Fig. 1. Structure diagram of 811 W single-frequency fiber laser^[8]

Fig. 2. Signal power and reflectivity versus pump power^[8]

Fig. 3. Structure diagram of 414 W single-frequency laser^[9]

Fig. 4. Experimental structure of all-fiber single-frequency PM fiber amplifier^[12]

Fig. 5. Structure diagram of linearly polarized single-frequency fiber amplifier based on 1018 nm fiber laser core-pumping^[13]

Fig. 6. Cross section of Panda type polarization-maintaining fiber

Fig. 7. Structure and output power curve of narrow-linewidth PM laser^[15]

Fig. 8. Experimental system of the 2.43 kW narrow-linewidth linearly polarized fiber laser^[21]

Fig. 9. Structure diagram of 2 kW narrow-linewidth linearly-polarized amplifier^[23]

Fig. 10. Laser power and PER versus pump power^[28]

Fig. 11. System structure of spectrum broadening by optimization algorithm^[54]

Fig. 12. Theoretical and experimental results^[54]. (a) Optimized signal waveforms; (b) optimized spectrum

Fig. 13. Output characteristics^[30]. (a) Laser power and PER versus pump power; (b) output spectra under different laser powers

Fig. 14. Experimental test results^[34]. (a) Output power curve; (b) forward spectrum; (c) backward spectrum; (d) beam quality

Fig. 15. Experimental system structure of narrow-linewidth linearly polarized fiber amplifier^[32]

Fig. 16. Output characteristics^[32]. (a) Output spectra under different laser powers; (b) 3 dB linewidth and RMS linewidth versus output power

Fig. 17. Output characteristics of 3.96 kW narrow-linewidth linearly polarized amplifier^[33]. (a) Output spectra under different laser powers; (b) linewidth versus laser power

Fig. 18. Output characteristics of 4.5 kW narrow-linewidth linearly polarized fiber amplifier^[35]. (a) Backward powers under different laser powers; (b) output spectra under different laser powers; (c) PERs under different laser powers

Fig. 19. Output characteristics of 5 kW narrow-linewidth linearly polarized fiber amplifier^[36]. (a) Backward powers under different laser powers; (b) PERs under different laser powers; (c) output spectra under maximum power

Fig. 20. Experimental system structure of 2 kW narrow linewidth linearly-polarized fiber amplifier^[37]

Fig. 21. Experimental system structure of narrow-linewidth linearly polarized fiber amplifier with output power of 5 kW and linewidth of 10 GHz^[38]

Fig. 22. Output characteristics^[38]. (a) PER versus laser power; (b) output spectrum at maximum laser power

Fig. 23. Experimental structure and results^[19]. (a) Structure of narrow-linewidth fiber amplifier based on adaptive polarization control; (b) P∕S polarized power and PER versus laser power

Fig. 24. Structure of narrow-linewidth fiber amplifier based on adaptive polarization control^[20]

Fig. 25. Experimental system of 3.1 kW narrow-linewidth fiber laser^[17]

Fig. 26. Linearly polarized fiber oscillator^{based onfiber Bragg grating (FBG) [55]}

Fig. 27. Theoretical simulation results of PM 15/130 μm fiber loss for different linear polarization modes^[56]

Fig. 28. Experimental system of kW 1030 nm narrow-linewidth linearly polarized fiber amplifier ^[43]

Fig. 29. Experimental system of 3 kW narrow-linewidth linearly polarized fiber amplifier ^[44]

Fig. 30. Experimental results^[44]. (a) Output spectrum under 3.08 kW power; (b) SRS spectrum

Fig. 31. Experimental system of 4.6 kW narrow-linewidth linearly polarized fiber amplifier^[46]

Fig. 32. Refractive index distribution of single-trench fiber^[88]

Fig. 33. Longitudinal profile for core diameter of biconical tapered fiber^[89]

Fig. 34. Cross section photograph and refractive index distribution of optical fiber^[90]. (a) Cross section of partially doped fiber; (b) refractive index distribution

Fig. 35. Device diagram of coherent beam combining system, output time domain signal, and spot patterns^[98]. (a) Device diagram of coherent beam combining system; (b) output time domain signal and spot patterns

Fig. 36. Experimental results of coherent beam combing with 20 kW level fiber laser^[100]. (a) Long exposure pattern of far-field spot pattern in open loop; (b) long exposure pattern of far-field spot pattern in close loop

Fig. 37. Optical path diagram of coherent beam combining^system[102]

Fig. 38. Experimental results before and after phase locking^[102]. (a) Before phase locking; (b) after phase locking

Fig. 39. Optical path diagram of coherent beam combining^system[95]

Fig. 40. Output power versus total laser power after beam combining ^[95]

Fig. 41. Power curves of four-channel amplifier and output power and efficiency curves after polarization coherent beam combining^[96]. (a) Power curves of four-channel amplifier; (b) output power and efficiency curves after polarization coherent beam combining

Fig. 42. Optical path diagram of coherent beam combining^{system [97]}

Fig. 43. Absorption characteristics of copper at different wavelengths ^[107]

Fig. 44. Schematic of green fiber laser based on single-pass SHG^[110]

Fig. 45. Output power and SHG conversion efficiency of 532 nm CW laser ^[110]

Fig. 46. Output power and SHG conversion efficiency of 532 nm QCW laser^[110]

Fig. 47. Schematic of sub-ns fiber laser and its harmonic conversion system^[111]

Fig. 48. SHG green laser power versus pump power at different linewidths^[112]

Fig. 49. Schematic diagram of experimental setup^[109]

Fig. 50. Calculated and experimental results under different crystal lengths^[109]. (a) Normalized power versus wavelength under different crystal lengths; (b) frequency doubling efficiency and green light power versus fundamental frequency light power

Fig. 51. Simplified diagram of LIGO gravitational wave detector ^[114]

Fig. 52. Single frequency fiber amplifier based on cascaded amplification structure^[115]

Fig. 53. Schematic diagram of 3 W single frequency fiber amplifier^[116]