Integrated coherent LiDAR: Advances in tunability, steering, and parallelization

Integrated coherent LiDAR has emerged as a promising route toward compact, low-cost, and high-performance ranging systems. This review focuses on coherent frequency-modulated continuous-wave LiDAR implemented on photonic integrated circuits, where system performance is jointly determined by source coherence and tunability, solid-state beam steering, and scalable parallelization. We highlight recent advances in three essential building blocks. First, narrow-linewidth and frequency-agile integrated lasers provide the coherence and tunability required for precise coherent detection. Second, solid-state beam scanners, including optical phased arrays and focal-plane switch arrays, enable reliable and scalable beam steering while overcoming the limitations of mechanical systems. Third, on-chip optical frequency combs supply mutually coherent wavelength channels that facilitate massively parallel detection, high-speed calibration, and enhanced ranging precision. A comparative analysis of material platforms, including silicon, silicon nitride, thin-film lithium niobate, and emerging heterogeneous integration, reveals the tradeoffs between low loss, fast modulation, and integration compatibility. Looking forward, progress in material innovation, co-designed photonic–electronic packaging, as well as comb amplification and spectral flattening will be essential to achieving coherent, agile, and scalable photonic LiDAR suitable for deployment in autonomous vehicles, robotics, and precision metrology.