Beam Fanning in Coupled-Wave Theory of 2-Beam Coupling,
ROCKWELL INTERNATIONAL THOUSAND OAKS CA
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Simple coupled-wave theory accounts for depletion of the pump beam due to energy transfer i.e., amplification to the probe beam. Ignoring absorption, the probe gain g can be expressed as g I sub 1z divided by I sub 1O 1r sub pp divided by 1r sub pp e to the -Gamma z where I sub 1O is the incident probe intensity, Gamma is the two-beam-coupling coefficient and r sub pp is the incident pump-to-probe beam ratio given by r sub pp I sub 2O divided by I sub 1O with I sub 2O being the incident pump intensity. Equation 1 remains valid even when the pump-to-probe ratio r sub pp is not sufficiently large only the approximation for the probe gain g approximates e to the gamma L becomes invalid when r sub pp is not much greater than e to the gamma l. Similarly, the pump depletion d can be expressed as d I sub 2z divided by I sub 2sO 1r sub pp divided by r sub pp e to the gamma z. These expressions for the probe gain and pump depletion have proven inadequate in describing the two-beam-coupling process, particularly when the coupling coefficient IF is very large recently, rs exceeding 60cm have been reported.