# Research on Narrow Linewidth External Cavity Semiconductor Lasers
## 4 Confocal F-P Cavity Structure of NLECSLs  
To further narrow the linewidth on the basis of the structure of the ECSL, it is necessary to use the mode selection element with a narrow bandwidth. The interference filter or F-P cavity and narrow-band filter are the structures of the external cavity optical feedback technology that are commonly used to narrow the linewidth [39]. Compared with the optical cavities used in the traditional fiber narrow linewidth laser, solid narrow linewidth laser and chip external cavity narrow linewidth laser, the high quality factor F-P cavity has an extremely low thermal effect, nonlinear effect and ultra-high temperature stability [40]. Confocal F-P cavity structures of NLECSLs are designed as a monolithic confocal F-P cavity and the focused laser beam is coupled with the tilted monolithic confocal F-P cavity. The tilt angle of the cavity, with regard to the optical axis of the laser system, prevents the non-resonant feedback from the cavity being re-injected into the emitter; this ensures resonant-only optical feedback in the laser diode, as shown in Figure 4. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Central** & **Tuning Range** & **SMSR** & **Line Width** & **Output** & **Publication** \\
**Wavelength** & **Tuning Range** & **SMSR** & **Line Width** & **Power** & **Time** \\ \hline
1565 nm & 1560–1570 nm & - & 31 Hz & 300 mW & 2021 [29] \\ \hline
1550 nm & 60 nm & 55 dB & 37 kHz & 11 mW & 2018 [30] \\ \hline
1540 nm & 45 nm & 60 dB & 100 kHz & 0.78 mW & 2019 [31] \\ \hline
1550 nm & - & 55 dB & 192 kHz & 312 mW & 2020 [32] \\ \hline
1559 nm & 160 nm & 55 dB & 30 kHz & - & 2020 [33] \\ \hline
1670 nm & 1647–1690 nm & 46.01 dB & 0.7 kHz & 31.1 mW & 2020 [34] \\ \hline
1550 nm & 58.5 nm & 70 dB & 2.5 kHz & 34 mW & 2021 [35] \\ \hline
1546 nm & 44 nm & 67 dB & 6.6 kHz & 23.5 mW & 2021 [36] \\ \hline
1542 nm & 1513–1564 nm & 42 dB & 3 kHz & 37.9 mW & 2021 [37] \\ \hline
1550 nm & 170 nm & 64 dB & 2.8 kHz & 24.8 mW & 2022 [38] \\ \hline \hline \end{tabular} Note: “-” denotes that the data are not available.  
\end{table} 
Table 4: Performance of silicon-based waveguide structure of NLECSLs. Figure 4: Confocal F-P Cavity structure of NLECSLs. A 657 nm ECSL system with stable output frequency was proposed in 2011 [41]. Through a narrowband high transmission interference filter, the instantaneous linewidth of the laser emitted by this new diode laser system was 7 kHz and the linewidth was 432 kHz. In the same year, [PERSON] et al. [42] proposed a wide-cavity ECSL with a linewidth of kilohertz using optical feedback from a single folded F-P cavity. The linewidth of the ECSL was successfully reduced to 6.8 kHz. In 2012, [PERSON] et al. [43] proposed a NLECSL with high-precision dual-mirror non-confocal cavity optical feedback. Through Lorentz fitting, the full width half maximum linewidth of the laser was reduced to 100 Hz, and the instantaneous linewidth was reduced to 30 Hz. In 2014, [PERSON] et al. [44] proposed an extended cavity diode laser with MHz linewidth. The optical feedback technology of the folded Fabry-Perot cavity was used to replace the mirror in the traditional ECSL configuration. The effective laser reduced the linewidth and stable frequency, and the linewidth of the laser was reduced from about 20 GHz to 15 MHz. In 2015, [PERSON] et al. [10] proposed a compact, ultra-narrow linewidth semiconductor laser based on a 780 nm distributed feedback diode laser, which was self-locked to an external single-chip confocal F-P cavity mode. When the output power exceeds 50 mW, the Lorentz linewidth corresponding to the resonant optical feedback laser is 15.7 Hz. In 2015, [PERSON] et al. [45] proposed a DFB based on 1.5 mm length and 780 nm with a single confocal Fabry-Perot cavity. Both surfaces of DFB are coated with anti-reflection coating. The tilt of the external resonator cavity relative to the optical axis of the laser system is 15\({}^{\circ}\), which can prevent the non-resonant feedback of the cavity from being reinjected into the transmitter.