Abstract
It is essential to understand the pump-induced lensing and aberration effects in solid-state lasers, such as Alexandrite, since these set limits on laser power scaling whilst maintaining high spatial TEM00 beam quality. In this work, we present direct wavefront measurements of pump-induced lensing and spherical aberration using a Shack-Hartmann wavefront sensor, for the first time, in a diode-pumped Alexandrite laser, and under both non-lasing and lasing conditions. The lens dioptric power is found to be weakly sub-linear with respect to the absorbed pump power, and under lasing, the lensing power is observed to decrease to 60 % of its non-lasing value. The results are inconsistent with a thermal lens model but a fuller theoretical formulation is made of a combined thermal and population lens model giving good quantitative agreement to the observed pump power dependence of the induced-lensing under non-lasing conditions and the reduced lensing under lasing conditions. The deduced value for the difference in excited to ground state polarizability is consistent with prior measurement estimates for other chromium-doped gain media. The finding of this paper provide new insight into pump-induced lensing in Alexandrite and also provides a basis for a fast saturable population lens mechanism to account for self-Q-switching observed recently in Alexandrite laser systems.
Highlights
Pump-induced lensing and refractive index aberration effects in the laser gain medium are primary problems that limit power scaling and maintenance of high spatial quality in many lasers, and especially in solid-state lasers
This paper provides an in-depth investigation of pump-induced lensing effects in diode-endpumped Alexandrite using a combined direct experimental measurement approach coupled to detailed analytical modelling and numerical finite element analysis (FEA)
The non-lasing lens dioptric power was found to grow slightly sub-linearly with pump power. This pump dependence is surprising as it is inconsistent with a theoretical prediction of nonlinear heating, due to pump excited state absorption (ESA) that is known to occur in Alexandrite, that should produce a super-linear lensing dependence with pump power
Summary
Pump-induced lensing and refractive index aberration effects in the laser gain medium are primary problems that limit power scaling and maintenance of high spatial quality in many lasers, and especially in solid-state lasers. Various experimental methods for thermal lens measurement have been devised based on different principles and implementations [9] These methods include: passing a probe beam through the laser medium and finding its focus distance to deduce the thermally induced lens dioptric power [10]; laser cavity stability and mode size measurements utilising standard laser cavity ABCD analysis [4]; wavefront measurements from classical fringe analysis; lateral shearing interferometry [11] and use of Shack-Hartmann wavefront sensors [12]
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