## 3 FMCW LiDAR Research Progress
### Progress in Narrow Linewidth Research
The Si\({}_{3}\)N\({}_{4}\) platform facilitated a low-loss external cavity, aiding in reducing the laser linewidth to below 4 kHz across the full tuning range. The chip-level laser output power reached 26.7 mW at a wavelength of 1550 nm, providing a broad tuning range and narrow linewidth, making it highly applicable in optical communications, sensing, and photodetection and ranging systems. In 2023, [PERSON] and colleagues at Shanghai Jiao Tong University developed a fully integrated solid-state LiDAR emitter [62], resulting in the creation of a high-performance LiDAR system. The team employed a multi-layer silicon nitride platform on a silicon substrate, integrating a widely tunable external cavity laser and a two-dimensional optical phased array based on a 256-channel aperiodic grating. The system achieved an on-chip laser output of 18 mw and high-fidelity beam control by leveraging the high-power handling capabilities and minimal thermal cross-talk of the silicon nitride external cavity lasers. Furthermore, a novel method for compensating for optical path differences was devised  
Figure 15: Schematic structure of the III–V/Si3N4 hybrid laser © APL Photonics Copyright 2022, AIP Publishing [61]. to ensure that the beam-forming angles of the phased array remained constant during wavelength scanning. In conclusion, the system emitted light into the free space through the emission aperture, thereby forming a highly directional far-field spot. This research addressed several key challenges inherent to LiDAR systems. Primarily, the employment of silicon nitride external cavity lasers resulted in the achievement of narrow intrinsic spectral linewidths, thereby enhancing the overall performance of the laser. Secondly, the design of the 256-channel optical phased array permitted precise beam control, thereby enhancing the accuracy of the measurements and the capacity to detect targets in the LiDAR system. Moreover, the compensation of optical path differences guaranteed the stability of the beam-forming angles, thereby enhancing the system's stability and reliability. A broadly tuned single-mode laser output in the 100 nm range was achieved with a 42 dB side-mode rejection ratio, 18 mW output power, and 2.8 kHz linewidth. Two-dimensional beam steering demonstrated a beam divergence of 0.051\({}^{\circ}\)\(\times\) 0.016\({}^{\circ}\) over a 140\({}^{\circ}\)\(\times\) 16\({}^{\circ}\) field of view. By employing innovative design and integration techniques, this study has resolved critical issues in solid-state LiDAR emitters, thereby providing significant technical support for the realization of high-performance LiDAR systems. Although narrow linewidth semiconductor lasers have made important progress in improving the resolution and accuracy of LiDAR, the realization of perfect spectral linewidth compression and ultra-high coherence still faces technical challenges. The stability of current technology in high-noise environments and sensitivity to small, reflected signals need to be further improved. Future research needs to focus on developing new semiconductor materials and more-precise frequency control technology to compress the spectral linewidth further and improve the overall performance of the system. These technological advances are expected to expand the application of narrow linewidth semiconductor lasers in FMCW LiDAR, especially in fields such as autonomous driving and environmental monitoring, significantly enhancing their performance and reliability by improving coherence and resolution.