## 3 FMCW LiDAR Research Progress
### Progress in Detection Accuracy Research
In 2021, researchers at Shanghai Jiao Tong University developed an innovative LiDAR system, leveraging a silicon dual-parallel Mach-Zehnder modulator (DP-MZM) for FMCW signal generation [40]. This system, distinguished by its use of generated triangular wave modulation signals, demonstrated the capability for concurrent ranging and velocity measurements. During experiments with a target positioned 2.2 m away, the system demonstrated high-precision measurement capabilities, achieving a ranging resolution of 13 mm and a velocity resolution of 0.5 m/s. In 2022, a team from the University of California designed a composite focal plane switch array [41]. This innovative array combined a grating antenna with light-switch array technology customized for microelectromechanical systems (MEMS), featuring an advanced structure with 128 \(\times\) 128 elements. Integrating a highly sophisticated chip with FMCW ranging technology, the research team accomplished precise distance measurements for targets within a 0.8 m to 10 m range, achieving a ranging accuracy of 1.7 cm. This study underscored the significant potential of focal plane switch arrays in enhancing ranging accuracy. Figure 11: Scheme of massively parallel coherent LiDAR: **(a)** Experimental setup. The amplified frequency-modulated LiDAR microcomb source is split into signal and local oscillator pathways. The signal is dispersed with a transmission grating (966 lines per millimeter) over the horizontal circumference of a flywheel mounted on a small direct-current motor. The reflected signals are spectrally isolated before detection. COL: fiber collimator. **(b)** Radio frequency spectrum of LiDAR back-reflection mixed with the local oscillator (sampling length 3.75 \(\upmu\)s) around 2.5 us (upward ramp) and 7.5 \(\upmu\)s (downward ramp). **(c)** Optical spectrum of comb lines after amplification. Blue shading highlights 30 comb lines with sufficient power (>0 dBm) for LiDAR detection. **(d)** Schematic illustration of the flywheel section irradiated by the frequency-modulated soliton microcomb lines indicating the projection of the position \(x_{\mu}\) and velocity \(v_{\mu}\) of the wheel onto the comb lines. ©Springer Nature. Copyright 2021 Nature [39]. In 2023, researchers from Pengcheng Laboratory in Shenzhen, in collaboration with Jilin University, developed an innovative FMCW LiDAR system based on optical frequency shifting [42]. By employing the linear scanning of optical frequencies and frequency shifts induced by an AOM, the system is capable of accurately measuring both the distance and velocity of targets. The distance is calculated from the frequency difference in the beat signal generated during operation. The introduction of frequency shifts by the AOM enables the differentiation between static and dynamic targets, thereby enhancing the precision of distance measurements. Furthermore, the relationship between the mean frequency of the beat signal and the frequency shifts introduced by the AOM enables the calculation of the target's radial velocity. The direction of rotation of the target is determined by comparing the magnitude of the average frequency to the frequency shift caused by the AOM. Experimental demonstrations demonstrated the real time measurement of a disk rotating at 26.43 m/s at a distance of 1.267 m, achieving distance and velocity measurement accuracies of 16 mm and 0.037 m/s, respectively. The system is not only capable of accurately measuring the distance and velocity of targets, but it is also able to determine their rotation direction directly, thus improving measurement efficiency, as shown in Figure 12.  
Although current FMCW LiDAR technology has made remarkable achievements in ranging accuracy, it still faces challenges in maintaining such high accuracy in extreme environments. In particular, harsh climates and changing conditions have become key factors restricting its widespread application.