Multimode waveguide-cavity sensor based on fringe visibility detection

Fringe visibility detection of the interaction of two bus spatial eigenmodes with a resonant cavity is investigated for the purpose of achieving a sensor platform with high sensitivity. The power distribution between the bus waveguide eigenmodes is modulated by the interaction with the cavity and is detected via fringe visibility lineshapes produced by twin-fiber interferometry. A test device is fabricated in a polymer-silica material system by a photolithographic process and is characterized by measuring the fringe visibility change as a function of analyte refractive index. Fringe visibility modulation from a straight two-mode waveguide coupled to a single mode ring resonator exposed to an aqueous glucose solution demonstrates a visibility change of 1.57 per weight percent, compared to a transmission change of 0.19 per weight percent for a single mode waveguide critically coupled to a ring with similar intrinsic quality factor. The demonstrated change in fringe visibility is 8.2 times larger.