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 4+ :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 SAπ /σ ESAπ ) for π-polarized light to the S 4 local symmetry axis of Cr 4+ 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 -17 and 2.3 × 10 -19 cm 2 for π and for σ-polarized light to the S 4 local symmetry axis of Cr 4+ center, respectively. The value of the excited-state absorption cross section was also estimated correspond to 2.4 × 10 -18 cm 2 .