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Abstract
Optical biosensors based upon spectral modulation of fluorescent or absorbant chemical species suffer from well-known defects, including problems with signal-to-noise ratio and analyte specificity. All sources of amplitude variation (connectors, micro- and macrobends, thermal effects, source power variations) contribute signal modulation identical to that induced by changes in the measurand. Solutions to these problems often require a multiplicity of reagents, separation means, illumination wavelengths and deconvolution algorithms, all of which tend to detract from the intrinsic simplicity of the approach.
This work has developed an alternative, patented sensing modality based upon phase modulation of the orthogonal axial modes in highly-birefringent singlemode fiber (depressed inner-cladding, elliptical-core fiber with D-shaped outer cladding, Andrew Corporation, Illinois). Interaction with external analytes occurs via the evanescent wave present at the core-cladding boundary of an etched section of fiber. Because of the difficulty in determining effective mode index under end-fire coupling, analytical treatment of modal propagation is based upon a planar waveguide model which is shown to closely approximate the etched fiber structure. Closed-form equations derived from lossless planar waveguide analysis are presented, with a view to understanding generally applicable parameters such as effective mode field diameter, field intensity distribution, sensitivity to bulk index modulation and sensitivity to adlayer formation.
A linearized, vectorial signal processing scheme is presented which increases dynamic range several hundred times over comparable interferometric devices, while maintaining the highest possible phase resolution. The technique is applicable to steady state measurands, a feature which distinguishes it from standard homodyne and heterodyne approaches. Numerical simulation and comparison to experimental testing is described, and the technique used to follow the etching process in real time. Results of exposure to polar and non-polar analytes are presented, along with detection sensitivity calculations and noise factors. The effects of surface modification, bulk index modulation and adlayer formation are evaluated and compared to theoretical predictions. Polarimetric-mode resolution, sensitivity and linearity in aqueous flow-injection experiments are determined, and techniques to confer analyte specificity and selectivity are discussed. Finally, directions for future work are suggested, along with detailed modifications of the system for improved performance.
