Fig. 1. Relationship between emission spectrum of HHG and driving laser wavelength under the condition of perfect phase matching^[6]

Fig. 2. Schematic of two kinds of 2-μm OPCPA system front-ends based on difference frequency process. (a) One kind of front-end includes a femtosecond Ti∶Sapphire oscillator, the 2-μm seed pulses are generated by a DFG process directly^[29-31]; (b) another kind of frond-end is the pump laser based on OPCPA system, this pump laser is a picosecond Yb-doped laser amplifier, which is used to generate the 2-μm seed laser with the assistance of SCG^[14]

Fig. 3. Transmittance windows and the maximum effective nonlinear coefficients of several typical nonlinear crystals^[4]

Fig. 4. Transmittance of several typical solid mediums

Fig. 5. GVD and TOD of solid mediums at different wavelengths. (a) GVD; (b) TOD

Fig. 6. Output parameters of ~800-nm high repetition rate (≥1 kHz) OPCPA systems^{[11-12, 25-27,41-48]}

Fig. 7. Output parameters of ~1.5-μm high repetition rate (≥1 kHz) OPCPA systems^{[13,17,28,49-57]}

Fig. 8. Output parameters of ~2-μm high repetition rate (≥1 kHz) OPCPA systems^{[14,29-30,58-68]}

Fig. 9. Output parameters of ~3-μm high repetition rate (≥1 kHz) OPCPA systems^{[31-33,53,69-78]}

Table 1. Damage thresholds of several typical nonlinear crystals

Table 2. Output parameters of high repetition rate (≥1 kHz) OPCPA systems operating beyond 4 μm wavelength

Table 3. Optimal output parameters of the OPCPA systems operation in different wavelengths