Temperature Dependence of Emission Cross Section and Fluorescence Lifetime of Tm:YLF in the 78–300 K Range

High-average power and high output pulse energy lasers at the region of $2\ \mu \mathrm{m}$ wavelength are becoming of great interest to the scientific community. Many processes such as high-harmonic generation, frequency down conversion and single cycle THz generation benefit greatly from the use of a $2\ \mu \mathrm{m}$ laser pump source. Tm:YLF crystals attract our attention because of their strong emission bands in the $2\ \mu\mathrm{m}$ region and well-developed host crystal with a negative thermo-optic coefficient (dn/dT) helping to achieve high-average power output. Room-temperature Tm:YLF lasers have already demonstrated average output power levels of hundreds of watt or pulse energies of a few joules [1]. In our laser development, we have applied cryogenic laser technology to simultaneously achieve high average power and high pulse output energy for Yb:YLF [2] and we believe that this is the right path for Tm:YLF lasers as well. Unfortunately, the lack of detailed spectroscopic data at cryogenic temperatures makes the design of Tm:YLF lasers/amplifiers challenging. In this work, we have investigated temperature dependence of spectroscopic properties of Tm:YLF crystals in detail to uncover its potential at cryogenic temperatures.