Fig. 1. Schematics of (a) front end, (b) regenerative amplifier, (c) cascaded-quadratic compressor, and (d) mid-infrared (MIR) intrapulse difference-frequency generation (IPDFG) stage. The front end consists of an Yb-fiber oscillator and a designed spectral shaper. The regenerative amplifier employs an Yb:CALGO crystal. HR, high reflection mirror; PBS, polarization beam splitter; L, lens; FR, Faraday rotator; HWP, half-wave plate; PC, Pockels cell; QWP, quarter-wave plate; DM, dichroic mirror; LD, laser diode; LPF, low-pass filter; G, high-efficiency transmission grating with 1600 lines/mm; LGS, LiGaS2 crystal.

Fig. 2. (a) Comparison of the seed spectrum with (red) and without (black) the spectral shaper. The single reflection (blush dash) and overall response (blue dot) of the spectral shaper as a function of the wavelength are also included. (b) Comparison of the amplified spectrum with (red) and without (black) spectral shaping. The spectra correspond to the output power of ∼15.5  W with 90 round trips. (c) The amplified spectral bandwidth with (red) and without (black) the spectral shaper, and the output power (blue) as a function of the round trips in the regenerative amplifier, with the absorbed pump power of 150 W. (d) The amplified output power in the regenerative amplifier as a function of the absorbed pump power with 90 round trips.

Fig. 3. Measured output beam profiles from (a) the Yb:CALGO regenerative amplifier and (b) the cascaded-quadratic compressor.

Fig. 4. SHG-FROG measurement of the 95 fs pulses from the regenerative amplifier. The (a) measured and (b) retrieved FROG traces of the 95 fs pulse. The FROG error is 0.8%. (c) The retrieved temporal profile and the transform-limited temporal profile. (d) The retrieved FROG spectral intensity and phase of the 95 fs laser pulse, compared to the spectrum independently measured using a spectral analyzer.

Fig. 5. SHG-FROG measurement of the 50 fs pulses from the cascaded-quadratic compressor. The (a) measured and (b) retrieved FROG traces of the 50 fs pulse. The FROG error is 0.95%. (c) The retrieved temporal profile and the transform-limited temporal profile. (d) The retrieved FROG spectral intensity and phase of the 50 fs laser pulse, compared to the spectrum independently measured using a spectral analyzer.

Fig. 6. (a) Measured spectra of driving pulses plotted in logarithmic scale from the regenerative amplifier (black), after the BBO cascaded-quadratic compressor (red), and after the 8 mm thick LGS crystal (blue). (b) The measured MIR spectra with different phase-matching angles tuning in the wavelength range of 7.5–11.2 μm.