Optical waveguides – With disengagable mechanical connector – Structure surrounding optical fiber-to-fiber connection
Reexamination Certificate
2000-08-03
2002-07-09
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With disengagable mechanical connector
Structure surrounding optical fiber-to-fiber connection
C385S071000
Reexamination Certificate
active
06416234
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to optical fibers, and more particularly to optical fiber probes that use manipulated delivery and reception regions to improve sensitivity to specific light-matter interactions.
BACKGROUND OF THE INVENTION
In recent years, the use of optical fibers has become increasingly widespread in a variety of applications. Optical fiber probes have been found to be especially useful for analyzing materials by employing various types of light-scattering spectroscopy.
Optical fibers offer numerous advantages over other types of source/detection equipment. In short, the fiber provides a light conduit so that the source-generating hardware and the recording apparatus are stationed independently of the subject under investigation and the point of analysis. Thus, analyses are conducted remotely in otherwise inaccessible locations. Previously unattainable information is acquired in situ, often in real time. This capability is sought in numerous industrial, environmental, and biomedical applications. The laboratory is moved on line in the industrial realm, to the field in the environmental sector, and in vivo in the biotechnical arena. Additionally, hardware and measurements are more robust, quicker, less intrusive, more rugged, less costly, and many other advantages are realized.
LIGHT SCATTERING SPECTROSCOPY
While transmission spectroscopy analyzes light passing through a substance, light-scattering spectroscopy entails illumination of a measurand and analyzing light that is scattered at angles relative to the incident source. The photon-matter interactions of the scattering events may be either elastic or inelastic. In an inelastic event, a photon's energy (wavelength) changes as a result of the light-matter interaction. In an elastic event, a photon's energy (wavelength) does not change. Absorption, the phenomena in which a fraction of photons are entirely absorbed, also plays a role in light-scattering spectroscopies. Raman, diffuse, reflectance, and fluorescence spectroscopies are of particular interest as they relate to vibrational and nonvibrational photonic responses of a material.
The Raman effect describes a subtle light-matter interaction. Minute fractions of light illuminating a substance are Raman-scattered in random directions. Raman-scattered light is color shifted from the incident beam (usually a laser). The color (frequency) shifts are highly specific as they relate to molecular bond vibrations inducing molecular polarizability changes. Raman spectroscopy is a powerful technique for chemical analysis and monitoring. The resulting low light levels require sophisticated, expensive instrumentation and technical complexity. Suitable technology and products for on-line analysis of processes and environmental contaminants are just becoming available.
Specular reflectance relates to a surface's mirror-like aspects. Diffuse reflectance relates to light that is elastically scattered from a surface of material at diffuse angles relative to the incident team. For example, a projector screen diffusely reflects light while a glossy, new waxed car has a high specular component. Diffuse reflectance spectroscopy is important for chemical analysis as well as measuring visual perception. Among other things, it is based on particulate-scattering and absorption events.
Fluorescence relates to substances which absorb light at one wavelength then re-emit it at a longer wavelength as a result of electronic transitions. As an example, a “highlighter” felt-tip marker appears to “glow” green as it absorbs blue and ultraviolet light then emits it as green. Fluorescence provides a powerful technique for chemical monitoring.
Raman spectroscopy is a well-established laboratory technique and is generally recognized as having enormous potential for on-line monitoring and sensing. With the advent of stable lasers, cheap computing power, efficient detectors, and other new technological advancements, Raman spectroscopy is primed for widespread industrial monitoring deployment. In addition to process control monitoring, it will be utilized in specialized monitoring and sensing devices ranging from neuroimaging to environmental monitoring, to in vitro and in vivo medical testing.
Raman spectroscopy involves energizing a sample with a high-power, narrow-wavelength energy source, such as a laser. The laser photons induce low intensity light emissions as wavelengths shift from the laser's. The Raman effect is an elastic scattering of photons The emitted Raman light is collected and analyzed with a specialized instrument.
The spectral positions (colors) of the shifts provide fingerprints of the chemicals in the sample. Thus, Raman spectroscopy provides a means for chemical identification. The intensity of the shift (the spectral peak height) correlates to chemical concentration. Thus, a properly calibrated instrument provides chemical content and concentration. In practicality, Raman spectroscopy is technically complex and requires sophisticated, expensive instrumentation.
Raman spectroscopy is well suited to aqueous-based media without sample preparation. From this standpoint, it is an ideal tool for process control medical testing and environmental applications. Thus, Raman spectroscopy has great potential for real-time monitoring and is being vigorously pursued.
The basic concept for a probe-based, on-line Raman instrument is simple. Laser light is directed down an optical fiber to a remote probe. The laser light exits the fiber and illuminates the sample medium. Another fiber picks up the Raman-emitted light and returns it to the instrument for analysis.
In practicality, the engineering challenges for a robust physical probe implementation are substantial. In addition to the optical performance expected by laboratory instruments, a probe must be hardened to withstand extreme physical and chemical conditions. Optical characteristics must also remain constant as dynamic conditions change.
Optical aspects of probe engineering require particular design finesse. The Raman effect involves very weak signals. Raman emissions may be one trillionth as intense as the exciting radiation. Subsequently, the probe must be incredibly efficient in collecting and transmitting Raman-emitted light. And, the signal must not be corrupted by extraneous influences. As an example of the sensitivity, Raman instruments typically feature cosmic ray filters. The mechanisms identify and discard measurement data samples influenced by passage of a single cosmic ray photon through the detector.
A phenomenon known as the silica-Raman effect has proven especially troublesome for those engaged in remote Raman spectroscopy. As laser light is transmitted over optical fibers, a subtle light-matter interaction inherently occurs. The laser light and the silica in the glass fiber interact generating “silica-Raman” light. The extraneous silica-Raman light becomes waveguided in the fiber and hopelessly mixed with the laser light. The purity of the laser light is corrupted. Fiber fluorescence causes similar problems.
Remote Raman spectroscopy employs optical fiber between the base instrument and the remote probe or process interface. Optical fiber delivers laser light from its source to the probe. Separate fiber returns sensed light from the probe to an instrument for analysis. In both delivery and return, undesirable silica-Raman light travels in the fibers concurrently with desirable laser and sensor light. A major obstacle in fiber-optic-based Raman spectroscopy has been in separating the desirable light from the undesirable silica-Raman light.
FLAT FACE, PARALLEL FIBER PROBES
Standard optical fibers deliver and receive light within narrow angular confines. Consider a “probe” that is formed by mounting two standard, flat-face fibers (i.e., a source fiber and a collection fiber) in parallel. The functionality, operation, and limitations of this probe will be analyzed to present relevant technical requirements. The technical discussion addresses, among other things, issues of optical ef
Marple Eric Todd
Wach Michael Leonard
Cirrex Corp.
King & Spalding
Palmer Phan T. H.
LandOfFree
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