Fig. 1. Schematic of the rep-rate tunable µJ-level femtosecond fiber laser system. SM-LD, single-mode laser diode; WDM, wavelength-division-multiplexer; YDF, Yb-doped fiber; COL, collimator; HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarization beam splitter; ISO, isolator; HR, highly reflective mirror; PM, polarization-maintaining; CIR, circulator; BPF, bandpass filter; AOM, acousto-optic modulator; MM-LD, multi-mode laser diode; DM, dichroic mirror.

Fig. 2. Characteristics of the mode-locked fiber oscillator. (a) Mode-locked spectrum, (b) RF spectrum, (c) pulse train waveform, and (d) pulse auto-correlation trace.

Fig. 3. (a) CFBG reflected spectrum (blue curve) and core-pumped fiber amplifier spectrum (orange curve). (b) Stretched pulse temporal waveform. (c) Picked pulse train waveform at a rep-rate of 2 MHz. (d) Picked pulse train waveform at a rep-rate of 200 kHz.

Fig. 4. Characteristics of final output pulse. (a) 2 MHz: output spectra from power amplifier (blue curve) and pulse compressor (orange curve). (b) 2 MHz: output pulse AC trace (orange curve) and FTL AC trace (light green curve). (c) 200 kHz: output spectra from power amplifier (blue curve) and pulse compressor (orange curve). (d) 200 kHz: output pulse AC trace (orange curve) and FTL AC trace (light green curve).

Fig. 5. (a) Pulse energy (blue square) and pulse duration (orange triangle) corresponding to rep-rate. (b) Slope efficiency (SE) of the fiber power amplifier at 2 MHz (blue solid square) and 200 kHz (orange solid square). (c) Power stability at 2 MHz (blue line) and 200 kHz (orange line). (d) M² measurement for both x- and y-directions of the compressed pulse beam (10 µJ). Inset: beam profile.