## 3 FMCW LiDAR Research Progress
### Progress in Narrow Linewidth Research  
Narrow linewidth semiconductor lasers have become the ideal light source for the new generation of high-precision LiDAR systems due to their small size, light weight, high efficiency, long life, direct current drive, narrow spectral linewidth and excellent coherence. This type of laser usually integrates a frequency selective structure in the resonant cavity or couples it with an external mode-selective device to accurately control the gain and loss of different wavelengths to achieve effective compression of the spectral linewidth. With the rapid development of technologies such as autonomous driving and remote sensing, the performance requirements for laser sources are becoming increasingly stringent, and research on narrow-linewidth lasers is also constantly deepening. Optical feedback narrow-linewidth technology is primarily focused on the continual design optimization of various novel devices based on their underlying physical mechanisms to optimize the design for different laser structures, i.e., DFB, DBR, and external cavity lasers that correspond to a different narrow-linewidth implementation method. We utilized two methods with the status of the devices involved in the research to launch a specific introduction and discuss the different technical characteristics and research issues that must be addressed, as shown in Table 5. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Year** & **Institution** & **Modulation Type** & **Lasers** & **Spectra Linewidth** & **Ref.** \\ \hline
2014 & University of Ottawa & & Multi-electrode surface & 140 kHz & [56] \\ \hline
2014 & University of California, Santa Barbara & Internal cavity optical feedback technology & Narrow-linewidth laser using a slot-coupled cavity & 720 kHz (room temperature) & [57] \\ \hline
2017 & Tampere University of Technology in Finland & & Narrow-linewidth DBR & 250 kHz & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Progress in the study of the spectral linewidths of lasers with different structures. In 2014, the University of Ottawa conducted research on multi-electrode surface grating DFB lasers [56], as shown in Figure 14. The structure has three electrodes on the top layer and uses surface gratings for feedback and wavelength selection. Traditional buried grating DFB lasers require secondary growth of the overlying cladding and ridge waveguides using metal-organic chemical vapor deposition (MOCVD) after grating etching, which, to some extent, reduces the single-mode yield and reliability of the device. The advantage of surface grating structures is that they do not require secondary growth processes after grating fabrication. However, challenges include considerations of lateral etch width, Bragg period constancy, and the roughness of the etched surfaces. Step-and-repeat photolithography or electron beam lithography (EBL) is required to achieve high etch precision. The multi-electrode DFB allows for flexible control of the current ratios across the three electrodes and achieves a spectral linewidth of 140 kHz and a single-mode suppression ratio of more than 50 dB in the 1550 nm band, with an output power approaching 7 mW. These results provide important guidance for the fabrication of high-power narrow-linewidth lasers and offer valuable tools for communications and other applications. In 2014, UCSB developed a slot-coupled cavity narrow-linewidth laser [57]. A deep slot was etched into the ridge waveguide on one side of the cavity to provide sufficient feedback to balance the intracavity losses. The deep-slot waveguide and cleaved facets formed the front and rear mirrors to achieve resonant lasing. The slot parameters, including etch depth, width, and period, were optimized using a two-dimensional scattering matrix approach.