Fourth harmonic generation apparatus

Abstract

A fourth harmonic frequency generating method and apparatus is provided. The system includes within the optical cavity an active laser medium, a second harmonic generator for generating second harmonic frequency of the fundamental frequency emitted by the laser. The fundamental beam is directed to a second harmonic generation crystal where a portion of the fundamental beam is converted to a second harmonic beam. Both the second harmonic and unconverted fundamental beams are directed back across the second harmonic generator by reflective surfaces for a second pass prior to any conversion of second harmonic beam to a higher harmonic beam. Preferably the second harmonic and fundamental beams are reflected by one of the laser cavity reflective surfaces for the second pass. Optionally, there can be separate reflective surfaces to reflect each of the fundamental and second harmonic beams. A third harmonic generator for generating third harmonic frequency of the fundamental frequency, a fourth harmonic generator for generating a fourth harmonic frequency of the fundamental frequency and a wavelength selective coupling to facilitate the removal of the fourth harmonic beam from the optical cavity are provided in optical communication with the reflected second harmonic and fundamental beam. Optionally the unconverted second harmonic beam can be directed through the laser medium as additional pumping. Optionally substantially all UV beams are removed from the laser cavity preferably 99% or greater. The resulting laser has improved conversion efficiency and an improved mode quality.