Single mode condition for shallow silica-titania rib waveguides

A single mode condition for silica-titania rib waveguides is proposed in this work. It is applicable to rib waveguides having rib height sufficiently small to sustain fundamental modes in slab waveguides adjacent to the rib. The relationships binding rib waveguide dimensionless morphological parameters for which the secondary order, HE and EH, horizontal modes leak outside the rib, rendering rib waveguides single mode, were determined by using the film mode matching method. A new criterion function is proposed for approximation to these relationships. It is shown that the proposed single mode condition gives significantly better results than the one proposed by Soref. Furthermore, it is shown that the fundamental EH mode can also leak outside the rib. This requires to complement the single mode condition, what can be accomplished by using the proposed criterion function. The studies also showed that a Soref's correction factor significantly depends on rib waveguides aspect ratio.