Use of phase masks for reducing divergence of Gaussian beams

In the composite method of obtaining Gaussian beams with low divergence, the spherical component of the wave front is compensated for by passing the beam through a lens with suitable radius of curvature, and phase jumps between mode spots are removed by placing a phase mask in the beam path, the segments of which repeat the configuration of the mode spots and provide a field phase shift of pi radians between neighboring spots. Two conditions must be satisfied for successful implementation of this method: (1) stable generation of the Gaussian beam, and (2) efficient energy conversion of the corrected beam. An experimental setup in which these two conditions were realized is described.