Optimization of Pump Pulse Duration and Cr:ZnSe Crystal Length for Efficient Generation of Tunable Mid-IR Laser Radiation Pumped by Laser Diode at Room Temperature

Abstract

The spectral and laser output characteristics of Cr:ZnSe single crystals optically polished at four different lengths of 2, 3, 5, and 10 mm combined with or without antireflection (AR) coatings were investigated under pulse and continuous wave (CW) laser diode excitation at the wavelength of ~1.7 μm. The absorption and fluorescence spectra of Cr²⁺ ions were measured at room temperature. Optimization of laser system efficiency was performed step-by-step in dependence on the active length of the crystal sample, output coupler reflectivity, the duration of the pump pulse, and the repetition rate. For a given 1 : 1 transferring optics, a 3-mm thick AR-coated Cr:ZnSe crystal was chosen as optimal in pulse mode of operation: The ~10% higher maximum slope efficiency of ~39% with respect to optical-to-optical power conversion was obtained with the 3-mm thick AR-coated sample compared to the uncoated one. The laser system was characterized by an optimal repetition rate of 10 Hz and a pump pulse duration of 20 ms selected for its best efficiency performance. In CW mode, better results were observed with a thicker sample (5 mm, AR-coated). Specifically, the output power of ~2.7 W at the wavelength of ~2.4 μm with a slope efficiency of ~24% was obtained. In this case, the M² beam parameter was measured as low as ~1.1 ± 0.1. Using a 0.8 mm thick MgF₂ birefringent filter plate, the laser oscillations were continuously tunable in the range of ~2.1–2.7 μm within narrow spectral linewidths (5–10 nm FWHM) and a Gaussian beam profile in both the pulse and CW regimes.