Optical Networking Best Practices Handbook

7.1. FREE-SPACE OPTICAL COMMUNICATION

While surface micromachining generally does not simultaneously offer large scan angles and large mirror sizes, microelectromechanical system (MEMS) micromirrors based on silicon-on-insulator (SOI) and deep reactive ion etching (DRIE) technology provide attractive features, such as excellent mirror flatness and high aspect-ratio springs, which yield small cross-mode coupling. There have been many efforts to make scanning micromirrors that employ vertical comb-drive actuators fabricated on SOI wafers [1]. Although vertical comb-drive actuators provide high force density, they have difficulty in producing two-axis scanning micromirrors with comparable scanning performance on both axes. One way to realize two-axis micromirrors is to utilize the mechanical rotation transformers [1]. The method of utilizing lateral comb drives to create torsional movement of scanning mirrors is by the bidirectional force generated by the lateral comb-drive actuator, as it is transformed into an off-axis torque about the torsional springs by the pushing/pulling arms. One benefit of this concept is the separation of the mirror and the actuator, which provides more flexibility to the design. A large actuator can be designed without contributing much moment of inertia due to this transforming linkage, and therefore the device can have higher resonant frequency, compared with a mirror actuated by the vertical comb drive. This design also offers more shock resistance. ...