All solid-state oscillator-amplifier tunes over 70 nm
Chromium-doped lithium strontium aluminum fluoride (Cr:LiSAF) is the active medium in a tunable, all-solid-state oscillator-amplifier recently demonstrated by researchers at Imperial College (London, England). The system has produced 200-fs pulses at a 25-kH¥repetition rate, with output energies exceeding 1 µJ over a tuning range from 805 to 875 nm. The system consists of a regenerative amplifier seeded by a femtosecond oscillator. The Cr:LiSAF oscillator is pumped at 670 nm by four 300-mW GaInP/AlGaIn¥prototype diode lasers (Philips Optoelectronics Center, Eindhoven, Netherlands). Kerr lens modelocking yields 70-mW average output power pulses as short as 24 fs. Output from the oscillator seeds the diode-pumped Cr:LiSAF regenerative amplifier, which uses chirped pulse amplification to produce femtosecond pulses of increased power. The pulses are stretched via a diffraction grating prior to injection into the amplifier cavity for multiple passes. When the gain medium reaches saturation, the optical power is switched out of the cavity with a Pockels cell and the resultant pulse is recompressed. Precompression pulse energies were as high as 2.6 µJ.