Surface-micromachined mirrors for scalable fiber optic switching applications

We present design, analysis and characterization of surface-micromachined mirrors developed for fiber-optical cross-connects (OXCs). These mirrors are controlled by electrostatic microactuators, and are optimized for our beam-steering OXC switch architecture. Their geometry leads to high-density switch matrices, and the absence of frictional hinges in their actuation mechanism allows highly repeatable operation. This mirror structure features an adjustable maximum deflection angle that can be set during initial assembly or dynamically varied by integration with a standard surface-micromachined linear actuator. A commercial three-layer polysilicon surface-micromachining process is used for fabrication of the micromirrors. The OXC architecture presented has the advantage of scalability in both the spatial and wavelength dimensions and is ideally suited For wavelength division multiplexed (WDM) switching applications. In our design, the OXC can be scaled to an N input, N output switch using 2N mirrors while maintaining full cross-connect functionality. Assuming a nominal initial mirror tilt angle of 25 degrees and an incident optical beam perpendicular to the chip surface, we calculate that the switch would accommodate a maximum of 19 channels using these micromirrors. The torsional micromirrors measure 460x430x1.5 mum and are actuated using wedge-gap style electrostatic actuators. The resonant frequency of the mirror is 1.31 kHz. The system is underdamped with a damping coefficient of 0.3 and a switching time of approximately 6 ms. The maximum deflection is dependent on the initial tilt angle of the mirrors and is about 7 degrees for an initial mirror tilt of 20 degrees. We develop a wedge-gap electrostatic model for the micromirrors and show close correlation between the theoretical and experimental results.