1. A laser resonator having a cavity for the propagation of a laser beam having a wavefront in which a lasing medium is positioned, comprising: (a) a retro-reflector means including: (1) optical means disposed in said cavity for expanding the wavefront leaving the lasing medium; and (2) aspherical concave mirror means disposed in said cavity facing said optical means for reflecting the expanded wavefront toward the lasing medium again and correcting wavefront aberrations induced when the laser beam passes through the lasing medium; and (b) output coupler means arranged facing said retro-reflector means with the lasing medium positioned within an intracavity formed between said output coupler means and said optical means, for providing feedback for regenerative action of the laser beam.