## 3 FMCW LiDAR Research Progress
### Research Progress on Integration
This technological breakthrough not only transcends the Nyquist limitation for precise distance measurement but also considerably expands the measurable distance range, offering more flexibility and possibilities for FMCW LiDAR applications by adjusting the measurement distance range and resolution with different sweep bandwidths. In 2023, Zhuhai Yingxun Xinguang Technology Co., Ltd. (Zhuhai, China) disclosed a chip-integrated FMCW LiDAR system [66] that employs an external-cavity tunable laser composed of an emitting gain chip, first and second integrated optical waveguides, a broadband optical feedback structure, a waveguide phase tuning control area, an adjustable waveguide filter, a collimating optical lens, and an optical filter feedbacker. This design features a simple, reliable, and rapid laser beam wavelength tuning mechanism. By incorporating an optical converter and multiple free-space optical feedback channels, the system achieves wavelength tuning and directional selection over a broader wavelength region and range. Dispersion optical components are used to enable solid-state laser beam angle scanning driven by wavelength tuning without mechanical movement. Wavelength tuning linearity or chirp of the external-cavity tunable laser is monitored, calibrated, and controlled using a waveguide interferometer and a photonic balanced detector. FMCW coherent laser ranging is achieved using a focusing optical lens, a waveguide interferometer, and a photonic balanced detector. This semiconductor chip-based LiDAR solution offers high cost-effectiveness for advanced ranging and 3D sensing applications such as autonomous driving. In 2024, SiFotonic Corporation (Hangzhou, China) demonstrated its independently developed four-channel dual-polarization FMCW LiDAR receiver module based on a silicon photonic coherent receiver chip [67]. Each channel is capable of processing light signals with dual polarization. This module is a joint package of silicon photonic coherent receiver chips and TIA chips, integrating edge couplers (EC), polarization beam rotation splitters (PBRS), 180\({}^{\circ}\) optical mixers, and balanced Ge/Si waveguide PD pairs. In terms of performance, the common-mode rejection ratio (CMRR) of each channel exceeds 30 dB, and the polarization extinction ratio (PER) surpasses 31.5 dB, which are significantly advantageous within the wavelength range of 1540 nm to 1560 nm. Importantly, the module exhibits a high sensitivity better than \(-\)80 dBm, enabling precise ranging operations up to 81.9 m. This work also demonstrates the potential of higher linearity FM lasers to further enhance the performance of FMCW LiDAR systems. In 2024, researchers [PERSON] and [PERSON] from the Institute of Semiconductors, Chinese Academy of Sciences, developed a novel focal plane array (FPA) chip design [68]. The design incorporates a single antenna for the emission of laser beams and an antenna array for the reception of return signals. The deployment of a single antenna for emission, in conjunction with the array for signal reception, enhances the directivity of the antenna system, achieving a theoretical directivity of 91.6%. The routing of light beams is managed via an array of micro-ring switches, which control the beam by aligning the resonant wavelength of the micro-rings with the incoming light wavelength, thus enabling precise beam manipulation. The FPA chip is integrated with an auxiliary lens system that collimates and deflects the beam for two-dimensional scanning. The integrated antenna array markedly enhances the range-finding capabilities of the FPA, as evidenced by the high SNR sustained over a distance of up to 11 m, thereby improving the precision of distance measurement. The implementation of an all-solid-state structure negates the necessity for mechanical components, simplifies the system design, and reduces both complexity and cost.