Determination of refractive indices and thicknesses of silica double layer slab waveguides on silicon

The authors present a method which simultaneously determines the refractive indices and thicknesses of double layer silica films grown on silicon independent of the index difference between the two layers. It is based on an earlier theory for double layers on high index substrates and uses the propagating and radiating mode indices obtained from prism coupling measurements (e.g. at wavelengths of 876, 1304 or 1547 nm). The method is useful for analysing single mode waveguides at low infrared wavelengths used in telecommunication applications. The theoretical mode indices obtained from a proper approximation of the rigorous eigenvalue equation with initial assumptions on refractive indices and thicknesses are compared with the measured mode indices, and, in an iterative fashion, the procedure is repeated with different parameters until the mode indices match. Good agreement is found between the results of single and double layers with about 20 μm buffer and 7 μm core layer thicknesses, respectively, with index difference in the range 10 ^-3-10^-2 using prism coupling and reflectometric measurements. Typical accuracies within the constraints of the measurement for the refractive indices and thicknesses are beyond 10^-4 and 0.2 μm, respectively