Readout instrumentation for Fabry-Perot sensors

Abstract

Continuous monitoring of blood glucose levels (BGL) on intensive care unit (ICU) patients opens up for the possibility of keeping insulin at normal levels. This method is called intensive insulin therapy (IIT) and reduces both recovery time, chances for complications and death, thus saves the hospital resources. Before IIT can be employed as the standard treatment, better products are needed for continuous glucose monitoring (CGM). GlucoSet A/S is currently developing a CGM system based on a Fabry-Pérot (FP) hydrogel sensor that responds to BGL by changing length. By sending a light spectrum to the sensor and measuring the interference pattern returned, information about the length of the sensor cavity is achieved; the frequency of the interference pattern directly depends on the length of the cavity. In this thesis an accurate quadrature demodulation technique (QDT) used by GlucoSet today in a scientific setup, it transfered over to a 32-bit microcontroller unit (MCU) based platform. Simulation of critical parameters such as superluminescent light emitting diode (SLED) light source bandwidth is conducted and performance improving alterations are made on the algorithm to make it perform faster calculations. The results showed that the QDT can be implemented on a 32-bit MCU; when the SLED bandwidth is 80nm the phase estimate calculates cavity length with a <0.21% error(~±50 nm), while the QDT phase estimate tracks changes in cavity length with a <0.013% error(~±3 nm). If real sensor measurements yields the same accuracy as simulated in this thesis, less accurate components are suggested tested to benefit from an accuracy/price trade-off as further work.