Three-wave mixing processes doubling, sum and difference frequency

Three-wave mixing processes doubling, sum and difference frequency 

Al generate one (outgoing) photon of wavelength A2 (energy is conserved). 

Otherwise, the two fundamental and second-harmonic waves come out of phase and destructive interference diminishes the conversion efficiency. 

However, because of the beam divergence of the focused beam, the coherence length is reduced because the direction of the wave vector k( j)) spreads as well. Careful matching of the beam waist of the focused beam and the NLO-crystal length minimizes this effect. 

Jurdik et al. (2002)). It should be noted that, for a number of fixed-wavelength lasers (such as NdrYAG lasers), frequency doubling is implemented in intra-cavity configurations, which exhibit the benefit of automatic resonator-enhanced circulating power of the pump wave conversion efficiencies of more than 50 per cent are routinely achieved in commercial green CW Nd YAG laser systems, and values of close to 90 per cent are not uncommon (e.g. see Schneider et al. (1996)). 

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