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