Abstract

In this talk, we present the first demonstration of cw mode-locked operation of wavelength tunable Cr³⁺ doped LiSrAlF₆ (Cr:LiSAF) and LiSr_0.8Ca_0.2AlF₆ (Cr:LiSCAF). The Czochralski grown crystals, with a nominal chromium concentration of 2% in the melt, were Brewster cut and polished to 15mm lengths and mounted in a four mirror X-fold cavity. Cr:LiSAF¹ and Cr:LiSCAF² crystals showed similar cw and mode-locked performance. Pumping with a cw krypton laser (Laser Ionics) gave a threshold power of 210mW using the red doublet. The output power peaked at around 300mW with 1.5W input. The tuning range was 800 to 900nm using a birefringent filter, limited by the mirror bandwidths. To achieve mode-locked operation, a NEOS high-Q, acousto-optic modulator with broadband anti-reflection coatings centered at 800nm was used. An intracavity two prism sequence was inserted to compensate for group velocity dispersion. A stable, 82MHz mode- locked train was achieved with 1W of krypton pump. When fully optimized, intensity and interferometric autocorrelation and pulse bandwidth measurements confirmed chirp-free pulses of 150fsec duration. Tuning across the 800 to 900nm range was achieved using an intracavity slit. It is likely that self-phase modulation assists the spectral broadening and pulse shortening. The pump wavelength, lack of concentration quenching for the chromium doping and the low threshold makes these crystals good candidates for diode and flashlamp pumped ultrashort pulse sources.

© 1991 Optical Society of America