## 3 FMCW LiDAR Research Progress
### Advances in FM Linearity Research  
The performance of an FMCW LiDAR is also limited by its FM linearity. Optimization of FM linearity not only improves the accuracy of range and speed measurement but also is critical to the stability and reliability of the overall system. With the advancement of technology and the increase in application requirements, improving FM linearity hasbecome a necessary means of achieving higher system performance. Therefore, in-depth study and optimization of FM linearity is the key to ensuring the best performance of FMCW LiDAR in automation and intelligent applications. Researchers have endeavored to enhance linearity through innovative techniques and approaches, such as the use of novel tuned light sources and improved FM strategies, to meet the increasingly stringent technical standards and application requirements. In 2018, researchers at Tianjin University [48; 49] introduced a novel approach that combines cyclic frequency shift loop technology with swept-frequency tuning of a light source. Utilizing a continuous laser with an accuracy of up to 20 kHz as the seed light source, this method employs an optical switch to generate square-wave pulse light with a 5.5 \(\upmu\)s period and a 343.75 ns duration. The technique stabilizes the pulse frequency in the cycle to a 12.5 GHz shift, creating a series of 16 optical pulses that incrementally vary over time. This approach achieves continuous frequency conjunction in the frequency domain, facilitating linear frequency tuning up to 200 GHz. External modulation is employed to preserve the narrow-linewidth characteristics of the seed light, maintaining the linewidth of the emitted light below 50 kHz and ensuring high coherence. This development of a narrow-linewidth, wide-band light source presents new prospects for high-resolution, long-distance imaging in FMCW LiDAR applications, boasting distance resolutions of less than 1 mm and detection ranges of over 1 km, thus paving the way for advanced long-distance high-resolution LiDAR technologies. In 2019, the University of California, USA [50], pioneered a new technique aimed at enhancing the linearity of laser frequency scanning. This technique uses iterative learning control and predistortion methods to improve laser frequency scanning's linearity. Demonstrated with a commercial vertical-cavity surface-emitting laser (VCSEL) and DFB lasers, the technique significantly minimized the relative residual nonlinearity of frequency scanning to less than 0.005%, marking a substantial improvement in the precision and reliability of frequency scanning processes. In 2020, the Shanghai Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences developed a high-precision 3D FMCW LiDAR system in which state-of-the-art MEMS scanning technology was used [51]. The system utilizes a self-developed arbitrary waveform generator, optoelectronic modulator, and optical filter to generate a linear FMCW signal with a bandwidth of 2.0 GHz and a pulse repetition frequency of 100 kHz. By combining outlier coherent detection with MEMS scanning techniques, the system greatly improves the sensitivity. In addition, in order to obtain distance and fast imaging of targets with a large dynamic range, a multiphase FFT is employed to calculate the full frequency band. The multiphase FFT algorithm can make full use of the hardware resources of FPGA to improve the real-time performance, thus effectively increasing the number of channels and realizing the parallel processing of FFT. Rigorous testing of static and dynamic target imaging scenarios shows that the system is capable of capturing 3D images at 40 fps, which meets the high-speed imaging requirements of various applications. To demonstrate the performance of the system, the research team sampled the frame data images at 1 s (10 frames/s) intervals, as shown in Figure 12(a). At this point, the motion state at different moments can be seen more clearly. The speed of human movement measured at the same time was 1.5 m/s.