Recent advancements in micro-optical and micro-mechanical resonator technologies haveallowed researchers to exploit coupling between the optical field and mechanical motionin an optical cavity to affect cooling or amplification of mechanical motion. Cooling the mechanical motion of microscale objects has been of high scientific interest, since it facilitates observation and exploration of certain quantum phenomena, e.g., the standard quantum limit of detection. On the other hand, amplification of the mechanical motion allows realization of microscale devices for practical applications, such as light driven low phase noise signal generation by radiation pressure driven Opto-Mechanical Oscillators (OMO’s).
The ability to achieve self-sustained oscillation with no need for feedback electronics makes an OMO compelling for on-chip applications where directed light energy, e.g., from a laser, is available to fuel the oscillation, such as Chip Scale Atomic Clocks (CSAC’s). Indeed, an OMO can substantially reduce power consumption of a CSAC by replacing itspower-hungry conventional quartz-based microwave synthesizer but this requires that theOMO output is sufficiently stable, as gauged over short time spans by its phase noise.
Pursuant to identify phase noise mechanisms, this thesis presents a new phase noise model for OMO’s by deriving an OMO oscillator model with intuitive engineering understanding of its operation consistent with the established OMO theory. Phase noise theory suggests that attaining high mechanical-Q(Qm) is crucial to lower the phase noise while high enough optical-Q(Qo) is required for reasonably low-power operation. This motivates a focus on achieving a high-QmOMO to have low phase noise while maintaining a high enough Qo for low power operation–a challenge in previous OMO’s that had to trade-off Qm and Qo mainly because they use a single material that sets both.
The work in this thesis demonstrates integrated MEMS-cavity optomechanical oscillators that combine the best properties of optical and MEMS resonators in single composite multi-material OMO structures to simultaneously optimize mechanical and optical Q’s. The multi-material coplanar ring OMO structure using a high-Qo silicon nitride optical ring and a high-Qm polysilicon ring simultaneously achieves high Qm>22,000, which is more than 2×higher than that of previous-best silicon nitride OMO, and high Qo>280,000 on par with single silicon nitride ring demonstrations. With its highQm, the coplanar ring OMO exhibits a best-to-date phase noise of -114 dBc/Hz at 1 kHz offset and -142 dBc/Hz at 1 MHz offset from its 52-MHz carrier–a 12 dB improvement from the previous best by an OMO constructed of silicon nitride alone. The doped polysilicon structure and electrodes additionally allow tuning of the OMO’s oscillation frequency via voltage control and harmonic locking to an external source, enabling future deployment of the multi-material OMO as a locked oscillator in a target low-power CSAC application. A second integrated OMO structure, dubbed stacked-ring OMO, is also demonstrated using similar silicon nitride and polysilicon ring resonators but this time coupled in a vertical fashion, allowing easy integration with sidewall sacrificial layer defined gap MEMS process technology to achieve high electromechanical coupling in the composite OMO.
Enabled by the MEMS integration that allows electrically coupled intput-outputs, a newoptical communications application based on an OMO is introduced. A super-regenerative optical receiver detecting on-off key (OOK) modulated light inputs has been demonstrated that harnesses the radiation-pressure gain of the electrically-sustained integrated OMO to render its oscillation amplitude as a function of the intensity of light coupled into the oscillator. Unlike previous electronic super-regenerative receivers, this rendition removes the need to periodically quench the oscillation signal, which then simplifies the receiver architecture and increases the attainable receive bit rate. A fully functional receiver with a compact∼90μm OMO comprised only of silicon-compatible materials demonstrates successful recovery of a 2 kbps bit stream from an OOK modulated 1550 nm laser input. By removing the need for the expensive III-V compound semiconductor materials often used in conventional optical receivers, this OMO-based receiver offers a lower cost alternative for sensor network applications.
August 31, 2016
Beyazoglu, T. B. (2016). Integrated MEMS Cavity Optomechanical Oscillators for Wireless and Optical Communications. United States: University of California, Berkeley.