Solid state saturable absorbers for Q-switching at 1 and 1.3μm: investigation and modeling

Yttrium and Lutecium garnets (YAG and LuAG) doped by Chromium or Vanadium ions (Cr4+ or V3+) were investigated as saturable absorbers potentially useful for passive Q-switching at wavelengths 1 mu m and/or 1.3 mu m. For comparison also color center saturable absorber LiF:F-2(-) and Cobalt doped spinel (Co:MALO) were studied. Firstly, low power absorption spectra were recorded for all samples. Next, absorbers transmission in dependence on incident energy/power density was measured using the z-scan method. Crystals Cr:YAG, Cr:LuAG, V:YAG, and LiF:F-2(-) were tested at wavelength 1064 nm. Therefore Alexandrite laser pumped Q-switched Nd:YAG laser was used as a radiation source (pulse length 6.9ns, energy up to 1.5mJ). Crystals V:YAG, V:LuAG, and Co:MALO were tested at wavelength 1338 nm. So diode pumped Nd:YAG/V:YAG microchip laser was used as a radiation source (pulse length 6.2 ns, energy up to 0.1 mJ). Using measured data fitting, and by their comparison with numerical model of a "thick" saturable absorber transmission for Q-switched Gaussian laser beam, following parameters were estimated: saturable absorber initial transmission T-0, saturation energy density w(s), ground state absorption cross-section sigma(GSA), saturated absorber transmission T-s, excited state absorption cross-section sigma(ESA), ratio gamma = sigma(GSA)/sigma(ESA), and absorbing ions density. For V:YAG crystal, a polarization dependence of T-s was also investigated. With the help of rate equation numerical solution, an impact of saturable absorber parameters on generated Q-switched pulse properties was studied in plane wave approximation. Selected saturable absorbers were also investigated as a Q-switch and results were compared with the model.