Fig. 1. Schematic of seed laser pulse generation for the 2 μm OPCPA system (pulse delay setup is not drawn in this figure)

Fig. 2. Visible (dashed line) and near-IR (solid line) supercontinuum spectra generated in YAG crystal 　by loose focusing and tight focusing

Fig. 3. Gain bandwidths of different nonlinear crystals around 2 μm wavelength region under the same amplification factor. (a) BBO; (b) BiBO; (c) LiNbO_3;(d) YCOB

Fig. 4. Schematic of second order optical parametric amplification of the 2 μm seed laser

Fig. 5. Measured power of the 2 μm seed pulse within 30 min after amplification

Fig. 6. Beam characteristics of the amplified 2 μm seed laser. (a) Beam profile of seed laser after amplification; (b) intensity distribution in horizontal direction x and vertical direction y (solid line: measured data; dotted line: 　Gaussian fitted line)

Fig. 7. Characteristics of amplified seed pulse laser in frequency and temporal domains. (a) Pulse spectrum；(b) autocorrelation curves