Intracavity adaptive optics for a powerful Nd:YAG laser

Thermal focusing of the Nd :YAG rod is the most important factor that limits both stability widths of a source and constancy of the related beam parameter product. Due to its radial temperature profile, the laser rod acts as a thick lens whose dioptric power increases with increasing input power. This behavior leads to a large evolution of the resonator characteristics. Extensive work on compensating for the thermal defocusing effect has been accomplished by designing stable resonators that are relatively insensitive to variations of the dioptric power. Unfortunately, such resonators have some restrictions. The intracavity active-optic technology is able to continue progress in the development of high-power solid-state lasers with an output power of more than 1 kW. The optical quality of laser radiation in these methods will be optimized in real time by means of continuous adjustments of the resonator configuration with corrective optical elements. There are two ways that intracavity adaptive-optic technology can compensate for the harmful influence of a thermal lens (TL) on the laser radiation quality. The first one corresponds to the TL fluence compensation by means of implementing a phase-conjugation control. This technology has been used with a solid-state laser plane-parallel resonator. It was established that the initially plane-parallel configuration of the laser resonator should be shifted in a slightly unstable region for optimum conditions of TL compensation. The control of active unstable resonator configuration allows one to preserve an optical quality of solid-state laser radiation in conditions of strong thermal lensing. To implement both approaches, a new type of mirrorlike phase corrector, an adaptive deformable mirror (ADM), was developed. In this design, the ADM can alter the reflective surface shape due to mechanical stresses developed in the mirror substrate when the controlled value of the external fluence is applied to it. As the shape of the ADM surface can be altered, it is possible to adjust it with the TL. For effective resonator configuration control, a wide range of ADM focal-length alteration, of the same order as the range of the TL optical-power alteration, should be provided. Such characteristics may be obtained with an ADM fabricated from the traditional optical materials, such as glass or quartz.