Suresh Sridaran

A monolithic radiation-pressure driven, low phase noise silicon nitride opto-mechanical oscillator

Cavity opto-mechanics enabled radiation pressure (RP) driven oscillators shown in the past offer an all optical Radio Frequency (RF) source without the need for external electrical feedback. However these oscillators require external tapered fiber or prism coupling and non-standard fabrication processes. In this work, we present a CMOS compatible fabrication process to design high optical quality factor opto-mechanical resonators in silicon nitride. The ring resonators designed in this process demonstrate low phase noise RP driven oscillations. Using integrated grating couplers and waveguide to couple light to the micro-resonator eliminates 1/f(3) and other higher order phase noise slopes at close-to-carrier frequencies present in previous demonstrations. We present an RP driven opto-mechanical oscillator (OMO) operating at 41.97 MHz with a signal power of -11 dBm and phase noise of -85 dBc/Hz at 1 kHz offset with only 1/f(2) noise down to 10 Hz offset from carrier.

Electrostatic actuation of silicon optomechanical resonators

Cavity optomechanical systems offer one of the most sensitive methods for detecting mechanical motion using shifts in the optical resonance frequency of the optomechanical resonator. Presently, these systems are used for measuring mechanical thermal noise displacement or mechanical motion actuated by optical forces. Electrostatic capacitive actuation and detection have been shown previously for silicon micro electro mechanical resonators for application in filters and oscillators. Here, we demonstrate monolithic integration of electrostatic capacitive actuation with optical sensing using silicon optomechanical disk resonators and waveguides. The electrically excited mechanical motion is observed as an optical intensity modulation when the input electrical signal is at a frequency of 235 MHz corresponding to the radial vibrational mode of the silicon microdisk.

Nanophotonic devices on thin buried oxide Silicon-On-Insulator substrates

We demonstrate a silicon photonic platform using thin buried oxide silicon-on-insulator (SOI) substrates using localized substrate removal. We show high confinement silicon strip waveguides, micro-ring resonators and nanotapers using this technology. Propagation losses for the waveguides using the cutback method are 3.88 dB/cm for the quasi-TE mode and 5.06 dB/cm for the quasi-TM mode. Ring resonators with a loaded quality factor (Q) of 46,500 for the quasi-TM mode and intrinsic Q of 148,000 for the quasi-TE mode have been obtained. This process will enable the integration of photonic structures with thin buried oxide SOI based electronics.

Opto-acoustic oscillator using silicon MEMS optical modulator

We show operation of a silicon MEMS based narrow-band optical modulator with large modulation depth by improving the electro-mechanical transducer. We demonstrate an application of the narrowband optical modulator as both the filter and optical modulator in an opto-electronic oscillator loop to obtain a 236.22 MHz Opto-Acoustic Oscillator (OAO) with phase noise of −68 dBc/Hz at 1 kHz offset.

Phase noise modeling of opto-mechanical oscillators

We build upon and derive a precise far from carrier phase noise model for radiation pressure driven opto-mechanical oscillators and show that calculations based on our model accurately match published phase noise data for such oscillators. Furthermore, we derive insights based on the equations presented and calculate phase noise for an array of coupled disk resonators, showing that it is possible to achieve phase noise as low as −80 dBc/Hz at 1 kHz offset for a 54 MHz opto-mechanical oscillator.

Motional impedance analysis: Bridging the ‘Gap’ in dielectric transduction

This paper presents an analytical model to estimate the motional resistance for partial air gap capacitively-transduced MEMS resonators. This model serves as a link between the well formulated analytical models for conventional air gap and internal dielectric transduction schemes, thereby helping decide which scheme is optimal for a given design frequency. Using this model, we simulate and experimentally verify the motional resistance for a 303MHz polysilicon disk resonator within a 5% range of accuracy.

A silicon nitride optomechanical oscillator with zero flicker noise

We present an integrated chip-scale Radiation-Pressure driven Opto-Mechanical Oscillator (RP-OMO) in silicon nitride with excellent close-to-carrier phase noise. We illustrate a process to micro-fabricate optomechanical resonators, waveguides and grating couplers in silicon nitride and demonstrate an RP-OMO operating at 41.95MHz, with phase noise of -85dBc/Hz at 1kHz offset. The phase noise does not show 1/f3 or other higher order slopes all the way down to 10Hz offset from carrier. Using a lower optical quality factor resonance, we demonstrate improvement of 6dB in phase noise.

1.12GHz Opto-Acoustic Oscillator

We report on the development of a GHz Opto-Acoustic Oscillator (OAO) using an air gap capacitively actuated silicon optomechanical resonator. An optical sensing technique using optomechanical ring resonators in silicon allows for observation of the mechanical resonances of electrostatically excited resonators up to 3.5GHz. Using amplifiers to compensate for losses, we demonstrate an oscillator operating at 1.12GHz with 8.8dBm output RF power and a phase noise of -85 dBc/Hz at 10 kHz offset.

Silicon monolithic acousto-optic modulator

This paper reports on the co-fabrication of RF MEMS radial contour mode resonators and photonic whispering gallery mode disk resonators on the same SOI substrate. By mechanically coupling the MEMS and photonic resonators, we have demonstrated a silicon acousto-optic modulator (AOM) which can modulate a 1550 nm laser at 288 MHz. An output RF power of −83.7 dBm is observed corresponding to a modulation of ±415 nW around a biased optical output of 3.98 µW. The AOM has a footprint of 50 µm × 30 µm and bandwidth of 24.2 kHz in vacuum due to a high mechanical quality factor (Q) of 11,890.