Fig. 1. (a) Schematic of 2.1 μm few-cycle OPCPA system; (b) Measured (blue) and retrieved (red) spectral intensity and phase (dashed black), and (c) measured temporal intensity and phase. Inset: measured spatial intensity profile after the third stage^[7]

Fig. 2. (a) 2.2 μm OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured spectrum; blue-dashed line, retrieved spectrum; orange line, retrieved phase^[8]

Fig. 3. (a) Layout of the 3.9 μm OPCPA system; (b) Spectra of the signal and idler pulses after the last OPCPA stage measured, respectively. The dotted green curve is the transmission spectrum of the KTA crystal^[14]

Fig. 4. (a) Setup of the high-power, MIR OPCPA system. The seed is generated by a two-color fiber front-end in combination with a DFG stage. Afterward, the MIR pulses are stretched and consecutively amplified in a preamplifier and two booster amplifiers. Maximum conversion efficiencies are achieved by multiple use of the pump beam and by individually tailored seed-to-pump pulse durations. The MIR output is compressed in a bulk stretcher and (b) the final compression to a single optical cycle is performed using an Ar-filled ARR-PCF. Output characteristics of the MIR OPCPA system. SHG-FROG retrieval of the MIR output pulses, showing (c) the spectral amplitude and phase, and (d) the temporal amplitude and instantaneous frequency. (e) The pulse-to-pulse power stability measured over 30 min. The inset shows the output beam profile^[13]

Fig. 5. (a) Schematic of the 4 μm OPCPA and postcompression system; (b) Pulse temporal profile of 21.5 fs FWHM duration and (c) reconstructed spectrum^[15]

Fig. 6. (a) Schematic of flat-top beam shaping of the high-energy and high-average-power 3 µm OPCPA. The MIR pulses centered at 3 µm are generated and amplified to 300 µJ from 3-stage OPCPA preamplifiers via periodically poled lithium niobate (PPLN) and KTA crystals. The 4^th OPCPA stage is designed to boost up the MIR output and enhance the parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4^th-stage OPCPA is sent to a flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging plane of the lens. The Gaussian idler beam generated from the first-3 OPCPA stages is amplified with a flat-top pump, producing a high-energy and high-average-power flat-top-like 3 µm output. The measured pump beam profiles (b) with and (c) without the flat-top beam shaper, on the KTA crystal. The cross section beam profiles on the x and y axes are included too. (d) The pulse energy measurements of the 3 µm idler pulse from the OPCPA with flat-top (red) and Gaussian (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy are obtained from the flat-top and Gaussian pump, corresponding to 7% and 13.5% pump-to-idler efficiency for the 4^th-OPCPA stage, respectively^[10]

Fig. 7. Experimental setup of a MIR DC-OPA laser system with MgO:LiNbO₃ crystals^[16]

Fig. 8. Schematic layout of the 2.8 µm laser system^[17]

Fig. 9. Schematic drawing for a proof-of-principle experiment for demonstrating DC-OPA^[18]

Fig. 10. Schematic diagram of the 2.1 µm OPCPA system. The 500 W Yb:YAG thin disk laser acts as both pump and signal generation source^[19]

Fig. 11. Layout of the tunable mid-IR OPCPA system^[20]

Fig. 12. (a) Setup of the mid-IR OPCPA source pumped at 2 μm. The main parts are the seed source, the 2 μm Ho:YLF CPA amplifiers, DFG, the SLM, and the three OPA stages based on ZGP crystals. Regen. amp., regenerative amplifier; Booster, power amplifier; CVBG, chirped volume Bragg grating; SC, supercontinuum; HNLF, highly nonlinear fiber; TFP, thin-film polarizer. (b) DFG spectrum (gray), signal spectrum after the first (blue) and second OPA stage (green); (c) Idler spectrum after the third OPA stage measured (black) and calculated (purple). TFL, Fourier-transform-limited^[21]

Fig. 13. (a) Layout of the 7 μm OPCPA. The MIR seed is generated using the two broadband femtosecond outputs from a three-color fiber frontend via DFG. Afterward, the MIR pulses are stretched in a dielectric bulk and consecutively amplified in a pre-amplifier and a booster amplifier separated with a chirp inversion stage. Maximum efficiency of the OPCPA is achieved by tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broadband high-energy mid-IR pulses are recompressed using a dielectric bulk rod of BaF₂. (b) The retrieved pulse envelope with 188 fs FWHM duration, and (c) measured (filled gray) and retrieved spectrum (red line) and phase (green line)^[22]

Fig. 14. (a) The schematic of the 9 μm OPCPA. YAG, Yttrium aluminum garnet; ZnSe, Zinc selenide window; HR, High reflective mirror; TFP, Thin film polarizer; BS, Beam splitter; LGS, LiGaS₂ crystal; Ge, Germanium window. For TFP, the reflectance of the S-polarized pump and the transmittance of the P-polarized signal are measured as > 99% and 91% respectively. (b) The spectra of signal pulses after SC generation (blue dotted), the pre-amplification stage (red) and the main-amplification stage (black dashed); (c) The measured (black) and simulated (red dashed) spectra of the output idler pulse ^[23]

Table 1. Relevant parameters of 2-4 μm OPCPA system

Table 2. Parameters of long wave MIR-OPCPA system