OPA exponential gain: |A_s(z)| = |A_s(0)|*cosh(|kappa|*z); asymptotic exp(|kappa|*z)/2

OPA exponential gain theorem. From framework F7 (variation-of-parameters on coupled-wave ODEs), the signal amplitude grows as |A_s(z)| = |A_s(0)|*cosh(|kappa|*z). For large z this asymptotes to (1/2)*|A_s(0)|*exp(|kappa|*z) - exponential…