FUNDAMENTAL TECHNOLOGIES FOR THE OPTO MECHANICAL GYROSCOPE

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Opto-mechanics is the convergence of optics and mechanics towards the understanding of novel physical phenomenon and the achievement of cutting edge applications. From the effects of radiation pressure on the study of gravitational waves in a LIGO to the study of the quantum mechanical ground state of a simple harmonic oscillator, opto-mechanics spans a very wide range of the physical world. Optical systems with their capability of high sensitivity have been evaluated for applications as sensors like heart rate monitors, gas sensors and LIDARs. There has also been a growing demand for more robust, sensitive, low cost and easy to integrate gyroscopes for applications ranging from defense to autonomous driving. While there are existing technologies for high performance gyroscopes like the ring laser gyroscope and the machined hemispherical resonator gyroscope, these can be large, expensive and unwieldy rendering them unsuitable for a wide variety of applications. MEMS based gyroscopes are low-cost, come in smaller packages and are easy to integrate into systems but they might not have the high performance desired in these new demanding applications. Opto-mechanics has the potential to bridge the gap by combining MEMS and optics to leverage the high sensitivity of optics with the low cost and ease of integration of MEMS devices. The work presented in this thesis showcases some of the fundamental technologies needed for an opto-mechanical gyroscope (OMG). First is the (\mu m) scale hemispherical opto-mechanical resonator that is meant to incorporate the highly desirable mechanical properties of the cm scale hemispherical resonator gyroscope. These silicon dioxide resonators were fabricated in a MEMS centric process flow. The mechanical modes were optically sensed with a tapered optical fiber coupled to a toroidal optical cavity fabricated on the lip of the hemispherical resonator. Next is the development of silicon opto-mechanical wineglass mode resonators with on-chip waveguides. The silicon-on-insulator (SOI) platform employed here allows for better integration of the electro-static actuation and opto-mechanical sensing of the wineglass modes. The two orthogonal n=2 wineglass modes can be used for gyroscopy. Here, I first present the path towards closing the loop on the gyroscope drive wineglass mode. The differential electro-static actuation scheme achieved an 18db relative suppression of the undesired radial mode relative to the wineglass mode. The differential opto-mechanical sensing achieved a 5.4db enhancement of the drive wineglass mode detection and a further 11dB attenuation of radial mode. The suppression of the spurious modes relative to the desired wineglass mode allows a path for closing the oscillator loop on the wineglass mode without the need for additional filters. Next, I present the electro-static actuation of the drive wineglass mode and the opto-mechanical sensing of the orthogonal sense wineglass mode around 22MHz. The resonator was specially designed to achieve a controlled frequency mismatch between these two wineglass modes of only 120KHz and the design allows for tuning this further as needed. Measurements in vacuum showed an optical Q of 61,000 for the optical ring cavity mode and a mechanical Q of 11,500 for the drive wineglass mode and 7,670 for the sense wineglass mode. Last is the work with Lithium Niobate (LN) resonators. LN is of interest for its piezoelectric properties where the large piezoelectric coupling coefficient ((k_t^2)) allows for efficient electrical actuation of its mechanical modes. There is a potential for leveraging the high efficiency mechanical mode actuation to actuate the drive mode of a gyroscope device. Typically these resonators are actuated with gold but gold has been found to be mechanically lossy. Aluminum was chosen to be the replacement of gold in this work for its lower mechanical losses compared to gold. However, aluminum is very reactive with hydrofluoric acid (HF) which is used as part of the fabrication process of these devices. This necessitated an exploration of passivation methods to protect the aluminum electrodes from HF. Among the various films that I evaluated, a protective film called ProTEK PSB was found to be the most promising in resisting HF while protecting the aluminum electrodes. ProTEK PSB also had a process for its removal after the HF processing. In summary, this work presents some of the fundamental technologies such as a (\mu m) scale high mechanical performance device, a chip scale platform for selective transduction of the wineglass modes and a path towards high efficiency actuation of mechanical modes, all of which could contribute towards building an opto-mechanical gyroscope.

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138 pages
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2022-05
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Gyroscope; MEMS; Optomechanics; Photonics; Resonator
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Pollock, Clifford R.
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Lal, Amit
Kan, Edwin C.
Degree Discipline
Electrical and Computer Engineering
Degree Name
Ph. D., Electrical and Computer Engineering
Degree Level
Doctor of Philosophy
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dissertation or thesis
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