Saturation of green absorption in titanium-doped sapphire laser crystals

Abstract

We observed a reduction in the absorption of 532-nm laser radiation by Ti:sapphire with increased laser fluence up to 3.5 J/cm2. This effect was observed for all Ti:sapphire samples studied, six crystals as of now, and is attributed to saturation of the optical transition of the Ti3+ ions. Since 532-nm radiation is used to pump Ti:sapphire lasers, it is important to know its absorption characteristics to understand completely the laser performance. However, this saturation effect is more important as an accurate method of measuring dopant ion density and absorption cross section independently. The transmittances of Ti:sapphire at 532-nm were measured with respect to incident laser fluence and fitted to a numerical saturation model using a nonsteady-state rate equation analysis based on the Franz-Nodvick approach. Upper-state fluorescence was ignored since absorption occurred on a time scale (≈10 ns) short compared to the fluorescence lifetime (≈3.2 μs). The fitted parameters were the weak-signal transmittance T0 and the saturation flux Esat (hν/σ) from which the absorption cross section and dopant ion density can be determined. For example, using a 2.1-cm Ti:sapphire sample having a weak-signal absorption coefficient of 1.58 cm−1 at 532 ran, the Esat was 7.7 J/cm2 which gave a cross section of 4.8 × 10−20 cm2 and a Ti3+ density of 3.3 × 1019 cm−3, consistent with estimates of the total Ti density.