Abstract

We report on first successful subpicosecond color center laser operation in the 1.85 – 2.30 μm range. As active materials we use color centers in oxygen-doped potassium bromide crystals, and in oxygen and sodium co-doped potassium bromide crystals, respectively. These F₂⁺-like high-gain color centers have their <110>-polarized lowenergy absorption transitions near 1.6 μm and their Stokes-shifted <110>-polarized emissions in the 1.8–2.3 μm range^[1]. Efficient excitation is accomplished with a mode locked NaCl color center laser delivering ~ 6 ps wide pulses at 76 MHz repetition rate and ~ 2 W average power. Matching the potassium bromide laser length to that of the NaCl pump, purely synchronously mode locked pulses shorter than 600 femtoseconds could be generated in the peak of the crystals’ tuning ranges near 1.9 μm, and 2.15 μm, respectively, with average output power levels reaching ~ 150 mW. Spectrum and autocorrelation measurements show that it is possible to obtain nearly transform-limited sech² pulses when spectrally narrowing the gain widths with a single-plate birefringent filter of appropriate thickness. Using the same filter as tuning element, subpicosecond purely synch-pumped laser operation could be achieved over wide (~200 nm) portions of the respective gain curves. The observed behavior of pulse width narrowing with increasing pump power levels and the formation of satellite pulses under cavity length detuning is found to be similar to other synchronously mode locked color center lasers^[2]. It is in qualitative agreement with theory^[3] for synchronously mode locked lasers in which the cavity round trip time is short compared with the spontaneous lifetime of the active medium. The described system consisting of two cascaded synchronized color center lasers promises to be useful for two-color picosecond time-resolved pump-probe experiments in the near IR.

© 1992 IQEC