Laser system with highly efficient, single-pass, harmonic generator with round output beam

A laser system comprising:

• a seed source configured to produce seed radiation used to form a nominally round, non-astigmatic, diffraction limited input fundamental beam;
• and
• an extra cavity harmonic generator system configured to produce a round, non-astigmatic third harmonic output beam from a nominally round, non-astigmatic, diffraction limited input fundamental beam, the system comprising:
• a second harmonic generation (SHG) crystal having an input face oriented at an angle of incidence with respect to the input fundamental beam wherein the fundamental beam size is expanded in a non-walkoff direction for the SHG crystal at the input face, the SHG crystal being configured to generate a second harmonic beam from the input fundamental beam;
• a third harmonic generation (THG) crystal having an output face oriented at an oblique angle of incidence in a non-walkoff direction for the THG crystal such that the third harmonic beam size is contracted in this direction, the THG crystal being configured to generate a third harmonic beam from the second harmonic beam and a residual fundamental beam from the SHG crystal;
• wherein an expansion of the input fundamental beam at the input face of the SHG crystal exceeds a reduction of a third harmonic beam at the output face of the THG crystal.