Generation, and nonlinear Parameter Estimation, of Reflection Spectra from an Intravascular Fabry-Perót Sensor, for detecting Temperature and Glucose Concentrations
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GlucoSet AS has developed an intravascular Fabry-Perót (FP) glucose sensor. The sensor consists of a hydrogel cast on an optical fiber cut, representing a low finesse FP cavity. The cavity changes length with respect to varying glucose concentrations. Nevertheless, the glucose measurements are temperature sensitive. In addition, the current sensor read out system is inaccurate in the absolute length estimate. The Master s thesis objective is to measure temperature in an FBG fiber and estimate absolute length of the FP cavity. A simulation program for a hybrid FBG-FP sensor was developed and used in simulation experiments. For verification, a nonlinear parameter estimation algorithm was coded to estimate the FBG and FP sensor reflection spectra, to estimate temperature and absolute length of the FP cavity. The same algorithm was included in a developed user interface program for empirical prototype testing. The user program tracks relative length changes and absolute lengths in real time.In simulations, the estimated uncertainty in temperature was approximately 0.084 Celsius, but the FP sensor interfered with the FBG readings which gave misrepresented temperature changes of 0.66 Celsius. The uncertainty in the absolute length was 10.8 nm, when the algorithms nonlinear model matched the spectra.In practical experiments, the estimated uncertainty in temperature was approximately 0.078 Celsius, and the estimated uncertainty in the absolute length gave values from 11.7-20 nm. However, the models rarely matched which often resulted in systematic offsets in the absolute length estimate, up to 140 nm.The temperature estimate is satisfactory but can be improved by a stronger FBG reflection. The hybrid FBG-FP sensor has temperature interference issues from the FP sensor but can be compensated for in future work due to its predictive nature. The absolute length estimate is significantly improved but a good signal is required to maximize the model match and minimize the offset. In all, the hybrid FBG-FP sensor proved to be a feasible device for intravascular temperature control, with some pending improvements.