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

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[Ultrahigh-speed videography of fiber fuse propagation: a tool for studying void formation](https://mdr.nims.go.jp/datasets/bf4df251-0294-4670-a954-3369546bfae8)

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Ultrahigh-speed videography of fiber fuse propagation:a tool for studying void formationShin-ichi TodorokiAdvanced Materials Laboratory, National Institute for Materials Science,Namiki 1-1, Tsukuba, Ibaraki 305-0044, JapanABSTRACTUltrahigh-speed videography of fiber fuse propagation through single mode silica fiber revealed that asymmetric, or tailed,optical discharge pumped by more than 2.0 W of CW 1480nm light generates periodic voids, whereas symmetric one givesa thin continuous void. A number of optical micrographs showing front part of fiber fuse damage pumped by same energywere collected and sorted in the order of increasing distance between the top of the first big void and the top of the firstregular void. The sorted sequence suggests the periodic void formation by the optical discharge; the discharge forms anasymmetric void and casts off its tail which shrinks to be one of regular voids. This process helps us to understand whythe regular void looks like a bullet considering the internal pressure of the optical discharge and the temperature gradientalong the fiber.Keywords: Fiber fuse, Ultrahigh-speed videography, Laser-induced damage1. INTRODUCTIONFiber fuse effect was discovered in late 1980s,1–6 initiated by local heating of optical fiber to generate an optical dischargerunning along the fiber to the light source (∼W) resulting in catastrophic and strange destruction of core region, i.e. peri-odic and bullet-shaped void formation. Ever since, nearly 40 papers has been published concerning this phenomenon.7–37On the one hand, recent growth of available laser power (>kW) gives rise to an practical need for fiber fuse termina-tion.6, 22, 25, 31 On the other hand, the mechanism of void formation is not fully elucidated, because only few experimentaldata was available directly from dazzling and rapidly moving (∼m/s) optical discharge, such as spectrum of backscatteredlight4 and single snapshot.10, 19 Therefore, former discussions18, 29 were mainly based on static images of fused damageand macroscopic properties including propagating speed.Recently, the author reported ultrahigh-speed videography of fiber-fuse propagation and mentioned the relation be-tween the shape of running optical discharge and the morphology of generated voids.34, 35 This paper presents furtherinvestigation of this topic to obtain a sequence of photographs demonstrating periodic void formation.2. EXPERIMENTALFigure 1 shows the experimental setup in this study.FiberLaser1480nmND filterSMF-28Zoom lensUltrahigh-speedcameraFigure 1. Experimental setup for observing fiber fuse propagation.One end of a commercial single-mode silica glass opticalfiber (SMF-28, Corning, core diameter: 9µm) was con-nected to a Raman fiber laser (PYL-10-1480, IPG Laser,1.48µm, 10 W max.). The other end was folded and incontact with a metallic plate in order to initiate a fiberfuse when the laser light entered. The optical dischargewas observed in a stripped section of the fiber through aCCD (Charge Coupled Device) camera (ultima APX-RS,monochrome version, Photron Ltd., sensitivity range: 380-790 nm) with an appropriate zoom lens. Pictures with aresolution of 128×16 were taken every 4µs with 1-µs-exposure time through ND (neutral density) filters (x16 or x32). The fusing phenomenon was terminated by switching offthe pumping laser. The time for extinction is less than 100µs, which is the minimum time resolution of the power meter.Damaged sites were examined by an optical microscope.E-mail: TODOROKI.Shin-ichi@nims.go.jp, Facsimile: 81 298 54 9060, URL: http://www.geocities.com/Tokyo/1406/Figure 2. Photograph and contour map of optical discharge propagating through single mode silica glass fiber pumped by 9.0 W light(upper) and their intensity profiles along the dashed lines on the photo at every 4µsec (lower). Photographs of fused fibers under thesame condition are shown in Fig. 8.Figure 3. Pumping power dependence of fusing speed (N) and void interval (̈ ).Figure 4. Pumping power dependence of the length (¥) and radius (•) of the first big voids, some of which are shown in Fig. 5. Theradius values are larger than the actual size,∼ 9µm, (see Fig. 6) because the fiber acts as a cylindrical lens.3. RESULTSFigure 2 shows a typical results of ultrahigh-speed videography. The lower half shows a time-varying intensity profile ofthe optical discharge along the dashed line shown in the upper photos. The depression nearx = 100 is due to a dust on thefiber surface blocking the emission. Propagation speed of the optical discharge was calculated from this result. Pumpingpower dependence of the speed is plotted in Fig. 3 as closed triangles.Periodic void generation was observed for the damaged fibers pumped by more than 2.0 W, and their intervals areplotted in Fig. 3. Only thin continuous void was observed in the fiber pumped by 1.5 W, and both periodic voids and thincontinuous voids were observed for the 2.0 W condition. Thus, pumping power of 2.0 W is the lowest limit for periodicFigure 5. Optical micrographs showing front-void of fused fibers (left column), captured images of optical discharge by the ultrahigh-speed videography (middle column), and intensity profiles along the dashed line in each image (right column). Pumping powers are (a)5.0 W, (b) 3.5 W, (c) 2.0 W, and (d) 1.5 W. The scales given in the bottom left are also valid for the photographs on the left.void generation.Figure 5 shows optical micrographs of the front part of generated damage and captured video images of optical dis-charge. Pumping power dependence of the length and radius of the front void is plotted in Fig 4. The actual radius of thefront-void pumped by 5.0 W is determined to be about 9µm from the cross-sectional view shown in Fig. 6.Figure 6. A cross-sectional view of a front-void generated by 5.0-W-pumping, showing the radius is about 9µm.Figure 7.Overexposed images of 9.0-W-pumped optical dischargepropagating a single mode silica siber.35 Exposure time is 4µs.(a) t = 0 µs, (b)t = 20µs, and (c)t = 180µs. The scale given inthe bottom in Fig. 2 is also valid for these.Figure 8. A series of optical micrographs showing fiber fuse dam-age in 9.0-W-pumped fibers. The interval of vertical lines is 22µm. The photo in the bottom is the same as the top, shifted by 22µm to the left. The scale given in the bottom in Fig. 2 is also validfor these.(1)(2)(3)Distance from the top (4)(5)(6)(a)(b)(c)Figure 9. An illustration showing a scheme of transformation ofthe generated void during fiber fuse.4. DISCUSSION4.1. Optical discharge and front voidAs shown in Fig. 4, the length of the front-void increases with an increase of the pumping laser power whereas its radiusremain nearly constant. Similar tendency is also found in the snapshots of optical discharge shown in Fig. 2 and Fig. 5.Thus, it is reasonable to conclude that the optical discharge used to be located in the first big void. This is also supported byanother result of overexposed ultrahigh-speed videography35 (see Fig. 7) showing that discrete scattering points are clearlyseen immediately after 9.0-W-pumped optical discharge whereas no scattering points are seen in front of the discharge.Moreover, the length of this optical discharge and the interval of scattering points coincide with those of voids shown inFig. 8, which is generated by 9.0-W-pumped fiber fuse. (Note that Fig. 7 and Fig. 8 are printed in the same scale.)Constancy of the void radius means that optical discharge is strongly enclosed in core region. Thus, the length of thefront void, i.e. that of optical discharge, increases linearly with increasing pumping power (see Fig. 4). As shown in Fig. 5,the shape of the optical discharge becomes asymmetric along the fiber length with an increase of pumping power. As forthe front void, its shape changes from simple ellipsoid to tailed cylinder (see left half of Fig. 5). Thus, the appearance of tailis the origin of this asymmetric shape. In addition, periodic voids are generated when the front void has a tail. Therefore,the tail must be a key player to form periodic voids.4.2. Sequence of periodic void formationLet us consider the period for one void formation. This is calculated from the propagating speed of optical discharge andthe void interval, which are shown in Fig. 3. The values vary from 31.0µs (2W) to 18.7µs (9W), and are much larger thana cycle of ultrahigh-speed videography, 4µs. Under the 9.0-W-pumping condition, optical discharge runs at a constantspeed during the period of one void formation, as shown in lower half of Fig. 2. This is also valid for other conditions.What is clear as of now is that the top of optical discharge moves at constant rate whereas a small void is generatedevery few tens ofµs. Then, the photographs shown in left half of Fig. 5 are only one snapshot during the period of onevoid formation. Therefore, one can capture other moments by further sample preparation. A number of photographs werecollected and sorted in a manner of increasing the distance between the top of the first big void and the top of the firstregular void37 (see Fig. 8). This sorting operation corresponds to a rearrangement in chronological order within the voidformation cycle.The sorted sequence seems to suggest a void formation process; the first big void casts off its tail which shrinks to bethe top of regular voids. In this process, the tail acts as a source of regular voids. Thus, it is natural that optical dischargewith no tail, pumped by less than 2.0 W, produces a continuous thin void.We have to notice that there is a possibility that these shapes were modified within the quenching period immediatelyafter stopping the pumping laser. The author thinks, however, that it hardly undermine the void formation process discussedabove, because of the following reason. If the time for decreasing the pumping energy to zero were much longer than theperiod of one void formation, the void interval near the big void would decrease gradually, since the interval has beenreported to decrease when reducing the pumping power.18 This is also shown in Fig. 3. Such a reduction of the intervalwas not observed in the present study. The author confirmed that at least for 20 mm from the big void, the interval wasconstant. This distance corresponds to be few tens of ms for travelling. Thus, the modification during quenching is likelyto occur within the last cycle of void formation, few tens ofµs. Moreover, the viscosity of silica glass is known to increasesteeply with decreasing temperature (for example, see Yakovlenko’s paper29). Thus, the modification is expected to besmaller in scale than the one-void formation.4.3. Mechanism of periodic void formationThe sequence of void formation helps us to understand qualitatively why the regular void looks like a bullet when we thinkof the glass bridge generated in the tail of the big void. Figure 9 shows a simplified model of shape-modification near thetop of regular voids and the glass bridge, which is extracted from Fig. 8. From a viewpoint of the regular void, it is pinchedoff from the big void at (3) in Fig. 9, and shrinks (4,5) to be fixed (6). On the other hand, the glass bridge changes its shapein the sequence of (a), (b) and (c) in Fig. 9.This action is governed by the internal pressure of the optical discharge and the temperature gradient, i.e., a changein viscosity of the glass, along the fiber. Once a glass bridge appears in the tail, it is pushed backward by the pressure ofoptical discharge (note that the discharge is strongly enclosed in the core region as discussed above). As the distance fromthe top of the optical discharge increases, the temperature decreases and the viscosity of the glass increases. Therefore,the displacement of the interface between the glass and the void decreases, as shown by the horizontal arrows in Fig. 9.Consequently, the front end of the pinched-off void solidifies after its backward is fixed. This time lag brings about theshape of bullet.The appearance of the glass bridge is related with a creation of new free surface at the front end of the optical discharge.The discharge keeps its total surface area in balance by these actions.The present results can provide a concrete model to the recent works of theoretical approach and computer simula-tion20, 27, 29, 32 toward a deeper elucidation of this phenomenon.5. CONCLUSIONUltrahigh-speed videography of fiber fuse propagation and microscopic observation of fiber fuse damage revealed thatasymmetric, or tailed, optical discharge generates periodic voids. A sequence of periodic void formation during fiber fuseis generated by sorting a number of optical micrographs showing front part of the damage. The sequence seems to suggesta void formation process; a glass bridge appears in the tail of optical discharge and is pushed backward to form a bulletshaped void. The origin of this shape is the time lag of solidification between the front and back end of the generated voidunder the internal pressure of the optical discharge and the temperature gradient along the fiber.ACKNOWLEDGEMENTThe author is grateful to Mr. Kazuhide Hanaka, Mr. Akira Sakamaki, and Joji Kuwabara (Photron Ltd.) for helping theexperiment of ultrahigh-speed videography and Dr. Satoru Inoue (National Institute for Materials Science) for continuoussupport.REFERENCES1. R. Kashyap and K. J. Blow, “Spectacular demonstration of catastrophic failure in long length of optical fibre viaself-propelled self-focusing,” inEighth National Quantum Electronics Conference, p. 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