## 3 FMCW LiDAR Research Progress
### Progress in Detection Accuracy Research  
The ranging accuracy of FMCW LiDAR systems constitutes a pivotal aspect of their technical performance. Accurate distance measurement not only significantly impacts the system's responsiveness and decision-making accuracy but also serves as a crucial element in enhancing overall safety and reliability. As automation and intelligence levels advance, the high-precision ranging capabilities of FMCW LiDAR are increasingly becoming the cornerstone of technological progress in these domains. Therefore, rigorous investigation and enhancement of FMCW LiDAR's ranging accuracy are imperative not only to meet the evolving technical demands but also to comply with increasingly stringent industry standards and application requirements. In 2018, [PERSON] et al. [37], from Tianjin University, proposed an innovative amplitude modulation technique to correct the nonlinear error introduced by the frequency sweep source in FMCW LiDAR systems. By implementing this method, the research team successfully developed a ranging system, as shown in Figure 10, which consists of two cascaded fiber Mach-Zehnder interferometers to improve measurement accuracy and stability. The first interferometer generates a sine wave signal with equidistant frequency peaks, while the second interferometer decomposes these carrier signals into multiple components and recombines them into a mixed beat signal, which allows for the mixed beat signal to be amplitude-modulated according to the signal returned from the measurement path. The system operates in a sweep bandwidth range of 1540 nm to 1560 nm, ensures adequate frequency resolution to correct nonlinear frequency sweep errors effectively and improve measurement accuracy. Experimental results show that the system achieves a range resolution of 69 \(\upmu\)m, a stability of 2.9 \(\upmu\)m, and an excellent ranging accuracy of 4.3 \(\upmu\)m within a range of 1 m. This method can effectively suppress nonlinear errors and eliminate phase differences caused by the mismatch between the clock and measurement signal sampling time. This achievement not only demonstrates the potential of amplitude modulation methods in enhancing the performance of FMCW LiDAR systems but also provides a feasible method for precise ranging. In 2019, [PERSON] and colleagues at the Massachusetts Institute of Technology significantly expanded the application of FMCW LiDAR by introducing OPA-based technology for coherent ranging and velocity measurements, marking a crucial shift from laboratory experiments to practical applications. The team innovativel  
Figure 10: Schematic of amplitude modulation system. © Optical Society of America. Copyright 2018 Optics Express [37]. scale optical phased array chips to construct a prototype FMCW LiDAR system. This system demonstrated its capability by simultaneously measuring the range and velocity of a rotating umbrella at a distance of 25 m, showcasing the first successful application of OPA-based technology in coherent LiDAR operations. Furthermore, the team conducted a one-dimensional OPA ranging experiment, successfully detecting pedestrians at a distance of 185 m. Additionally, by employing two active OPA chips, the researchers achieved three-dimensional depth detection up to 12 m, with the point cloud results of a person at 7 m clearly illustrated, further proving the system's efficient performance and potential for broader applications [38]. In 2020, [PERSON] et al. from the Swiss Federal Institute of Technology in Zurich introduced a new FMCW LiDAR technology [39]. This novel technology employs micro-resonator optical soliton frequency combs, enabling large-scale parallel FMCW measurements. As demonstrated in Figure 11, the system excelled in simultaneous ranging and velocity assessments. During trials targeting an object 11.5 m away, the system achieved exceptional ranging precision, attaining an accuracy of 1 cm.