The present invention relates to a fiberoptic sensor system for biomedical and other uses. More particularly, the present invention relates to a fiberoptic sensing system which utilizes a comparison of phase of one light beam relative to another, with one of the beams being sensitive to an analyte, and the other beam being used as a reference. The phase comparison is accomplished by an interference technique.
In conventional fiberoptic sensing systems, light at one or more wave lengths is transmitted from an optoelectronic base unit to a distal sensing element through a fiberoptic light guide. The distal sensing element is situated within an environment to be analyzed. The sensing element modulates incoming light in known proportion to the presence of one or more analytes that are situated within the environment. The modulated light is then returned to the base unit through the same input fiberoptic fiber, or through one or more alternative fibers. The level of the analyte or analytes may be inferred by quantitative assessment of the optical intensity returned from the sensor to the base unit with respect to the optical intensity transmitted to the sensor. Systems employing quantitative optical intensity assessment are herein referred to as intensity modulated sensing systems.
In one prior art intensity modulated sensing system, the sensing element consists of a reactive chemistry that is disposed substantially entirely within a hole formed inside a distal end of a multimode optical fiber itself. Light at one or more wavelengths is transmitted from the proximal end of the fiber to the perforated distal end where the light interacts with the reactive chemistry which can be composed of a fluorescent or optically attenuative material. The magnitude of the fluorescent emission, or the extent of the optical attenuation by the attenuative material, is generally proportional to the concentration of a particular analyte that is in contact with the distal end of the fiber. The fluorescent intensity, or the degree of optical attenuation, can be measured by instruments located at the proximal end of the fiber.
One problem associated with conventional intensity modulated sensing systems of this type is that the modulation of the light can be affected by factors other than the concentration of the analyte. If the multimode optical fiber is flexed during transduction, some of the light transmitted to the sensor may be lost by conversion of guided modes within the fiber to radiation modes. Additionally, some of the light returning from the sensor may be lost by the same mechanism. This loss of light may be incorrectly interpreted as a change in analyte concentration because a correlation is being made between the level of the analyte and the degree of light modulation. In fact, all factors affecting the absolute magnitude of the optical energy travelling within the fiber may be incorrectly interpreted by the system as changes in analyte concentration. These factors include, but are not limited to, variations in illumination intensity, changes in transmission at fiber connection points, and intrinsic changes within the sensor such as optical bleaching of the illuminated material.
Another problem associated with some intensity modulated sensing systems is the very low overall operating efficiency, often less than 10.sup.-10. This extremely low operating efficiency puts severe constraints on the systems. In order to maintain an adequate signal-to-noise ratio under these adverse conditions, high intensity illumination (e.g. laser or arc lamp) and high efficiency detectors (e.g. photomultiplier tubes) must be used. Complex and expensive wavelength selection devices are often required in order to optimally match the illumination to the sensing material. Because of the spectral energy and efficiency characteristics required of source and detector, attempts to convert conventional intensity modulated sensing system components to solid state optical devices have not been particularly successful in the biomedical sensing arena.
Some of the problems associated with some intensity modulated sensing systems can be overcome by employing a single-mode polarization-preserving fiberoptic waveguide to form an interferometric sensor system such as is shown in U.S. Pat. No. 4,697,987. In such a system, light from a coherent source such as a laser is directed through a beam splitter which sends half of the light into a reference fiber and the other half into a sensor fiber. The sensor fiber is coupled to the environment sought to be measured so that the phase of the light is modulated by an environmental signal. The light in both fibers is then recombined by a second beam splitter and fed to a photodetector responsive only to the amplitude of the combined signal. The system is adjusted to generate a sharp null with any change in environmental condition being reflected in a non-null signal. While such a system enjoys significantly enhanced sensitivity to environmental signals over intensity modulated sensing systems, the system as a whole exhibits other problems due to the Mach-Zehnder configuration of the system such as the need for optical detectors situated at the distal ends of the fibers, and the need for very complex signal processing algorithms to provide the desired quantification of the non-null signals.
It is therefore one object of the present invention to provide a fiberoptic sensing system which does not rely on intensity modulation to determine the concentration of one or more analytes.
Another object of the present invention is to provide a fiberoptic sensing system which is insensitive to environmental and other noise sources.
Yet another object of the present invention is to provide a fiberoptic sensing system that has an efficiency level substantially greater than conventional fiberoptic sensing systems.
Still another object of the present invention is to provide a fiberoptic sensing system that employs very simple signal processing algorithms to quantify the measured changes.