
Propagation mechanism of polymer optical fiber fuse. Sci. Rep. 4, 4800; DOI:10.1038/
srep04800 (2014).
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