Diffraction-based integrated optical readout for micromachined optomechanical sensors
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Highly sensitive optical displacement detection methods implemented in a small volume and with reduced power consumption have a potential to compete with commonly used capacitance based methods in micromechanical sensor systems. This dissertation presents the design, implementation, and characterization of a miniaturized optomechanical displacement sensor system heterogeneously integrated with a coherent laser source and optoelectronic readout as a step in realizing this potential. The sensor uses a phase-sensitive diffraction grating built on a transparent substrate to achieve interferometric sensitivity in a small volume. The device sensitivity is actively optimized via the built-in electrostatic actuation capability, which may be utilized for self calibration and force feedback operation. Optical interconnect through the backside of the sensor enables compact integration with optoelectronic components. For optical readout, a custom-designed silicon photodiode array has been fabricated including deep reactive ion etching of through-wafer holes. The hybrid-integrated system has been implemented and characterized in an acoustic sensor application using both continuous wave and pulsed lasers to show reduced power consumption potential. Comprehensive diffraction analysis has been carried out for optical design of the integrated sensor. Furthermore, a fully-vectorial method has been formulated for general multilayered grating structures and compared with the scalar diffraction approach to investigate the effects of polarization and grating periods. In addition, a grating-assisted resonant-cavity-enhanced (GARCE) detection method has been proposed to improve the displacement sensitivity in optomechanical microsensors. Fabrication of the GARCE structures based on both metallic and dielectric mirrors has been successfully demonstrated, and preliminary experimental results have shown a good agreement with theoretical predictions. The concepts developed and demonstrated in this thesis form a technology platform which already had an impact in a variety of applications including optical microphones, micromachined ultrasonic transducers and transducers arrays, micromachined inertial sensors, and scanning probe microscopy.