Fig. 1. Experimental setup of laser and output index measurement systems. (a) Experimental setup of laser; (b) linewidth measurement system; (c) polarization extinction ratio measurement system; (d) beam quality measurement system

Fig. 2. Experimental results. (a) Output powers versus pump power under different gain fiber coiling diameters, and backward laser power versus pump power under 8-cm gain fiber coiling diameter; (b) laser time domain distribution under different output powers and gain fiber coiling diameters with frequency domain distribution shown in inset

Fig. 3. Calculation results. Power of LP01 mode and LP11 mode in amplifier versus fiber length when gain fiber coiling diameter is (a) 12 cm, (b) 9 cm and (c) 8 cm; (d) LP01 mode power in amplifier versus fiber length when LP11 mode injection power and LP01 mode bending loss are ignored

Fig. 4. Spectrum of full power output laser

Fig. 5. 3 dB linewidth values measured by spectrum analyzer at different seed source modulation depths with output spectra measured by spectrum analyzer when seed source modulation depth is 11 dB‒16 dB

Fig. 6. Measurement results. (a) Beam quality measured at full power output; (b) PER measured at different laser powers