Crosstalk Caused by Scattering in Slab Waveguides

In an earlier paper1 I discussed the crosstalk between optical dielectric waveguides based on the directional coupler mechanism. In that case, crosstalk was caused by the fact that the exponentially decaying field of one guide reaches the region of a neighboring guide. This type of crosstalk exists even if both guides can be considered to be perfect dielectric cylinders. The present paper is devoted to a crosstalk mechanism of a different 1817 1818 T H E BELL SYSTEM TECHNICAL JOURNAL, J U L Y - A U G U S T 1971 type. A dielectric waveguide or optical fiber loses power by radiation if the guiding structure is imperfect in any way. This imperfection may consist of deviations in the perfect cylindrical geometry of the core-cladding interface or it may consist in random variations of the refractive index distribution. Some of the radiation, causing loss to the guided mode of one fiber, may reach a neighboring fiber and can there be scattered back into one of its guided modes. This type of crosstalk is thus caused by waveguide imperfections. However, just as in the case of the directional coupler effect, it is possible to link predictions of the effectiveness of this type of crosstalk mechanism to the loss it causes to the guided mode. This makes it possible to give very simple rules that link crosstalk to radiation loss (see equation (50)). It turns out that scattering crosstalk is no serious problem provided that the radiation loss of the guided mode (that is caused by the same mechanism) remains within acceptable limits.