In the past, Faraday based optical polarimetry approaches have shown considerable potential for the measurement of optical activity with application towards the noninvasive measurement of physiological glucose concentration. To date, most reported closed-loop systems incorporate separate Faraday components for modulation and compensation requiring two optical crystals. These systems have demonstrated significant stability and sub-millidegree rotational sensitivities; however, the main drawbacks to this approach are the optical materials (e.g., terbium gallium garnet) can be quite expensive and often custom fabricated induction coils are required. In this investigation, we propose a new design for the Faraday components capable of achieving both modulation and compensation in a single crystal device. The design is more compact and is capable of achieving similar performance with low cost commercially available inductive components. To facilitate prototype optimization, our group has developed a finite element model (FEM) that can simulate various physical parameters such as geometry, inductance, and orientation with respect to the optical rod in order to minimize power consumption and size while maintaining appropriate field strength. Performance is comparable to existing nonintegrated approaches and is capable of achieving modulation depths < 1° under similar operating conditions while attaining sub-millidegree linear polarization sensitivity. There is also excellent correlation between the FEM and experimental prototype with operational performance shown to be within 1.8%. The use of FEM simulations allows for the analysis of a vast range of parameters before prototypes are fabricated and can facilitate custom designs as related to development time, anticipated performance, and cost reduction.
In recent years, significant advances have been made in the development of noninvasive polarimetric glucose detection systems, salutary for the treatment of our rapidly increasing diabetic population. This area of research utilizes the aqueous humor as the detection medium for its strong correlation to blood glucose concentration and highlights three major features: the optical activity of glucose, minimal scattering of the medium, and the ability to detect sub-millidegree rotation in polarized light. However, many of the current polarimetric systems are faced with size constraints based on the paramount optical components. As a step toward developing a low cost hand-held design, our group has designed a miniaturized integrated single-crystal Faraday modulator/compensator. This device is capable of replacing the traditional two component arrangement that has been widely reported on in many Faraday-based polarimetric configurations. In this study, the newly designed prototype is compared with a theoretical model and its performance is evaluated experimentally under both noninvasive static and dynamic glucose monitoring conditions. The combined rotator can achieve modulation depths above 1°, and when operating in a compensated closed-loop configuration, it has demonstrated glucose prediction errors of 1.8 mg/dL and 5.4 mg/dL under hypoglycemic and hyperglycemic conditions, respectively. These results demonstrate that such an integrated design can perform similar if not better than its larger two-part predecessors. This technology could also be extended to facilitate the use of multispectral polarimetry by considerably reducing the required number of physical components. Such multispectral techniques have demonstrated usefulness for in vivo and multi-analyte noninvasive sensing.