Overcoming Laser Phase Noise in FMCW LiDAR via Dual-Sideband Residual-Carrier Modulation

Abstract

Frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) enables high-sensitivity detection and simultaneous range–velocity estimation by converting round-trip time delay into a beat frequency. However, at long distances, laser phase noise causes decorrelation between the reference and echo fields, degrading measurement precision beyond the laser’s coherence length. To overcome this limitation, we propose a carrier-aided dual-sideband residual-carrier modulation (DSB-RCM) scheme. After coherent detection, the extracted residual-carrier tone and the de-chirped beat are spectrally isolated and multiplied in the electrical domain to suppress common laser phase noise, yielding a phase-noise-cancelled beat. We demonstrate a proof-of-concept system using a single Mach–Zehnder modulator (MZM) driven by a 2 GHz sawtooth chirp and a distributed feedback (DFB) laser with 12.5 MHz linewidth. Using fiber delays to emulate long-distance propagation, the system achieves ranging precision approaching the range-bin spacing (10 cm) at an equivalent distance of 80 km—exceeding the laser’s coherence length by more than 15000 times. By relaxing the stringent linewidth requirements of coherent FMCW LiDAR, this approach enables the use of low-cost, MHz-linewidth DFB lasers in long-range systems, thereby providing a compact and scalable pathway to high-precision ranging and imaging.