Temperature sensors and refractometers using liquid-core waveguide structures monolithically integrated in silica-on-silicon

Integrated optofluidic devices have many potential applications for on-chip analysis and sensing. The fabrication of single-mode liquid-core waveguides in an integrated format allows the implementation of robust and very sensitive interferometers that combine long optical paths (on the cm scale) with small volumes (less than a nanoliter). We have demonstrated the monolithic integration of microchannels and liquid-core waveguides with planar silica lightwave circuits, which allows a number of refractometer devices to be implemented. Of these, we demonstrate experimentally a monolithic Mach-Zehnder interferometer (MZI) comprising a 20 mm-long liquid-core waveguide. The liquid-core waveguide is quasi single mode at 1550 nm when filled with a liquid of nominal index of ~1.47 (such as toluene or an index matching fluid). In these conditions, the output of the MZI is a pure cosine function, as a function of a linear progression of the refractive index of the liquid medium. Furthermore, the high contrast ratio experimentally observed in the output function allows a precise monitoring of refractive index changes by tracking the position of the transmission minimum in the spectral domain. Refractive index variations can be measured to a precision on the order of 4x10-6. The large differential in thermo-optic coefficients between liquid media and silica allows the structure to function as a temperature sensor with a precision on the order of 10-2 degrees Celsius. The measurement of the spectral fringe spacing of the interferometer response allows absolute-value measurements of temperature and refractive index.

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