Optical waveguide sensors

Details Optical waveguide sensors Optical waveguide sensors

1999-08-24

2002-03-19

Ngo, Hung N. (Department: 2874)

Optical waveguides

Optical waveguide sensor

C385S127000, C385S038000

Reexamination Certificate

active

06360031

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to optical sensors.
2. Background Art
Fiber optic sensors have been used for analytical purposes for a number of years. In their usual realizations, they use either the optical and physical properties of the fiber core material or that of the lower index internally reflecting cladding to develop a response. Responses that have been used include: direct optical absorption of transmitted light by the core material; absorption produced in the reflective cladding material that attenuates the transmitted light in the core by reducing interface reflectivity; changes in the refractive indices of the cladding or core leading to changes in light transmission sensed by interferometry; or the development of fluorescence in the core material or some reagent situated at the core terminus.
In some of these realizations, strain induced by sorption of the analyte in either the cladding or the core lead to responses measured interferometrically. Fiber optic strain gauges have been constructed in which the material properties are altered mechanically.
Generally, fiber optic sensors are constructed by altering the optical core, the cladding material, or sometimes even the outer protective layer to give an optically detected response in some defined region of the optical fiber's length. Sensors that result in a modification of the core material's optical properties are the most sensitive, but are also the most difficult to manufacture. Doping the core with a sensing reagent that indicates an environmental change necessitates fabricating the light guide from doped, bulk material and providing a cladding that transmits the analyte of interest. U.S. Pat. No. 5,286,777, to Schoeler et al., entitled “Preparing a dye-containing polymer,” discusses a method for producing doped core material for incorporation into a fiber optic sensor element. Changes in the optical properties of the sensing reagent induced by the environmental analyte are then measured as changes in light transmission of the fiber.
As mentioned above, custom fabrication of the fiber optic core can be difficult and consequently drives up sensor costs. Utilizing sensing reagents incorporated in the cladding material is considerably less expensive, and works by altering the optical properties of the fiber by a process called “frustrated” or “attenuated” internal reflection. These sensing layers are directly exposed to the environment and can be applied after fiber manufacture. Smardzewski (Talanta, Vol. 35, No.2, pp. 95-101 (1988)) discusses such a sensor in which the fibers are replaced by optical waveguides externally coated with an analyte sensitive cladding. U.S. Pat. No. 5,268,972, entitled “Aromatic Hydrocarbon Optrodes for Groundwater Monitoring Applications,” to Tabacco, et al., issued Dec. 7, 1993, discusses a sensor constructed with porous cladding material whose refractive index is modified by absorption of aromatic hydrocarbons resulting in reduced transmission by attenuation of the sensing light. However, choices of sensing reagent are limited by the rigorous optical requirements for cladding materials. For most organic fibers or fused silica, cladding with either fluorocarbons or silicones is necessary to provide a lower index of refraction. These materials are poor solvents for most complex organic sensing reagents or analytes.
In addition to the aforementioned materials considerations, there are other issues involved in fiber optic sensor production. Often, special fibers (either modified core or modified cladding) must be produced for the active sensor region and then coupled to inactive fiber optic lead(s) following production. The coupling of the sensing segment to the leads often is a limiting factor in sensitivity, reproducibility or device fabrication. Typical of this type of sensor, is the “Real Time Sensor for Therapeutic Radiation Delivery”, U.S. Pat. No. 5,704,890, to Bliss, et al., issued Jan. 6, 1998. In this device, the fiber optic leads are coupled to a scintillator (or scintillator segments) and serve to collect and transmit the scintillations to the detector. A limitation of this device is the necessity to carefully adjust the refractive indices of the scintillator material to that of the fiber optic leads to ensure efficient collection of light. Guthrie et al. (Talanta, Vol. 35, No. 2, pp. 157-159 (1988)) also describe an extrinsic sensor where the sensing element is a film sensitive to the pH of the solution in which it is immersed. The fiber optic leads serve to read changes in the color of the pH sensitive film.
Device calibration may also be an issue. U.S. Pat. No. 5,307,146, entitled “Dual-Wavelength Photometer and Fiber Optic Sensor Probe,” to Porter, et al., issued Apr. 26, 1994, and U.S. Pat. No. 5,446,280, entitled “Split-Spectrum Self-Referenced Fiber Optic Sensor,” to Wang, et al., issued Aug. 29, 1995, teach the necessity to analyze the sensing light at more than a single wavelength in order to achieve long term stability and calibration of the sensor. This places additional requirements on the sensor optical properties if the sensing and reference functions are confined to a single optical waveguide core. U.S. Pat. No. 5,563,967, entitled “Fiber Optic Sensor Having a Multicore Optical Fiber and an Associated Sensing Method,” to Haake, issued Oct. 8, 1996, uses two optical waveguides in a single fiber cable to overcome this problem in a device to measure mechanical differences between the sensor and reference elements.
The aforementioned references collectively contain a variety of limitations. Some require complex measurement systems while others require features that limit noninvasiveness. Therefore, a need exists for sensors that are easily instrumented and can operate in a relatively noninvasive manner.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
The present invention is of an optical waveguide sensor comprising a waveguide core; a reflective cladding; and at least one intermediate layer positioned between the waveguide core and the reflective cladding comprising a material responsive to at least one environmental stimulus; and wherein refractive indices of the core (n
1
), the at least one intermediate layer (n
2
) and the reflective cladding (n
3
) obey the relationship n
2
≧n
1
>n
3
. The at least one intermediate layer comprises a material that produces a response to at least one environmental stimulus that is detectable by electromagnetic absorption and/or electromagnetic transmission. The environmental stimulus is, for example, chemical concentration, ultraviolet radiation and/or ionizing radiation.
The waveguide sensor of the present invention can comprise a segment of a waveguide. In most instances, a waveguide comprises a waveguide core and a reflective cladding; however, any medium in contact with the fiber core that has a refractive index according to n
3
of the above-mentioned equation will act, to some degree, as a reflective cladding. For example, when a fiber core is immersed in a medium having a refractive index that is less than that of the fiber core, a wave internal to the fiber core will have a pronounced reflective component. Of course, the present invention is not limited to use of a “fiber” and it is understood that other waveguide geometric configurations are possible, such as, but not limited to, planar waveguides. Furthermore, waveguide sensors of the present invention are useful in a variety of geometric operational configurations, such as, but not limited to, transmission and reflective configurations. In transmission operational configurations, the sensor comprises, for example, at least one segment of a waveguide. In reflective operational configurations, the sensor comprises, for example, a terminal end of a waveguide. In such a configuration, the sensor comprises a reflective cap and at least one intermediate layer positioned between the terminal end of the waveguide core and the reflective cap. As mentioned previously, for operati

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