Beam Quality Dynam Ical Improvement In Diode-end-pumped Laser Amplifiers

Beam quality is a very critical factor for laser practical application. However, because of the thermal effect caused by the gain medium, the beam quality usually get worse and worse in amplifier stages. By researching thermal effect of Nd:YVO4 and spherical aberration self-reproduction inside the cavity, a method of beam quality dynamical improvement in diode-end-pumped laser amplifiers is developed and validated.This paper mainly divides into two parts. The first part focuses on beam wavefront propagation theory inside the cavity. The theoretical model is verified by simulation and experimental measurement. The second part is the main content of the work. Theoretical model of wavefront dynamical compensation by amplifier stage is proposed, which is also verified by the experimental measurement. Finally the principles and methods to achieve beam quality dynamical improvement is conducted.In the first part, the thermal distribution of Nd:YVO4 is obtained from heat conduction equation. Then the optical path difference (OPD) and spherical aberration caused by the thermal effect is calculated. We studied the relationship of spherical aberration and Gauss beam quality deterioration and found that spherical aberration become the most crucial factor when laser power get high. So wavefront compensation is an effective way to improve the beam quality. We also find that the positive spherical aberration can change its sign when passing through the beam waist. Based on this conclusion the theoretical model of wavefront self-reproduction inside the cavity is proposed, which is then verified by simulation and experimental measurement.In the second part, a method to achieve beam quality dynamical improvement by amplifier stage is raised, that is, imaging the place with negative spherical aberration to the amplifier stage. By using the positive spherical aberration caused by amplifier stage to compensate the negative one. Also this method is verified by experimental measurement. And finally when the pump power in amplifier stage is 63W, the M2=4.2 of the oscillation output is compensated to 1.6. The beam quality is improved while enlarging the output power.