Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-07-22
2004-12-14
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S123000, C385S141000, C385S142000, C385S144000
Reexamination Certificate
active
06832026
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical fiber for use in making various fiber devices, such as long-period gratings, of optical communication systems, and deals more particularly with an improved fiber and fiber devices having improved performance in response to changes in temperature. The invention also relates to methods for making such optical fiber and fiber devices.
BACKGROUND OF THE INVENTION
In the development of fiber optic communication systems it has been found possible and often desirable to form certain circuit components or devices with the fibers themselves. Known in the art are amplifiers, couplers, lasers and filters formed with optical fibers. For example, a filter can be formed by introducing a periodic structure, such as a grating to the fiber. It is known to form a filter, such as, for example, a Bragg grating filter, by controllably varying the index of refraction of the core, and also possibly the index of refraction of the cladding layer immediately surrounding the core, along the length of the fiber. In particular, photosensitive fibers are known wherein at least the core of the fiber includes dopant materials allowing the initial or base index of refraction of the core to be changed from its base value to some other value by the application of actinic radiation to the fiber, and it is also known to form a Bragg filter or grating in such a photosensitive fiber by changing the index of refraction of the core from its base value at a plurality of spaced points along the length of the core, which points of changed index then reflect or reject light of a wavelength which resonates with the points of changed index. As another example, it is also known to form a filter by periodically thinning the fiber with a microburner, as is described in U.S. Pat. No. 5,708,740, issued on Jan. 13, 1998 and assigned to Northern Telecom Limited.
In the making and use of fiber optic system components from specialty fibers it is known that the components customarily have response characteristics dependent on the temperature at which the components are used.
To obtain fixed responses despite varying environmental temperatures various complex and expensive solutions have been proposed in the past, such as housing the components in temperature controlled containers or adding temperature sensitive mechanical stressing features to the fiber. Other attempts at solving the problem have been directed to the selection and proportioning of the constituents of the core and cladding materials of fibers, used in making the components, whereby components made from the fibers inherently have good temperature insensitivity and can hopefully be used in many applications without the need for temperature controlled containers, stressing mechanisms or the like.
In regard to optical communication fibers with filters as described above, it has been found that such filters made in the past have a peak transmission loss wavelength which varies with temperature, and therefore if the operating temperature of the filter changes significantly the ability of the filter to efficiently reject or pass the wavelength(s) for which it was designed becomes impaired. In certain components or devices this problem can arise because the time required for light of a given wavelength to move from one point to another along the length of the core varies with the temperature, with the result that with changes in temperature the light of the wavelength to be rejected or passed by the filter loses its exact resonance with the spaced points of changed refractivity and causes the points of changed refractivity index to reflect or transmit light of the given wavelength with less efficiency.
Prior efforts toward providing an optical fiber with high temperature insensitivity by core and cladding constituent control are revealed by U.S. Pat. Nos. 5,703,978, 5,949,934 and 6,201,918. These patents suggest that changes with respect to temperature of a long-period grating filter made from a given fiber are directly related to changes in the fiber's effective refractive index n
eff
with respect to temperature. These patents still further suggest that a fiber with high temperature insensitivity can be made by formulating the constituents of the core and cladding materials such that the characteristic curve of the core index vs. temperature and the characteristic curve of the cladding index with respect to temperature have substantially the same shape (see
FIG. 5
of the '978 patent) so that at every reasonable temperature the rate of change of the core index,
ⅆ
n
core
ⅆ
T
,
is equal to the rate of change of the cladding index,
ⅆ
n
clad
ⅆ
T
,
to accordingly make the rate of change of the fiber effective index acceptably low or zero at all temperatures.
In regard to the disclosures of the foregoing patents, applicants have found that it is impractical, and perhaps impossible, to in all cases formulate core and cladding materials such that a grating or other device made with the fiber has the requisite temperature performance.
Accordingly, it is an object of the present invention to address one or more of the foregoing disadvantages or deficiencies of the prior art.
SUMMARY OF THE INVENTION
In one aspect, the invention resides in the finding by applicants that for a fiber having a core co-doped with selected constituents, such as, for example, germanium and boron, the temperature sensitivity characteristic of a long period grating formed in the fiber core, as expressed by a plot of the absolute change in grating wavelength vs. temperature, is a characteristic curve having a temperature T
m
at which the sensitivity of the grating wavelength (i.e., the wavelength at which the transmission loss through the grating is a maximum, also referred to as &lgr;
B
) to changes in temperature is a minimum (e.g., the slope of the curve is zero), with the curve increasing smoothly with both increases and decreases in temperature away from the temperature T
m
.
Another aspect of the invention resides in that applicants have found that by controlling the constituents of the core and cladding materials of a fiber in certain ways the characteristic temperature T
m
can be made to fall at any desired temperature within a useful range of temperatures for the fiber. U.S. Pat. No. 6,201,918 shows, in
FIG. 6
, a grating having a T
m
lying within a range of −5° C. to +5° C., and an article entitled
Temperature Insensitive Long
-
Period Fiber Grating
by K. Shima, S. Okude, T. Sakai, A. Wada, and R. Yamauchi in Fujikura Technical Review 1998, pages 1 & 2, shows, in
FIG. 2
, a grating having a T
m
lying within a range of +15° C. to +25° C. Neither of these publications, however, suggests the idea of intentionally controlling the constituents of the core and cladding materials to cause the T
m
of a fiber, or of a grating or other optical system component made from the fiber, to fall at a wanted temperature, and especially no suggestion is made for intentionally causing the T
m
of a fiber to fall at a useful temperature of the fiber not within the range of −5° C. to +5° C. or the range of +15° C. to +25° C.
In keeping with the above, the invention resides in part in an optical fiber for use in making optical communication system components, especially long-period gratings, in said components, and in the method for making such fiber and components, wherein the core material and the cladding material of the fiber are so formulated that their characteristic curves of index vs. temperature are of dissimilar shapes and yet are such that at a selected temperature, which may be approximately the mean or average temperature at which a component made from the fiber is expected to be used, the curves of rate of change of index with respect to temperature of the core
(
i
.
e
.
,
ⅆ
n
core
ⅆ
T
v
s
.
T
)
and the cladding
(
i
.
e
.
,
ⅆ
n
clad
ⅆ
T
v
s
.
T
)
materials cross one anot
Carter Adrian
Farroni Julia
Jacobson Nils
Tankala Kanishka
Healy Brian
Huber Donald
McCormick, Paulding and Huber LLP
Nufern
Petkovsek Daniel
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