Fig. 1. Different laser configuration models

Fig. 2. Linear main cavity lasers. (a) Schematic views of the slant trench F-P laser in three dimensions and from above^[38]; (b) VFR linear cavity fiber laser^[41]

Fig. 3. Ring main cavity lasers. (a) Ring-cavity fiber laser based on EYDSF and SA^[22]; (b) ring-cavity fiber laser using a WGM filter and photograph of WGM sandwiched between two prisms^[53]; (c) geometrical-optical description and mode-field distributions of light propagation in a microsphere WGM^[19]; (d) the WGM lasing system and the higher-order laser modes^[90]

Fig. 4. NPRO laser. (a) NPRO laser structure diagram^[91]; (b) the NPRO-based pre-stabilized laser system in LIGO^[92]

Fig. 5. Mechanical design of external cavity laser^[100]. (a) Mechanical design; (b) isolated section of the laser head consisting of a fiber-coupled gain chip and collimated optics

Fig. 6. Typical DBR and DFB laser structures^[25]. (a) DBR; (b) DFB

Fig. 7. DBR configuration lasers. (a) E-DBR laser configuration^[112]; (b) DBR fiber laser configuration^[121]

Fig. 8. DFB configuration lasers. (a) Schematic diagram of the sidewall transverse DFB integrated laser and scanning electron microscopy of the ridge waveguide cross-section^[103]; (b) DFB fiber laser structure^[133]

Fig. 9. Littman and Littrow external cavity feedback laser structures^[25]. (a) Littman; (b) Littrow

Fig. 10. Various external-cavity feedback narrow linewidth lasers. (a) Self-injection feedback structure fiber laser ^[159]; (b) self-injection-locked fiber laser with linewidth compression using a WGM ^[163]; (c) four-wavelength narrow linewidth laser array based on WGM self-injection locking^[168]; (d) Si₃N₄ material on-chip external-cavity feedback laser^[174]; (e) photoelectric oscillation feedback laser system^[184]

Fig. 11. Principle of laser spectral purification based on adaptive distributed weak feedback^[188]. (a) Adaptive distributed weak feedback laser configuration; (b) evolution of laser phase fluctuation and noise coupling strength at different round trips; (c) spectral distribution at different noise levels

Fig. 12. Principle of spectral evolution of distributed weak feedback structure^[193]. (a) Spectral evolution model; (b) Rayleigh scattering spectral width evolution process; (c) Rayleigh scattering spectral width trend with increasing number of scattering sources

Fig. 13. Experimental investigation of spectral evolution in distributed feedback structures^[194-195]. (a) Experimental setup; (b) spectral evolution of Rayleigh scattering with increasing pump power; (c) 3 dB spectral width trend

Fig. 14. Theoretical model of distributed weak feedback mechanism^[26]. (a) Schematic of the distribution of effective feedback planes in one-dimensional waveguide structure; (b) amplitudes at adjacent planes out of a stack of N_k planes

Fig. 15. Spectrum evolution of different reflection coefficients and spectrum evolution of different feedback surfaces in distributed feedback structure^[198]. (a) Spectrum evolution of different reflection coefficients; (b) spectrum evolution of different feedback surfaces

Fig. 16. Power spectrum evolution process^[26]. (a) Two-dimensional pseudocolor map of the spectra varying with the number of round trips;(b) localized enlargement corresponding to the blue box in Fig. 16(a); (c) spectra of different wavelengths at the same round trips

Fig. 17. Simulation results of laser linewidth evolution^[188]. (a) Laser linewidth evolution with the feedback length under different feedback coefficients; (b) linewidth curve at the different feedback ratios;(c) (d) two-dimensional pseudocolor maps of the spectra varying with the length and with feedback ratio

Fig. 18. Self-adaptive compression process of laser linewidth^[188]. (a) (b) Transient spectrum and corresponding Lorentzian linewidth when switching on the feedback; (c) (d) transient spectrum and corresponding Lorentzian linewidth when tuning the frequency of the main laser cavity

Fig. 19. Self-adaptive fiber lasers based on distributed weak feedback^{[197, 200]}

Fig. 20. External cavity weak distribution feedback ultra-narrow linewidth single-frequency fiber laser^[201]

Fig. 21. Continuously linewidth-tunable distributed feedback external cavity DBR fiber laser^[202]

Fig. 22. Distributed weak feedback chip external cavity^[188]. (a) Distributed weak feedback chip external cavity structure; (b) comparison curves of the frequency spectrum from beat frequency signal; (c) Lorentz fitting curve of the linewidth with a distributed feedback; (d) comparison curves of the frequency noise PSD; (e) comparison curves of the RIN spectra, where a red curve indicates the compressed result

Fig. 23. Ring cavity structure wavelength tuning/wavelength sweeping ultra-narrow linewidth laser^{[26, 196, 205-206]}

Fig. 24. Wavelength-tuned ultra-narrow linewidth linear cavity laser ^{[188, 207-208]}

Fig. 25. Other narrow linewidth lasers based on distributed weak feedback^[209-216]

Fig. 26. Coherent envelope method for measuring laser linewidth^[217-218]. (a) (b) Normalized power spectral density for different delay line lengths and different laser linewidths ; (c) second peak-to-valley variation curve with laser linewidth