Mid-infrared Diode Pumped Cr:ZnSe Laser Continuously Tunable from 2.1 mu m up to 2.7 mu m Operated at Room Temperature

The spectroscopic and laser characteristics of optically polished Cr:ZnSe single crystals in four different lengths of 2, 3, 5, and 10 mm were investigated when pumping with a laser diode generating radiation at the wavelength of similar to 1.69 mu m. The optimization from the point of laser efficiency was done in dependence on the crystal samples active length, effect of antireflection (AR) coatings, pumping pulse duration, and repetition rate. As a result, a 3 mm thick anti-reflection coated Cr:ZnSe crystal pumped by a laser diode with a pulse length of 20 ms and repetition rate of 10 Hz was chosen as the optimal. Using a 0.8 mm thick MgF2 birefringent filter the laser oscillations were continuously tunable from 2.1 mu m up to 2.7 mu m with the narrow spectral linewidth of similar to 5 nm and Gaussian beam profile. Maximum mean output power of similar to 0.35 W and maximum optical-to-optical slope efficiency of similar to 39% was obtained for 3 mm thick AR-coated sample which was similar to 28% and similar to 10% higher than that for uncoated sample of the same thickness, respectively. Continuously tunable laser oscillation wavelength was measured in the range of similar to 2.1-2.7 mu m with the mean output power of P >= 150 mW within a similar to 5 nm linewidth. The absorption and fluorescence spectra as well as fluorescence lifetime were also measured. The Cr2+ fluorescence lifetime of similar to 7.2 mu s measured for the 1 ns pulse excitation at room temperature (RT) was constant within the whole absorption band of 1.35-2.25 mu m.