Directional Coupler Biosensor Based on Thermooptic Detection Mechanism in Silica Waveguide

We propose a directional coupler (DC) biosensor to detect the concentration of glucose based on the thermooptic detection mechanism in silica waveguide. We discuss the design principles using the coupled-mode theory of DC and deduce the conditions for a valid biosensor model. The coupling length, the detectable refractive index range, and the sensing resolution for different silica widths and separation distances of the DC are presented by analytical calculation. By a tradeoff between coupling length and resolution of refractive index, we can obtain a sensor with a compact size of
$48\ \mu\text{m}\times 30\ \mu\text{m}$
. The biosensor shows a resolution of about
$1\times 10^{-6}$
for glucose concentration ranging from 1 to 200 mg/dL.