RF Oscillators Based on Piezoelectrically Driven Optical Modulation

This paper investigates the use of piezoelectrically actuated acousto-optic modulators as a building block for generating electronic frequency references at radio frequencies (RFs). Similar to the concept of an optoelectronic oscillator, this approach leverages the strengths of multiple domains such as strong photonic confinement with optical resonances and the inherent compact RF filtering capabilities of piezoelectric resonators. An initial oscillator demonstration is constructed around an integrated aluminum nitride acousto-optic modulator, with a center frequency of 653 MHz. Phase noise of this device is analyzed and matched with the measured results, including the various contributors to input noise power density. This model allows us to determine anticipated figures for phase noise performance on par with the state of the art based on future improvements to the acousto-optic modulator.

I. Introduction

Oscillators are relatively universal elements in all modern communication and navigation systems. Since the performance of these systems is often dictated by the accuracy and stability of the frequency references they employ, a variety of technologies have been developed to generate periodic signals. In particular, photonic radio frequency (RF) systems utilize optical waves as a carrier to transport RF signals through fiber. This technology has numerous potential applications in wireless networks and distributed antenna systems, among other areas [1]–[3]. An ideal oscillator for a photonic RF system should be able to produce RF signals in both optical and electrical domains. Generation of an RF signal in the optical domain can be accomplished by modulation of a diode laser or electrooptic modulator with the signal from a local oscillator [4], [5], or by mixing the output of lasers operating at different optical frequencies. The former approach may require several stages of multipliers and amplifiers leading to a bulky system, while the frequency stability of the latter is subject to the drift in the optical frequencies of the lasers. Over the past two decades, a popular alternative has been the use of the optoelectronic oscillator (OEO) [6]. The OEO is based on a feedback loop that converts modulated light from a pump laser to RF frequencies. All oscillator feedback loops are typically constructed around a resonant element with a high quality factor (), corresponding to a long energy decay time. In the case of the OEO, the energy-storage function is satisfied by a long (on the order of kilometers) fiber-optic delay line, while the frequency of operation is determined by a filter in the electronic segment of the loop. By incorporating both optical and electronic elements, this allows for versatility in the design, and OEOs have been demonstrated with very high spectral purity [7]. The use of optical fiber, however, limits these devices to implementation in rack mountable units and thus more compact OEO devices have incorporated whispering-gallery mode (WGM) optical resonators [8], [9]. These WGM resonators have been demonstrated with electrooptic modulation in bulk crystalline materials such as CaF₂ and LiNbO₃ with input/output coupling via prism or fiber taper. Thus far, however, these systems have been limited in their capacity for optoelectronic integration on-chip, revealing a need for alternative approaches.