[A miniaturized, highly sensitive polarimeter as detector in an implantable glucose probe. II. Opto-electronic...

There is a clinical requirement for an implantable telemetric probe for monitoring glucose levels in humans. This probe can measure the glucose content of the intercellular tissue fluid, which reflects glucose levels in the blood. The lifespan of such an implantable probe should be maximal, so that presumably only physical measuring detectors, but not aging-sensitive bio-sensors can be considered. We are in the process of developing a very sensitive miniaturised detector based on polarimetry, capable of determining the measuring parameter--the spatial orientation of the in-plane vibration of a polarised light beam--with high accuracy. This is necessary for our purpose, since the physiological and pathological glucose levels modify in in-plane vibration by only a tiny angle of rotation. The high level of accuracy is achieved by various specific mechanisms both of the measuring parameters and the electric signal. Two suitable optoelectronic amplification methods are described. The first makes use of the ratio of the signal provided by the intensity of two consecutive beams, derived from the original light beam with the aid of a beam splitter. In this way, the sensitivity of determining the spatial position of the in-plane vibration of the polarised light beam can be increased by up to 50-fold in comparison with a "simple" polarimetry. The second method requires two very closely approximated (quasi united) or actually united beams from two sources, which are both "fixed-phase" time-coupled and quantitatively periodically intensity-modulated in opposite sense. Together with the already-mentioned ratio of the intensity signals of two consecutive beams, a periodically modulated signal generated from the individual signals is derived from this quasi-unified beam that enables the use of a phase sensitive rectifier-amplifier (lock-in amplifier) with its enormous amplification factor and noise elimination in the following circuitry.