Fig. 1. Schematic diagram of the three structures of fiber lasers: (a) fiber laser oscillator based on the FP-fiber cavity; (b) fiber laser amplifier based on the MOPA structure; (c) OAIFL.

Fig. 2. Schematic of the bidirectionally pumped OAIFL: (a) experimental structure; (b) schematic diagram of the fiber groove.

Fig. 3. Experimental results of co-pumping: (a) variation curves of the output laser power and the O–O efficiency with the pump power; (b) signal of the PD at the maximum output power and their FFT results (inset); (c) result of the beam quality M² factor at the power of 1537 W (inset: the beam profile); (d) spectra at different output powers.

Fig. 4. Experimental results of bidirectional pumping: (a) variation curves of the output laser power and the O–O efficiency with the pump power; (b) signal of the PD at the highest output power and their FFT results (inset); (c) spectra at different output powers; (d) measured 3-dB bandwidth at the different output powers.

Fig. 5. Experimental results of bidirectional pumping after shortening the active fiber of the amplifying section: (a) spectral comparison before and after the shortening of the fiber; (b) result of the beam quality M² factor at the power of 3517 W. Inset: a beam profile of the output laser.