All-solid-state femtosecond Cr:LiSAF and Cr:LiSGaF lasers

Abstract

This thesis describes the development of all-solid-state self-modelocked Cr:LiSAF and Cr:LiSGaF lasers. Diode-pumped colquiriite lasers compare favourably with mainframe-pumped Ti:sapphire lasers, but the conflicting requirements of self-modelocking and pumping by broad-area diodes can cause problems. Two alternatives were investigated. Firstly, high Cr3+ doping permitted pumping in the Cr:LiSAF green absorption trough. Frequency-doubled Nd:YLF and Nd:YAG minilasers offer high-power, diffraction-limited beams in the green. The 'green problem' plagued both minilasers, however, and an Ar-ion pump laser was therefore also used. A laser utilising a 10%-doped AR-coated Cr:LiSAF crystal produced 30 mW output power at 320 mW pump, and 90 fs pulses at 859 nm and 86 MHz repetition rate. The self-modelocking threshold was 280 mW. A laser utilising a 22%-doped Brewster-angled crystal produced 120 mW output power at 1.1 W pump, and 72 fs pulses at 76 mW output. The self-modelocking threshold was 360 mW. 160 mW output was obtained by pumping the crystal at both ends to overcome thermal problems. Secondly, factors affecting CW and self-modelocking thresholds were considered; in particular, the effects of waist size, beam brightness, cavity configuration and intracavity dispersion. A Cr:LiSAF laser, pumped by a self-injection-locked diode, produced sub-100-fs pulses for just 73 mW pump. Its tuning curve was modulated by birefringence effects. The excellent free-running noise properties of the laser permitted a sub-picosecond streak camera evaluation. Two low-threshold lasers utilising low-loss Cr:LiSGaF were also developed. 87 fs pulses at 11.5 mW output power were produced from a Z-cavity laser for 170 mW pump. A laser utilising a compact 'retroreflector' three-minor cavity produced 84- fs pulses at 173 MHz repetition rate for 116 mW pump. Subsequent optimisation led to the production of sub-100-fs pulses for 40 mW pump, with self-modelocked operation demonstrated at pump powers as low as 21 mW.