Study On Compensation Of Low Order Aberrations Based On Beam Shaping System

Solid-state lasers have in recent years become one of the most important research directions in the laser field. This kind of lasers has many merits, such as compact structure, long lifetime, high energy transferrate and better output beam quality. With the increase of average output power, the thermal effect of laser crystal becomes a fatal problem. The PV(peak to valley) value of the wavefront that contains low order aberrations could be up to 100 microns, which will lead to the serious deterioration of the beam quality. In the paper, the compensation of low order aberrations based on reflective beam shaping systems is studied to find out an effective way to compensate the low order aberrations so that the residual aberrations are small enough to be compensated by deformable mirrors, and thus the output beam quality could be improved by using the two-step correction strategy.The technical characteristics and limitations of the existing strategies to compensate low order aberrations are analyzed in details in the thesis. Then two new kinds of reflective beam shaping systems are designed respectively to correct the low order aberrations of chemical lasers and slab lasers. The system designed for chemical lasers is consisted of three cylindrical mirrors and one spherical mirror and the one designed for slab lasers is consisted of two cylindrical mirrors and one spherical mirror. Then the configuration of system is optimized to achieve the lowest static aberrations.The configuration parameters of the system model are controlled by Matlab based on the DDE technology, and after analyzing the system sensitivity, we conclude that: the defocus and 0-degree astigmatism aberrations are sensitive to the displacement of the spherical mirror and the X-direction cylindrical mirror; the 45-degree astigmatism aberration is sensitive to the rotation around the optical axis of the X-direction cylindrical mirror.PID algorithm is used to control the system to compensate the low order aberrations automatically. The system is tested and verified to be effective, stable and fast enough to compensate defocus aberrations, 0-degree astigmatism aberrations and 45-degree astigmatism aberrations. After compensation, the PV value of the laser beam could be reduced to 0.5λ.The effectiveness of the reflective beam shaping system is verified experimentally. In the experiment, a Hartmann sensor is used to detect the wavefront distribution of the system’s output beam. The experiment results show that the simulation results are accurate and the reflective beam shaping system is effective to compensate defocus aberrations, 0-degree astigmatism aberrations and 45-degree astigmatism aberrations.