Opto Acoustic Oscillator Using Silicon Rf Mems Based Optical Modulator

Abstract

Mechanical resonators have been used for the last few decades as the frequency selection element of high frequency oscillators and radio frequency filters due to their high quality factors. Mechanical resonators scaled to the micro scale, called micromechanical resonators, offer the promise of integration of these high precision frequency selection elements along with microelectronics on the same substrate. Scaling to the micro scale allows micromechanical resonators operate at desired higher frequencies compared to their macroscopic counterparts. This along with the advantage of lower manufacturing cost due to the microelectronic fabrication process used for the their fabrication have made them attractive candidates for use in modern wireless radio devices. Micromechanical resonators excited and sensed using electrostatic air gap capacitive transduction have been shown to have very high quality factors close to the material loss limit. While electrostatic air gap transducers are easy to co-fabricate with microelectronics in a shared process, it suffers from lower sensitivity at higher frequencies making it difficult to use in high frequency applications. Cavity optomechanical systems, where a mechanical resonator is also an optical resonance cavity, has been shown to be one of the most sensitive methods for detecting mechanical motion. Such systems use shifts in the optical resonance frequency of the optomechanical resonator to sense mechanical motion. Presently, these optomechanical systems are used for measuring mechanical thermal noise displacement or mechanical motion actuated by optical forces. In this dissertation, a monolithic scheme for integration of electrostatic capacitive actuation of mechanical resonators with optical sensing using silicon optomechanical disk resonators and waveguides is presented. To obtain an optically sensed electrostatically actuated mechanical resonator, a coupled disk geometry is used, where one disk acts as the sensing optomechanical resonator while mechanical vibrations are excited through electrodes around the other disk. The electrostatically actuated optomechanical resonator combines the frequency filtering response of a mechanical resonator with the optical amplitude modulation property of the optomechanical resonator thereby creating an integrated narrowband optical modulator.This narrow band optical modulator called the acousto optic modulator and fabricated on the silicon device layer of a silicon on insulator substrate modulates output light when the electrical input is around the mechanical resonance. For disks of 10[MICRO SIGN]m radius, the radial vibrational modes are observed as optical modulation around 236MHz with an extinction ratio of 12dB for a DC bias of 20V, RF input power of 5dBm and optical quality factor of 53,000. Scaling the radius of the disks to 3.8/mum increases the observed frequency of the fundamental mode resonance to 706MHz along with the second radial vibrational mode to 1.93GHz. An alternate geometry using ring resonators shows multiple mechanical modes up to 3.5GHz making this one of the highest observed mechanical frequencies with air gap electrostatic actuation. An important application of mechanical resonator is in frequency selection as part of an oscillator loop. Implementing the acousto optic modulator in an oscillator is similar to the opto-electronic oscillator (OEO), which is the current state-of-art oscillator in the few GHz regimes that uses optical feedback in the oscillation loop. The optical output from the modulator is converted back to the electrical domain using a high speed optical detector and then amplified and fed back into the modulator. Employing this technique, an opto-acoustic oscillator (OAO) has been demonstrated at the mechanical frequency of 236MHz of the disk resonator with an output power of 6.5dBm and a phase noise of 65dBc/Hz at 1kHz offset. Additionally, an OAO operating at 1.12 GHz with an output power of 8.8dBm and -65dBc/Hz at 10kHz offset is demonstrated using the ring resonator based modulator. In summary, this work presents a combined electrical-optical micromechanical system fabricated on a CMOS compatible process thereby opening up the possibility of novel devices for future electo-optic-mechanical multi domain systems.