Leveraging Opto-Mechanics To Design High Performance Rf Oscillators
Reference oscillators are ubiquitous elements used in almost every electronics system today. The need for miniaturized, batch manufacturable oscillators as chipscale timing references arises from the quest to replace the well-established, high performance yet expensive quartz-based oscillators, without compromising performance. The electrical specifications of an oscillator depend on the application it serves, which has resulted in a variety of different mainstream oscillator technologies. Consumer RF applications can broadly be classified mechanical oscillators and electrical oscillators. In mechanical oscillators, the frequency selective element is a mechanical resonator, and typically quartz is the material of choice for high-end applications. For less demanding and cost-sensitive applications, micro-electromechanical (MEMS) resonators, being CMOS compatible and high quality factor resonators, offer a unique set of parameters well-suited for oscillator design. Electrostatic capacitive transduced silicon resonators and piezoelectric transduced thin film bulk acoustic resonators (FBARs) are commercially available for MHz range and GHz range applications respectively. Scaling MEMS oscillators to higher frequencies presents challenges in terms of reduced transduction efficiencies and material limitations on quality factors. Opto-mechanical transduction offers higher sensitivity and opens up possibilities to interrogate high frequency mechanical resonances hitherto inaccessible. The focus of this thesis is to leverage opto-mechanical transduction to design high frequency high performance MEMS oscillators and exploring various designs and fabrication techniques to realize these devices. This disertation explores two classes of oscillators, namely the opto-mechanical and opto-acoustic oscillators. The former oscillator type exploits parametric amplification and does not require external electrical feedback to sustain oscillations, thus doing away with a dominant noise source. To eliminate coupling environmental noise to the oscillation signal, the opto-mechanical resonator was fabricated on a silicon nitride chip with waveguides and grating couplers integrated on to the same chip. The device was used to demonstrate self-sustained mechanical oscillations at 41MHz with phase noise -91dBc/Hz at 1kHz offset from carrier. The integrated design results in immunity of the oscillation signal from environmental flicker noise. Designing low phase noise opto-mechanical oscillators for GHz range frequencies is very challenging, the limitations being mainly imposed by the efficiency of the optical drive scheme. A design worth exploring to overcome this limitation is the acousto-optic modulator designed and developed in the OxideMEMS lab, which marries the highly sensitive optical sense scheme with electrostatic capacitive transduced drive scheme. Operating the modulator in a feedback loop as an opto-acoustic oscillator has been realized by using the device as an intensity modulator. However the two-coupled opto-mechanical resonator design cannot be successfully scaled to design an oscillator at frequencies beyond GHz. An alternative efficient transduction scheme of interest for GHz range MEMS resonators is partial air gap capacitive transduction. Exploiting partial air-gap transduction using alumina in addition to designing an array of resonators employing a micromechanical displacement amplifier, an opto-acoustic oscillator employing a higher order radial mode at 2.1GHz was demonstrated. The oscillator has RF output power of +18dBm and phase noise -80dBc/Hz at 10kHz offset from carrier. The inherent non-linearity of the opto-mechanical modulation based sensing generates oscilllation harmonics all the way up to 16.4GHz with greater than -45dBm signal power. A detailed phase noise model for such oscillators was derived and insights derived from the model were followed to identify appropriate photo-detectors to lower the far-from-carrier phase noise by 15dB. Fabrication techniques developed along the way were also used to design other interesting opto-mechanical devices for electromechanical detection of optical modulation and to study acousto-optic frequency modulation. In summary, the overall focus of this work is to bring together MEMS and photonics techniques and devices in ways that address long-standing needs in both communities.
Bhave, Sunil A.
Gaeta, Alexander L.; Rana, Farhan; Afshari, Ehsan
Ph. D., Electrical Engineering
Doctor of Philosophy
dissertation or thesis