Research On Beam Cleanup Of Slab Lasers

Solid-state lasers are currently the dominant class of lasers. Recent years havewitnessed rapid growth in both average and peak powers attainable from solid-statelasers including slab lasers. One of the most important issues in slab lasers’applications is achieving high beam qualities. However, a lot of negative factorsdegrade the beam quality of slab lasers, for example, thermal distortions andnon-uniform distribution of the pump light. To solve these problems, a series oftheoretical and experimental researches on laser beam cleanup with the help ofadaptive optics are presented, especially using deformable mirrors to compensate forthe aberrations of laser beam.Because of the high aspect ratio of slabs, the shape of output laser beam isrectangular. As orthogonal polynomials necessary for representing phase aberrationsare lacking in such rectangular areas, Zernike polynomials and Gram-Schmidtorthogonalizing method are used to create the orthogonal polynomials. Threedifferent deformable mirrors are used to compensate for the rectangle aberrationswith simulations. Though some aberrations can only be partly corrected, thecorrection capabilities of deformable mirrors are nearly fully used. In order to makea better correction, expansion of the output laser beam in the narrow direction isconsidered. The results of simulations indicate that better corrections are obtainedwith the beam expansion.The output beams of a slab Master Oscillator Power Amplifier (MOPA) laserchain in laboratory are measured at different output power. The uniformities ofoutput beams are calculated and the results indicate that adaptive optics systems withHartmann-Shack wave-front sensors are improper for this laser beam. In order to getthe wave-front aberration corrected, a wave-front sensor-less adaptive optics systemis introduced. However, some problems need to explain. Simulation results point outthat in these systems, displacement between the far field spot detector and the focalplane of a focusing lens would introduce extra wave-front to deformable mirrors. Awave-front sensor-less beam cleanup system for continuous wave and pulse slablaser is built, and a series of experiments are done at different output power. Theeffectiveness of this beam cleanup system is proved.In beam cleanup experiments, a single deformable mirror was found unable to fully compensate the aberrations of laser beam. Therefore, an optimization-basedmethod is developed to control two deformable mirrors simultaneously in awave-front sensor-less beam cleanup system. Stochastic parallel gradient decentalgorithm is chosen as the optimization algorithm. In this control method, differentaberrations are assigned to each deformable mirror according to their differentcorrection qualities. The method is proved to be effective by numerical simulationsas well as experiments.