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[ol_ffuse.pdf](https://mdr.nims.go.jp/filesets/e67f121b-9db5-4967-9067-04b6cb1921f1/download)

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[TODOROKI, Shin-ichi](https://orcid.org/0000-0003-3986-1900)

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[Animation of fiber fuse damage, demonstrating periodic void formation](https://mdr.nims.go.jp/datasets/793e709c-b1c6-455c-bb0b-f51dfa0fe901)

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Animation of fiber fuse damage, demonstrating periodic void formationAnimation of fiber fuse damage,demonstrating periodic void formationShin-ichi TodorokiAdvanced Materials Laboratory, National Institute for Materials Science,Namiki 1-1, Tsukuba, Ibaraki 305-0044, JapanReceived April 15, 2005; revised manuscript received June 1, 2005; accepted June 8, 2005;published as Opt. Lett. 30 [19] 2551–2553 (2005)http://www.opticsinfobase.org/abstract.cfm?URI=ol-30-19-2551http://www.geocities.com/Tokyo/1406/node2.html#Todoroki05OLA series of optical micrographs showing the front region of the fiber fusedamage were obtained to reveal the periodic void formation process. Theywere collected from a number of samples and were sorted in order of increasingdistance between the top of the first large void and the top of the first regularvoid. The micrographs clearly show that the first large void sheds its tail,which shrinks to form a regular void. This mechanism leads to the formationof bullet-shaped regular voids as the result of the balance between the internalpressure of the optical discharge and the increasing viscosity of the surroundingglass that occurs during pinching off. c© 2006 Optical Society of AmericaOCIS codes: 060.2290, 060.2400, 3140.3330, 3140.3440, 350.5340, 350.5400.The fiber fuse effect has been a familiar phenomenon since the late 1980s. It is initiated bythe local heating of an optical fiber, which generates an optical discharge running along thefiber to the light source (approximately a few watts). This results in catastrophic destructionof the core region, i.e. periodic and bullet-shaped void formation.1,2 The recent increase inavailable laser power (to greater than a kilowatt) has led to a practical need to terminatethe fiber fuse.3,4, 5 On the other hand, the void formation mechanism is not fully understoodand has only been discussed from a theoretical point of view6,7 and by means of a fewdirect observations near the bright and rapidly moving (approximately a meter per second)optical discharge.8,9, 10 This Letter describes two types of experimental results that suggestthe mechanism behind periodic and bullet-shaped void formation.The first result is a direct observation of the optical discharge obtained with an ultrahigh-speed CCD camera8,10 as shown in the upper half of Fig. 1. One end of a commercial1http://www.opticsinfobase.org/abstract.cfm?URI=ol-30-19-2551http://www.geocities.com/Tokyo/1406/node2.html#Todoroki05OLsingle-mode silica glass optical fiber (SMF-28, Corning) was connected to a Raman fiberlaser (wavelength: 1.48 µm). The other end was folded and brought into contact with ametallic plate in order to initiate a fiber fuse when a 7.0 W laser light was launched intoit. The optical discharge was observed in a stripped section of the fiber through the CCDcamera (ultima APX-RS, Photron Ltd., sensitivity range: 380–790 nm) with an appropriatezoom lens. Images with a resolution of 128×16 were taken every 4 µs with a 1-µs-exposuretime through neutral density (ND) filters (×16). Since the fiber acted as a cylindrical lens,the images were expanded in the vertical direction. The speed of the optical discharge wascalculated to be 1 m/s, as shown in the lower half of Fig. 1. The damaged fiber was examinedwith an optical microscope to measure the interval of the generated voids, which was foundto be 20.2 µm (see the right half of the photographs in Fig. 2). Thus, a void is generatedabout every 20 µs, i.e., one void per five photographs. A series of cross-sectional views of thefront part of the optical discharge, shown in the lower half of Fig. 1, clearly shows that theoptical discharge runs at a constant speed during the formation of one void.The second result is a collection of micrographs showing the front part of the damagetrain. The samples were obtained by switching off the 7.0 W pump laser after a fiber fusewas generated. It required no more than 7 µs for the emission from the optical discharge todrop to zero. This value is near the resolution of the CCD camera. A typical view is shownat the top of Fig. 2. The vanished optical discharge was originally in the first large void forthe following two reasons. Firstly, the asymmetric shape and length of the void (∼ 120 µm)coincide with those in the images shown in Fig. 1. This relationship is also confirmed forsamples with different pump powers (1.5–9.0 W).11 Secondly, the micrographs of a runningoptical discharge and following voids pumped at more than 5.9 W, which were reported byBufetov et al.9 and Todoroki,10 also show similar geometry, that is, an asymmetric luminousregion and periodic voids.The micrograph at the top of Fig. 2 is only one snapshot taken during 20-µs void for-mation sequence. Other moments can be captured by preparing further samples. A numberof micrographs were collected and sorted in order of increasing distance between the top ofthe first large void and the top of the first regular void (see the rest of Fig. 2). This sortingoperation corresponds to a rearrangement in chronological order within the void formationcycle, since the optical discharge runs at a constant rate during the cycle as described above.This rearrangement was also applied to other groups of micrographs with different pumppowers (3.5, 5.0 and 9.0 W) to produce similar sequences.The sorted sequence seems to suggest a void formation process; the first large void shedsoff its tail, which shrinks to become the top of regular voids. We have to notice, however,that the shape of the large void may change during the quenching period immediately afterstopping the pump laser. However, this does little to undermine the above void formation2process for the following reason. It takes less than 7 µs for the emission from the opticaldischarge to drop to zero. In addition, the viscosity of silica glass is known to increase steeplywith decreasing temperature. Since the heated area near the core region is surrounded bya cold and thick cladding layer and polymer coating, the temperature is expected to dropimmediately after the laser is switched off. Therefore, considering the period of 20 µs neededfor the formation of one void at elevated temperature, the amount of modification in alarge void during this shorter quenching period is expected to be smaller in scale than thatduring one void formation. Consequently, although Fig. 2 is not an in-situ observation, thecharacteristics of these shapes are sufficient for a discussion of the periodic void formationprocess.The void formation sequence helps us to understand qualitatively why the regular voidlooks like a bullet when we consider the glass bridge generated in the tail of the large void.Figure 3 shows a simplified model of the shape modification of the voids and the glass bridge,which is derived from Fig. 2. As regards a regular void, it is pinched off from the large voidat (3) in Fig. 3, shrinks (4,5) and becomes fixed (6). On the other hand, the glass bridgechanges its shape in the sequence (a), (b), (c) in Fig. 3.This action is governed by the internal pressure of the optical discharge and the tempera-ture gradient, i.e., the change in the viscosity of the glass along the fiber. Once a glass bridgeappears in the tail, it is pushed backward by the pressure of the optical discharge. As thedistance from the top of the optical discharge increases, the temperature decreases and theviscosity of the glass increases. Therefore, the displacement of the interface between the glassand the void decreases, as shown by the horizontal arrows in Fig. 3. Consequently, the frontend of the pinched-off void, i.e. the side of the first large void, solidifies after its back end isfixed. This time lag results in the bullet shape. The appearance of the glass bridge is relatedto a creation of a new free surface at the front end of the optical discharge. The total surfacearea surrounding the discharge is kept in balance by these actions.The present results can provide a concrete model for recent work on theoretical approachesand computer simulations7,12,13,14 and thus help to provide a clear understanding.In summary, this work has successfully reconstructed the sequence of the periodic voidformation that occurs during a fiber fuse from a number of optical micrographs showingthe front part of fiber fuse damage. These micrographs were collected from single-modesilica fibers with the pump laser (1.48 µm, 7.0 W) switched off after fiber fuse had beengenerated. They were sorted in order of increasing distance from the top of the first large void,where there had been an optical discharge, to the top of the first regular void. Although thephotographs were not obtained in-situ, the sorted sequence clearly suggests a void formationprocess; the first large void sheds its tail, which shrinks to become a bullet-shaped regularvoid. The origin of this shape is the time lag of the solidification between the front and3back end of the pinched-off void under the internal pressure of the optical discharge and thetemperature gradient along the fiber.The author is grateful to Mr. Kazuhide Hanaka, Mr. Akira Sakamaki, Mr. JojiKuwabara, and Mr. Keisuke Aizawa (Photron Ltd.) for helping with the ultrahigh-speedvideography experiment, Dr. I. A. Bufetov (General Physics Institute of the RussianAcademy od Sciences) for valuable discussion, and Dr. Satoru Inoue (National Institutefor Materials Science) for continuous support. S. Todoroki’s e-mail address and URL areTODOROKI.shin-ichi at nims.go.jp and http://www.geocities.com/Tokyo/1406/.References1. R. Kashyap and K. J. Blow, “Observation of Catastrophic Self-Propelled Self-Focusingin Optical Fibres,” Electron. Lett. 24, 47–9 (1988). URL http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=8155.2. D. P. Hand and P. S. J. Russell, “Solitary thermal shock waves and optical damagein optical fibers: the fiber fuse,” Opt. Lett. 13(9), 767–769 (1988). URL http://www.opticsinfobase.org/abstract.cfm?URI=ol-13-9-767.3. D. P. Hand and T. A. Birks, “Single-mode tapers as ’fibre fuse’ damage circuit-breakers,”Electron. Lett. 25(1), 33–34 (1989). URL http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=19651.4. S. Yanagi, S. Asakawa, M. Kobayashi, Y. Shuto, and R. Naruse, “Fiber fuse terminator,”in The 5th Pacific Rim Conference on Lasers and Electro-Optics, vol. 1, p. 386 (2003).(W4J-(8)-6, Taipei. Taiwan, 22-26 Jul. 2003), URL http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=1274838.5. E. M. Dianov, I. A. Bufetov, and A. A. Frolov, “Destruction of silica fiber cladding by thefuse effect,” Opt. Lett. 29(16), 1852–1854 (2004). URL http://ol.osa.org/abstract.cfm?id=80825.6. R. M. Atkins, P. G. Simpkins, and A. D. Yablon, “Track of a fiber fuse: a Rayleighinstability in optical waveguides,” Opt. Lett. 28(12), 974–976 (2003). URL http://ol.osa.org/abstract.cfm?id=72607.7. S. I. Yakovlenko, “Plasma behind the front of a damage wave and the mechanism oflaser-induced production of a chain of caverns in an optical fibre,” Quantum Electron.34(8), 765–770 (2004).8. S. Todoroki, “In-Situ Observation of Fiber-Fuse Propagation,” in Proc. 30th EuropeanConf. Optical Communication Post-deadline papers, pp. 32–33 (Kista Photonics ResearchCenter, Stockholm, Sweden, 2004). (Th4.3.3).9. I. A. Bufetov, A. A. Frolov, E. M. Dianov, V. E. Fortov, and V. P. Efremov, “Dy-namics of Fiber Fuse Propagation,” in Optical Fiber Communication Conference, 2005.4http://www.geocities.com/Tokyo/1406/http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=8155http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=8155http://www.opticsinfobase.org/abstract.cfm?URI=ol-13-9-767http://www.opticsinfobase.org/abstract.cfm?URI=ol-13-9-767http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=19651http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=19651http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=1274838http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=1274838http://ol.osa.org/abstract.cfm?id=80825http://ol.osa.org/abstract.cfm?id=80825http://ol.osa.org/abstract.cfm?id=72607http://ol.osa.org/abstract.cfm?id=72607Technical Digest. OFC/NFOEC, vol. 4 (Anaheim, CA, 2005). (OThQ7), URL http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1501536.10. S. Todoroki, “In-Situ Observation of Fiber-Fuse Propagation,” Jpn. J. Appl. Phys.44(6A), 4022–4024 (2005).11. S. Todoroki, “Origin of periodic void formation during fiber fuse,” Optics Express13(17), 6381–6389 (2005). URL http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-17-6381.12. Y. Shuto, S. Yanagi, S. Asakawa, M. Kobayashi, and R. Nagase, “Simulation of FiberFuse Phenomenon in Single-Mode Optical Fibers,” J. Lightwave Tech. 21(11), 2511–2517(2003).13. Y. Shuto, S. Yanagi, S. Asakawa, M. Kobayashi, and R. Nagase, “Fiber Fuse Phenomenonin Step-Index Single-Mode Optical Fibers,” IEEE J. Quantum Electronics 40(8), 1113–1121 (2004).14. R. I. Golyatina, A. N. Tkachev, and S. I. Yakovlenko, “Calculation of Velocity andThreshold for a Thermal Wave of Laser Radiation Absorption in a Fiber Optic Waveg-uide Based on the Two-Dimensional Nonstationary Heat Conduction Equation,” LaserPhysics 14(11), 1429–1433 (2004).5http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1501536http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1501536http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-17-6381http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-17-6381List of Figures1 Photograph and contour map of optical discharge propagating through asingle-mode silica glass fiber pumped by 7.0 W light (top) and their intensityprofiles every 4 µs along the dashed line in the photo (bottom) . . . . . . . . 72 Series of optical micrographs showing the damage generated in 7.0 W pumpedfibers, focusing on the voids inside the fiber. The interval of the vertical linesis 20 µm. The photograph at the bottom is the same as that at the top, shifted20 µm to the left. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Transformation of the void generated during fiber fuse. . . . . . . . . . . . . 96Fig. 1. Photograph and contour map of optical discharge propagating through a single-modesilica glass fiber pumped by 7.0 W light (top) and their intensity profiles every 4 µs alongthe dashed line in the photo (bottom)7Fig. 2. Series of optical micrographs showing the damage generated in 7.0 W pumped fibers,focusing on the voids inside the fiber. The interval of the vertical lines is 20 µm. The photo-graph at the bottom is the same as that at the top, shifted 20 µm to the left.8(1)(2)(3)Distance from top (4)(5)(6)(a)(b)(c)Fig. 3. Transformation of the void generated during fiber fuse.9