Si3N4 rib optical waveguides suitable for evanescent-wave photonic biosensors are analyzed to derive the conditions for single-mode propagation. A photonic biosensor structure at a wavelength of 0.63 µm based on two-mode interference is proposed, and the rib width, rib thickness and rib-core thickness for single-mode and two-mode waveguides (corresponding to sensing region) are proposed to be 3 µm, 2 nm, and 150 nm, and 3 µm, 2 nm, and 340 nm, respectively. The optical characteristics and propagation of each guided-wave mode are investigated utilizing the film-mode-matching and eigenmode-expansion methods.
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