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

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[Formation of optical coupling structure between two ends of silica glass optical fibers by inserting tellurite glass melt](https://mdr.nims.go.jp/datasets/56638cd6-4185-4e41-a3d9-8e15dc488d43)

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「インテグレーション特集号用原稿」a) 投稿責任者連絡先〒 305-0044 つくば市並木 1-1 物質・材料研究機構 物質研究所轟 眞市Tel: 0298-58-5648 Fax: 0298-54-9060E-mail: TODOROKI.Shin-ichi@nims.go.jpb) 報文種類 論文c) C1-a C2 C3d) シリカガラス光ファイバ先端間をテルライトガラスで満たした光結合構造の形成Formation of optical coupling structure between two ends of silicaglass optical fibers by inserting tellurite glass melte) 轟 眞市・貫井 昭彦・井上 悟物質・材料研究機構 物質研究所Shin-ichi Todoroki, Akihiko Nukui and Satiru InoueAdvanced Materials Laboratory, National Institute for Material Sciencef) Several nano liters of tellurite glass melt was inserted and quenched between two ends ofsilica glass optical fibers to form a optical coupling structure, whose length was several hundredµm. Dispite the large gap of thermal expansion coefficient between these glass materials, neitherfracture nor bubbles were observed, which usually lead to a large optical propagation loss. Theinsertion loss was less than 10 dB, which was mainly due to the lack of an optical waveguidestructure in the tellurite glass segment. Further loss decrease is expected to be possible byintroducing a refractive index modulation.g)数 nlのテルライトガラス融液を２本のシリカガラス製光ファイバの間の数百μmの空間に挿入後急冷した光結合構造を作製した。両ガラスの熱膨張係数が大きく異なるにもかかわらず、光散乱の原因になるようなヒビや気泡は発生しなかった。挿入損失は 10dB以下であり、この原因はテルライトガラス部分に導波構造を持たないことによる。屈折率変調を導入すれば、この損失値を低下させることが可能と考えられる。h) optical fiber, tellurite glass, thermal expansion注意)写真の原本は JPEGファイルである。(Fig.2: 1600x600, Fig.6: 1600x800)Typeset using jjap.cls <ver.1.0.1>Formation of optical coupling structure between two ends of silica glassoptical fibers by inserting tellurite glass melt.Shin-ichi TODOROKI, Akihiko NUKUI and Satiru INOUEAdvanced Materials Laboratory, National Institute for Material Science1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan(Received )Several nano liters of tellurite glass melt was inserted and quenched between two ends ofsilica glass optical fibers to form a optical coupling structure, whose length was several hundredµm. Dispite the large gap of thermal expansion coefficient between these glass materials, neitherfracture nor bubbles were observed, which usually lead to a large optical propagation loss. Theinsertion loss was less than 10 dB, which was mainly due to the lack of an optical waveguidestructure in the tellurite glass segment. Further loss decrease is expected to be possible byintroducing a refractive index modulation.KEYWORDS: optical fiber, tellurite glass, thermal expansion11. IntroductionConstructing integrated photonic circuit needs the technologies to connect various opticalmodules each other, such as light sources, modulators and detectors, via optical waveguides.The most typical material for optical waveguide is silica glass because its transmission loss isso low that it is used for optical fibers and planar lightwave circuit (PLC), which is made ofdeposited a-SiO2 thick film on Si substrate.1) The connection between PLC and semiconductor-based optical modules is easily accomplished, because their fabrication technique is common,i.e. deposition, lithography and etching.As for the modules made of inorganic glasses, except silica glass, it is not so easy becausethese glasses are mainly fabricated via liquidus state at higher temperature. Optical fiber is thetypical example; hot glass melt is poured into a mold to make a fiber preform, which is then re-heated to draw fibers.2) In order to introduce non-silica glass materials into existing fabricationprocess for semiconductors, some deposition techniques are being investigated.3)In this study, we propose another approach to connect non-silica glass device and silica-glass-based waveguide, that is, shaping the glass melt directly on the waveguide by introducingspot-heating and manipulating technique of small amount of hot melt. This is possible in theoryif the hot melt does not react with the waveguide and the quenched glass acts as an opticaldevice itself. The first limitation is satisfied by using glasses with low softening temperature.Tellurite glasses are appropriate for this purpose because their softening temperature is about350 ◦C, much lower than that of silica glass, and is known to show active properties such asnon-linear optical effect,4) acousto-optics effect5) and broad band amplification for 1.55µmband when Er3+ ions are doped.6) In this study, tellurite glass melt is inserted between two endsof silica glass optical fibers to form an optical coupling structure. Furthermore, transmittanceand reflectance of this device is evaluated.2. ExperimentCommercial optical fiber cables (single mode, 3m, with FC connectors) are used in thisstudy. Bare fibers were cut by a diamond blade with ultrasonic vibration which was placedon the surface of the fiber stretched along its length. Two fibers were placed on fiber holdersso that their ends face each other, as shown in Fig. 1. A Pt plate with a small heater was2set between the two ends of the fibers. Their relative positions were controlled by a personalcomputer. The heater was kept at a constant temperature of about 400◦C which was monitoredthrough a thermo couple placed on the back. A small piece of 80TeO2-20ZnO (mol%) glasswas melted on the Pt plate. The glass transition temperature for this glass is 307◦C, which wasdetermined by DTA measurement (heating rate: 10 K/min). The droplet was observed throughvideo cameras placed from its top and side. Two fibers were inserted into the droplet from itsside. Then, the plate is lowered to leave a small amount of the melt between the two ends. Thefibers were immediately moved to an appropriate position before the melt was solidified. Themovement of the fibers described above ends within 1 seconds.Reflection from the optical coupling structure was measured by a High-Resolution Reflec-tometer (AQ7410A, Ando Electric Co.,Ltd.) which consists of a Michelson interferometer anda laser of 1.31µm. Transmittance was measured by a halogen lamp and spectral analyzer (AQ-6315B, Ando Electric Co.,Ltd.).3. ResultFig. 2 shows a side view of the optical coupling structure. The diameter of the fiber is 125µmand the distance between the two fiber end is about 0.6mm. Fig. 3 is the distribution of reflectedlight along the light path of the optical coupling structure and an empty fiber pair. There isonly two sharp peaks which correspond to the reflection from the fiber ends. The fine structurebelow−65dB is due to the noise of light source. Since this measurement assumes the refractiveindex of the whole path to be 1.5, the length between the two peaks is not correspond to its truelength, that is, the length seems to be shorter for glass (n ∼ 2) and longer for air (n ∼ 1).Transmittance spectrum of the same structure and an optical fiber only are shown as thedashed lines in Fig. 4. The subtraction of these spectrum corresponds to the insertion loss ofthis optical coupling structure, which is less than about 10 dB. The hole located near 1100nm ap-pears because there is a difference in the cut-off wavelength of single mode propagation amongthese two fiber cables. Since these cables are commercial products, their cut-off wavelengthsare not guaranteed to be the same value as their propagation losses are. Thus, the subtractionspectrum is reliable except at around the hole.34. DiscussionOn splicing two materials each other, we have to be careful for the gap between their prop-erties such as refractive index and thermal expansion coefficient. The former gap bring aboutFresnel’s reflection and the latter an internal stress. After discussing these effects, the merits ofthis process are discussed.4.1 Origin of the insertion lossAccording to Fresnel’s laws of reflection, reflectance and transmittance in energy at normalincidence are given asR =(n1 − n2n1 + n2)2(1)andT =4n1n2(n1 + n2)2, (2)wheren1 andn2 are refractive indices of incidence and transmittance sides respectively. In thissection, we assume the refractive indices of the glasses used in this study, i.e. silica glass andtellurite glass, as 1.46 and 2.00. Thus,R = 0.024 andT = 0.976 are obtained. Then, under anideal condition that this optical coupling structure has no loss except Fresnel’s reflection loss,its insertion loss is calculated asT 2 = 0.95 = −0.2 dB (see Fig. 5 top).On the other hand, the actual transmittance for 1.31µm light is−8.6 dB ∼ 0.14 (see Fig. 5bottom) as shown in Fig. 4. The gap appears because there is no waveguide structure in thetellurite glass segment, which bring about low optical coupling. Assuming the coupling ratiox,the actual transmittance is written asxT 2. Thus,x = −8.4 dB∼ 0.14 is obtained.In order to observe light propagation in the glass segment, we made a coupling structure byusing Er3+-doped tellurite glass melt, which is known to show upconversion fluorescence whenexcited by 800nm light.7) The Er3+ ions were excited by an irradiation of CW Ti:Sapphirelaser light (800 nm, 10 mw) which is propagated in multi-mode through one of the fibers. Asshown in Fig. 6, the propagation is recognized as a expanding beam of green upconversionfluorescence.To increase the optical coupling efficiency in this structure, the beam expansion in the glasssegment should be restricted. This can be performed by TEC (Thermal-diffusion ExpandedCore) fiber, where refractive index distribution near the end of the fiber is modified so that its4beam propagation is controlled. It may be also effective if a refractive index modification isinduced inside the glass segment by an irradiation of high-energy laser pulse (∼fs).8)4.2 Effect of thermal expansion coefficientWhen we splice two materials by melting, it is desirable if their thermal expansion coefficientare same value. Because, if the amount of shrinkage on cooling is different each other, aninternal stress is generated along the interface of these two materials, which may bring aboutsome cracks and/or precipitation of dissolved gases.Between the two glasses used in the present coupling structure, there is a large gap in thermalexpansion coefficient as listed in Table I. The gap is one order larger than that for a-SiO2/Si pair.This pair is used in commercially available PLC. In spite of this two-order-gap, no fracture andbubble are observed in the glass segment. This is supported by the reflection data shown inFig. 3 that no reflection is found between the two interfaces. The reason is considered that thearea of the interface is so small that the induced internal stress is under the critical point ofappearing cracks and bubbles. The absence of reflection also shows that there’s no precipitationof crystals which causes light scattering.4.3 Merits of the process proposedGenerally, optical fibers made of non-silica glasses are fabricated by preform method or dou-ble crucible method. By the former method, the glass melts are once quenched to room tem-perature, reformed to make a fiber preform, and re-heated to draw fibers. During this process,crystallization may occur because the glass stays at just above the glass transition temperature,at which nucleation rate is its maximum. By the latter method, the glass melts stay long time atits softening temperature in the crucible, which may also bring about precipitation of crystals.As for the proposed process, the glass segment is connected with optical waveguides by amechanical operation in a few seconds. Since the volume of the glass segment is several nanoliters, the melt can be quenched so quickly to prevent precipitation of crystals. Therefore, thismethod can be applied for even the glass materials not suitable for fiber drawing due to its poorthermal stability.55. ConclusionWe fabricated an optical coupling structure in which two silica glass optical fibers are splicedby tellurite glass. This was made by manipulating small amount of the glass melt through theoptical fibers. In spite of a large gap in thermal expansion coefficient among these glasses, nofracture and bubbles are observed in the tellurite glass segment. Although the insertion loss isabout 10dB, this can be reduced by modification of refractive index profile of the structure.6References1) Kawachi, M., Yasu, M. and Edahiro, T.,Electron. Lett., 19, 583–584 (1983).2) Todoroki, S. and Sakaguchi, S., “Drawing of oxide glass optical fibers,” inProperties ofGlasses and Rare-earth doped glasses for optical fibers(D. Hewak, ed.), EMIS DatareviewSeries, ch. B.4.2, pp. 138–141, London, UK: INSPEC, IEE, 1998.3) Sp̈alter, S., Lenz, G., Slusher, R. E., Hwang, H. Y., Zimmermann, J., Katsufuji, T., Cheong,S.-W. and Lines, M. E.,2000 OFC/Technical Digest Series Conference Edition, ThI4–1,137–139 (2000).4) Tanaka, K., Kashima, K., Hirao, K., Soga, N., Mito, A. and Nasu, H.,Jpn. J. Appl. Phys.,32, L843–L845 (1993).5) Yano, T., Fukumoto, A. and Watanabe, A.,J. Appl. Phys., 42, 3674–3676 (1971).6) Mori, A., Ohishi, Y. and Sudo, S.,Electron. Lett., 33, 863–864 (1997).7) Tanabe, S., Hirao, K. and Soga, N.,J. Non-Cryst. Solids, 122, 79–82 (1990).8) Davis, K. M., Miura, K., Sugimoto, N. and Hirao, K.,Opt. Lett., 21, 1729–1731 (1996).7Figure captionsFig. 1. Experimental setup.Fig. 2. Sideview of an optical coupling structure. The spacing between the two silica fibers isabout 0.6 mm.Fig. 3. Distribution of reflection along the light path of the coupling structure and an fiber pairwhich is separated by air.Fig. 4. Transparent spectra of the coupling structure and an silica glass fiber withoutair-spacing.Fig. 5. Illustration of analysis in loss factors(see text).Fig. 6. Sideview of an optical coupling structure with Er3+-doped tellurite glass. Upconver-sion fluorescence is observed by exciting 800nm laser light.8Pt plate + HeaterFiber HoldersVideo CamerasFig. 1 (TODOROKI)9Fig. 2 (TODOROKI)10Fig. 3 (TODOROKI)11Fig. 4 (TODOROKI)12         95%(=-0.2dB)2.4%Fresnel reflection lossCoupling ratio2.4%Experimental value-8.6dB = 14%                     x          -8.4dB = 14%0.6 mm×n=2.00, 1.46Fig. 5 (TODOROKI)13Fig. 6 (TODOROKI)14Table I. Thermal expansion coefficient for several materials taken from some databooks(×10−7/◦C).silica glass ∼6Silicon 26.380TeO2-20ZnO glass (mol%) 17015