Crosstalk caused by scattering in slab waveguides

Light scattered in one optical fiber can be scattered back into a guided mode in a neighboring optical fiber. This type of scattering crosstalk is investigated in this paper for slab waveguides. However, the results for the slab waveguide case are upper limits on the crosstalk between round optical fibers. Scattering crosstalk is expressed in terms of guided mode radiation loss whose cause is the same scattering mechanism. It is thus not necessary to know the details of the scattering mechanism as long as the scattering loss, which is a measurable quantity, is known. In addition to the total radiation loss the crosstalk depends on the width of the radiation lobe in which the scattered energy escapes from the waveguide. The crosstalk is inversely proportional to the width of the radiation lobe and thus is larger when the radiation lobe is narrow. Scattering crosstalk can become serious if both guides have a systematic sinusoidal imperfection of the same mechanical frequency resulting in a radiation lobe of nearly zero width. In the absence of a systematic sinusoidal imperfection it can be concluded that scattering crosstalk between dielectric waveguides (optical fibers) is negligible if the radiation losses are tolerable. In this case it appears unnecessary to suppress crosstalk by means of a lossy medium between the waveguides.