Nonlinear frequency conversion efficiency depends sensitively on the phase mismatch parameter σ defined in Eqn. 4.17. For collimated light beams, the dependence is [sin(σ/2)/(σ/2)]2 which is maximum at σ = 0 and falls to half its maximum value at σ equal to 2.8. For optimally focused incident beam(s), the dependence of the conversion efficiency on is similar, although the maximum is achieved at a nonzero value of σ (as shown in Ref. [2]). Since the ratio of the length of the nonlinear material (typically centimeters) to that of light wavelength (between 0.3 and 10 μm) is large, even small values of dispersion in the material can make σ quite large, and thereby reduce the conversion ...
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