A Low-noise Photonic Heterodyne Synthesizer and Application to Millimeter-wave Radar

Microwave photonic systems have created new paradigms for ultra-wideband electronic signal processing and frequency synthesis with record-low phase noise levels. Despite the intrinsic bandwidth of optical systems operating at ~200 THz carrier frequencies, many existing schemes for high-performance photonics-based microwave generation lack wideband tunability, and experience tradeoffs between noise level, system complexity, and operation frequency. An alternative approach uses direct frequency down-mixing of two tunable semiconductor lasers on a fast photodiode. This form of optical heterodyning produces widely-tunable operation, but experimental realizations have been hindered by the relatively high noise of free-running lasers. Here, we demonstrate a heterodyne synthesizer based on ultralow-noise self-injection-locked lasers, enabling highly-coherent, photonics-based microwave and millimeter-wave generation. Continuously-tunable operation is realized from 1-104 GHz, with constant phase noise of -109 dBc/Hz at 100 kHz offset from carrier across this frequency range. This tuning range is limited only by photodiode availability. To explore its practical utility, we leverage this photonic source as the local oscillator within a 95-GHz frequency-modulated continuous wave (FMCW) radar. Through field testing, we observe dramatic reduction in phase-noise-related Doppler and ranging artifacts as compared to the radar's existing electronic synthesizer. These results establish strong potential for coherent heterodyne millimeter-wave generation, opening the door to a wide variety of future applications including high-dynamic range remote sensing, wideband wireless communications, and THz spectroscopy.