WGM-Based Sensing of Characterized Glucose- Aqueous Solutions at mm-Waves

Abstract

A newly-developed commercial coaxial probe kit (DAK-TL) is used to characterize the electromagnetic properties of glucose-loaded samples prepared at a range of concentrations 0.7-1.2 mg/ml similar to Type-2 normal diabetics in a broad range of frequencies from 300 MHz to 67 GHz using two different $50~\Omega $ open-ended coaxial probes. The objective is to determine the spectrum portion of the most sensitivity to slight variations in glucose concentrations and to identify the amount of change in the dielectric permittivity and loss tangent due to different concentrations of interest. The millimeter-wave range 50-67 GHz has shown to be promising for acquiring both high sensitivity and sufficient penetration depth for the most interaction between the glucose molecules and electromagnetic waves. Subsequently, the relative permittivity of the glucose-water mimicking samples are modeled in this mm-wave band using a single-pole Debye model with independent coefficients. The fitted Debye model is used to design a simple low-cost highly-sensitive integrated mm-wave sensing structure that utilizes the travelling-wave Whispering Gallery Modes (WGMs) launched in a dielectric disc resonator (DDR) when coupled to a dielectric image waveguide (DIG). The proposed sensor is used for continuous monitoring of glucose levels in blood mimicking aquatic solutions by tracking the variations in the magnitude and phase of the WGM transmission resonances in the mm-wave spectrum. This happens in reaction to the strong interactions of the coupled WGM evanescent field with the glucose samples loaded on top of the DDR inside a container. The sensor exhibits a high sensitivity performance (2.5-7.7 dB/[mg/ml]) for the two proposed DIG layouts, straight and curved, at the lower-order modes WGH600 and WGH700 of its five pure WGH modes supported in the frequency range 50-70 GHz as demonstrated by numerical simulations in a 3D full-wave EM solver and validated through proof-of-concept measurements.