Unidirectional operation of rectangular dielectric single-mode ring waveguide lasers

Since most fabricated waveguide lasers have approximately rectangular guiding cross sections, a qualitative description of propagation modes in such structures is given. Having developed the concept of the effective mode index to be used in Fresnel reflection coefficient calculations, the nonreciprocal mechanism due to the tilted output facet is described. This is followed by an estimate of the reciprocal-coupling loss introduced between the arms of the ring waveguide due to the tilted output facet. It is shown that with incidence angles approaching the critical angle, small tilt angles of the output facet can produce a drastic difference between the intensity reflectivities of the two oppositely sensed waves circulating in the ring, thus creating a nonreciprocal mechanism which favors unidirectional operation.>