ABSTRACT
The power transfer, that is, the optical or mode coupling is caused by any coupling mechanism between two waves. If such couplings, are not there the multimode fibers would be used as single-mode transmission media by using only one mode to transmit a narrow pulse without any broadening. This chapter describes the basics of the coupling. In multimode waveguides, for example, multimode fibers, even if single mode is excited, after the propagation, the power distributes over multiple modes by the mode coupling. The cause of the coupling is imperfect homogeneity of the waveguide structure along the axial direction, for example, as a result of bending of the fiber axis. Such a coupling mechanism varies at random along the axial direction. To suppress the coupling between two orthogonal polarization in a single-mode fiber, anisotropy in the refractive index is formed by inroducing the anisotropic stress.
