## 2 FMCW LiDAR
### FMCW LiDAR Key Parameters
#### 2.3.7 Narrow Linewidth of Light Source  
For FMCW LiDAR, the linewidth of the laser has a great impact on the range performance of the radar. This parameter represents the coherence of the laser and determines the theoretical detection range and accuracy. The use of a narrow-linewidth light source reduces the complexity of the processing circuitry, improves the stability of the optical communication system, and reduces the receiver bit error rate (BER) [29; 30]. Therefore, the light source used for FMCW LiDAR requires narrow linewidth, frequency stability, low power consumption, and a long lifetime. Narrow linewidth semiconductor laser technology is mainly divided into two categories: internal cavity optical feedback technology and external cavity optical feedback technology [31], as shown in Figure 8. Internal cavity optical feedback mainly includes distributed feedback laser (DFB) lasers and distributed Bragg reflector (DBR) lasers with a feedback mechanism. The purpose of this technology is to inhibit the spectral linewidth of the laser. The Bragg grating of the DFB laser is distributed along the length of the cavity, and the intracavity optical field is influenced by the grating to form a forward field by part of the transmission and a backward field by part of the reflection. During transmission, the two optical fields continuously couple with each other, acquiring gain and generating stimulated emission of radiation for light amplification. DBR lasers are designed with a high-reflectivity Bragg grating at one end face of the resonant cavity, the gain longitudinal mode wavelengths are selected to obtain strong feedback by optimizing the Bragg spectra, and single mode excitation is formed inside the resonant cavity. In uniform grating DFB lasers, the two longitudinal modes have the same low threshold gain near the Bragg wavelength, so there is usually a problem of dual-mode degeneracy. To solve this problem, the researchers proposed quarter-wavelength phase-shifted DFB lasers, which allow for single-mode excitation [32]. However, the introduction of a quarter-wavelength phase-shifted region in the uniform grating is equivalent to adding defects to the periodic structure, where the strong coupling of the optical field leads to the concentration of longitudinal photons in the phase-shifted position, producing a more severe spatial hole burning effect. This uneven distribution of photons makes the intracavity refractive index have a large perturbation, which ultimately causes the longitudinal mode frequency noise increases and spectral line broadening and other problems [33; 34]. To solve this problem, researchers have conducted a number of investigations. For example, the narrow linewidth DFB laser structure can be divided into multi-phase shift-coupled DFB narrow-linewidth laser, multi-electrode injection DFB, and chirp grating DFB, among others. Narrow linewidth DFB lasers are compact in structure and achieve single longitudinal mode excitation with the help of Bragg grating frequency selection, which has  
Figure 8: Narrow linewidth semiconductor laser realization. high side mode rejection ratio and high-speed direct tuning capability. By overcoming the chirp effect of the direct tuning frequency, narrow-linewidth DFB lasers are the first choice for building high-performance optical communication systems. Arrays of multiple DFB lasers combined with couplers and amplifiers enable the design of complex on-chip light sources. The use of high-speed external modulation in combination with narrow-linewidth DFB lasers enables the design and implementation of high-performance coherent optical transmission and reception systems. External cavity feedback technology utilizes external optical components to provide feedback and wavelength selection to the emitted light from semiconductor laser chips, thereby extending the effective length of the resonant cavity. Optimization of the passive region enhances the laser's quality factor (Q-value) and reduces spectral linewidth.