A laser for generating deep ultra-violet (DUV) continuous wave (CW) light includes a second-harmonic generator and a fourth-harmonic generator. The fourth-harmonic generator includes a plurality of mirrors as well as first and second non-linear optical (NLO) crystals. The first NLO crystal generates the light having the fourth harmonic wavelength, and is placed in operative relation to the plurality of mirrors. The second NLO crystal is placed in operative relation to the first NLO crystal such that the light having the second harmonic wavelength passes through both the first and the second NLO crystals. Notably, the second optical axes of the second NLO crystal are rotated about a direction of propagation of the light within the second NLO crystal approximately 90 degrees relative to the first optical axes of the first NLO crystal. The second NLO crystal provides no wavelength conversion.