Refractive index as the new water quality monitoring standard
Optiqua’s patented optical chip is based on an integrated version of the Mach-Zehnder Interferometer (MZI). This is a highly sensitive sensor for Refractive Index (“RI”) changes. Refractive index is a useful generic indicator of water quality as any substance, when dissolved in water, will change the refractive index of the water matrix in proportion to its own RI as well as its concentration. The generic Optiqua sensor chip operates at a sensitivity level equivalent to parts per million (ppm) levels for any chemical contaminant.
The relationship between the refractive index and the concentration of a contaminant is linear. The Optiqua sensor accurately measures changes in refractive index in the order of magnitude of 10-7 RI, which corresponds to low ppm level concentrations for most chemicals. This makes it an extremely useful parameter for detection and monitoring purpose. In combination with assays, as used in the MiniLab, analyte detection of specific substances down to the low ppb levels is possible.
Optiqua’s patented optical sensor technology is based on an integrated version of Mach-Zehnder Interferometer (MZI). The basic layout of the MZI consists of an input channel wave-guide (#1 in figure below) that splits into two identical branches (#3), a sensing branch exposed to the water sample (#4) and a reference branch, and are then combined again to form the output wave-guide. The MZI works as an optical scale, measuring minute differences in refractive index as seen by the sensing branch versus the reference branch.
Mach-Zehnder Chip Layout
The wave-guides are so called buried wave-guides in which the light travelling through the wave-guide is shielded from the environment via a top cladding. By using etching techniques, the top cladding is locally removed at a well-defined position above the waveguide in the sensing branch. In this so-called sensing window, the evanescent field of the light that travels through the under-laying channel wave-guide, extents into the environment above the sensor and becomes susceptible to changes in refractive index of the sample solution on top of the sensor window.
The resulting change of the effective refractive index leads to a change of the speed of the light in the sensing branch and a change in the relative phase between light that has travelled through the sensing branch as compared to the reference branch. This change in relative phase leads to a change in the interference between light coming from the sensing and reference branch at the combining section and manifests itself as change in the output intensity of the MZI. The patented Optiqua sensor is an adaptation of the basic MZI design to improve the overall performance in terms of sensitivity, robustness and temperature independence.
Patented interferometric sensor using an integrated optical waveguide architecture:
- Europe, Patent number EP 2214049 B1
- Japan, Patent number JP 2012517023 A
- Singapore, Patent number SG173446 A1
- China, Patent number CN102292665 A
- Other patents pending