In conclusion, we have demonstrated, to our knowledge, the first OPCPA high-gain preamplifier at 1053 nm in PPKTP. OPCPA in one PPKTP crystal produced a gain of 1.1x10~5, with a conversion efficiency of 13% to signal and idler, or 65% of the theoretical maximum of this configuration. The obtained beam profile is closest to the diffraction limit of all OPCPA outputs reported to date. We have measured the angular acceptance bandwidth of our crystal and determined it to be two orders of magnitude broader than the acceptance bandwidth of a bulk crystal such as BBO. We have not observed any grey-tracking degradation of the PPKTP crystal during ～24 hours of operation, but longer lifetime experiments are in progress. Several improvements are suggested to increase the output energy of the PPKTP preamplifier. The use of a broad bandwidth (16 nm) oscillator should allow greater stretching ratio. A shorter PPKTP crystal would allow us to operate at an intensity of 100 MW/cm~2 without significant parametric fluorescence, increasing the output pulse energy by a factor of ～3. This intensity level has been previously reported as the limit for safe operation without crystal damage or degradation. Finally, periodic poling of KTP with larger crystal apertures (～3 mm) would allow scaling of this device to millijoule energies. Beam quality, spectral characteristics, and simplicity make OPCPA in PPKTP a desirable choice as a part of the front end of short-pulse Nd:glass laser. High conversion efficiency opens up a possibility of use of OPCPA in PPKTP for scaling of short pulses to high average power.
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