Diode-pumped, single-frequency Cr:LiSrAlF6 ring laser.

The Ti:sapphire laser has become an established source for cw, watt-level, single-frequency power from 680 to 1050 nm, but the commonly used Ar-ion laser pump source is a disadvantage for many applications. Tunable, external-cavity, single-frequency diode lasers are available for portions of this wavelength region, but a given diode laser can cover a range of only 15– 25 nm. Cr:LiSrAlF6 (Cr:LiSAF) is a possible alternative to Ti:sapphire for wavelengths beyond 780 nm, and the broad 600–700-nm absorption band permits pumping by InGaAlP diode lasers emitting near 670 nm. A single-frequency, coupled-cavity Cr:LiSAF laser emitting 5 mW of power at 870 nm was first demonstrated by Zhang et al. Sutherland et al. reported the first diode-pumped, single-frequency Cr:LiSAF composite microlaser, which produced 1 mW of output power. By modifying the microlaser cavity configuration, they achieved close to 10 mW of output power, but multifrequency operation was frequently observed. Another method of achieving single-frequency operation, which utilized a coupled cavity/grating resonator, was demonstrated by Vilain et al. They produced 5 mW of single-frequency output power when the laser was pumped 30% over threshold. At higher pump powers the output was multifrequency, which, according to the authors, was probably due to spatial hole burning. Knappe et al. utilized an 850-nm, singlestripe diode laser to injection seed a Cr:LiSAF ring laser and achieved 20 mW of single-frequency output over a wavelength range limited by the diodelaser tuning. We report what is to our knowledge the first demonstration of a diode-pumped, broadly tunable, single-frequency, unidirectional ring Cr:LiSAF laser. To achieve stable, continuously tunable, singlefrequency operation of Cr:LiSAF, we have chosen to employ an optical design similar to that used in commercial single-frequency ring Ti:sapphire lasers. All other parameters being equal, the peak cw gain of Cr:LiSAF should be approximately 2.6 times higher than for Ti:sapphire, given the ratio of the products of the peak effective gain cross sections (seff ) and lifetimes (t), where we assume seff values of 2.5 3 10219 and 0.32 3 10219 cm2 for Ti:sapphire and Cr:LiSAF, respectively, and t values of 3.2 and 67 ms for Ti:sapphire and Cr:LiSAF, respectively. With conventional multimode diode pump sources the pump intensity available in the crystal is well below that from a TEM00-mode ion-laser pump, and thus even