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[jncs_tec.pdf](https://mdr.nims.go.jp/filesets/f7b0a9af-e760-4048-953c-424c08162486/download)

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

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[Low loss optical coupling structure between two ends of silica glass optical fibers by inserting TeO2 melt](https://mdr.nims.go.jp/datasets/bda55d20-d362-4986-b75e-c3f1720df49c)

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Low loss optical coupling structure between two endsof silica glass optical fibers by inserting TeO2 meltS. Todoroki∗ and S. InoueAdvanced Materials Laboratory, National Institute for Materials Science,Namiki 1-1, Tsukuba, Ibaraki 305-0044, JAPANAbstractLess than 1.5dB of insertion loss was realized in an optical coupling structure in which twoTEC (Thermal-diffusion Expanded Core) fibers are spliced via quenched TeO2 melt whoselength was 0.5mm. The quenched melt seems to be free of precipitates because they wouldbring about larger loss if existed. The loss due to imperfect optical coupling between thefibers is estimated to be about 1 dB, which can be reduced by introducing some refractiveindex modulation into the present structure.Key words: optical fiber, tellurite glassPACS:42.81.i, 42.82.Fv, 81.05.Kf1 IntroductionMaking a low-loss optical connection among photonic components is an essentialart for integrating photonic circuit but also a challenging work when the modules∗ Corresponding author.Email address:TODOROKI.Shin-ichi@nims.go.jp (S. Todoroki).URL: http://www.geocities.com/Tokyo/1406/ (S. Todoroki).Preprint submitted to Elsevier Science 19 September 2003are made of different kind of materials, such as silica fiber and non-silica glass hav-ing low softening temperature. Recently, the authors succeeded to make an opticalcoupling structure between two ends of silica glass optical fibers by inserting sev-eral nano litters of tellurite glass melt (80TeO2-20ZnO in mol%[1] and TeO2[2]).In spite of the large gap in thermal expansion coefficient among these glasses, nofracture and bubbles were observed in the tellurite glass segment. Its insertion losswas, however, quite high as about 10dB for 0.5mm-long tellurite glass segmentbecause there is no waveguide structure in the segment.In order to decrease the insertion loss, we tried to make this structure by using TEC(Thermal-diffusion Expanded Core) fibers in which the diameter of the core at thefiber-end is expanded so as to make the outgoing beam collimated.2 Experimental proceduresCommercial single-mode fiber cables with TEC treatment are used in this study.Their expanded core diameters are about 30µm. Two fibers were placed on fiberholders so that their ends face each other. A gold plate with a small heater was setbetween the two ends of the fibers. A small amount of TeO2 powder (5N, ShinkoChemical Co., Ltd.) was melted on the heater, which was kept at a constant tem-perature of about 460◦C. The end of the fibers were inserted into the glass meltfrom its side(see Fig. 1(1)). Then, the plate is lowered to leave a small amount ofthe melt between the two ends(Fig. 1(2)). The fibers were immediately moved to anappropriate position before the melt was solidified(Fig. 1(3)). It takes few secondsto perform all the movements of the fibers and the heater described above, whichare controlled by a personal computer. We made two set of this coupling structuresand also made another group of sets by using normal single-mode(SM) fiber cables,2whose core diameter is about 10µm, for reference.The location of scattering points along the light path of the optical coupling struc-ture was determined by a high-resolution reflectometer (AQ7410A, Ando ElectricCo.,Ltd.) which consists of a Michelson interferometer and a laser of 1.31µm.This information gives us not only the distance between the two fiber ends butalso whether or not some crystals and/or bubbles precipitated in the glass segment.Transmittance of the laser light through the optical coupling structure was measuredby an optical multimeter (AQ-2140, Ando Electric Co.,Ltd.). These measurementsalso performed on an empty fiber pair varying the distance of two fiber ends.3 ResultsFig. 2 shows a typical side view of the coupling structure in which any visibleprecipitation is not observed. The reflection measurement also shows that there’sno reflection along the light path except at the two interfaces between the fiber andthe glass.Fig. 3 shows the insertion loss values of the optical coupling structures (closedmarks) and empty fiber pairs (open marks) as a function of the distance betweenthe two fiber ends,d. 0 dB of the insertion loss corresponds a configuration wheretwo fiber ends are physically contacted to give a minimum transmission loss.The loss values of empty fiber pairs increase withd since the transmitted lightbetween the fibers is not completely collimated even for TEC fibers. The loss valuesof the coupling structures with SM fibers are smaller than that of the fiber pairbecause the outgoing beam from the fiber end refract to inner angle in the glasssegment compared with that in the air. This situation is not clearly observed for3TEC fibers because the outgoing beam is already collimated.There are variations in loss value because of a dis-alignment of facing fiber centers,which is due to an accumulated displacement of the fiber holders during their mo-tions in fabrication. The lowest values for SM fibers in the present trial is 7.63 dBwhile the values for TEC fibers are 1.33 and 1.48 dB, which are less than one-fifthof the former value.4 DiscussionThese insertion loss values includes Fresnel reflection loss at the interface of silicaand TeO2 glasses or air, which is estimated as 0.18 dB or 0.16 dB per an interface,respectively, on the assumption that their refractive indices are 1.46, 2.19 (extrap-olated value in [3]) and 1.00, respectively. Scattering loss due to precipitates at theTeO2 glass segment is not probable because insertion loss values of the couplingstructure with SM fibers for TeO2 glass and 80TeO2-20ZnO glass, which is morethermally stable, are nearly the same[2]. Thus, loss factors intrinsic to TeO2 glass,i.e. absorption and scattering during 0.5mm segment, are expected to be very small.Consequently, the residual loss for the TEC fiber pair is about 1 dB, which is mainlydue to the optical coupling efficiency between the TEC fibers.These reflection loss and coupling loss can be reduced by refractive index mod-ulation at the interface and the glass segment. The reflection is suppressed if therefractive index gap at the interface become decreased by introducing refractiveindex gradient at the end of the fibers[4]. The coupling efficiency is expected toincrease when waveguide structure is induced inside the glass segment by an irra-diation of high-energy laser pulse (∼fs)[5].4The quenching rate of the inserted melt is expected to be more than 103K/sec onthe assumption that the quenched TeO2 melt completely vitrified[2]. This impliesthat even the melts with poor stability, i.e. which can vitrify only by twin rollerquenching method, can be spliced to silica fibers without precipitation. Thus, thisfabrication technique has a possibility of introducing new active function of non-silica glasses into passive silica waveguides.TeO2 has the following advantages as bonding agent in the present fabricationmethod. 1) It forms glassy state only in single component, 2) TeO2 is non-hygroscopic,and 3) melting time is only less than ten seconds so as to avoid volatilization prob-lems.5 ConclusionLow loss optical coupling between two TEC fibers is demonstrated via quenchedTeO2 melt. The lowest insertion loss is 1.33 dB, whose main loss factors are Fres-nel loss (about 0.35 dB) and imperfect coupling between two fibers. Further lossreduction can be possible by modulating refractive index at the coupling structure.Moreover, this fabrication method is useful to make a connection between silicawaveguides and non-silica glasses with poor thermal stability.References[1] S. Todoroki, A. Nukui, S. Inoue, Formation of optical coupling structure between twoends of silica glass optical fibers by inserting tellurite glass melt, J. Ceram. Soc. Jpn.110 (5) (2002) 476–478.5[2] S. Todoroki, A. Nukui, S. Inoue, Formation of optical coupling structure between silicaglass waveguides and molten tellurite glass droplet (invited), in: The InternationalSymposium On Photonic Glasses, Vol. 5061 of SPIE Proceedings, Shanghai, China,2002, (to be submitted).[3] N. Mochida, K. Takahashi, K. Nakata, S. Shibusawa, Properties and structure of thebinary tellurite glasses containing mono- and di-valent cations, J. Ceram. Soc. Jpn.86 (7) (1978) 317–326, (in Japanese).[4] T. Anzaki, K. Mori, T. Kunisada, K. Nakama, K. Nakamura, M. Honda, K. Enjoji,M. Oikawa, T. Fukuzawa, The new optical coupling with the super wide rangeAR by the GRIN-coat, in: Proceedings of the 22nd European Conf. on OpticalCommunication, Copenhagen, Denmark, 2002, P1.12.[5] K. M. Davis, K. Miura, N. Sugimoto, K. Hirao, Writing waveguides in glass withfemtosecond laser, Opt. Lett. 21 (1996) 1729–1731.Figure CaptionsFig. 1 Illustration showing an experimental setup and a procedure to make anoptical coupling structure(see text).Fig. 2 Sideview of an optical coupling structure. The diameter of the fiber is 125µm and the distance between the two fiber end is about 0.5mm.Fig. 3 Insertion loss vs. distance between the two fiber ends for the optical couplingstructure (closed marks) and an empty fiber pair (open circles).6Au plate + HeaterFiber HoldersCCD Cameras(1)(2)(3)Fig. 1. Illustration showing an experimental setup and a procedure to make an optical cou-pling structure(see text).Memo to the publisher: Original figure is an EPS file.7Fig. 2. Sideview of an optical coupling structure. The diameter of the fiber is 125µm andthe distance between the two fiber end is about 0.5mm.Memo to the publisher: Original photo is a JPEG file(1400 x 700).8Fig. 3. Insertion loss vs. distance between the two fiber ends for the optical coupling struc-ture (closed marks) and an empty fiber pair (open circles).Memo to the publisher: Original figure is an EPS file.9