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Optical ring micro-resonators (OMR) can be integrated onto chips to obtain sensitive, robust, low cost and portable sensor systems. They are used for in-situ real time detection of specific molecules by specialized or non- specialized persons. Target analytes, homogeneously spread in the cladding layer, induces a complex refractive index variation Δncl of the OMR waveguides upper cladding.
In this study, we propose an optimized analytical approach to OMR designs in terms of bulk sensitivity.
Those type of sensors are based on the evanescent field sensing. Interaction between the evanescent field and the analytes induces resonance wavelengths modifications. The main sensing strategy is based on resonant wavelength shift measurement. However, contrast variation, due to the absorption coefficient linked to analytes concentration, can also be measured. Colorimetric reactions, used to obtain a specific sensor, change significantly the light intensity in a specific peak of the transmission spectrum. This is due to the complex formation between a specific ligand and a heavy metal, such as hexavalent chromium and 1,5 diphenylcarbazide.
From the well-known ring resonator’s transmission expression, we can establish an analytical model of sensitivity’s dependence on geometric dimensions. Sensitivity in influenced by the round-trip attenuation coefficient a, the auto-coupling coefficient τ, the optical path and the ratio of guided power into the cladding Γcl. We validated our approach with FDTD simulation of OMR’s response for a 15 μm radius. This analytical approach makes it possible, from the waveguide propagation structure and propagation losses, to obtain both the optimal ring radius and the resonator gap in order to obtain maximum sensitivity.
Based on optical characterization of OMR, measured variations of 1% power drop at resonance should allow variation measurement on the extinction coefficient of ∆ni = 10−6.
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