Recently, the SYLOS optical parametric chirped pulse amplification (OPCPA) system at ELI-ALPS set new records in terms of combination of peak and average power (5.5TW at 53W average power), as well as energy and CEP stability for a TW-class OPCPA system . The system delivered sub-9fs pulses, but further reduction of the pulse duration is necessary to maximize the potential of the system as a light source attosecond science experiments. Although sub-5fs pulses from a multi-TW OPCPA system have been achieved by combining BBO-based amplification stages pumped at 532nm and 355nm , this approach cannot be used at 1kHz repetition rate due to fast degradation of UV optics at high average power. Therefore, we developed a new technique of combining different nonlinear crystals (BBO and LBO) and different phase matching angles in sequential amplification stages. This new technique enables our OPCPA system to produce 2.1 optical cycle (6.3fs) pulses with 5-terawatt peak power and 35W average power, while maintaining excellent output stability.
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