An optoelectronic microwave synthesizer with frequency tunability and low phase noise

Abstract

Communication, navigation and radar systems rely on frequency-tunable and low-noise microwave sources. Compared to electronic microwave synthesizers, photonic systems that leverage high spectral purity lasers and optical frequency combs can generate microwaves with exceptionally low phase noise. However, photonic approaches lack frequency tunability and have substantial size, weight and power requirements, which limit wider application. Here we address these shortcomings with a hybrid optoelectronic approach that combines simplified optical frequency division with direct digital synthesis to produce tunable low-phase-noise microwaves across the entire X-band (8–12 GHz). This resulted in phase noise at 10 GHz of −156 dBc Hz−1 at 10 kHz offset and fractional frequency instability of 1 × 10−13 at 0.1 s. Spot-tuning away from 10 GHz by ±500 MHz, ±1 GHz or ±2 GHz yielded phase noise at 10 kHz offset of −150, −146 and −140 dBc Hz−1, respectively. Our synthesizer architecture is compatible with integrated photonic implementations and, thus, could be integrated in a chip-scale package. A synthesizer that combines a fixed low-noise photonic oscillator and a direct digital synthesizer—and is based on components that can all be integrated on chip—can create microwave signals that are tunable with low noise.