ABSTRACT
This chapter highlights the light propagation equations for three wave mixing in second order nonlinear optical media. It discusses both the optimized focusing and collimated cases for the processes of sum and difference frequency generations and for second harmonic generation, which is of course a special case of sum frequency generations. The nonlinear propagation equations contain the effects of diffraction, beam walk-off, linear absorption and nonlinear frequency mixing. Focusing light too tightly in the nonlinear material would cause damage to the medium and more frequency converted energy can be obtained with collimated beams. The actual beam distributions and temporal pulse shapes of the incident beams in nonlinear interactions can be experimentally determined and may or may not fit well with analytic expressions. The efficiency of three wave mixing processes depends on the irradiance(s) of the incident beam(s), so it would seem to be desirable to focus the beam tighter in the nonlinear optical medium to increase conversion efficiency.
