Picosecond pump dispersion management and jitter stabilization in a petawatt-scale few-cycle OPCPA system
The petawatt field synthesizer (PFS) is a high-power optical parametric chirped-pulse\namplification (OPCPA) system under development, which aims at generating fewcycle\npulses with high energies of several Joule. The availability of light pulses with\nthese unique parameters will enable an efficient generation of even shorter attosecond\npulses with significantly higher photon flux than achievable today [1]. Not\nonly the real-time observation, but also the control of charge transfer in molecular\nsystems will become feasible for the first time [2].\nThe technique for realizing the ambitious PFS specifications is short-pulse pumped\nOPCPA in mm-thin crystals. The reduced crystal thickness allows for ultra-broadband\namplification. The pump-pulse duration is reduced to a picosecond\u2014compared to\n100 ps to nanosecond pump-pulse duration in conventional high power OPCPA systems.\nThe shortened pulse duration facilitates higher pump intensities whereby an\nefficient amplification in the mm-thin crystals is achieved.\nThe demonstration of this novel scheme in the PFS project will allow its use in\nthe extreme light infrastructure (ELI)[3]\u2014a pan-European high-power laser project.\nBased on the PFS technology for the front end, the ELI will generate exawatt peakpower\npulses and therefore facilitate the study of laser-matter interaction in an unprecedented\nintensity range [4].\nThis work describes the CPA-aspects of a suitable chirped pulse amplification (CPA)\npump laser for the PFS OPCPA system. The diode-pumped Yb:YAG amplifiers up to\nan energy of 300 mJ (at 1030 nm) are presented in combination with the dispersion\nmanagement. The application of spectral-amplitude shaping in conjunction with\nan Yb:glass amplifier with broader bandwidth than Yb:YAG enables an unprecedented\nbandwidth of 3.5nm in the Yb:YAG amplifier at this energy level. Simulations\nshow that a similar bandwidth can be maintained for the full amplifier system.\nThe pulses with 200 mJ could be compressed to 900 fs, close to the transform limit.\nLater changes in the stretcher increase the bandwidth more and compression down\nto 740 fs is demonstrated. To date, these are the highest peak power pulses generated\nin Yb:YAG. For the application as OPCPA pump, the so generated pulses are\nfrequency doubled in a DKDP crystal.\nAnother key aspect of this work is the synchronization of the OPCPA pump and\nsignal pulses. In spite of optical synchronization of both pulses, a large timing fluctuation\nbetween these pulses is measured at the first OPCPA stage. The high accuracy\njitter measurement setup and a series of measurements, which showed that the\nstretcher/compressor setup is the main source of jitter, are presented. Theoretical\ninvestigations yield that the optical delay in a compressor is orders of magnitude\nmore sensitive to angle changes compared to free space propagation. This makes the\nstretcher and compressor extremely sensitive for timing jitter caused by turbulent\nair or mechanical instabilities. This novel insight helped us to significantly reduce\nthe jitter to 100 fs and to demonstrate the feasibility of the PFS concept with first\nbroad-band OPCPA experiments.