Fig. 1. Schematic diagram of linear main cavity laser^［39］

Fig. 2. 1 603 nm DBR fiber laser^［46］

Fig. 3. Schematic diagram of ring main cavity laser^{［49，55］}

Fig. 4. Narrow linewidth laser system based on microring resonator thermo-optic locking^［59］

Fig. 5. DFB narrow linewidth laser system based on FBG-FP self-injection locking^［62］

Fig. 6. Narrow linewidth laser system based on self-injection feedback^［70］

Fig. 7. Narrow linewidth fiber laser system based on fiber ring resonator ^［71］

Fig. 8. Narrow linewidth fiber laser system based on Add-Drop structure^［72］

Fig. 9. Principle of laser linewidth compression based on distributed weak feedback

Fig. 10. Principle of spectral evolution of distributed weak feedback structure^［77］

Fig. 11. Experimental investigation of spectral evolution in distributed feedback structures^［78-79］

Fig. 12. Principle and output characteristics of artificial distributed feedback short waveguide structures^［80-82］

Fig. 13. Ultra-high Q wedge resonator and output characteristics^［83］

Fig. 14. Lithium niobate microring with ultra-high Q factor and optical properties^［84］

Fig. 15. Self-adaptive fiber laser based on distributed weak feedback^{［88，90-91，31］}

Fig. 16. An on-chip laser system based on distributed feedback^［29］

Fig. 17. Narrow linewidth laser based on ultra-high Q micro resonator^［94-95］

Fig. 18. Distributed sensing system based on narrow linewidth laser^［99］

Fig. 19. Structure diagram of tunable laser based on silicon microring resonator^［101］

Fig. 20. Schematic diagram of photoelectronic system based on on-chip optical frequency comb^［104］