## 3 FMCW LiDAR Research Progress
### Advances in FM Linearity Research
The 3D image in Figure 12(a) is processed via simple filtering. Again, the colors represent the depth of the distance. Thereafter, the slow scanning time of the MEMS scanner was further reduced. The period of the slow scan was adjusted to 40 Hz; i.e., the 3D image was acquired at a rate of 0.25 s (40 frames/s). Figure 12(b) shows the result of a 3D image at a slow scanning period of 40 Hz with 50 pixels \(\times\) 50 pixels per frame. The colors reflect the reflectivity of the target. As can be seen from the results, the spatial resolution in the slow scanning direction is relatively low compared to 10 fps imaging. However, the final 3D imaging result is still clear enough. Similarly, the undisplayed 3D images were extracted and displayed at intervals of 0.25 s.  
In 2022, a research team from Shanghai Jiao Tong University [52] developed an all-solid-state FMCW LiDAR system, incorporating beam-steering with lens-assisted technology to achieve a ranging capability of up to 210 m. Utilizing directly modulated DFB lasers for high-bandwidth performance and cost-efficiency, the team enhanced frequency scanning linearity through precise predistortion drive voltage adjustments for the DFB lasers. Achieving a nonlinear residual ratio of 1\(-\)\(r^{2}\) at 5.19 \(\times\) 10\({}^{-8}\), the system demonstrated efficient performance. Operating at a wavelength of 1550 nm, the system was validated with 16 scanning directions, a beam-steering step of 0.35\({}^{\circ}\), and a total steering angle of 1.05\({}^{\circ}\). By optimizing the transmit array and lens configuration, the system can achieve a wider scanning range and larger scanning angle. Phase-noise compensation technology enables excellent spatial resolution even beyond the laser's coherence length, demonstrating its applicability in long-range measurement scenarios. In 2023, the Chinese University of Hong Kong's research team [53] developed a laser-ranging system utilizing dynamic FMCW technology and optical coupling through a rotating interface, addressing beat frequency issues in traditional FMCW LiDAR. By employing a fiber-optic rotary joint and real-time laser chirp nonlinearity corrections vis synchronous laser injection current and FMCW signal adjustments, the system eliminates rotational coupling-induced measurement deviations. This FMCW LiDAR system excels in high-precision ranging during low-speed rotation, achieving a rapid capture rate through the linearization of laser frequency-modulation in each frequency sweep, outperforming the rotational speed of the fiber-optic rotary joint. Capable of rotating clockwise and counterclockwise, it consistently generates clear, robust beat frequency signals over a 50 cm measurement distance. Additionally, the results during rotation demonstrate remarkable consistency with those from stable conditions, with average measured distances of 50.8 cm and 51.4 cm, both within the standard deviation of stable condition measurements. This study presents a new approach for applying FMCW LiDAR technology in dynamic environments. In 2024, a research team from Yanshan University [54] proposed a novel approach to mitigate the nonlinear frequency scanning issue in FMCW LiDAR light sources by utilizing a hybrid electro-optic phase-locked loop (HEO-PLL) for precise control over the frequency linearity of the DFB laser. The HEO-PLL proves exceptionally effective in controlling DFB lasers during continuous, rapid, and extensive frequency modulation via current tuning, enabling these lasers to function as high-performance light sources for FMCW LiDAR and significantly enhancing the system's resolution and distance measurement capabilities. The study meticulously discusses the challenges associated with loop bandwidth, center frequency tuning, and the sensitivity to temperature and voltage variations caused by charge pump phase-locked loops. Achieving a scanning speed of 6.78 THz/s and a 0.078% nonlinear triangular wave scan, the research demonstrated a notable 31.49 dB enhancement in beat frequency signal power.