Fig. 1. Simulation results of output spectra in OAIFL employing different kinds of SSYDFs.

Fig. 2. Schematic diagram of the longitudinal structure of the ASSYDF.

Fig. 3. Schematic diagram of the 6 kW fiber laser using ASSYDF. CLS, cladding light striper; HR/OC FBG, high reflector/output coupler fiber Bragg grating; YDF, ytterbium-doped fiber; SPC, side pump combiner; CTFBG, chirped and tilted fiber Bragg grating; ASSYDF, asymmetric spindle-shaped YDF; BPSC, backward pump/signal combiner; LD, laser diode; QBH, quartz beam head.

Fig. 4. Output characteristics of the amplification stage pumped by non-wavelength-stabilized 976 nm LDs. Measured (a) output powers and (b) conversion efficiencies under different pump powers and pump wavelengths; (c) measured time-domain signals and calculated Fourier spectra in the frequency domain under different output powers.

Fig. 5. Output characteristics of the amplification stage hybrid pumped by 976 nm LDs and DW LDs for the first time. Measured (a) output powers and conversion efficiencies under different pump powers, (b) spectra of the seed laser and 5 kW high-power laser, (c) time-domain signals and Fourier spectra in the frequency domain, and (d) beam quality (M² factors) of the 5 kW high-power laser.

Fig. 6. Output characteristics of the amplification stage hybrid pumped by 976 nm LDs and DW LDs for the second time. (a) Output powers and conversion efficiencies under different pump powers, (b) laser spectra under different output powers, (c) time-domain signals and Fourier spectra in the frequency domain, (d) beam quality (M² factors) of the 6 kW high-power laser, and (e) M² factors and STDs of normalized PD signals under different output powers.