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Summary form only given. Down-conversion processes can be divided into degenerate and nondegenerate. In the former case idler and signal photons are identical while in the latter they differ in frequencies and/or polarizations. It has been shown that transverse patterns in nondegenerate OPO and soliton dynamics in nondegenerate free propagation have qualitative differences from their degenerate counterparts. The difference can be formally identified as due to an additional symmetry, in the differential phase of signal and idler fields. This symmetry is suppressed in the degenerate case. As a result the frequency of the signal component of any solution, including solitons, is exactly half the pump frequency, /spl omega//sub s//spl equiv//spl omega//sub i/=/spl omega//sub p//2, on the other hand, in the nondegenerate case any arbitrary frequency difference 2/spl Omega/ between the idler and signal fields still satisfies the condition /spl omega//sub p/=/spl omega//sub s/+/spl omega//sub i/. This raises the question of whether there are any physical constraints on /spl Omega/.