Propagation of radiation in two identical coupled waveguides

The exact solution is obtained of the problem of propagation of radiation in two coupled fiber waveguides. The characteristic equations valid for an arbitrary difference between the refractive indices are obtained for the first time. It is shown that the characteristic equation for the case of weak waveguiding ability yields the familiar approximate expressions for the coupling constants. An analytic method for the calculation of cutoffs of the modes in the coupled waveguides is considered. The first analysis is reported of the influence of the induced birefringence on the cutoff frequency. The waveguide parameters are calculated for a quartz–air waveguiding interface. The spectral dependences are obtained for the transfer and transmission coefficients of single-mode couplers near the cutoff frequency of an antisymmetric mode. Experimental observations are reported of the cutoff of orthogonally polarized normal modes in the coupling region of a single-mode fused coupler.