Tm-Doping Concentration Influence on Tm:GGAG Lasing and Tenability at 2 μm Spectral Region

Thulium doped solid state lasers are well-known sources of radiation tunable around 2 pm. Obtainable wavelengths cover important spectral regions such as absorption lines of water, characteristic spectra of some gases or the atmospheric transmission bands. Tunable Tm
³⁺
lasing was accomplished using various matrices, such as YAG, YAP or YLF [1]. Recently an interest re-emerged in the mixed garnets as host materials. Their multi-site structure induces broadening of spectra of the active ions, improving overlap with pumping diode emission and extending emission spectra and tunability [2]. In this submission, Tm
³⁺
doped mixed gadolinium-gallium-aluminium-garnet Gd
₃
(Ga
_5-x
Al
_x
)O
₁₂
(GGAG) is investigated. Focus is given to the Tm
³⁺
concentration dependence of lasing and tunability parameters as a follow-up work to the first Tm:GGAG lasing report with one Tm
³⁺
concentration sample [3]. The Tm:GGAG crystal was grown using Czochralski method in a slightly oxidative atmosphere. Five plane-parallel face-polished Tm:GGAG samples with concentrations varying from 1.4 at.% to 8.4 at.% (Tm/Gd) and thicknesses ranging from 2.7 mm to 3.6 mm were cut from the crystal. The samples were longitudinally pumped by fiber coupled (core diam. 400 μm, NA = 0.22) laser diode at wavelength 785 nm in QCW regime (pulse length 5 ms, repetition rate 10 Hz). The 145 mm resonator consisted of a flat pumping mirror (HR @ 1.85-2.11 μm , HT @ 0.78 μm) and concave output coupler (r = 150 mm, R = 97.5 % @1.8-2.10 μm ). For the wavelength tuning, 1.5 mm thick birefringent SiO
₂
plate was inserted in the cavity at the Brewster's angle.