Fig. 1. Structural diagram of 5 kW near-single-mode fiber laser oscillator

Fig. 2. Output characteristics of fiber laser oscillator under forward pump condition. (a) Output power and optical-to-optical conversion efficiency varying with pump power; (b) optical spectrum at pump power of 1.644 kW; (c) time-domain signals recorded by photoelectric detector at P₁ and P₂; (d) Fourier spectra in frequency domain corresponding to time-domain signals at P₁ and P₂

Fig. 3. Output characteristics of fiber laser oscillator under backward pump condition. (a) Output power and optical-to-optical conversion efficiency varying with pump power; (b) optical spectrum at pump power of 4 kW; (c) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at pump power of 4 kW

Fig. 4. Output characteristics of fiber laser oscillator under bidirectional pump condition. (a) Output power and optical-to-optical conversion efficiency varying with pump power; (b) optical spectrum at pump power of 5 kW; (c) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at pump power of 5 kW

Fig. 5. Measurement result of beam quality of output laser with different powers under bidirectional pump condition

Fig. 6. Output characteristics of fiber laser oscillator under full power working. (a) Output power of fiber laser oscillator varying with operating time under full power working; (b) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at stageⅠ; (c) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at stageII; (d) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at stage Ⅲ

Fig. 7. Comparison of output characteristics of fiber laser oscillator before and after long-time full power working. (a) Output power varying with optical-to-optical conversion efficiency varying with pump power before and after long-time working; (b) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at pump power of 6.512 kW; (c) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at pump power of 6.556 kW

Fig. 8. Output characteristics of fiber laser oscillator under bidirectional pump condition after adjusting forward and backward pump power ratio. (a) Output power and optical-to-optical conversion efficiency varying with pump power; (b) optical spectrum at pump power of 5 kW; (c) time-domain signal recorded by photoelectric detector and corresponding Fourier spectrum in frequency domain at pump power of 5 kW

Fig. 9. Output power of fiber laser oscillator varying with operating time under second full power working

Fig. 10. Comparison of output power and conversion efficiency of fiber laser oscillator before and after photodarkening effect. (a)(d) Output power and optical-to-optical conversion efficiency varying with pump power; (b)(e) time-domain signals; (c)(f) Fourier spectra in frequency domain corresponding to time-domain signals