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[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/46bea2e9-f87a-424f-bfb3-895116d116a8)

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Private preprint: SPIE Proceedings Vol. 5601, pp.50–58 1FORMATION OF OPTICAL COUPLING STRUCTUREBETWEEN SILICA GLASS WAVEGUIDES ANDMOLTEN TELLURITE GLASS DROPLETS. Todoroki1, A. Nukui and S. InoueAdvanced Materials Laboratory, National Institute for Material Science1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanAbstractSeveral nano liters of tellurite glass melt (xTeO2-(100 − x)ZnO, x = 80, 90, 100 in mol%) were insertedand quenched between two ends of silica glass optical fibers to form a new optical coupling structure, whoselength was several hundred microns. No visible precipitates were found even in the quenched melt of 100%TeO2. On the basis of reflection and insertion loss measurements and a bending test, it is proved that there’s nomicro crystals in the quenched melt segment which cause light scattering and/or stress concentration.Few tens nano liters of the melt were also inserted into a silica glass capillary tube with the interior diameterof 126 µm, in order to examine their tolerance to the residual stress induced on cooling due to the large gapin thermal expansion coefficient between the two glasses. Neither fracture nor bubbles were observed in thequenched melt inside if its length is less than 2mm. This implies that tellurite melt can be introduced into voidsof sub-mm in size to integrate hybrid lightwave circuits.KEYWORDS: optical fiber, tellurite glass, insertion loss, thermal expansion coefficient1 INTRODUCTIONConstructing integrated photonic circuit needs the technologies to connect various optical modules each other, suchas light sources, modulators and detectors, via optical waveguides. One of the most important materials for opticalwaveguide is silica glass because its transmission loss is so low that it is used for optical fibers and planar lightwavecircuit (PLC), which is made of deposited a-SiO2 thick film on Si substrate. The connection between PLC andsemiconductor-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 because these glasses aremainly fabricated via liquidus state at higher temperature. Recently, the authors succeeded to make an opticalcoupling structure between two ends of silica glass optical fibers by inserting several nano litters of tellurite glassmelt (80TeO2-20ZnO in mol%)[1]. In spite of the large gap in thermal expansion coefficient among these glasses(table 1), no fracture and bubbles were observed in the tellurite glass segment. Since there is no waveguide structurein the glass segment, its insertion loss was about 10dB.In this fabrication process, the melt is expected to be quenched very rapidly because the volume of the meltis only several nano litters and most of its surface is exposed to open air. Thus, this process may be applicable tothe glasses which has poor stability against crystallization. Glass forming regions of binary zinc tellurite systemare determined by B̈urgeret al.[2] as a function of cooling rate, which is shown in Fig. 1 on a phase diagram ofthis system[3]. From this figure, the stability of the melt is found to decrease drastically from 80TeO2-20ZnO to100TeO2. In the first half of this study, we tried to make optical coupling structures with TeO2 melt and examinedwhether precipitation occurred or not through optical measurements.In order to introduce non-silica glass materials into silica-based PLC by this fabrication method, small voidsshould be made inside the silica glass layer to be filled by the melt. In this configuration, the surface area of theglass melt exposed to air becomes smaller compared with the case for a fiber pair. This means that the stressinduced at the interface between the two glasses is not negligible. In the latter half of this paper, we introduced themelt into a silica glass capillary tube to see under what condition the quenched glass is damaged.1Email: TODOROKI.Shin-ichi @nims.go.jpPrivate preprint: SPIE Proceedings Vol. 5601, pp.50–58 2Table 1: Properties of the glasses used in this study. Data source: O.V. Mazurinet al., Handbook of glass data.thermal expansion coefficient (×10−7/◦C) refractive indexSiO2 ∼6 1.4680TeO2-20ZnO (mol%) 170 2.08Pt plate + HeaterFiber HoldersCCD Cameras(1)(2)(3)Figure 1: Phase diagram[3] and glass forming regions of TeO2–ZnO system with different cooling rates[2].N:cooled in a carbon mold,�: cooled in a copper mold, andF: quenched by pouring into a rotating twin roll.Figure 2: Illustration showing an experimental setup and a procedure to make an optical coupling structure(seetext).2 EXPERIMENTAL2.1 Fiber splicingCommercial optical fiber cables (single mode, core diameter: 10µm, 3m-long with FC connectors) are used inthis study. Bare fibers were cut by a fiber cleaver (York FK11-4). Two fibers were placed on fiber holders so thattheir ends face each other, as shown in Fig. 2. A Pt plate with a small heater (width: 10mm) was set between thetwo ends of the fibers. Their relative positions were controlled by a personal computer. The heater was kept at aconstant temperature of about 440◦C which was monitored through a thermo couple placed on the back.The glass melt was supplied by putting a small piece of glass (xTeO2-(100− x)ZnO, x = 80, 90 in mol%) ora small amount of TeO2 powder (5N, Shinko Chemical Co.,Ltd.) on the Pt plate. These glass pieces were preparedby the following procedures. The corresponding glass melt is melted in a Pt crucible heated at 800◦C. Then themelt is sucked into a Pyrex glass capillary tube (inner diameter: 1.5mm) and cooled to room temperature[4, 5].Finally, the inner glass bar is taken from the outer tube.The droplet on the heater was observed through video cameras placed from its top and side. Two fibers wereinserted into the droplet from its side((1) in Fig. 2). Then, the plate is lowered to leave a small amount of the meltbetween the two ends(2). Lastly, the fibers were immediately moved to an appropriate position before the melt wassolidified(3). All the movements described above ends within few seconds.Reflection from the optical coupling structure 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 resolutionis 20 µm. Transmittance of the laser light through the optical coupling structure was measured by an opticalmultimeter (AQ-2140, Ando Electric Co.,Ltd.). These measurements also performed on an empty fiber pair varyingPrivate preprint: SPIE Proceedings Vol. 5601, pp.50–58 3Figure 3: Photographs of an optical coupling structure, in which the composition of the quenched melt is 100%TeO2. (a, left) The diameter of the fiber is 125µm and the distance between the two fiber end is about 0.4mm. (b,right) The fiber is bended in order to test its toughness. The diameter of the coin is 22mm.the distance of two fiber ends.2.2 Capillary fillingA silica glass capillary tube (OD 1.8mmφ × ID 0.126mmφ × 20mm) is placed in a vertical ring furnace kept at800◦C after an silica glass optical fiber (OD 0.125mm) is inserted into the capillary from its lower hole. A smallPt crucible containing tellurite glass melt (80TeO2-20ZnO) is also placed beside the capillary in the furnace. A Ptwire is dipped into the melt and then a droplet of the melt at the top of the wire is placed over the upper hole ofthe capillary tube. Next, the inserted fiber is pulled and removed from the capillary in order to introduce the meltinside. Then, the capillary is taken from the furnace to cool it.3 RESULTS3.1 Fiber splicingFor each of the melt compositions, optical coupling structure was made without any apparent precipitation. Fig. 3(a)shows a side view of the structure in which TeO2 melt was quenched. The diameter of the fiber is 125µm and thedistance between the two fiber end is about 0.4mm. This structure is not so fragile if properly treated that the fibersegment can be bended without fracture as shown in Fig. 3(b).Fig. 4 is a typical example of the distribution of reflected light along the light path of the optical couplingstructure. There are only two sharp peaks which correspond to the reflection from the fiber ends. The fine structurebelow−50dB is due to the noise of light source. No sample is found to exhibit extra peaks other than these twopeaks. Since this measurement assumes the refractive index of the whole path to be 1.5, the distance between thetwo peaks,dnominal, is not correspond to its true value,d. The relation between the two lengths is described asd/n = dnominal/1.5, where the refractive index of the glass segment is aboutn ∼ 2.1.Fig. 5 shows the insertion loss values of the optical coupling structures (closed stars for TeO2 glass and openpolygons for 80TeO2-20ZnO glass) and an empty fiber pair (open circles) as a function of the distance between thetwo fiber ends. Each distance is calculated from the reflection measurement described above. The refractive indicesof 80TeO2-20ZnO and 100TeO2 glasses are assumed to be 2.08 and 2.19(extrapolated value)[6], respectively. 0 dBof the insertion loss corresponds a configuration where two fiber ends are physically contacted to give a minimumtransmission loss. There are variations in insertion loss because of an error in disalignment of facing fibers whichis due to an accumulated displacement of fiber holders during their motion in fabrication[7].Since the transmitted light between the fibers is not collimated, the insertion loss values increase withd. Theloss values of the optical coupling structures are smaller than that of the fiber pair because the transmitted lightrefract more strongly in the glass melt compared with in the air[7].3.2 Capillary fillingIn the quenched melt inside the capillary, some fractures and/or bubbles were observed if the length of the meltis larger than 2mm. Figure 6 shows a typical damage-free glass segment, in which residual stress is found by thePrivate preprint: SPIE Proceedings Vol. 5601, pp.50–58 4Figure 4: Distribution of reflection along the light path of a coupling structure shown in Fig. 3(a), in which thecomposition of the quenched melt is 100% TeO2.Figure 5: Insertion loss vs. distance between the two fiber ends for the coupling structure (open polygons for80TeO2–20ZnO glass and closed stars for TeO2 glass) and an empty fiber pair (open circles).Figure 6: Sideview of quenched melt (80TeO2–20ZnO) inside a silica glass capillary. The outer diameter of thecapillary is 1.8mm.Figure 7: An idea to utilize the present fabrication technique to make a hybrid PLC(see text).observation of a strain inspection scope.4 DISCUSSION4.1 Quenching rateThere seems to be no precipitates in the quenched melt of TeO2. Although the absence of crystals is not provedby XRD measurement, it is possible to conclude that there’s no precipitate which scatter the incident light in thisstructure. In fact, since the insertion loss values for TeO2 glass are comparable to those for 80TeO2–20ZnO glass,attenuation factors for light transmission among them are much the same. Moreover, considering that fractures ofglass fibers are generally caused by a stress concentration at a micro crystal precipitated on their surface[8], thebending test shown in Fig. 3(b) also supports to be free of precipitates. Thus, there’s no precipitation which isharmful to optical devices.The quenching rate of the melt in this fabrication technique is considered to be as large as that in twin-rollerquenching method, which corresponds to be 103 K/s[2] (see Fig. 1). This implies that even the melts with poorstability which can vitrify only by twin roller quenching method can be spliced to silica fibers without precipitation.This means that we can use considerably wider range of glass compositions to make these optical coupling structurePrivate preprint: SPIE Proceedings Vol. 5601, pp.50–58 5compared with the range for making optical fibers, in which glasses have to survive heating process for a long timewithout precipitation.4.2 Possible applicationsThe present fabrication technique has a potential to be applied to form a hybrid PLC. For example, a hybrid PLCmay be fabricated by the following procedure illustrated in Fig. 7. A small amount (∼n`) of hot melt is captured atthe top of two fibers and to is transmitted to an appropriate void on the PLC in order to be coupled with embeddedwaveguides. Since the void in PLC is rigid compared with the void between the ends of the fiber pair, the variationin insertion loss value is expected to be small[7]. Tellurite glasses are appropriate for hybrid PLC because theirsoftening temperature is about 350◦C, much lower than that of silica glass, and is known to show active propertiessuch as non-linear optical effect[9], acousto-optics effect[10] and broad band amplification for 1.55µm band whenEr3+ ions are doped[11].Although the thermal expansion coefficient of tellurite glass is 2-orders bigger than that of silica glass (table 1),the capillary filling test in this study suggests that the fracture due to residual stress can be suppressed if the voidsize is in the order of sub-mm.Another possible application is to make microcavity devices, in which lasing with ultra-low threshold is pos-sible. For example, Spillaneet al. demonstrated recently that Raman lasing with 60µW pumping is possible in asystem of silica glass microspheres attached with a thin silica glass optical fiber[12]. It will have a considerableimpact on this field if microcavity devices made of non-silica glasses are realized. Some methods of making non-silica glass microspheres are already proposed such as re-heating raw glass powder in a furnace[13] and pouringglass melt into liquid nitrogen[14]. By these methods, however, it is very hard to control the cavity size and toconnect the microsphere to existing optical waveguides.Since the present fabrication method provides a technique to sample nano liters of hot melt and to quench itrapidly, it can help to make microcavities with further controllability. Moreover, high refractive index of telluriteglass enables to capture light into microcavities efficiently. We are now planning to make such microcavitiesconnected with optical fibers.5 SUMMARYSeveral nano liters of tellurite glass melt is captured between the ends of silica glass optical fibers and quenchedit in order to make optical coupling structures. The quenching speed is found to be so fast that even TeO2 glassmelt is solidified without any visible precipitation, light scattering and stress concentration leading to fracture.Although a large gap in thermal expansion coefficient among these glasses bring about some residual stress at theinterface, fractures can be prevented if the cavity size is within sub-mm. This fabrication technique can be appliedto make hybrid planar lightwave circuit and microcavity devices.REFERENCE[1] S. Todoroki, A. Nukui, and S. Inoue, “Formation of optical coupling structure between two ends of silicaglass optical fibers by inserting tellurite glass melt,”J. Ceram. Soc. Jpn., vol. 110, no. 5, pp. 476–478, 2002.[2] H. Bürger, K. Kneipp, H. Hobert, W. Vogel, V. Kozhukharov, and S. Neov, “Glass formation, properties andstructure of glasses in the TeO2-ZnO system,”J. Non-Cryst. Solids, vol. 151, pp. 134–142, 1992.[3] M. Marinov and V. KozhukharovC.R. Acad. Bulg. Sci., vol. 25, p. 329, 1972.[4] S. Todoroki, S. Inoue, and T. Matsumoto, “Combinatorial evaluation system for thermal properties of glassmaterials using a vertical furnace with temperature gradient,”Appl. Surface Sci., vol. 189, no. 3–4, pp. 241–244, 2002.[5] S. Todoroki, T. Matsumoto, and S. Inoue, “Rapid and quantitative determination of crystallization tendency ofzinc tellurite glass melt by using temperature-gradient furnace,” inCombinatorial and Artificial IntelligenceMethods in Material Science(I. Takeuchi, J. M. Newsam, L. T. Wille, H. Koinuma, and E. J. Amis, eds.),vol. 700 ofMaterial Research Society Symposium Proceedings, (Pennsylvania, USA), pp. 209–214, MaterialResearch Society, 2002.Private preprint: SPIE Proceedings Vol. 5601, pp.50–58 6[6] N. Mochida, K. Takahashi, K. Nakata, and S. Shibusawa, “Properties and structure of the binary telluriteglasses containing mono- and di-valent cations,”J. Ceram. Soc. Jpn., vol. 86, no. 7, pp. 317–326, 1978. (inJapanese).[7] S. Todoroki, A. Nukui, and S. Inoue, “Formation of optical coupling structure between two ends of silicaglass optical fibers by inserting tellurite glass melt,”J. Austral. Ceram. Soc.to be submitted.[8] K. Fujiura, “Strength of non-silica fiber for use in optical fiber amplifiers,”New Glass, vol. 15, no. 2, pp. 13–17, 2000. (in Japanese).[9] K. Tanaka, K. Kashima, K. Hirao, N. Soga, A. Mito, and H. Nasu, “Second harmonic generation in poledtellurite glasses,”Jpn. J. Appl. Phys., vol. 32, no. 6B, pp. L843–L845, 1993.[10] T. Yano, A. Fukumoto, and A. Watanabe, “Tellurite glass: A new acousto-optic material,”J. Appl. Phys.,vol. 42, no. 10, pp. 3674–3676, 1971.[11] A. Mori, Y. Ohishi, and S. Sudo, “Erbium-doped tellurite fiber laser and amplifier,”Electron. Lett., vol. 33,no. 10, pp. 863–864, 1997.[12] S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralow-threshold raman laser using a sperical dielectricmicrocavity,”Nature, vol. 415, pp. 621–623, 2002.[13] F. Lissillour, P. F́eron, N. Dubreuil, P. Dupriez, G. M. Stéphan, and M. Poulain, “Whispering-gallery modeof Er-ZBLAN microlasers at 1.56µm,” in Proc. SPIE Conf. on Laser Resonators II, vol. 3611, pp. 199–205,Jan. 1999.[14] K. Miura, K. Tanaka, and K. Hirao, “CW laser oscillation on both the4F3/2-4I11/2 and4F3/2-4I13/2 tran-sitions of Nd3+ ions using a fluoride glass microsphere,”J. Non-Cryst. Solids, vol. 213–214, pp. 276–280,1997.