Extreme Engineering of MEMS SLM for Laser Beam Steering

Abstract

Digital Micromirror Devices (DMDs) are reflective micro-electromechanical (MEMS) spatial light modulators originally designed for the visible spectrum, offering a low-cost platform for agile beam steering. DMD beam steering with pulsed laser exploits the short (microsecond-scale) mirror transition between their two bistable tilts. A short laser pulse, delayed with respect to the mirror reset, can be timed to meet the blaze condition of a chosen diffraction order, consequently the pulse is steered into one of the diffraction orders supported by the mirror array. Varying the delay selects the order. In contrast, continuous-wave (CW) illumination cannot be time gated to the transient sweep for conventional Application Specific Integrated Circuit (ASIC) driven operation. Because the mirrors spend most of each cycle in the latched states (bi-stable positions namely, ON and OFF), the energy throughput into any intermediate diffraction order is small. This restricts the use of DMD in CW applications such as free space optical communication, where modulated CW is used in fiber optic communications. This thesis proposes mechanisms that improve energy throughput for CW operation: 1) Continuous operation (fm ≥80kHz). By bypassing the on-board driver and directly addressing the Mirror Bias Reset (MBRESET) electrodes, continuous motion of mirror array is excited near its mechanical resonance. fd is the driving frequency which is a trigger signal quantity and fm is the mirror frequency which is the frequency with which mirrors tilt in a continuously operating system. Mirrors reach their full ±12◦ tilt at fd above 100kHz, fm is found to be half of the fd above 100kHz, yielding 17 usable diffraction orders. The continuous motion of the micromirrors improve the energy throughput into intermediate orders as the mirror has more transitions in a continuous moving configuration compared to the ASIC pattern update method. 2) Quasi-static steering. Quasi-static steering is the ability to hold the micromirror in its intermediate mirror tilt angles without reaching its bi-stable position by triggering the mirrors repeatedly mid-tilt using as external signal with a higher frequency than the mirror array resonance frequency. Access to intermediate mirror tilt angles for extended duration increases the energy throughput into the desired order for CW sources. Collectively, these results extend the functional envelope of off-the-shelf DMDs from conventional application speeds to high speed and quasi-static applications. The methods preserve the low cost and reliability of mass-produced DLP hardware and opens a pathway CW based beam steering applications.