Title:
Assessing Integrated Photonic Measurement Methods to Sense the pH of Saliva
Abstract:
Photonic integrated circuits (PICs) are a rapidly growing technology. Currently, PICs are being explored for sensing. Fiber-based optical sensors are a well-established and common technology. PIC sensors, however, are more compact, scalable, and consume less power. In addition, since the components are integrated onto a chip, integrated sensors are generally less affected by environmental factors like temperature and mechanical stress.
This thesis assesses methods of measuring the pH of a transparent substance in the visible spectrum using PICs for the eventual purpose of measuring the pH of saliva. Measuring the pH of saliva is important as a diagnostic factor, where the pH is the desired sensed parameter, and as an environmental factor, where the pH may interfere with accurately sensing other target analytes.
Measurements were taken by applying the analyte onto a passive integrated photonic component where the upper cladding was etched away to create a sensing trench. The analyte becomes the cladding of the waveguide, and the evanescent fields of the guided light interact with the analyte, affecting the spectra. Two main parameters’ correlation to pH were examined: absorption and refractive index. To assess absorption, analyte solutions of varying pHs were mixed with bromothymol blue, a pH-sensitive dye, and these solutions were deposited on trenched straight-through waveguides. A shift in the maximum loss correlated to pH was observed, although the measurement method demonstrated issues with resolution. Both of the refractive index measurements, Fresnel reflections on trenched straight-through waveguides and interferometry on trenched Mach-Zehnder interferometers, showed no correlation to pH, which may be due to the sensitivity and design of these structures.
In addition, an optimization-based program to assess fabrication errors was briefly explored. This could offer the chance to further enhance sensing capabilities, as fabrication errors could be estimated to account for changes in the spectra caused by them, as opposed to the desired sensing parameter.
This work provides insights into promising PIC measurement methods of pH. It also demonstrates hurdles in resolution and sensitivity, which may be of interest to future research.
Research Advisor:
Prof. Doug Petkie
Physics Department, and affiliation with ECE Department, WPI
Research Committee:
Prof. Ulkuhan Guler
ECE Department, WPI
Prof. Suat Ay
ECE Department, WPI