Research On Intracavity Aberration Analysis And Correction Technique Of Unstable Cavity Thin-film Laser

The wavefront aberration of laser gain medium will reduce both the output power and the output laser beam quality when thermal loading. This defect restricts the development of solid-state laser. Adaptive aberration correction technology is an effective method that solves the solid-state laser heat distortion problem, achieving high power and high beam quality laser output. In this paper, based on the high-power multi thin-disk unstable resonator laser, we numerically analyzed the wavefront aberration characteristics of the gain medium in large diameter Nd:YAG thin-disk, and studied the intra-cavity aberration correction in unstable resonator thin-disk laser. Moreover, we corrected intra-cavity aberration of high-power multi thin-disk unstable resonator laser in the experiment.We studied the wavefront aberration characteristics by measuring the wavefront aberration of laser gain medium under the condition of thermal loading in experiment. These thin-disks have large tilt aberration and large defocus aberration. We used the fast Fourier transform (FFT) to establish a numerical model, studying the effect on output laser beam of unstable resonator that induced by wavefront aberration of laser gain medium. The simulation result shows that the tilt aberration and defocus aberration reduce the output power of unstable resonator severely, which sometimes further result in the situation that the laser fails to work. Moreover, the defocus aberration and the high-order aberration deteriorate the output beam quality severely. To address this problem, we proposed the aberration correction strategy:correcting both intra-cavity tilt aberration and defocus aberration by closed loop system, and then correcting the high-order intra-cavity aberration by closed loop system.The intra-cavity tilt aberration tilts both laser optical axis and laser pupil. A common tilt correction method is based on the measurement tilt information of one-way wavefront. This correction method requires two mirrors together with complex control algorithms that is difficult to achieve. This paper presented a new intra-cavity tilt correction method based on round-trip probe, which corrects both of laser optical axis and laser pupil offset with only one mirror. This correction method is flexible, convenient to debug and applicable. In the experiment of tilt correction, the output power decreases 31.58% within 80s and still remains unstable in the open loop state. However, in the close loop state, the laser keeps stable working using the tilt aberration corrected method that proposed in this paper. This correction method is proved to be effective through the experiment.The effect of intra-cavity aberration correction based on nagation of phase in positive-branch unstable resonator is not ideal because of the different sizes of oscillation laser on optical elements and light deflection that caused by the aberration. This paper presented the high-order intra-cavity aberration correction method based on the relaxation iterative approximation algorithm using a collimated probe beam wavefront as the reference. This method overcomes the difficulties of correcting the non-conjugated intracavity aberration of evolution aberration. The reciprocating detection device is simple, has low insertion loss, and doesn't have selected polarization. In the experiment of intracavity aberration correction in seven thin-disk laser used in unstable resonator, the high-order aberration is corrected by deformable mirror. When intra-cavity high-order aberration is corrected, the laser keeps working stably. The output power of laser increases by 24%, and the beam quality of laser decreases from ?= 25.24 to ?=6.58.This correction method is proved to be effective for improving beam quality through the experiment.