Design considerations to realize differential absorption based optofluidic sensors for determination of ionic content in water

Integrated optofluidic absorption spectroscopy has the potential to be used in the real-time identification and quantification of ionic content in drinking water. Such an approach requires single mode operation in combination with low propagation and bend losses. The design criteria for the realization of an integrated Near-infrared (NIR) sensor platform fabricated using silicon waveguide based technology is presented. The cross-section of TriPleX waveguides was optimized for a spectral region between 940 and 1040nm. The waveguide structure provides single mode operation, high sensitivity combined with reasonable tolerance for fabrication deviations and <0.01 dB/cm bend losses for a radius of 500 μm. The optimum sensing length on chip was determined for propagation losses of 0.1 dB/cm, 0.5 dB/cm and 1.5 dB/cm. Furthermore, a Ξ coupler was designed for the spectral region of interest (0.94 to 1.04 μm). Multiple integrated photonic circuit layouts varying in functionality and complexity are reviewed. These optofluidic absorption sensors offer in-situ & in-line capabilities, potentially low-cost, remote signal data processing, and mechanical & thermal stability. This work provides a roadmap for maximizing the sensitivity of broadband spectroscopy using differential absorbance.