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Module Having 100 Input and 100 Output Ports. Tsuyoshi Yamamoto, Member, IEEE, Johji Yamaguchi, Nobuyuki Takeuchi, Akira Shimizu, Eiji Higurashi,.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 10, OCTOBER 2003

A Three-Dimensional MEMS Optical Switching Module Having 100 Input and 100 Output Ports Tsuyoshi Yamamoto, Member, IEEE, Johji Yamaguchi, Nobuyuki Takeuchi, Akira Shimizu, Eiji Higurashi, Renshi Sawada, and Yuji Uenishi, Senior Member, IEEE

Abstract—This paper describes an optical switching module based on microelectromechanical systems (MEMS) two-axis tilt mirror arrays and low-cost highly accurate free-space optics. The MEMS mirror arrays are integrated on single-crystal silicon wafers and provide reliable switching operation at a low driving voltage. The free-space optics consists of two-dimensional optical fiber and microlens arrays based on low-cost and highly accurate polymer-based components. They provide a compact switching 60 35mm [170 cc]) and are module (approximately 80 assembled passively by using dowel pins. A prototype switch module with 100-ch optical fiber I/O has a low coupling loss of 4.0 dB, a low crosstalk within 60 dB, and switching time of 3 ms. Index Terms—Free-space optics, microelectromechanical systems (MEMS), optical cross-connect (OXC), optical switch.

Fig. 1. Basic structure of the 3-D MEMS optical switch.

II. CONFIGURATION OF THE SWITCHING MODULE

I. INTRODUCTION

D

ENSE WAVELENGTH-DIVISION multiplexing (DWDM) is a mission-critical technology promising to construct a backbone network having a throughput of over several terbits per second. To achieve high robustness to traffic fluctuation and an optimum network configuration by arranging network resources, optical cross connects with wavelength-path capability are going to be introduced into the networks; the cross connect provides multiple restoration options needed for data-traffic management in the network. Recently, three-dimensional (3-D), microelectromechanical systems (MEMS) optical switches have been widely studied for constructing a large-scale, all-optical switching fabric in optical cross connects. MEMS switches are attractive as the number of optical input–output ports increases because of their high-density 3-D connectivity and several switching demonstrations have been received with keen interest [1], [2]. In this paper, we demonstrate a free-space optical switching module based on MEMS two-axis tilt mirror arrays and compact free-space optics. The MEMS mirrors that constitute the array are made of single-crystal silicon and are actuated by electro-static force in two degrees of freedom. The free-space optics consists of low-cost, highly accurate two-dimensional (2-D) optical fiber arrays, microlens arrays, and metal frames. We also present the basic characteristics of our 3-D MEMS optical switching module having 100 input and 100 output ports. Manuscript received January 31, 2003; revised June 12, 2003. The authors are with NTT Microsystem Integration Laboratories, Nippon Telegraph and Telephone Corporation, Kanagawa 243-0198, Japan (e-mail: [email protected]). Digital Object Identifier 10.1109/LPT.2003.818224

A. Basic Structure of the 3-D MEMS Optical Switch Fig. 1 shows the basic configuration of a 3-D MEMS optical switch using free-space optics. The optical beams passing through the optical fiber are collimated by the microlens array, and reflected twice by a two-axis tilt mirror array fabricated using MEMS technology. When mirrors are tilted two-dimensionally, the optical beam is deflected two-dimensionally as well. Therefore, any connection between input and output fibers can be achieved by controlling the tilt angle of each mirror. Additionally, the MEMS technology is suitable for mass production because it is based on silicon large-scale integration (LSI) fabrication processes. B. MEMS Two-Axis Tilt Mirror Array Fig. 2 shows a cross-sectional schematic diagram of the MEMS two-axis tilt mirror. The mirror consists of two single-crystal silicon substrates, the mirror, and driving electrode substrates. These substrates are processed independently and are bonded to each other. Each mirror is supported by folded torsion springs in two orthogonal axes and tilted two-dimensionally by electrostatic force. The tilt angle of the mirror can be changed by controlling the applied voltage between the two substrates. In this mirror, the driving electrode has a 3-D terrace structure and this effectively reduces the control voltage [3]. The fabricated MEMS mirror is shown in Fig. 3. The mirror, supported by the two-axes torsion springs is 600 m in diameter and 10- m thick. The torsion spring, shown in Fig. 3(b), has an aspect ratio of more than six. This high-aspect-ratio spring features strong bending capability relative to torsion and provides strong support for the mirror, which is needed for reli-

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YAMAMOTO et al.: THREE-DIMENSIONAL MEMS OPTICAL SWITCHING MODULE

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Fig. 2. Cross-sectional schematic of the MEMS two-axis tilt mirror.

Fig. 4. Structure of the 2-D optical fiber array.

TABLE I SUMMARY OF THE OPTICAL CHARACTERISTICS

Fig. 3. (a) MEMS mirror and magnified views of the (b) high-aspect torsion spring and (c) 3-D terrace electrode.

able switching operation. The driving electrode has a 3-D terrace structure, as shown in Fig. 3(c). The MEMS mirror arrays, having a 10 10 arrangement and 1.3-mm spacing, are bonded onto a conventional PGA-ceramic package. C. Free-Space Optics in the Switching Module Two-dimensional(2-D) optical fiber arrays are key components in free-space optical switches. However, their assembly has proven to be costly when high-precision positioning is required. Several assembly techniques have been reported. In one such technique, fibers are aligned two dimensionally by using stacked V-grooved substrates [4]. In another, the optical fibers are inserted into holes made in a silicon substrate by wet etching [5]. Such conventional assembly processes may be costly because they require highly accurate, time-consuming fiber handling. To overcome these problems, we have proposed a 2-D fiber array using single-mode optical fibers with metal-micro ferrules and a polymer substrate having an array of precisely aligned holes. Fig. 4 shows the configuration of the fiber array. Optical fibers with conventional metal-micro ferrules and two dowel pins are inserted into holes in the polymer substrate, respectively. Each ferrule end is polished and coated with antireflection material by vapor deposition in advance. The polymer substrate with guide holes, made of a thermosetting molding compound, was molded by a transfer-molding method based on a common technique for the fabrication of ferrules in MT-type optical connectors [6]. Dowel pins are used for alignment between fiber and microlens arrays. In this structure, the fiber ar-

rays are assembled in a series of simple steps with a passive assembly. The fibers in each array have 1.3-mm spacing with m. These results indicate that fiber displacements of within our 2-D fiber arrays provide highly accurate fiber positioning at low cost. The microlens arrays have two guide holes for the dowel pins and made from transparent polymer with a wavelength of 1550–1600 nm. They were fabricated by a precise injection-molding method. Both surfaces of the microlens array also have an antireflection coating to prevent multiple reflections in the free-space optics. The optical components in the module are assembled with a passive approach; they are also mounted on metal frames by using dowel pins. The switching module having 100 input and mm (excluding 100 output ports is approximately optical fibers). III. PERFORMANCE OF THE SWITCHING MODULE We first experimentally evaluated the optical characteristics of the switching modules. Table I shows evaluation results for the optical switching module at 1552-nm wavelength. The typical insertion loss in the ports is 4.0 dB, the return losses are greater than 30 dB, the polarization-dependent losses (PDLs) are less than 0.5 dB, and the crosstalk into adjacent ports is less than 60 dB. These results experimentally confirm that our switching module has good optical characteristics for practical applications. We next evaluated the switching operation of the switching module. Fig. 5 shows an example of the switching operation when driving voltages for the MEMS mirrors are within 50 V and this indicates that the switching time is about 3 ms. This result demonstrates the feasibility of a compact and low-cost 3-D MEMS optical switching module.

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and microlens arrays based on polymer devices, are fabricated with passive assembly. We experimentally confirmed that a prototype module with 100-ch optical fiber I/O provides a low coupling loss of 4.0 dB and switching time of 3 ms. ACKNOWLEDGMENT The authors would like to thank S. Nagashima of NTT Electronics Corporation for his technical assistance. REFERENCES

Fig. 5. Example of a switching operation.

IV. CONCLUSION We have presented a switching module based on MEMS two-axis tilt mirror arrays and low-cost highly accurate free-space optics. The MEMS-mirror arrays consist of single-crystal silicon mirrors supported by high-aspect ratio torsion springs and provide the highly durable mechanical structure needed for reliable operation. The free-space optics, which comprises low-cost and highly accurate 2-D optical fiber

[1] D. J. Bishop, C. R. Giles, and G. P. Austin, “The Lucent LambdaRouter: MEMS technology of the future here today,” IEEE Commun. Mag., vol. 40, pp. 75–89, 2002. [2] R. Ryf et al., “1296-port MEMS transparent optical crossconnect with 2.07 petabit/s switch capacity,” in Proc. Optical Fiber Communication Conf. (OFC 2001), vol. 4, 2001, p. PD28-1-3. [3] R. Sawada, E. Higurashi, A. Shimizu, and T.Tohru Maruno, “Single crystalline mirror actuated electrostatically by terraced electrodes with high-aspect ratio torsion spring,” in Proc. Opt. MEMS 2001, 2001, pp. 23–24. [4] C. M. Schroeder, “Accurate silicon spacer chips for an optical-fiber cable connector,” The Bell Syst. Tech. J., vol. 57, no. 1, pp. 91–97, 1978. [5] G. Proudley, C. Stace, and H. White, “Fabrication of two-dimensional fiber optic arrays for an optical crossbar switch,” Opt. Eng., vol. 33, no. 2, pp. 627–645, 1994. [6] T. Satake, N. Kashima, and M. Oki, “Very small single-mode ten-fiber connector,” J. Lightwave Technol., vol. 6, pp. 269–272, Feb. 1988.