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Direct Measurement of Temporal Transmission Distribution of a Saturable Absorber in a Passively Q-Switched Laser

An in situ temporal decay and spatial distribution of the transmission of a Cr<sup>4+</sup>:YAG saturable absorber in a passively Q-switched Nd:YAG laser were measured simultaneously by a scanning transmission focused laser beam method. An excited ion decay time of 4 μs and a top hat (saturated) transmission distribution were clearly observed. The ratio of the ground-state absorption cross section and the excited-state absorption cross section (σ<sub>SAπ</sub>/σ<sub>ESAπ</sub>) for π-polarized light to the S<sub>4</sub> local symmetry axis of Cr<sup>4+</sup> center was measured at approximately 4.1-4.2. Using a theoretical fitting of the transmission distributions, we found the ground-state absorption cross sections to be at 1 × 10<sup>-17</sup> and 2.3 × 10<sup>-19</sup> cm<sup>2</sup> for π and for σ-polarized light to the S<sub>4</sub> local symmetry axis of Cr<sup>4+</sup> center, respectively. The value of the excited-state absorption cross section was also estimated correspond to 2.4 × 10<sup>-18</sup> cm<sup>2</sup>.