Strictly Non-blocking NxN Thermo-Capillarity OpticalMatrix Switch using Silica-based WaveguideMitsuhiro Makihara, Fusao Shimokawa, and Kazumasa KanekoNTT Telecommunication Energy Laboratories, 3-9-11, Midori-Cho, Musashino-Shi, Tokyo 180-8585 Japantelephone:+81-422-59-2664, fax:+81-422-59-4622, email address:makihara@ilab.ntt.co.jpWe propose a silica-based strictly non-blocking NxN thermo-capillarity optical matrix switch. Thefeasibility of 2-dimensional batch oil injection and multi-layered wiring were confirmed by fabricating aprototype 16x16 optical matrix switch.
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`Strictly Non-blocking NxN Thermo-Capillarity OpticalMatrix Switch using Silica-based WaveguideMitsuhiro Makihara, Fusao Shimokawa, and Kazumasa KanekoNTT Telecommunication Energy Laboratories, 3-9-11, Midori-Cho, Musashino-Shi, Tokyo 180-8585 Japantelephone:+81-422-59-2664, fax:+81-422-59-4622, email address:makihara@ilab.ntt.co.jpWe propose a strictly non-blocking NxN thermo-capillarity optical matrix switch using a silica-based waveguide. A prototype 16x16 optical matrix switch was fabricated using 2-dimensional batch oilinjection and multi-layered wiring, thus confirming the feasibility of these important developmenttechniques.1. IntroductionIn the rapidly developing field of high-speed and high-density network systems using mainlyoptical wavelength division multiplexing technologies, space-division optical switches are indispensablefor future fiber-optic communication systems such as optical path cross-connect nodes, photonic inter-module connectors, and protection switches. We have been studying thermo-capillarity optical switchesfor such applications [1]. These switches have many advantages such as low insertion loss, polarization-and wavelength-insensitive operation, low consumption power by self-latching of the optical path, long-term stability, and suitability for large-scale integration. We propose a strictly non-blocking thermo-capillarity optical matrix switch, and report two key techniques in its fabrication and their feasibility.2. Structure of optical matrix switchFigure 1 shows the basic structure of the thermo-capillarity optical switch element. This switchelement consists of an upper substrate and an intersecting waveguide substrate that has a slit at eachcrossing point with refractive index matching oil in it and a pair of microheaters that produce a thermalgradient along the slit. The basic concept of the optical switching is that the matching oil within the slit isdriven by a decrease in interfacial tension of the air-oil interface caused by heating (thermo-capillarity).This switch element also has bi-stable self-latching achieved by capillary pressure that depends on the slitwidth. These basic characteristics have already been confirmed in practice [2].Fig. 1 Basic structure of thermo-capillarity optical switch element.Silica-based waveguideMicroheaterRefractive index matching oilCoreMain injection pathSub injection pathSubstrateUpper substrateSlit
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`The NxN optical matrix switch consists of switch elements that are configured like a grid (Fig. 2).To inject the oil into all the slits, one main injection path is fabricated on the upper substrate for all theslits in a row, and these connect with the main injection path through sub injection paths fabricated on theupper substrate (Fig. 2). This structure allows the desired amount of the oil to be injected into all the slitssimultaneously by using a conventional pressure control method [3]. After the oil in the main injectionpath has been removed, the main injection path is filled with a sealing agent and the sub injection pathsare sealed. As a result, all of the oil in the slits is independently sealed.Fig. 2 Structure of 8x8 thermo-capillarity optical matrix switch.Fig. 3 Structure of multi-layered and shared wiring to microheater.In our NxN optical matrix switch, wiring to 2 NxN microheaters is necessary in order to drive theoil in the slit. We applied multi-layered wiring in the semiconductor chip fabrication and shared wiring tofabricate flexible high-density wiring. Figure 3 shows their structures. The wiring is configured in rowsand columns, and the microheaters share a wire. A microheater is heated by applying a voltage betweenwires at the row and column. This structure does not allow the matrix switch to switch all optical paths atthe same time, but it does allow a very small large-scale optical matrix switch to be fabricated byexploiting the bi-stable self-latching feature.3. Prototype of optical matrix switchThe NxN optical matrix switch is intended for scales of 8x8 to 16x16. Figure 4 shows a prototype16x16 thermo-capillarity optical matrix switch fabricated using the 2-dimensional batch oil injection andthe multi-layered and shared wiring. Figures 4(a) and (b) show the appearance and switch elements of theoptical matrix switch, respectively. The waveguide chip size was 23 mm x 23 mm and the core pitch was500 mm. The chip size is about one sixteenth of a thermo-optic matrix switch of the same scale [4]. Theinsertion losses were measured at a wavelength of 1.55 mm using an ASE (Amplified SpontaneousEmission) light source with single-mode fibers butted to input and output waveguides. The losses were 4ii+1jj+1rtrttrtrMicroheaterWiring in rowWiring in column: Switch element1b2b3b4b5b6b7b8b2a4a6a8a1a3a5a7aOutput portInput portMain injection pathSub injection pathSlitUpper SubstrateWaveguide substrateCoreSilica-based waveguideRefractive index matching oilMain injection pathSub injection pathSubstrateUpper substrateSlit* The core and microheaters are not illustrated.
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`dB and 10 dB at the shortest and longest optical paths, respectively. These results confirm the feasibilityof 2-dimensional batch oil injection and multi-layered wiring, which are important techniques indepeloping an optical matrix. (a) Appearance of 16x16 optical matrix switch (b) Switch elements of optical matrix switchFig. 4 Prototype of thermo-capillarity optical matrix switch.4. ConclusionWe have proposed a strictly non-blocking NxN thermo-capillarity optical matrix switch using asilica-based waveguide with structures of main and sub injection paths for 2-dimensional batch oilinjection and multi-layered wiring for flexible high-density wiring. The prototype 16x16 optical matrixswitch with these structures was fabricated, and the result of the fabrication confirmed the feasibility of 2-dimensional batch oil injection and multi-layered wiring, which are important techniques for developingan optical matrix switch.References[1] M. Sato, F. Shimokawa, S. Inagaki, and Y. Nishida, Waveguide optical switch for 8:1 standby system of optical lineterminals, Technical digest of OFC’98, pp. 194-195, 1997.[2] M. Sato, M. Makihara, F. Shimokawa, and Y. Nishida, Self-latching waveguide optical switch based on thermo-capillarity,Technical digest of 23rd ECOC (IEE No. 448), pp. 73-76, 1997.[3] H. Togo, M. Sato, and F. Shimokawa, Multi-element thermo-capillary optical switch and sub-nanoliter oil injection,MEMS’99, Orlando, Florida, Jan. 1999.[4] T. Goh, M. Yasu, K. Hattori, A. Himeno, M. Okuno, and Y. Ohmori, Low-loss and high-extinction-ratio silica-based strictlynonblocking 16x16 thermooptic matrix switch, IEEE Photon. Technol. Lett., Vol. 10, pp. 810-812, Jun. 1998.
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