Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1998-12-04
2001-10-02
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000, C385S123000, C385S024000
Reexamination Certificate
active
06298184
ABSTRACT:
TECHNICAL FIELD
This invention relates to fiber Bragg gratings, and more particularly to forming a tube-encased Bragg grating.
BACKGROUND ART
It is known in the art of fiber optics that Bragg gratings (i.e., periodic or aperiodic variations in the refractive index of the optical fiber) exhibit a predetermined wavelength reflection profile. As is known, a fiber Bragg grating is the result of a photo-refractive effect. In particular, when the core of a photosensitive (e.g., germania-doped) optical fiber is exposed to ultra-violet radiation in a predetermined wavelength range, a permanent change in the refractive index is produced. The magnitude of the refractive index change is related to the intensity of the incident radiation and the time of exposure.
As is also known, a Bragg grating is impressed (or embedded or written or imprinted) into the core of an optical fiber by allowing two coherent nominally plane optical waves to interfere within the fiber core at a location along the fiber where the grating is desired. The resulting interference pattern is a standing wave which exists along the longitudinal axis of the fiber having an intensity variation which causes a corresponding spatially periodic or aperiodic variation in refractive index along the longitudinal axis of the fiber. For periodic variations, the grating has a peak reflection wavelength related to twice the spatial period (or grating spacing). The spatial periodicity or other spatial variations of the refractive index of the fiber, and the resultant reflectivity profile, are a function of the wavelength, amplitude, and/or phase of the two incident writing beams and/or their angle of intersection within the fiber.
The above described techniques are described in U.S. Pat. Nos. 4,807,950 and 4,725,110, entitled “Method for Impressing Gratings Within Fiber Optics”, both to Glenn et al and U.S. Pat. No. 5,388,173, entitled “Method and Apparatus for Forming Aperiodic Gratings in Optical Fibers”, to Glenn, which are hereby incorporated by reference.
It is also known that Bragg gratings embedded in the fiber may be used for parameter sensing or for wavelength filtering or tuning applications. Furthermore, a tube-encased grating (i.e., a grating in an optical fiber, where the grating and fiber are encased within and fused to at least a portion of a glass tube), may be used in numerous applications, such as is discussed in commonly-owned, co-pending U.S. patent applications, Ser. No. 09/399,404 (now abandoned), entitled “Tube-Encased Fiber Grating Pressure Sensor”, Ser. No. 09/400,364 (now abandoned), entitled “Tube-Encased Fiber Grating Temperature Sensor”, and Ser. No. 09/205,846 (now abandoned), entitled “Tube-Encased Compression-Tuned Fiber Grating”, all filed contemporaneously herewith. In those cases, the fiber is encased in the tube by heating the tube and heating and fusing the tube to the fiber, as is described in commonly-owned copendng U.S. patent applications Ser. No. 09/399,495 (now abandoned), entitled “Tube-Encased Fiber Grating”. When such a tube-encased grating is used, it may be desirable to write the grating into the optical fiber after it is encased in the tube, for ease of manufacturability, or to avoid having the grating affected by the heating and fusing process, or for other reasons.
SUMMARY OF THE INVENTION
Objects of the present invention include provision of a method and apparatus for writing a Bragg grating into an optical waveguide encased in a tube.
According to the present invention, an apparatus for impressing a grating in an tube-encased optical waveguide, comprises a photosensitive optical waveguide; a tube, having the optical waveguide encased therein along a longitudinal axis of the tube, the tube being fused to at least a portion of the waveguide; an interface medium, disposed adjacent to the tube; an intermediate material disposed between the medium and the tube; means for providing a pair of writing beams having a predetermined wavelength capable of writing a Bragg grating in the waveguide; the writing beams being incident on a surface of and passing through a portion of the interface medium, the intermediate material, and the tube, and the beams intersecting and interfering at a predetermined region of the waveguide; and the surface of the interface medium being optically flat.
According further to the present invention, the intermediate material is substantially transparent to the wavelength of the writing beams. According further to the present invention, the intermediate material has a refractive index that substantially matches the refractive index of the tube and the interface medium. According further to the present invention, the intermediate material comprises an oil. According further to the present invention, a beam width of the writing beams is no greater than a dimension of the intermediate material.
According to a second aspect of the present invention, a method for making a tube-encased Bragg grating, comprises the steps of obtaining a tube, having a photosensitive optical waveguide encased therein along a longitudinal axis of the tube, the tube being fused to at least a portion of the waveguide; placing an interface medium adjacent to the tube; placing an intermediate material between the medium and the tube; simultaneously exposing the waveguide, the tube, the intermediate material and the interface medium to two writing beams having a predetermined wavelength, the beams intersecting and interfering at a predetermined region of the waveguide so as to write a Bragg grating in the waveguide, the writing beams passing through the interface medium the intermediate material and the tube; and the surface of the interface medium having being optically flat.
According further to the second aspect of the present invention, the intermediate material is substantially transparent to the wavelength of the writing beams. According further to the second aspect of the present invention, the intermediate material has a refractive index that substantially matches the refractive index of the tube and the interface medium. According further to the second aspect of the present invention, a beam width of the writing beams is no greater than a dimension of the intermediate material.
The present invention provides a method and apparatus for writing a Bragg grating through a tube into an optical fiber which is encased in and fused to at least a portion of a capillary tube. The tube may be made of a glass material for encasing a glass fiber. Also, the tube may be fused to the grating area and/or on opposite axial sides of the grating area. The invention allows the grating to be written into the fiber through the tube after the tube is heated and fused to the fiber for ease of manufacturability, or to avoid having the grating affected by the heating and fusing process, or for other reasons. If desired, the tube-encased fiber may be hydrogen loaded prior to writing the grating to provide higher reflectivity and/or faster writing time.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.
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patent: 6075625 (2
Bailey Timothy J.
Davis Michael A.
Fernald Mark R.
Miller Matthew B.
Putnam Martin A.
CiDRA Corporation
Font Frank G.
Punnoose Roy M.
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