# Research on Narrow Linewidth External Cavity Semiconductor Lasers
## 3 Waveguide Structure of NLECSLs
### Silicon-Based Waveguide Structure of NLECSLs  
With the maturity of the silicon optical chip design and process platform, the external cavity feedback elements based on silicon, Si\({}_{3}\)N\({}_{4}\) and other materials endlessly emerge, and with the help of various microcavity structures, the linewidth of ECSL can be further compressed. It has the characteristics of high reliability and low power consumption of the monolithic integrated structure, as well as the narrow linewidth and wide tuning characteristics of the external cavity structure, and has gradually become a hotspot in the research field of NLECSLs [28]. [PERSON] et al. [29] studied the hybrid integrated mode-locked laser diode with silicon nitride expansion cavity in 2021, coupled the silicon nitride external cavity with the InP active chip, and obtained the line width of 31 Hz. In 2018, [PERSON] et al. [30] studied III-V/Si hybrid external cavity lasers. The Si\({}_{3}\)N\({}_{4}\) edge-coupled silicon chip is mixed into the spot size converter in the silicon chip. The maximum output power of the laser is 11 mW, the measured minimum linewidth is 37 kHz (maximum \(<\) 80 Hz), and the SMSR is 55 dB. In 2019, [PERSON] et al. [31] demonstrated a III-V/silicon nitride hybrid external cavity laser. The tuning range of ECSL is 45 nm, the SMSR is 60 dB, and the linewidth is about 100 kHz. In 2020, [PERSON] et al. [32] showed a high-power on-chip 1550 nm laser, which was integrated into a silicon nitride waveguide and distributed Bragg reflector grating photonic integrated circuit by a bending channel and a two-way InGaAsP/InP plate coupled with an optical waveguide amplifier. The driving current of the single-mode emission optical power of 312 kW was 2.5 A, and the linewidth of 192 kHz was integrated. In 2020, [PERSON] et al. [33] demonstrated hybrid lasers by using InP reflective semiconductor optical amplifier chips coupled with Si\({}_{3}\)N\({}_{4}\) tunable reflector chips. The laser wavelength tuning range was 160 nm, and the linewidth was 30 kHz. In 2020, [PERSON] et al. [34] adopted the hybrid integration of the III-V optical amplifier and extended, low-loss wavelength-tunable silicon cursor cavity, and first reported the III-V/silicon hybrid wavelength-tunable laser in the rich wavelength region of 1647-1690 nm. When the continuous wave operates at room temperature, the output power can reach 31.1 mW, the maximum SMSR is 46.01 dB, and the line width is 0.7 kHz. In 2021, [PERSON] et al. [35] reported a widely tunable III-V/Si\({}_{3}\)N\({}_{4}\) hybrid integrated external cavity laser. Under 500 mA injection current, the maximum output power was 34 mW. In the tuning range of 58.5 nm, the SMSR exceeds 70 dB. The laser linewidth is 2.5 kHz. The same structure was used in the optical fiber communication conference and exhibition next year, but its performance was higher than last year, reaching a record of about 170 nm tuning range. The linewidth of the laser decreased slightly less than 2.8 kHz [36]. In 2021, [PERSON] et al. [37] reported a wavelength tunable hybrid integrated external cavity laser for C-band. Two parallel reflective semiconductor optical amplifier gain channels are composed of Y branches in the Si\({}_{3}\)N\({}_{4}\) photonic circuit to increase the optical gain. The SMSR is about 67 dB and the pump current is 75 mA. The linewidth of the unpackaged laser is 6.6 kHz, and the on-chip output power is 23.5 mW. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Central** & **Tuning Range** & **Line Width** & **Output Power** & **Publication Time** \\ \hline
1550 nm & - & 1 kHz & 370 mW & 2011 [22] \\ \hline
1532.83 nm & 20 pm & 14 kHz & 9 mW & 2016 [23] \\ \hline
1550.4 nm & 62.5 GHz & 35 kHz & - & 2017 [24] \\ \hline
1565 nm & 2.9 GHz & 1 kHz & 37 mW & 2019 [25] \\ \hline
400 nm & - & 16 kHz & - & 2021 [26] \\ \hline
1572 nm & 22 GHz & 15 kHz & 25 mW & 2022 [27] \\ \hline \end{tabular} Note: “-” denotes that the data are not available.  
\end{table} 
Table 3: Performance of FBG structure of NLECSLs. In 2021, [PERSON] et al.