Octave wide tunable UV-pumped NOPA: pulses down to 20 fs at 0.5 MHz repetition rate

Femtosecond laser pulses, which are tunable from 440 to 990 nm, are generated at MHz repetition rates by noncollinear parametric amplification (NOPA). The pulses have durations of 20 to 30 fs over the major part of the tuning range and a high energy stability of 1.3% (rms). The NOPA is pumped with ultraviolet pulses from the third harmonic of an ytterbium doped fiber laser system and seeded by a smooth continuum generated in bulk sapphire. The residual second harmonic is used to pump an additional NOPA, which is independently tunable from 620 to 990 nm. Interference experiments show that the two NOPA systems have a precisely locked relative phase, despite of being pumped by different harmonics with a random phase jitter. This demonstrates that the phase of pulses generated by optical parametric amplification does not depend on the pump phase.