Preliminary study on the multidither coherent optical adaptive technique (COAT) system for CO2 laser

The wavefront of the laser beam propagating through the atmosphere is distorted temporally and spatially due to the atmospheric turbulence, resulting in beam spread, intensity fluctuation, and beam wander on the target. The adaptive optical system can compensate the wavefront distortions of the laser beam by deforming the mirror surface to match the conjugate wavefront. Thus, by using an adaptive optical system, the laser beam could be continuously directed to a certain distant position with the near diffraction—limited spot size under the turbulence. The multidither Coherent Optical Adaptive Technique (COAT) is one of several existing adaptive optical techniques, whose primary merit is to be able to correct the wavefront remotely with the signals returned from the target and need no complicated wavefront sensors. In the 1970's, Pearson et al.1'2 first fabricated the multidither COAT system for a visible (argon ion) laser, and demonstrated the effectiveness of the COAT system in compensating its wavefront distorted by atmospheric turbulence. However, since the first demonstration, considerable few experiments of atmospheric compensation using the COAT system for CO2 laser have been reported only except for several papers describing the usage as an intracavity adaptive optics.36 We fabricated the COAT system to correct the distorted wavefront of the CO2 laser beam,7 and performed the in—house experiments to evaluate the system performance and optimize it. In turn, preliminary experiments for atmospheric compensation were conducted, in which our COAT system was integrated with a large Cassegrainian type telescope and tried to correct the distortions of a focused CO2 laser beam. In this paper we describe the characteristics of our COAT system compensating the beam wander and the intensity fluctuation due to the atmospheric turbulence.