Adaptive optics system prototype for the automatic control of geometrical fluctuations in a laser beam in air

We present the results of reduction of laser beam geometrical fluctuations that we have experimentally measured using a prototype of Adaptive Optics (AO) system previously designed. The scheme of this prototype provides that the wavefront sensing function is not operated by a Shack Hartmann sensor as usual, but it is based on an interferometric technique which allows to detect the perturbed phase profile using the interference fringes pattern. We show that this technique is of particular interest when high sensitivity and large bandwidth are required for the correction of small perturbations. The architecture of the system is based on a typical Michelson configuration with He-Ne laser source. It is assumed that one arm of the interferometer is passed through by the reference beam while in the other one there is the aberrated wave. The output intensity produces a fringe pattern which is detected by means of a pixellated photodiode. Combining the output of each photodiode, we achieve signals which can be immediately interpreted as aberration coefficients and thus reintroduced into the digital control as error signals in order to calculate the correction commands to be sent to the deformable mirror. The results here presented show that we have obtained 2 decades of noise reduction in the 10 Hz bandwidth, and that the control has been operating 10 dB attenuation up to 200 Hz.