# Fileset

[icg_fuse.pdf](https://mdr.nims.go.jp/filesets/6df388c8-880c-41db-ae08-b34305528534/download)

## Creator

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

## Rights



## Other metadata

[Optical fuse made of silica glass optical fibers spliced through low-melting glass with carbon-coating](https://mdr.nims.go.jp/datasets/11dcf5cc-f4ac-47d7-ab27-c4a4e36f655b)

## Fulltext

Proceedings of XX Int. Cong. on Glass, 2004 Kyoto JAPAN O-14-010OPTICAL FUSE MADE OF SILICA GLASS OPTICAL FIBERS SPLICEDTHROUGH LOW-MELTING GLASS WITH CARBON-COATINGShin-ichi Todoroki∗ and Satoru InoueAdvanced Materials Laboratory, National Institute for Materials Science,Namiki 1-1, Tsukuba, Ibaraki 305-0044, JapanTODOROKI.Shin-ichi@nims.go.jpLight-induced breakdown of low-melting glass with thickness of 50µm is demonstrated, whichwas coated with carbon paint and formed between two end of single-mode silica glass opti-cal fibers. This phenomenon is useful to make irreversible optical limiting devices known asoptical fuse. The present structure breaks by 1.2–5.3 W of incident light (∼1.5µm), exhibitslow insertion loss of less than 1 dB, and is formed by dipping-up a small amount of hot glassmelt between fibers, aligning the fibers and quenching them. The relation between shape of thecaptured melt and its insertion loss is discussed.(Key words: optical fuse, optical fiber, low melting glass, telluria glass, light-induced deforma-tion)1 IntroductionOptical fuses/limiters are switching devices whoseFig. 1: Photographs of (1) a 50-µm-thick TeO2glass layer inserted in a silica glass fiber circuit, (2)carbon-coated spliced region, and (3) light-inducedbreakdown of this structure. The diameter of thefiber is 125µm. The light source is connected to theright side.transparency drops permanently/reversibly by an ex-cessive incident beam and are used for protecting op-tical components to be damaged by the beam. Theirimportance is growing considerably with the recent de-velopment of high power light sources, especially foroptical fiber circuits. Optical limiters are realized bythermal lensing effect[1, 2], self-focusing due to non-linear effect[1], or misaligning induced by thermal ex-pansion of waveguides[3]. Although they are practi-cally useful due to their recoverable properties, opti-cal fuses are also attractive in their simple structure.Several passive optical fuses have been proposed[4, 5]and they have a common structure in which some thinlayers are inserted in a optical fiber circuit and one orsome of the layers made of metal absorb propagatinglight to bring about a permanent loss increase there.Recently, we proposed a new structure of passiveoptical fuse[6], in which transparent soft glass segment(pure TeO2,∼150µm-long with a necked region) is in-serted in the circuit (insertion loss:∼2–3 dB) and iscoated with carbon-containing paint. In this structure,leaked light from the glass segment is absorbed by thecoating to generate heat that deforms the glass segment to reduce its transparency. Although its responsetime in theory is expected to be much slower than that of the former optical fuses, it is still importantto offer several options for constructing fail-safe optical systems. Here we report the improved versionof this structure with reduced insertion loss, less than 1dB[7] (See Fig. 1). On the basis of real-timeobservation of fusing action, its mechanism to lose transparency is discussed.From a viewpoint of fabrication, it is very important to splice fibers with low insertion loss. Thus,the effect of soft glass’ shape onto the splicing loss is investigated.– 1 –Proceedings of XX Int. Cong. on Glass, 2004 Kyoto JAPAN O-14-010Au plate + HeaterFiber HoldersCCD Cameras(1)(2)(3)Fig. 3: Maximum output values from thepresent optical fuses vs. their insertion lossfor 1.3µm.Fig. 2: Fabrication apparatus of splicing optical fibers vialow-melting glass.2 ExperimentalCommercial optical fiber cables (single mode, core diameter: 10µm, 3m-long with FC connectors) areused in this study. Bare fibers were cut by a fiber cleaver (York FK10). Two fibers were placed onfiber holders so that their ends face each other, as shown in Fig. 2[8, 9, 10]. A gold plate with a smallheater was set between the two ends of the fibers. Their relative positions were controlled by a personalcomputer with a resolution of 1µm. The heater was kept at a constant temperature over 700◦C whichwas monitored through a thermo couple placed on the back. The glass melt was supplied by puttinga small amount of TeO2 powder (5N, Shinko Chemical Co.,Ltd.) on the gold plate. The droplet wasobserved through video cameras placed from its top and side. Two fibers were inserted into the dropletfrom its side((1) in Fig. 2). Then, the plate is lowered to leave a small amount of the melt between thetwo ends(2). Lastly, the fibers were immediately moved to an appropriate position before the melt wassolidified(3). TeO2 melt easily vitrified in this method because of its high quenching rate, estimated tobe as large as that in twin-roller quenching method,∼ 103 K/s[9]. In spite of a large gap in thermalexpansion coefficient among these glasses, 2 orders of magnitude, no fracture was observed due to smallinterfaces.The thickness of the inserted glass was measured by a High-Resolution Reflectometer (AQ7410A,Ando Electric Co.,Ltd.) which consists of a Michelson interferometer and a laser of 1.31µm. Its reso-lution is 20µm. Transmittance of the laser light through the optical coupling structure was measured byan optical multimeter (AQ-2140, Ando Electric Co.,Ltd.).3 Results3.1 Optical fuseWe made 7 samples with a 50µm-thick soft glass layer (see Fig. 1(1)), whose insertion loss values areplotted along the horizontal axis of Fig. 3. The variation among these values must be mainly due to thatof a little tilted cut at the fiber ends. This fiber circuit was connected to an Er-doped fiber laser (ELD-33-1540, IPG Laser, 1.54µm, 2W max., the samples are plotted as× and¤ in Fig. 3) or a Raman fiberlaser (PYL-10-1480, IPG Laser, 1.48µm , 10W max., denoted as©) and an optical multimeter (8163B,Agilent Tech., averaging time: 1 msec) to measure the variation of its transmitted power. We confirmed– 2 –Proceedings of XX Int. Cong. on Glass, 2004 Kyoto JAPAN O-14-010Fig. 4: Time-varying output power from the samples plotted as¤ in Fig. 3, to which 1.54µm CW light from afiber laser is coupled. The beam intensity increased stepwise to a predetermined value. The dotted line in the leftrepresents the data for the sample without carbon-coating, which is proportional to the incident light. The solid lineis the one for coated. The dotted line in the right represents the light from carbon burning, the intensity of 650mn.The vertical arrows represent the time at which flash appeared or the video image of Fig. 1(3) was captured.that the soft glass layer withstands transmitting the laser power up to its maximum, increased stepwisein about 2 sec or 4 sec for the Er laser (see the dotted line in Fig. 4(1)) or the Raman laser, respectively.Then, the glass layer and adjacent fiber ends were coated with commercial black watercolor, whichconsists of fine carbon powder and gum arabic in general (Fig. 1(2)).The laser light was coupled to the device in the same way described above and its outer appearancewas recorded as a video movie whose sampling rate was 30 images per second. For the samples exceptthe ones plotted as× in Fig. 3, a flush suddenly appeared from the glass segment as shown in Fig. 1(3),and subsequently the coated carbon burned completely and the glass layer was found to be disappearedbetween the red-hot fiber ends. This flush burning of the coated carbon is brought about by the leakedlight from the adjacent glass layer which has no waveguide structure.The time-varying output power from the samples plotted as¤ are shown as the solid line in Fig. 4.In addition, light flux from carbon coating was monitored by a multichannel spectrometer (Soma Optics,S-2600) through a fiber probe placed at near the fuse. A broad spectrum of 550–1050nm is obtainedfrom the burning light and its time-varying intensity at 650nm is plotted as the dotted line in Fig. 4(2).The vertical arrows near the line represents the times at which flash appeared or the image in Fig. 1(3)are captured. The maximum output power for each measurement is plotted in Fig. 3, as a function ofinsertion loss.In the present experiment, so-called fiber fuse[11] was not observed in spite of carbon-painting nearthe fiber ends. The absence of fiber fuse is because the fiber core is shielded from the carbon particles bythe inserted glass layer.3.2 Splicing lossWe made several tens of splicing structure with various shape of joining glass, including pot-belly, nowaist and waisted as illustrated in the upper right of each figures in Fig. 5, and distances between fibers,whose insertion loss values are plotted in Fig. 5. For each groups, averaged slope is calculated by leastsquare method to be 28, 21 and 20 dB/mm, respectively.– 3 –Proceedings of XX Int. Cong. on Glass, 2004 Kyoto JAPAN O-14-010Fig. 5: Insertion loss values for spliced fibers via TeO2 glass as a function of distance between the fibers,d,classified into three groups according to the shape of inserted glass, (1) pot-belly, (2) no waist, and (3) waisted.Dashed lines are drawn by least square approximation, whose slopes are 28, 21 and 20 dB/mm, respectively.4 Discussion4.1 Optical fuseFrom the result of time-varying output power from the sample shown in Fig. 4, the following three factsbecome obvious. (1) The output power drop was observed about 0.1sec after the beginning of the flushburning; (2) after the flush, the output power varied irregularly, on which some fine structures weresuperimposed; and (3) before the flush, a slight reduction of the power had been observed and this trendgrew with time until the flush occurred.The first fact suggests that the observed power drop is not directly related with the disappearance ofthe glass layer which is brought about by the flush burning. The second fact means that the light fromthe burning enters to the output fiber. Therefore, it is reasonable that a loss increase due to the absence ofthe glass layer is buried in the transient light flux of burning, and the observed power drop must be due toan overlap of extinction of the burning and induced misalignment of the fiber ends. This, the maximumoutput power for each measurement plotted in Fig. 3 are not the maximum power of the propagated laserlight but the sum of the light flux from the laser and the carbon-burning. The third fact implies that thesoft glass layer is somewhat modified before the flush burning. This is brought about by the generatedheat at the carbon-coating absorbing the propagating light.For the samples whose insertion loss is less than 0.63, plotted as× in Fig. 3, a carbon-burning isobserved only after the second trial of laser irradiation (max. 2W), or not observed during two trials.Considering that these two samples show smaller insertion loss values compared with the other samples,it is reasonable to consider that an ignition of carbon-burning needs a certain amount of leaked lightfrom the glass layer. In fact, in a separate experiment, samples with thinner glass layer showed smallerinsertion loss and absence of carbon-burning with the use of the present Er-doped fiber laser (max. 2W),and samples having thicker glass layer are easy to be burned and showed higher loss value. Thus, thecritical input power to cause fusing action is expected to be increased by eliminating the amount of leakedlight from the glass layer. This trend is roughly observed in the samples including the ones connectedwith the Raman laser (max. 10W), plotted as© in Fig. 3.4.2 Splicing lossThe insertion loss of the present structure is affected by the following factors. (1) Fresnel reflection lossat the interface of silica fiber and the glass layer, which is estimated as 0.18 dB per an interface[8], (2)decoupling loss due to the absence of waveguide structure in the glass layer, and (3) another decouplingloss due to a misalignment between the two fiber pigtails and a little tilted cut at the fiber ends. The– 4 –Proceedings of XX Int. Cong. on Glass, 2004 Kyoto JAPAN O-14-010first loss is suppressed if the refractive index gap at the interface is reduced by using another soft glasswith lower refractive index and/or by introducing refractive index gradient coating at the end-face of thefibers[12]. The second loss can be reduced by decreasing the thickness of the glass layer or by usingTEC (Thermally Expanded Core) fibers to collimate the propagating light[8].The last factor can be eliminated by a matured fabrication technique. In fact, the results shown Fig. 5clearly shows that the shape of captured melt should not be pot-bellied to reduce the loss. The reasonturns out to be clear when we think of surface tension of the melt. The pulling strength between the twofibers in such a shape is expected to be smaller than that in other shapes, which aligns the fibers.5 ConclusionPassive optical fusing action was demonstrated in the devices having a structure of 50-µm-thick trans-parent TeO2 glass layer inserted in a single-mode silica glass fiber circuit with carbon coating. Thedevice is blown out by an incident CW beam of∼1.5µm, about 1–5W. On the basis of the time-varyingoutput power data and the simultaneous video recording of the device’s appearance, the mechanism oflosing its transparency is discussed before and after the flush burning. The critical power for blowingout is expected to be controlled by the thickness of the glass layer and/or the insertion loss of the device.The latter is found to be affected by the shape of inserted glass layer, which is related to the alignmentbetween the fibers in fabrication process.References[1] J. Ichikawa, H. Nagata, K. Higuma, J. Minowa, T. Ogata, and Y. Taneda, “Light intensity attenuatorand attenuating method.” US Patent: US 6134372, Oct. 2000.[2] M. E. DeRosa, S. J. Caracci, D. C. Bookbinder, T. M. Leslie, and S. L. Logunov, “Photothermaloptical signal limiter.” US Patent: US 6415075, July 2002.[3] R. Oron, D. Nevo, and M. Oron, “Optical limiter.” PCT Patent Application: WO 03/058338, July2003.[4] Y. Taneda, T. Ogata, H. Nagata, J. Ichikawa, and K. Higuma, “Optical fuse.” US Patent: US6218658, Apr. 2001.[5] A. Donval, D. Nevo, M. Oron, and R. Oron, “Optical energy switching device and method.” PCTPatent Application: WO 03/076971, Sept. 2003.[6] S. Todoroki and S. Inoue, “Optical fuse by carbon-coated TeO2 glass segment inserted in silicaglass optical fiber circuit (express letter),”Jpn. J. Appl. Phys., vol. 43, no. 2B, pp. L256–L257,2004.[7] S. Todoroki and S. Inoue, “Observation of blowing out in low loss passive optical fuse formed insilica glass optical fiber circuit,”Jpn. J. Appl. Phys., vol. 43, no. 6A, pp. L728–L730, 2004.[8] S. Todoroki and S. Inoue, “Low loss optical coupling structure between two ends of silica glassoptical fibers by inserting TeO2 melt,” J. Non-Cryst. Solids, vol. 328, pp. 237–240, 2003.[9] S. Todoroki, A. Nukui, and S. Inoue, “Formation of optical coupling structure between silica glasswaveguides and molten tellurite glass droplet (invited),” inInternational Symposium on PhotonicGlass (ISPG 2002)(C. Zhu, ed.), vol. 5061 ofSPIE Proceedings, (USA), pp. 50–58, SPIE, 2003.(ISBN 0-8194-4867-2).[10] S. Todoroki, A. Nukui, and S. Inoue, “Formation of optical coupling structure between two endsof silica glass optical fibers by inserting tellurite glass melt,”J. Ceram. Soc. Jpn., vol. 110, no. 5,pp. 476–478, 2002.– 5 –Proceedings of XX Int. Cong. on Glass, 2004 Kyoto JAPAN O-14-010[11] R. Kashyap and K. J. Blow, “Observation of catastrophic self-propelled self-focusing in opticalfibres,”Electronics Letters, vol. 24, pp. 47–9, 1988.[12] T. Anzaki, K. Mori, T. Kunisada, K. Nakama, K. Nakamura, M. Honda, K. Enjoji, M. Oikawa,and T. Fukuzawa, “The new optical coupling with the super wide range AR by the GRIN-coat,”in Proceedings of the 22nd European Conf. on Optical Communication, (Denmark), COM, Sept.2002. (P1.12).– 6 –