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Chapter I Brief Introduction...
several methods for obtaining SHG are shown schematically. The simplest
scheme for frequency doubling is extra cavity doubling, where the laser beam
passes through the nonlinear crystal only once. However, if the power density
of laser is low, focused beam, intra-cavity doubling and external resonant
cavity are normally used to increase the power density on the crystals.
Figure: 1.2 Extra cavity SHG. Figure: 1.3 SHG with focused beam.
Figure: 1.4 Intra-cavity SHG. Figure: 1.5 External resonant cavity SHG.
Sum and Difference Frequency Generation or Parametric Generation
The essential goal of practical nonlinear optical frequency conversion is
the efficient generation of radiation of sum, difference and second harmonic
frequencies. If two monochromatic optical fields, with frequencies ω1 and ω2
and amplitudes E1 and E2, respectively, are applied to a medium with non
vanishing second-order susceptibility (2) , the induced dipole response will
contain, in addition to the ordinary linear response terms at frequencies ω1
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