Saturation and excited-state absorption in neodymium laser glass

The existence of excited-state absorption of 1.06-μ radiation in transitions from the upper laser level upward (
⁴
F_{3/2}
-
⁴
G_{9/2}
) is suggested by the ground-state absorption spectrum of Nd
³⁺
in soda-lime glass. The strength of this absorption was measured as follows. In the unpumped material the upper laser level was populated at high temperatures (600 and 783°K) by saturating the laser transition with an intense 1.06-μ probe laser beam. The residual absorption was attributed to
⁴
F_{3/2}
-
⁴
G_{9/2}
transitions. The steady-state saturation behavior was calculated on the assumption of rapid spectral cross relaxation, using spectroscopic data appropriate to the elevated temperature. Comparison of theoretical and experimental saturation behavior yielded an effective excited-state absorption cross section equal to one-third the gain cross section of the laser transition. This sizable value may be expected to have a significant adverse effect on laser threshold and efficiency. Also, it provides a mechanism for internal fracture such that the fracture threshold decreases with increasing neodymium concentration and with increasing temperature.