Mid-infrared diode pumped Cr: ZnSe laser continuously tunable from 2.1 μm up to 2.7 μm 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 ~1.69 μm. 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 μm up to 2.7 μm with the narrow spectral linewidth of ~5 nm and Gaussian beam profile. Maximum mean output power of ~0.35 W and maximum optical-to-optical slope efficiency of ~39% was obtained for 3 mm thick AR-coated sample which was ~28% and ~10% higher than that for uncoated sample of the same thickness, respectively. Continuously tunable laser oscillation wavelength was measured in the range of ∼2.1-2.7 µm with the mean output power of P ≥ 150 mW within a ∼5 nm linewidth. The absorption and fluorescence spectra as well as fluorescence lifetime were also measured. The Cr2+ fluorescence lifetime of ∼7.2 µs measured for the 1 ns pulse excitation at room temperature (RT) was constant within the whole absorption band of 1.35–2.25 µm.