Fig. 1. Experimental setup of the U-band HML fiber laser system (WDM: wavelength division multiplexer; EDF: Erbium-doped fiber; PC: polarization controller; PD-ISO: polarization dependent isolator; SMF: single mode fiber; OC: output coupler; ISO: isolator; CPS: cladding power stripper; DSF: dispersion-shifted fiber; LPF: long pass filter)

Fig. 2. Repetition rate and SMSR versus pump power of the seed source

Fig. 3. The characteristics of 722.0 MHz HML seed source, amplifier system and after SSFS output pulses. (a) Pulse trains of seed source and inset is pulse train with the span of 200 ns; (b) Optical spectrum of seed source, preamplifier and main amplifier; (c) Autocorrelation traces of seed source and preamplifier; (d) In the main amplifier, the variation of output power and pulse energy with pump power; (e) Autocorrelation traces after the main amplifier; (f) Optical spectrum after SSFS; (g) Autocorrelation traces after SSFS

Fig. 4. The characteristics of 941.7 MHz HML seed source，amplifier system and after SSFS output pulses. (a) Pulse trains of seed source and inset is pulse train with the span of 200 ns; (b) Optical spectrum of seed source, preamplifier and main amplifier; (c) Autocorrelation traces of seed source and preamplifier; (d) In the main amplifier, the variation of output power and pulse energy with pump power; (e) Autocorrelation traces after the main amplifier; (f) Optical spectrum after SSFS; (g) Autocorrelation traces after SSFS

Fig. 5. The characteristics of 1224.2 MHz HML seed source output pulses. (a) Optical spectrum; (b) Pulse trains; (c) RF spectra; (d) Autocorrelation trace

Fig. 6. In the main amplifier, the variation of output power and pulse energy with pump power

Fig. 7. Under different pump powers of the main amplifier at 1224.2 MHz. (a) Autocorrelation traces after the main amplifier; (b) Optical spectrum after main amplifier and SSFS; (c) Autocorrelation traces after SSFS