Improved silicon micromachined 3-D mirror for acceleration sensing using an extra-short external cavity laser self-mixing interferometer

We have designed a new type of folding suspended polysilicon micro-mirror, integrating a new type of precision position-lock (top lock), a new compact design for the hinges and a bi-directional electrostatic actuator to bias the position of the mirror. The mirror is intended to be used in a very short external cavity laser configuration (≈10 μm) for acceleration/displacement sensing. An extensive theory has been build to guide the design and the fabrication of the sensor. After probe assembly, the mirror has been actuated with a low-noise comb-drive actuator and the λ/2 range (0.4 μm) required to bias optimally the external cavity laser, is obtained with a bit less than 10 V, compatible with CMOS circuitry. The spring constant of the mirror suspension is about 0.38 N/m while the mass of the suspended mirror is slightly less than 2 μg, yielding a mechanical sensitivity to acceleration of about 50 nm/g. Using the mirror in an external cavity configuration with a Fabry-Perot laser diode at 0.8 μm provides an intensity modulated signal with an expected displacement resolution of about 0.05 nm, limited by the mechanical structure. It will provide a sensitivity to acceleration in the μg range. We show theoretically that the linear range is about 3 orders of magnitude, but depends on the amount of light feedback in the laser cavity.