Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1999-04-20
2001-11-06
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06313909
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods and apparatus for detecting defects in optical lightguide fibers.
BACKGROUND OF THE INVENTION
The ability to detect defects in optical lightguide fibers is critical in providing high quality fiber and in devising manufacturing techniques that minimize the occurrence of such defects. Defects or inhomogenieties can affect the strength or transmission characteristics of the optical fiber. One class of defects, loosely defined as “bubbles” or “air lines”, can range from the sub-micron (&mgr;m) to several microns in diameter and multiple-meter lengths. Although the term “air line” is used, defects can take on many different shapes and geometries. Defects over 10 &mgr;m in diameter can cause a variety of problems, including proof test breaks in the manufacturing process and fiber splice problems in the installation process.
Techniques for detecting defects in fibers (and, incidentally, dealing with the effects of defects on fiber diameter measurements) are known. See, for example, U.S. Pat. No. 4,046,536, issued Sep. 6, 1977, to D. H. Smithgall (analysis of fringe counts in the presence of “dropouts” resulting from faults in the fiber); U.S. Pat. No. 4,501,492, issued Feb. 26, 1985, to N. Douklias (detection of fiber defects and testing of fiber diameters using a spatial filter prepared using diffracted/scattered light from a defect-free fiber); and U.S. Pat. No. 5,185,636, issued Feb. 9, 1993, to L. J. Butten, et. al. (detection of fiber defects using light scattered from a fiber diameter measurement unit and performing Fast-Fourier-Transform (FFT) to examining the spectrum).
Although these techniques can detect defects in optical fibers, they nonetheless have several significant limitations, including cost and complexity. The added cost and complexity of such methods are due, in large part, to computational requirements and expenses associated with analyzing the frequency spectrum of light scattered signals, for example, performing a FFT on the light scattered signal repeatedly.
With increased market competition and heightened customer expectations, it has become important to develop a low-cost method to detect defects in optical fiber as it is drawn in the manufacturing process and deal with such defects in the fiber accordingly.
SUMMARY OF THE INVENTION
Cost and complexity of determining defects in lightguide fiber is significantly reduced in accordance with the principles of the present invention, by removing certain components of a scattered light signal, which represents an interference pattern received from a lightguide fiber clad diameter measurement system, and analyzing the modified scattered light signal. The modified scattered light signal is analyzed to determine if additional components are present and, in particular, so called defect-related components, the presence of these defect-related components being indicative of the presence of a defect in the associated fiber. Unlike prior fiber defect detection systems, the present invention obviates the need to compute and analyze the frequency spectrum of the scattered light signal. Accordingly, unnecessary expensive signal processing of the scattered light signal, such as digitizing and frequency transforms, is eliminated.
In one illustrative embodiment, a scattered light signal is filtered and the resulting signal is compared to a defect detection threshold to determine the presence of defect-related components in the scattered light signal, which in turn represent defects in the associated fiber. Specifically, the embodiment includes a filter to remove components of the scattered light signal associated with (a) the fiber clad diameter measurement system, such as the scan repetition rate, and (b) the fundamental component associated with the fiber clad diameter; a defect sensitivity adjuster to provide a detection threshold adjustment based on a reference signal; and a comparator to compare the bandpassed signal to the detection threshold to determine if a defect-related component is present, which represent defects in the associated fiber. Once the presence of a defect-related component is determined, the comparator generates a defect detection output pulse for each such event. These pulses can then be monitored and recorded by a fiber draw control computer, thereby creating a position and length record of the defects. This information can be used to simultaneously address both (1) the identity and removal of defects following the draw operation and (2) to determine the material quality of the fiber.
REFERENCES:
patent: 4501492 (1985-02-01), Douklias
patent: 5469252 (1995-11-01), Doles et al.
patent: 5880825 (1999-03-01), Jakobsen et al.
Frazee, Jr. Ralph Edward
Smithgall David Harry
Font Frank G.
Lucent Technologies - Inc.
Nguyen Tu T.
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