We show that the set of two coupled differential equations governing a four-wave mixing (FWM) component and its associated idler in a one-pump fiber optical parametric amplifier are the same as for standard parametric amplification, but with additional source terms arising from FWM between already-calculated waves. We show how to solve this new set of equations in general: this involves multiplication by an exponential matrix and its inverse, and integration. Hence it is in principle possible to write the exact expressions for the cross-talk fields, in terms of a large number of exponentials. The results show that the power of the first- and second-order cross-talk terms scales, respectively, like the first and second power of the signal power. In the case of two input signals, the powers of the cross-talk terms are independent of the phases of the signals, hence phase control cannot be used for reducing cross talk. In a particular case, cross talk scales like the inverse of pump power, which confirms a general trend noted in numerical simulations, and experiments.
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