Free space laser communication is a promising candidate for a high bit-rate, interference-free data link at lower power consumption compared to radio and microwave frequency systems. On the other hand, an optical link has its own challenges: pointing, stabilization, and acquisition. Considering the directional laser beam, with 1 miliradian (.017 deg) divergence, we can say that pointing accuracy must be a fraction of 1 miliradian, which is the beamwidth.
Vibrations on the hosting platform can easily be large enough to disturb the beam heading and interrupt the link. A small airplane, with a 1 meter wing span, is our main candidate as the host vehicle for the laser transceiver. Inertial sensors have demonstrated effectiveness in detecting and canceling the effects of undesired vehicle vibrations. Stabilization as described above, however, needs a steering capability of +/- 10 degrees optical.
The third challenge, acquisition, needs a laser beam to scan an area that the opposite end is likely to be in. An estimation of the target's position narrows the scan area, and its dimensions are limited by the errors in the estimation. In this particular application, we need another +/- 10 degrees of beam steering for acquisition. So, the total range of the mirror needs to be +/- 20 degrees optical.
Considering the requirements above, the beam steering element in the system must be precise, fast, and have a large dynamic range. A 2-DOF MEMS mirror built on SOI wafers will do the beam steering. Small dimensions give the mirror a reasonably large bandwidth to meet the speed requirement.
The goal of the research summarized here is to design and implement a feedback loop around the mirror in order to achieve the resolution, which is more than 12 bits. The 20 degree range requires high voltages in the actuator. Position sense for the mirror is another essential component in the feedback system. In most of the cases, separate sense fingers are needed for a better performance. Additional fingers bring up layout and stability challenges for the mirror. Various methods for drive and the sense have been investigated. The current research direction implements the high voltage actuation without a need for high voltage circuitry. The same method also has a chance of using the same set of fingers for both sense and drive.
Figure 1: A 2 DOF mirror built on SOI wafer