A refractive-index biosensor based on directional coupling of silica waveguides

We propose a refractive-index biosensor based on directional coupler (DC) structure to detect physiological concentrations of glucose in water by sweeping of incident light wavelength. The sensing mechanism is by changing the incident light wavelength the refractive index change induced by glucose concentration can be compensated to keep the coupling length of DC unchanged. The influences of the central light wavelength, the waveguide width and spacing on the sensitivity and resolution of the proposed sensor are analyzed. By a tradeoff between the coupling length and resolution in a sample index range of 0.02, we can obtain a sensor with a size of 33.65 mm × 100 μm for d1 = d2 = 3 μm which shows a resolution of about 1.8 × 10−5 and a sensitivity of 5500 nm/RIU using a tunable laser with a precision of 0.1 nm.