Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1998-12-04
2001-06-19
Font, Frank G. (Department: 7877)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06249624
ABSTRACT:
TECHNICAL FIELD
This invention relates to fiber Bragg gratings, and more particularly to forming a Bragg grating with high-intensity light.
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.
For some applications, it is desirable to write a grating with a high intensity interference pattern. For example, a standard grating or “Type 1” grating, is typically written with either a standard intensity (about 50-500 mjoules/cm
2
) or high intensity (about 500-800 mjoules/cm
2
) light. For a Type 1 grating, the higher the intensity of the light, the faster the grating is written and, in general, the higher the resultant reflectivity. Also, a high temperature grating or “Type 2” grating is written with very high light intensities (about 0.9-1.5 joules/cm
2
or higher), such as is discussed in J. L. Archambault et al., “100% Reflectivity Bragg Reflectors produced in Optical Fibers by Single Excimer Laser Pulses”, Electronics Letters 29(1993) pp 453-455. However, with a high intensity light (e.g., greater than about 800 mjoules/cm
2
for average surface quality glass), the electric field at the air-to-glass interface at the surface of the fiber would be so high that it would cause ablations (i.e., melting, evaporation, or particle discharge) at the surface of the fiber which cause damage to the outer surface of the fiber and reduce the tensile strength of the fiber. The maximum intensity depends on the glass surface quality, e.g., the amount of contamination, such as dust, dirt, oils, etc. Thus, for poor surface quality glass the maximum intensity may be lower than 800 mjoules/cm
2
(e.g., 500 mjoules/cm
2
or lower). The cleaner the glass, the higher the intensity that the glass can withstand before surface damage occurs. Thus, it would be desirable to be able to write gratings with such high intensity light without damaging the fiber outer surface.
SUMMARY OF THE INVENTION
Objects of the present invention include provision of a method and apparatus for writing a Bragg grating into an optical fiber with a high intensity light without damaging the surface of the fiber.
According to the present invention, an apparatus for impressing a grating in an optical waveguide, comprises a photosensitive optical waveguide; an interface medium, disposed adjacent to the waveguide; means for providing a pair of focussed 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, and the beams intersecting and interfering at a predetermined region of the waveguide; the surface of the interface medium being optically flat; and the writing beams having an intensity at the fiber of greater than a predetermined high intensity and the medium having a predetermined thickness such that the intensity sufficient to cause surface damage to said waveguide in the absence of said medium of the writing beams at the surface of the medium is less than a predetermined surface damage intensity such that surface damage does not occur on the medium and on the fiber when the writing beams are writing the grating.
According further to the present invention, an intermediate material is disposed between the medium and the waveguide. According to the present invention, the intermediate material is substantially transparent to the wavelength of the writing beams. According to the present invention, the intermediate material has a refractive index that substantially matches the refractive index of the waveguide and the interface medium. According to the present invention, the intermediate material comprises an oil. According further to the present invention, each of the writing beams is focussed to a line near the waveguide. According further to the present invention, the waveguide comprises an optical fiber.
The present invention provides a method and apparatus for writing a Bragg grating using a high intensity interference pattern in the fiber. In particular, the invention eliminates ablations that typically occur on the surface of the optical fiber when high intensity beams interfere to form a high intensity interference pattern. The invention uses a UV transparent interface medium (e.g., a glass block with a hole or a slit in it, or collar) adjacent to and contiguous with the outer surface of the fiber, which positions the air-to-glass interface at a predetermined distance from the focal point (or line) of the writing beams such that the intensity of the writing beams at the air-to-glass interface is low enough to not cause damage to the surface of the glass. A substantially index-matching transparent oil or gel may be used between the fiber and the medium to insure no air-to-glass interface exists at the surface of the fiber. If desired, the 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.
REFERENCES:
patent: 4636031 (1987-01-01), Schmadel, Jr. et al.
patent: 4725110 (1988-02-01), Glenn et al.
patent: 4915467 (1990-04-01), Berkey
patent: 5007705 (1991-04-01), Morey et al.
patent: 5042898 (1991-08-01), Morey et al.
patent: 5235659 (1993-08-01), Atkins et al.
patent: 5388173 (1995-02-01), Glenn
patent: 5399854 (1995-03-01), Dunphy et al.
patent: 5469520 (1995-11-01), Morey et al.
patent: 5512078 (1996-04-01), Griffin
patent: 5574810 (1996-11-01), Byron et al.
patent: 5578106 (1996-11-01), Fleming, Jr. et al.
patent: 5680412 (1997-10-01), DeMaria et al.
patent: 5682453 (1997-10-01), Daniel et al.
patent: 5684297 (1997-11-01), Cable
patent: 5691999 (1997-11-01), Ball et al.
patent: 5719974 (1998-02-01), Kashyap
patent: 5745626 (1998-04-01), Duck et al.
patent: 5841131 (1998-11-01), Schroeder et al.
patent: 5953472 (1999-09-01), Boschis et al.
patent: 6005877 (1999-12-01), Gaeta et al.
patent: 6160935 (2000-12-01), Gaeta et al.
patent: 0359351 (1992-09-01), None
Title: “100% Reflectivity Bragg Reflectors Produced in Optical Fibres by Single Excimer Laser Pulses”, Author(s): J.L. Archambault, L. Reekie and
Brucato Robert N.
Putnam Martin A.
CiDRA Corporation
Font Frank G.
Natividad Phil
LandOfFree
Method and apparatus for forming a Bragg grating with high... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for forming a Bragg grating with high..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for forming a Bragg grating with high... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2508086