Temperature influence on Er:GGAG crystal spectroscopic properties and lasing at 3 μm

The spectroscopic and laser properties of Er:GGAG active medium, in temperature range from 80 K to 300 K, are presented in this work. The sample of Er:GGAG (20 at. % of Er³⁺) crystal had face-polished plan-parallel faces without anti-reflection coatings (thickness 3.6 mm). During all experiments the Er:GGAG was attached to temperature controlled copper holder and it was placed in a vacuum chamber. The transmission and emission spectra together with the fluorescence decay time were measured depending on temperature. The excitation of Er:GGAG was carried out by a fibre-coupled laser diode radiation (pulse duration 2 ms, repetition rate 25 Hz, wavelength 964 nm). Laser resonator was hemispherical, 100 mm in length with flat pumping mirror (HR @ 2.95 μm) and a spherical output coupler (r = 100 mm, R = 97.5 % @ 2.65 - 2.95 μm). The tunability of laser at 300 K was tested using MgF₂ birefringent filter and several laser lines at several spectral bands 2800 – 2822 nm, 2829 – 2891 nm, and 2917 – 2942 nm were obtained. The fluorescence decay time of manifold ⁴I_11/2 (upper laser level) is slightly increasing with rising temperature from 482 μs (80 K) to 466 μs (300 K) on the other hand the intensity of up-conversion radiation was increasing with decreasing temperature. In the pulsed laser regime, the highest slope efficiency with respect to absorbed energy 9 % and the maximum output energy 2.6 mJ were reached at 5 % duty cycle. The laser radiation generated by Er:GGAG laser (2.94 μm) is close to the absorption peak of water (3 μm) thus this wavelength can be used in medicine or spectroscopy.