Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
1999-04-19
2002-05-07
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S013000, C356S432000
Reexamination Certificate
active
06385380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the detection of low concentrations of dissolved materials in solvents and, particularly, to optical analysis procedures which may be performed in a continuous manner. More specifically, this invention is directed to improvements in sensors comprising hollow optical waveguides and, especially, to liquid core waveguide sensors with enhancements in the means for delivery of analysis/excitation light to the core liquid. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
It is known to analyze fluids in cuvettes formed from optically transparent material, usually quartz, which have been drawn into the form of cylindrical, thin-walled capillaries. The fluid to be analyzed is confined in the capillary, i.e., forms a liquid core. Materials dissolved in such a confined liquid core can, even at low concentrations, be detected due to their characteristic optical absorbance of analysis light, their characteristic fluorescence when excited by analysis light of the appropriate wavelength or their Raman spectra. The coupling of light into and out of a capillary employed in such prior art analysis procedures has usually been accomplished at the opposite end faces of the capillary with the analysis light being directed through the cuvette axially thereof. The solvent with dissolved compounds is customarily delivered to and exhausted from the cuvette via radially oriented liquid channels.
It is also known, in the interest of limiting light leaving a cuvette in directions transverse to the axis thereof, to employ a coating on the inner wall of the capillary. When such a coating has a refractive index which is lower than that of the commonly used solvents in the visible and ultraviolet wavelength spectra, total light reflection will occur at the liquid/coating interface. This reflection decreases light loss in the liquid core and longer optical path lengths, which increase the sensitivity of a sensor cell comprising such a liquid core waveguide, are thus possible. The coating on the inner wall of the capillary, where the core liquid is to be an aqueous solution, will preferably consist of an amorphous fluorinated polymer, such as Teflon AF 1600 or Teflon AF 2400. These coating materials respectively have refractive indices of 1.31 and 1.29 in the wavelength region of the sodium-D-line.
The ultraviolet, visible and infrared regions of the light spectrum have long been used in spectroscopy for liquid and gas analysis. Commonly used analysis methods are transmittance and, as noted above, absorbance and fluorescence. To accomplish the desired measurements employing these methods, liquid filled cuvettes are positioned in the path of analysis light generated by an appropriate source. In addition, as disclosed in U.S. Pat. No. 4,260,257, rotational symmetric liquid-flushed, flow-through cells, with small diameters to reduce the sample volume, have been proposed for liquid and gas analysis.
Fiber optics have been developed and improved in recent years, and fiber optic coupling between light sources and cuvettes and flow-through cells has been accomplished. Thus, light from a source may be efficiently coupled into a fiber optic element, consisting of either a single fiber or a fiber bundle, and transmitted thereby into or away from a cuvette. The fibers employed for such purposes preferably consist of glass or quartz, depending on the wavelength of the light to be guided. Light emitted from the end of a fiber optic element, within the angle dictated by the numerical aperture thereof, may be converted by a micro-lens into parallel light which is guided through a cuvette. After such light passes through the relatively short optical path length of a standard cuvette, typically 10 cm, the light is coupled, by means of a focusing lens, into a fiber optic element connected to a light detector. The light detector may be a wavelength-selective system, e.g., a polychromator, or a detector, e.g., a silicon photo-diode, or a photomultiplier tube which may be provided with an analyte-adapted narrow or broad-band optical filter.
In the interest of signal enhancement, quartz and glass tubes with small outer diameters and thin walls, so-called capillaries, have been used as flow-through cells. The choice of quartz and glass tubes, in part, has been based upon the fact that such tubes are chemically inert against many liquids and solvents. These capillaries can be provided with an internal, and possibly also an external, metal coating in an environment where there would be negligible corrosion. However, metal-coated capillaries have the disadvantage of high light loss. Thus, the optical path length is relatively small for a pre-defined signal-to-noise ratio.
As noted above, by providing an optically transparent capillary with a coating of a suitable amorphous fluorinated polymer, an efficient liquid core optical waveguide may be produced. On this point, the following references may be taken into consideration: “Optical Characteristics of Teflon AF Fluoroplastic Materials”, by J. H. Lowry et al, Optical Engineering, Volume 31, page 1982 (1992); U.S. Pat. No. 5,184,192; “Raising the Sensitivity Benchmark in Diode Array Detection with Optical Improvements”, by P. DeLand, Internat. Laboratory 12C-12H (July 1998); “A Cylindrical Liquid Core Waveguide”, by P. Dress et al, Appl. Phys. B63, page 12 (1996) and U.S. Pat. No. 5,570,447. By insuring that the liquid core has a higher refractive index than a coaxial layer of material, as discussed above, coupled light is mainly guided in the liquid core because of the total reflection which occurs at, for example, the liquid/coating interface. In other words, optical losses resulting from transverse emission through the wall of the capillary are substantially eliminated through the use of low refractive index amorphous fluorinated polymers. Liquid core waveguides of the type described, wherein the low refractive index polymer forms either an interior or an exterior coating on the capillary, or forms the capillary itself, can be used as absorption sensors with high resolution. In such absorption sensors, light is axially coupled into and out of the capillary at the end face(s). Such light coupling has typically been accomplished by utilizing some type of focusing device between the light source and the capillary.
The problems concerned with transmission of light at wavelengths below 250 nm has been discussed in the article “UV-stabilized silica based fiber for applications around 200 nm wavelength”, by K.-F. Klein et al., Sensors and Actuators B, Vol. 39-123, 305-309 (1997). This article suggests that optical fibers capable of stable transmission of light with wavelengths below 250 nm would allow the field of fiber optic applications to be significantly expanded. For example, field-usable sensors for water pollution by detection of nitrate, nitrite and residual chlorine, which have strong absorption bands below 250 nm, have long been needed.
The principals of capillary-like coated liquid core waveguides have been surveyed in the articled entitled “Capillary Waveguide Sensors”, by O. S. Wolfbeis, Anal. Chem, Vol. 15, page 225 (1996). Side illumination of optical waveguides has also been previously suggested. The sensoric properties of such a side illuminated light guide are based on chemical or physical changes in its specific inner coating by the influence of the detectible substance. These changes include the fluorescence excitation of the inner coating by detectible substances. In such case, advantage is taken of the fact that the magnitude and spectral composition of a fluorescence signal changes with the detectible substances diffusing in and out of the coating. In such a sensor, the generated fluorescent light would be detected at the end of a short capillary.
A disadvantage of the conventional technique of axial coupling of light into a liquid core optical waveguide resides in the
Belz Mathias
Dress Peter
Friedrich Karl
Alix Yale & Ristas, LLP
Cherry Euncha
Spyrou Cassandra
World Precision Instruments, Inc.
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