Optical waveguides – Optical fiber bundle
Reexamination Certificate
1999-07-02
2003-05-13
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber bundle
C385S012000, C385S031000, C250S227110, C250S227140
Reexamination Certificate
active
06563992
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Present Invention
The present invention relates generally to diffuse reflectance spectrometry and, more particularly, to diffuse reflectance probes of the type that uses a fiber-optic bundle for transmitting radiation to and then gathering radiation reflected from a diffusely reflecting substance.
2. Description of the Related Art
Diffuse reflectance spectroscopy (“DRS”) has been used for many years to analyze a wide variety of materials. The sample interfacing techniques used have fallen into three broad categories depending on whether the optical spectrum used is in the visible region, the mid infrared region, or the near infrared region.
In the visible region of the spectrum, DRS devices have historically used the integrating sphere technique. Integrating spheres are attractive because they can collect virtually all of the radiation reflected from a surface, independent of direction. They are quite practical in the visible region due to the availability of large area highly sensitive optical detectors and extremely reflective diffuse white coatings for use on the inside of the spheres.
In the mid infrared region (“mid-IR”) of the spectrum (fingerprint), integrating spheres have found only occasional use because the reasonably fast mid-IR detectors that are available are relatively small and insensitive and because the diffusely reflective coatings that are available for use in this region do not have the desired reflectance. Instead, most mid-IR DRS sampling systems have used specularly reflecting optical elements such as cassagrains, off-axis paraboloids, or ellipsoids to image the illuminated target directly on a small detector with a minimum number of reflections.
In the near infrared (NIR) region, the history of DRS is quite distinct from both visible and mid-IR DRS. NIR region devices have, in fact, followed a completely different development path. NIR instruments were first developed in the 1970's to inspect grains and other agricultural products. The operation of these early NIR devices was based on switching between relatively small number of fixed frequency optical filters so as to acquire what amounts to a very low resolution spectrum. As a result of the low resolution, the response speed required of the IR detectors was quite modest, allowing large detectors to be used. A-typical NIR grain analyzer would thus have a modulated beam of radiation illuminating a small cup of ground up grain. The region above the cup would be occupied by hemispherical array of large area detectors all hooked together so as to collect the maximum possible amount of scattered radiation. This approach is somewhat similar to the methods used to study visible diffuse reflectance except that an array of detectors is used rather than a single detector and an integrating sphere.
Since the development of these early instruments, NIR systems have become more sophisticated, with dispersive spectrometers arriving first, followed by fourier transform infrared (“FTIR”) spectrometers being applied in the NIR region of the spectrum.
NIR is used in many fields due to the availability of near-IR transmitting fiber optics for coupling a spectrometer to a remote measurement location. However, sampling optics for use with these newer systems have tended to develop out of the “brute force” tradition started with the earlier NIR region grain analyzers. The most common approach to diffuse reflectance analysis in the NIR region uses a large bundle of fibers that may be one inch (1″) in diameter, or more. In some cases, a few of the fibers are used to illuminate the sample with spectrally modulated radiation while the rest of the fibers are used to collect the radiation scattered over a wide range of angles, the collected radiation being conveyed to a single large detector or an array of detectors. Others have conversely illuminated the sample with a large bundle of fibers which obtain their radiation from a large IR source and then used a smaller bundle or even a single fiber to collect a sample of the reflected radiation and route it to the spectral modulator and a small area detector.
The use of a large fiber bundle has a number of drawbacks. Among these are the high cost of large fiber bundles for use at wavelengths much longer than 1 &mgr;m, the limited areas of fast response detectors for use at longer wavelengths, and the fact the throughput of FTIR spectrometers does not allow the use of large bundles. The latter drawback is especially unfortunate because FTIR instruments are becoming increasingly popular for use in the NIR region due to their high frequency stability.
Even when small bundles or single fibers are used for both sample illumination and reception, most workers in the field have continued to use brute force, simply pointing the fibers at the sample. The most successful device of this nature, as shown in
FIG. 1
a
and
1
b
, uses a bifurcated fiber bundle
90
and terminates that bundle directly next to the target
50
without any other optics. The bifurcated fiber-bundle
90
has a number of individual transmitting or illumination fibers
91
and a number of individual receiving or detector fibers,
91
,
92
(eg. eighty fibers total) that are randomly distributed within a tip bundle
95
that splits into two smaller leg bundles (eg. forty fibers each)
93
,
94
. One leg bundle
93
carries radiation from a radiation source
70
to the target
50
while the other leg bundle
94
carries radiation reflected from the target to a detector
80
.
In an earlier patent application (Ser. No. 08/784,823), I disclose a diffuse reflectance probe featuring low stray light, detachability from a fiber-optic bundle, and the ability to analyze samples either in contact with the probe or displaced from it. Since filing that application, I have become familiar with a need for diffuse reflectance probes suitable for the analysis of molten polymers at high temperatures and pressures and have developed a new probes for such use based on some of the principles disclosed in my earlier application. However, it has also become apparent that the applicability of diffuse reflectance to hot polymer melt analysis would be much more useful if such a probe could be provided in a structure compatible with certain types of fittings available on many polymer extruders. The external threads that engage two such fittings are specified as or “½-20 N.F. THD” and “M18×1.5 THD”, being only about ½″ and ⅝″ in diameter, respectively. These fittings mandate a very limited diameter in which to house the probe optics. While it is possible that the earlier design could be adapted to such a small diameter situation, it is likely that this would result in a significant reduction in usable signal level. I have thus developed a new and innovative design which is more inherently compatible with the small available diameter.
SUMMARY OF INVENTION
The invention may be regarded as a diffuse reflectance probe for connection to a illumination fiber for carrying radiation from a remote source and illuminating a target and for connection to a return fiber for carrying target-modified radiation that is diffusely reflected from the target back to a remote detector for diffuse reflectance analysis, the diffuse reflectance probe comprising: a solid light guide having a fiber end with a fiber-side refractive surface, a target end with a target-side refractive surface, and an optical axis extending between the fiber end and the target end; and a fiber coupling structure optically interfacing the fiber-side refractive surface to the illumination fiber and to the detector fiber; the target-side refractive surface being oriented relative to the normal of the optical axis to minimize stray radiation that comes from the remote fiber and does not reach the target, but rather is reflected back toward the detector fiber via the solid light guide and the fiber coupling structure, from entering the detector fiber and overpowering the target-modified radiation.
In a mo
Andras Joseph C.
Axiom Analytical, Inc.
Healy Brian
Myers Dawes & Andras
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