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
$\mu \text{s}$
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 (
$\sigma _{\mathrm {SA\pi }/}\sigma _{\mathrm {ESA\pi }})$
for
$\pi $
-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
$1\times 10^{-17}$
and
$2.3\times 10^{-19}$
cm
²
for
$\pi $
and for
$\sigma $
-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
$2.4\times 10^{-18}$
cm
²