Basic parameters for Cr-, Tm-, Ho:YAG 2.1-μm lasers

Laser emission at 2.1 μm from holmium-doped YAG at liquid-nitrogen temperatures was first reported in 1965.1 Recent performance improvements at room temperature using chromium and thulium sensitization have renewed interest in the practical application of this laser material. The potential efficiency, energy storage, and atmospheric and silica fiber transmission properties of this laser, combined with its eye-safe wavelength, have encouraged research into military, medical, materials-processing, and remote-sensing applications. As an aid to the design of Cr-Tm-Ho:YAG laser systems, we have measured the basic laser parameters and spectral and thermal characteristics of a free-running, room-temperature, flashlamp-pumped Cr-Tm-Ho:YAG system. Birefringent tuning of this system revealed at least nine distinct low-threshold transitions. Simple analysis of the performance data yields the basic parameters of transition energy levels, emission, and absorption cross sections, saturation fluences, storage lifetime, and heat deposition. The dynamics of the chromium- and thulium-sensitized holmium laser are complicated and are not yet fully understood. Our progress toward understanding and modeling the temperature, concentration, and excitation dependence of the pumping efficiency and lifetime of the holmium laser system will also be reported.