Optical waveguides – Optical waveguide sensor
Reexamination Certificate
1999-06-21
2001-05-15
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S010000, C385S037000, C356S451000, C356S478000
Reexamination Certificate
active
06233373
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical spectrometry, and more particularly to optical spectrometers for use with distributed arrays of fiber optic Bragg gratings (FBGs), for use in such applications as strain sensing, temperature sensing, and other sensing systems for which distributed FBG arrays may be used.
2. Description of the Related Art
Optical spectrometers analyze light signals by measuring the intensity of the light over a range of wavelengths. Optical spectrometers are often used in conjunction with FBG arrays to detect the various light wavelengths that are reflected by the individual gratings in an FBG array.
An FBG array is a length of optical fiber with reflective gratings written into the fiber at intervals. Typically, the gratings in an array will have various peak reflectance wavelengths (the wavelength at which the grating reflects the maximum amount of light back to the source). The peak reflectance of a grating will change, however, as the grating stretches or contracts, typically due to thermal expansion or mechanical strain. By monitoring these changes, one can monitor how the environment is affecting the FBGs, and thereby deduce such things as temperature or strain at the FBG. The wavelengths of light reflected from an FBG array is therefore an optical signal to be analyzed. Typical optical spectrometers for use in conjunction with FBG arrays in these applications are slow and optically inefficient, requiring strong light signals, and have only moderate wavelength resolution.
Two types of optical spectrometers are generally available: serial spectrometers and parallel spectrometers.
Serial spectrometers scan sequentially through a range of wavelengths. They are inherently slower, and at risk for time-dependent errors, but may be more compact than parallel spectrometers.
However, they are inherently inefficient, because only a tiny portion of the employed wavelength range is collected at any time.
Parallel spectrometers have the advantages of optical collection efficiency (in that a large wavelength band is analyzed at any given time) and speed (i.e., data sampling rate), in that they can analyze a broad range of optical wavelengths simultaneously. A typical spectrometer is shown in U.S. Pat. No. 5,719,672 to Chien et al.
There are several drawbacks to applying conventional parallel spectrometers for use with FBGs. One of the disadvantages is the tradeoff between size and resolution. Another is the relatively poor optical-to-electrical efficiency (electrical signal/photon of input light), dynamic range, and speed that most of them achieve.
Previously, the data from parallel spectrometers has been analyzed by using a simple weighted average, or a simple variation of a simple weighted average, to identify spectral peak positions. This can result in appreciable errors in the presence of noisy data. A better method, preferably one permitting resolution down to {fraction (1/100)}th to {fraction (1/1000)}th of a pixel, and less sensitive to noise, is desired.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide an optical spectrometer with an improved geometry, to allow for a more compact configuration.
It is a further object of this invention to provide a fast, sensitive, and optically efficient spectrometer, with sub-pixel resolution and high dynamic range.
These and additional objects of the invention are accomplished by the structures and processes hereinafter described.
An aspect of the present invention is an optical spectrometer for detecting a spectral signature of an input optical signal, including: (a) a highly corrected lens, for collimating the input optical signal and for focusing dispersed light from the grating onto the detector array; (b) an input optical system, for directing the input optical signal to the collimating lens, in the focal plane and near the optical axis of the lens; (c) a diffraction grating, for dispersing the collimated input optical signal, positioned in the path of collimated light from the collimating lens, oriented to diffract a majority of the dispersed light back through the collimating lens; and (d) a detector, in the focal plane of the collimating lens, and preferably centered on its optical axis.
Another aspect of the invention is a fiber-based sensing system using the optical spectrometer of the invention. Such a system will include (a) an optical fiber having an array of fiber Bragg gratings, where each of the gratings is reflective over a narrow wavelength band, typically less than {fraction (1/10,000)} of the center wavelength, more typically on the order of about {fraction (1/16,000)} of the center wavelength; (b) a light source, for radiating light that encompasses each of the selected wavelengths, optically coupled to the optical fiber; and (c) the optical spectrometer of the invention, optically coupled to the optical fiber, for detecting a spectral signature of reflected light from the fiber Bragg gratings. The FBG-based sensing system may be used to measure strain, temperature, or any other measurand for FBG-based sensing systems. Selection of individual gratings in the array may be achieved by time-domain selection, frequency-domain selection, or combinations of the two. Typically, the spectrometer output will be fed to a data capturing system, and subsequently to a data analysis system, which may combined in a single computer system for control, data capture, and analysis.
Another aspect of the invention is a method for analyzing stage signal data, such as data from a spectrometer, to achieve sub-pixel resolution, by computing a compounded centroid for this data.
Another aspect of the invention is a time division multiplexed method of operating the fiber-based sensing system of the invention.
REFERENCES:
patent: 5680489 (1997-10-01), Kersey
patent: 5719672 (1998-02-01), Chien
patent: 5784507 (1998-07-01), Holm-Kennedy et al.
patent: 5801831 (1998-09-01), Sargoytchev
patent: 5838437 (1998-11-01), Miller et al.
patent: 5987197 (1999-11-01), Kersey
patent: 6061129 (2000-05-01), Ershov et al.
patent: 6097487 (2000-08-01), Kringlebotn et al.
patent: 6118530 (2000-09-01), Bouevitch et al.
Zhou et al., “High pressure fiber optic light scattering spectrometer”,Rev. Sci. Instr.69 (5) 1955-60 (May 1998).
Davis et al., “Application of a Fiber Fourier Transform Spectrometer to the Detection of Wavelength-encoded Signals from Bragg Grating Sensors”,J. Lightwave Tech. 13 (7) 1289-95.
Kersey et al., “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter”,Optics Lett.18 (16) 1370-72 (Aug. 15, 1993).
Askins Charles G.
Putnam Martin A.
Doan Jennifer
Karasek John J.
Marciniszyn Jane B.
Sanghavi Hemang
The United States of America as represented by the Secretary of
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