Prism coupling into clad uniform optical waveguides

Abstract

The theory of prism coupling into multilayered dielectric slab waveguides is presented. In addition to including the possibility of high index cladding, the present theory also extends the region of validity of previously reported work to cover the regime of ’’strong coupling’’. The limiting conditions for validity of the present theory are that both αm/k and αm±p/k be much smaller than either unity or ‖βm−βm±p‖, where m is the mode under consideration, αm is the leakage of that mode, and ‖βm−βm±p‖ is the separation of the effective indices of adjacent modes. A numerical example is presented in which the coupling efficiency into a uniform or slab waveguide with Δn=0.002 is calculated for various cladding thicknesses with a cladding index of 2.5. The introduction of cladding is found to reduce coupling efficiency in this example due to increased phase mismatch between the incident and ’’ideal’’ beams.