A novel offset vertical comb-driven micromirror as an optical phase modulator

[Display omitted] ► The optical phase modulator using the out-of-plane piston motion of a micromirror was proposed. ► A non-overlapping vertical comb actuator was designed to drive the micromirror. ► A TLM MEMS process was designed to fabricate the OPM. ► The structure parameters were optimized by the FEM method. ► The “spring softening” effect was observed in dynamic response of the OPM. This paper presents an offset comb-driven micromirror with out-of-plane piston motion as an optical phase modulator (OPM) with high modulating accuracy. The upper and lower comb fingers of the comb-driver initially do not overlap, with a vertical offset of the dielectric layer thickness. A three-layer-mask (TLM) Micro-electro-mechanical systems (MEMS) process is employed to achieve self-alignment fabrication of vertical comb fingers and micromirror compatibility. The fabricated micromirror has a high-quality optical surface with an average surface roughness (Ra) of 5.14nm. The static and dynamic responses of the OPM are measured by a Michelson interferometer. The results show that the displacement of the comb is 325nm (λ/2) with a 28V dc driving voltage, and the offset vertical comb actuator can be actuated to multiple displacement wavelengths. The dynamic displacement of the comb at its mechanical resonant frequency of 7.3kHz is 9.4μm under a 6V ac driving voltage and a 30V dc bias voltage. A finite element method (FEM) simulation of the offset vertical comb actuator is performed to evaluate the electrostatic force and optimize its structural parameters. The simulation results agree well with the experimental data.