(a) Schematic representation of the 3-layer slab waveguides family; the guiding layer yields a refractive index n ₁ and a height 2h, the upper cladding n ₃ and the substrate n ₄ are both considered as semi-infinite layers. (b) Schematic representation of the family of 4-layer slab waveguides; the index n ₁ and n ₂ are associated with respectively the guiding layer and the first upper cladding whose height are respectively 2h and 2e. The upper cladding and the substrate of index n ₃ and n ₄ are considered as semi-infinite layers.