Optical waveguides – With optical coupler – With alignment device
Reexamination Certificate
1999-11-02
2001-09-18
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
With alignment device
Reexamination Certificate
active
06292610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fiber optic probes for spectrophotometry. In particular, the present invention relates to a rugged, mechanically stable coupler assembly for fiber optic probes, and to methods for making the assembly and aligning a plurality of optical fibers therein.
2. Discussion of Background
Recent developments in fiber optics, coupled with the availability of multichannel array-type spectrometers and multiplexing technology, have generated renewed interest in the use of remote spectroscopic techniques for in-line monitoring and process control, environmental monitoring, and medical applications. Signal transmission via optical fibers allows the placement of sensitive equipment in locations remote from industrial process streams, making remote sensing particularly attractive in harsh environments. Multiplexing—the capability of transmitting signals from a plurality of sources to a single instrument—facilitates the efficient use of complex and expensive instrumentation. Optical analysis techniques also improve the quality of the data. Data obtained from a sample is not always truly representative of the source of that sample, since the mere act of taking the sample can alter its properties; frequently, removing a sample can perturb the source as well. Optical analysis techniques can usually be undertaken without removing samples for laboratory analysis elsewhere; therefore, data from optical analyses is frequently more reliable than data obtained by other analytical techniques.
Remote fiber optic probes are essential for in-line monitoring and process control in corrosive and radioactive process environments. In the environmental field, fiber optic probes are used for in situ measurements of fluids in wells, boreholes, storage and process tanks, and so forth. Applications include monitoring groundwater flow, studying the migration of subsurface contaminants, evaluating the progress of remediation operations, and detecting toxic or explosive substances. Fiber optic probes can be used with absorption, diffuse reflectance, and Raman spectroscopy.
The absorbance of a substance is defined as A=−log
10
T, where T=I/I
0
, I is the transmitted light intensity, and I
0
the incident light intensity. The absorption spectrum of a substance—the frequency distribution of the absorbance—is used to identify its composition; the amount of light absorbed at different frequencies depends on the concentration of each constituent. Spectrophotometry is the measurement of this absorption spectrum. A typical spectrophotometer includes these basic components: a light source, a probe containing light-transmitting and light-receiving fibers, and a detector. Light from the source is directed to the substance of interest by the transmitting fiber. The light is transmitted through the substance to the receiving fiber and the detector, which produces an output signal proportional to the absorbance of the substance over a range of frequencies. Measurements taken from a suitable reference sample are compared to measurements taken from the test sample to help determine the concentrations of various constituents in the test sample.
Absorption spectroscopy requires samples that are optically translucent or transparent in the range of frequencies being studied. Other techniques based on analysis of the light scattered by the sample, such as diffuse reflectance, fluorescence, and Raman spectroscopy, are useful for in situ analysis of solids or slurries (as used herein, the term “scattered light” includes both elastic (Rayleigh) scattering and inelastic (Raman and fluorescence) scattering). In probes designed for these types of measurements, light is directed to the sample through a transmitting fiber; scattered light is collected by the receiving fiber and returned to the detector. Probes designed for Raman spectroscopy can also be used for fluorescence. For purposes of the following discussion, the terms “Raman spectroscopy” “Raman spectrophotometry,” and “Raman measurements” include all forms of inelastic scattering phenomena as well.
Raman spectrophotometry is a sensitive analytical technique based on the inelastic scattering of light (typically, monochromatic light from a laser) by an atom or molecule. While most of the scattered light has the same frequency as the incident light (Rayleigh scattering), a portion is frequency-shifted by an amount equal to one of the resonant frequencies of the molecule. Therefore, in addition to elastically-scattered light having the same frequency as the incident light, the scattered light contains small amounts of light with different frequencies. The pattern of frequency shifts is characteristic of the constituents of the sample; the intensity depends on the concentrations of each constituent in the sample. Raman spectrophotometry provides an excellent indicator, or fingerprint, of the types of molecules present in a sample.
Vibrational and rotational Raman spectra are typically in the visible or near infrared (NIR) region, therefore, Raman spectra are less severely attenuated than infrared (IR) absorption spectra by transmission over optical fibers. Therefore, Raman spectrophotometry can be done with normal silica fiber optic cables instead of the more expensive and fragile types of fibers needed for IR absorption spectrophotometry. In addition, Raman spectrophotometry is particularly useful for identifying the constituents of a substance since Raman spectra generally contain more spectral lines—and sharper lines—than other types of spectra.
A problem encountered in Raman spectrophotometry is the small scattering cross section, that is, the very low intensity of the Raman-scattered light compared to the intensity of the incident light (also termed the “exciting light”). Like absorption spectroscopy, Raman spectrophotometry requires a light source, an optical probe with light-transmitting and light-receiving fibers (also termed exciting and collecting fibers, respectively), and a detector. In addition to Raman-scattered light, some of the exciting light and some elastically-scattered light are reflected back to the receiving fiber. Light may also be reflected to the receiving fiber by the interior surfaces of the probe. In addition, monochromatic light transmitted by an optical fiber excites the fiber molecules, causing fluorescence and Raman scattering within the fiber itself. This “self-scattering” or “silica scattering” generates a signal that interferes with the Raman signal collected from the sample of interest.
When making Raman measurements with fiber optics, it is therefore necessary to reduce the amounts of nonshifted sample-induced scattered and reflected light returning to the spectrometer, as well as reduce fluorescence and silica Raman scattering generated in the fibers themselves. To filter out this noise, light from the transmitting fiber may be directed through a narrow bandpass filter at the fiber tip that transmits the laser frequency but rejects signals arising from the fiber (known as fluorescence and silica scattering) and extraneous light from the laser source (such as plasma lines, fluorescence, or superluminance). Light returning through the receiving fibers passes through a long-pass optical filter that rejects elastically-scattered light and reflected laser light but transmits Raman signals from the sample. High-intensity laser sources and sensitive detectors with high light gathering power and high stray light rejection are needed to isolate and measure the low intensity Raman signal due to the sample. Chemometric techniques are also used to help factor out background noise and identify the signal of interest. Instrumentation for Raman spectrophotometry is costly and delicate, requires high-precision, high-maintenance optical components, and is not well suited for use in many industrial process environments.
Presently-available fiber optic Raman probes include a probe having slanted tips (McLachlan, et al., U.S. Pat. No. 4,573,761). The transmitting fiber
O'Rourke Patrick E.
Toole, Jr. William R.
Equitech Int'l Corporation
Reichmanis Maria
Ullah Akm E.
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