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The coupled line equations for two modes traveling in the same direction are considered. The covariances of the mode transfer functions are calculated in the case of random coupling. Exact results are obtained when the coupling is a function of a finite state Markov chain, and also when the coupling is white noise. Perturbation results are obtained in the case of weak, zero mean, wide sense stationary coupling. It is also shown that perturbation results are valid in the case of strong coupling, if the correlation length is short.
Morrison et al. (Sat,) studied this question.