A5-volt operated mems variable opticxlattenuator ... - CiteSeerX

4D1.2 A 5-VOLT OPERATED MEMS VARIABLE OF'TICALATTENUATOR H. Toshiyoshi"', K . Isamoto'", A. Morosawai2',M. Tei'"', and H. Fujita'" Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan i2' Santec Co., Japan Contact: Hiroshi Toshiyoshi,Tel: +81-3-5452-6276, Fax: +81-3-5452-6250, e-mail: hiroG3iis.u-tokyo.ac.jp

ABSTRACT We report a micro mechanical variable optical attenuator (VOA) using an electrostatic micro torsion mirror combined with a fiber-optic collimator. The VOA operates at low voltages (DC 5V or less) for large optical attenuation (40dB) and fast response time (5 ms or faster). The mirror has been designed to .be shock-resistive up to IOOG without causing operation error and up to 5ooG without any mechanical failure. We also have suppressed temperature dependence of VOAs optical performance by mechanically decoupling the parasitic bimorph effect from the electrostatic operation.

Figure 1. Variable oprical artenuators (VOA) in jiber nerwork Shum bae"?

INTRODUCTION Variable optical attenuators (VOAs) are indispensable components in the fiber optic network, and they control the optical signal intensity before/after laser diodes, fiber optic amplifiers, and photo detectors. A large number of VOA's are needed in particular in the wavelength division multiplexing (WDM) system, where intensity of multiple channels of different wavelengths are individually controlled at the wavelength multiplexing (MUX) and demultiplexing.(DEMU) nodes, as shown in Fig. 1. Most conventional VOA's are made up with an assembly of a prism or a mirror driven by a solenoid coil or a motor. Despite excellent optical performance, they have not fully satisfied the escalating demand of customers in terms of device size, power consumption, mechanical reliability and cost. Several MEMS (micro electro mechanical systems) approaches, therefore, have been proposed to meet their requirement. Figure 2 shows three different architectures of MEMS VOAs. Shutter insertion type 11-31 shown in Fig. 2 (a) can be easily integrated with surface / bulk micromacbined actuators but polarization dependent loss (PDL) is generally greater than those of other types. Rotation or sliding mirror near a coupled pair of lens fiber 14-61 (Fig. 2 (b)) can also control coupling efficiency but it occupies relatively large area to hold the fibers on the substrate. Besides mirror, electromechanically operated Fabry-Perot interferometers [7] can control attenuation but complete block out (over 40 dB) is difficult. Hence we employed a torsion mirror with fibers coupled through a collimator as shown in Fig. 2 (c). The advantages of this optomechanical design are small PDL, small package size, and compatibility to arrayed multi-channels. In this work, we also developed a MEMS design for low voltage operation (DC 5Vor less) in spite of large shock tolerance and temperature insensibility.

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DESIGN AND FABRICATION Considering electromechanical stability and high production throughput as well as low operation voltage, we have concluded that the simple gap-closing electrostatic torsion mirror would be the most suitable design. Figure 3

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4D1.2 (a) shows the schematic view of the mirror. A circular minor with gap-closing electrostatic plates is suspended with torsion bars over a shaped through-hole. The whole upper structure is made of a silicon-on-insulator (Sol) wafer. Only the right-hand side p a t s of the SO1 actuator are backed up with the GND substrate via a 2-um-air-gap, where electrostatic attraction torque rotates the actuator plates by a small angle. The optical design including the fibers and the collimator requests a mechanical rotating angle of.only 0.5 degrees for 40-dB attenuation. As shown in Fig. 3 (b), the maximum angle i s determined by the contact angle of the actuator, g I W. There is a large through-hole under the mirror to prevent the mirror from hitting the substrate. Simplicity of actuation mechanism and fabrication process has been our first priority in designing the device for the sake of low production cost and high mechanical reliability. Figure 4 illustrates fabrication steps using two deep M E (DRIE) processes. The active layer of SO1 wafer (30-um-silicon I 2-urn-oxide / 400-um-Si substrate) is first pattemed into the mirror shape by DRIE, and the backside is etched to the buried oxide (BOX) using aluminum mask. After releasing the SUUCture in hydrofluoric acid, chromium and gold layers are deposited by sputtering through a stencil mask. Finally, individual chips are snapped off the wafer to be ready for packaging. Figure 4 (a) shows a scanning tunneling microscope (SEM) view of the finished torsion mirror chip (2.4mm x x 0.5 mm) mounted on a package stem. The center 2.4" mirror is 600 microns in diameter, and the actuator plates on the sides are 700 microns long. Figure 4 (b) is a close-up view of the suspension (2 microns wide and 400 microns long), whose supporting parts xe intentionally rounded (R = SO microns) for mechanical reinforcement. Thanks to this shape, the suspension would not break even after several mechanical shocks of 500 G. Tiny bars extending out from the torsion bar are for preventing the SO1 membrane from rupturing during the fabrication process.

sacrificial etching. In spite of such delicate torsion bars supporting the relatively large mirror, fabrication yield was nearly 100 % for 250 device pieces out of a 4-inch SO1 wafer. Figure 5 (c) is a photograph of assembled device (5.6 mm in diameter and 23 mm in length), which has been already commercially released. A pair of optical fibers is coupled into a collimator in the housing and aligned to the mirror. Thanks to the smallness of the mirror chip, it can be easily expanded to multiple-channel versions. Single-crystalline silicon has orientation-dependent elastic constants [SI. In the 410> direction, the Young's

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401.2 modulus along the direction is larger (-160 GPa) than that along the direction (-130 GPa). To the contrary, the shear modulus of rigidity of silicon around the direction is small (-62 GPa) than that around the direction. We have taken advantage of the anisotropic feature and have designed the torsion ban in parallel with the 45 dB

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4D1.2 This effect is serious when the entire SO1 surface is finished with the metal layers. Therefore, we confined the metalfmished area to the mirror part only to minimize the temperature dependent drift to less than 0.5 dB at a 20 dB attenuation in the temperature range of 0 degree C to 70 d e 5 e e C asshowninFig.9. Tolerance to mechanical vibration is another critical issue of VOA's reliability. We have designed the mirror and actuator to minimize the mass while maximizing the suspension stiffness in the up-and-down motion, which is directly combined with the electromechanical performance of the mirror, by aligning the suspension bar along the At the same time, we have silicon 410> direction. designed suspension length as shot as possible for the same purpose. Figure IO shows fluctuation of optical attenuation 5

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around 5 dB when a mechanical shock of 25 G is given to the assembled VOA on a test bench. Oscillation of +/-I dB is fairly good compared with the conventional VOA's, and it could be further reduced by optimizing the suspension design.

minor controlling the coupling efficiency of optical fihers through a collimator lens. Driving voltage to producing 40-dB attenuation is lower than dc 5 V. Even at such low voltage operation, dynamic response as fast as several ms has been achieved. Temperature dependent loss has been suppressed by decoupling the hi-metal effect from the electrostatic tilting operation of the mirror. Fabrication process of the VOA is very simple, and it requires only W O photolithographsteps. References

[I] V. Aksyuk, B. Barber, C. R. Giles, R. Ruel, L Stulz, and D. Bishop, "Low insertion loss package and fibre connectorised MEMS reflective optical switch," Electronics Lett., vol. 34, No. 14 (1998), pp. 1413[2] C.-H Ji, Y. Yee, J . Choi and 1 . 4 Bu, "Electromagnetic Variable Optical Attenuator," 2002 IEEELEOS Int. C o d . on Optical MEMS, 20 - 23 Aug. 2002, Lugano, Swiss,pp. 49-50. [3] S . S . Yun,Y.-Y. Kim, H.-N. Kown, W:H. Kim, 1:H. Lee, Y.-G. Lee, S.-C. Jung, "Optical characteristics of a micromachined VOA using successive partial transmission in a silicon optical leaker," 2002 IEEELEOS Int. Conf. on Optical MEMS, 20 - 23 Aug. 2002, Lugano, Swiss, pp. 51-52. [4] C.-H Kim, N. Park, and Y.-K Kim, "MEMS Reflective Type Variable Optical Attenuator Using Off-Axis Misalignment," 2002 IEEELEOS Int. Conf. on Optical MEMS, 20 - 23 Aug. 2002, Lugano, Swiss,pp. 55-56. [SI C. Marxer, B. de long, and N. de Rooij, "Comparison of MEMS Variable Optical Attenuator Designs," 2002 IEEELEOS Int. Conf. on Optical MEMS, 20-23Aug.2002,Lugano, Swiss,pp. 189-190. [6] W. Noell, P.-A. Clerc, L. Dellmann, B. Guldimann, H.-P. Herzig,O.Manzardo, C. R. Marxer,K. J. Weible, R. Dindliker, and N. de Rooij, "Applications of SOIBased Optical MEMS", IEEE J. Selected Topics in Quantum Elec., Vol. 8, No. I , (2002) pp. 148 - 154. [7] 1. E. Ford and J. A. Walker, "Dynamic Spectral Power Equalization Using Micro-Opto-Mechanics," IEEE Photon. Tech. Lett., vol. 10, No. IO (1998). pp. 1440[8] Marc A. Meyers and Krishan K. Chawla: MECHANICAL BEHAVIOR OF MATERIALS, 1999 Prencite-Hall, Inc., New Jersey. [9] H. Toshiyoshi, W. Pipawananametha, C.-T. Chan, and M. C. Wu, "Linearization of Electrostatically Actuated Surface Micromachined 2D Optical Scanner," IEEWASME J. Microelectromech.Syst. volume IO, June, 2001,pp. 205-214.

CONCLUSIONS We proposed a very reliable design for MEMS variable optical attenuator made by DRIE of an SO1 wafer. The attenuation mechanism uses an electrostatic tilting TRANSDUCERS '03 The l a h International Conference an Solid State Sensors. Actuators and MicrosystemS. Boston, June 8-12.2003

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