## 3 FMCW LiDAR Research Progress
### Progress in Narrow Linewidth Research
This structure is compatible with conventional photolithographic fabrication processes, yielding a slope efficiency of 0.11 mW/mA over a 750 \(\upmu\)m cavity length, with a minimum linewidth of 720 kHz at room temperature and linewidth maintained within 1 MHz between 10 \({}^{\circ}\)C and 60 \({}^{\circ}\)C. The team also proposed an integrated electro-absorption modulated coupled-cavity laser, which, through optimized slot design, achieved a slope efficiency of 0.12 mW/mA and a large signal modulation rate of 3 Gb/s over a 650 \(\upmu\)m  
Figure 14: Three-dimensional cutaway structure for the split-contact corrugated ridge waveguide with three electrodes. © Optical Society of America. Copyright 2014 [56]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Year** & **Institution** & **Modulation Type** & **Lasers** & **Spectra Linewidth** & **Ref.** \\ \hline
2021 & Italy & Narrow-linewidth lasers & & 300 kHz & [59] \\ \hline
2021 & \
\begin{tabular}{c} Xi’an Institute of Optics \\ and Precision Mechanics \\ at the Chinese Academy \\ of Sciences \\ \end{tabular} & External cavity optical &
\begin{tabular}{c} External-cavity narrow \\ linewidth mode-locked \\ semiconductor laser \\ \end{tabular} & 5.4 kHz & [60] \\ \hline
2022 & \
\begin{tabular}{c} Shanghai Jiao Tong \\ University \\ \end{tabular} &
\begin{tabular}{c} Chip-scale external \\ cavity lasers \\ \end{tabular} & 4 kHz & [61] \\ \hline
2023 & \
\begin{tabular}{c} Shanghai Jiao Tong \\ University \\ \end{tabular} &
\begin{tabular}{c} Hybrid-integrated tunable \\ external cavity laser \\ \end{tabular} & 2.8 kHz & [62] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._cavity length, achieving a single-mode suppression ratio of 52 dB near a wavelength of 1500 nm. In 2017, Tampere University of Technology in Finland designed a 1180 nm narrow linewidth DBR laser [58]. A high-order surface grating with a 2 mm length and a 50% duty cycle was employed as the Bragg mirror, effectively eliminating the need for the regrowth process typically associated with buried grating structures. The Bragg spectral bandwidth was designed around the peak wavelength of the multiple-quantum-well gain, ensuring single-mode lasing. A 3 mm active region was designed to provide sufficient optical gain, and a ridge waveguide width of 3.2 um was chosen to ensure single transverse mode operation, achieving a 50 dB side mode suppression ratio and effectively preventing mode hopping in the lasing wavelength. The design achieved high output power with a spectral linewidth of 250 kHz. In 2021, [PERSON] et al., in Italy, proposed a 780 nm narrow-linewidth laser based on a Littrow configuration, which overcame the disturbances in cavity length caused by grating angles compared to previously reported structures of the same type [59]. As a result, using piezoelectric ceramics to adjust the resonant frequency did not directly affect the external feedback unit and the grating selection mechanism, enhancing the ability to suppress mode hopping. This design ultimately achieved a spectral linewidth of 300 kHz. In 2021, the Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, designed an external-cavity, narrow-linewidth, mode-locked semiconductor laser based on a similar structure. Leveraging the strong four-wave mixing effect in the active region and the prolongation of photon lifetime by the external cavity, the device achieved a pulse repetition rate of 10 GHz and an ultra-narrow linewidth of 5.4 kHz [60]. In 2022, Shanghai Jiao Tong University demonstrated a hybrid integrated chip-level external cavity laser, composed of a wide bandgap III-V reflective semiconductor optical amplifier and a low-loss Si\({}_{3}\)N\({}_{4}\) wavelength-selective reflector, as illustrated in Figure 15 [61]. Enhanced laser mode selection was achieved through the tunable Sagnac loop reflector, enabling a record-setting laser wavelength tuning range of 172 nm beyond the free spectral range of the Vernier ring filter, with more than 40 dB single-mode suppression ratio.