Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2000-03-09
2001-03-06
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
Reexamination Certificate
active
06198868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature compensated long period optical fiber grating filter.
2. Description of the Related Art
Generally, a long period optical fiber grating filter is an optical device for coupling modes propagated in the core of an optical fiber to modes propagated in the cladding of the optical fiber. Such a long period optical fiber grating filter provides an advantage in terms of the gain flatness of erbium-doped optical fiber amplifiers (EDFAs) in that it is of a mode-coupling type other than a reflective mode-coupling type. Such a long period optical fiber grating is manufactured to exhibit a periodic variation in refractive index at its core. The periodic variation in refractive index is obtained by periodically exposing the core of an optical fiber, sensitive to ultraviolet rays, in the optical fiber grating to ultraviolet rays in the process of manufacturing the optical fiber grating. That is, an increase in refractive index is exhibited at portions of the core exposed to ultraviolet rays whereas no variation in refractive index occurs at the remaining portions of the core not exposed to ultraviolet rays. Thus, a periodic variation in refractive index is exhibited in the core. In such a long period optical fiber grating, a mode coupling occurs in a state in which a phase matching condition expressed by the following Expression 1 is satisfied.
&bgr;
co
−&bgr;
cl
(m)
=2&pgr;/&Lgr; [EXPRESSION 1]
where, &bgr;
co
represents a propagation constant of a core mode, &bgr;
cl
represents a propagation constant of an m-th cladding mode, and A represents a grating period.
Where &bgr;=2&pgr;n/&lgr; is substituted into Expression 1 (“n” represents a refractive index, and &lgr; represents a wavelength), the refractive index difference between the core and cladding modes is derived which corresponds to n
co
−n
cl
(m)
=&lgr;/&Lgr;. Accordingly, a change of light with a certain wavelength into a cladding mode can be achieved by appropriately determining a desired grating period &Lgr; and a desired refractive index difference n
co
−n
cl
(m)
.
A desired refractive index difference can be obtained by appropriately radiating an ultraviolet laser onto the optical fiber which is sensitive to ultraviolet rays. That is, the optical fiber sensitive to ultraviolet rays is masked by a mask having a particular period. When a laser is then radiated onto the mask, the opto-sensitive optical fiber generates a reaction resulting in a variation in the refractive index of the core. In order to obtain a desired spectrum, that is, a desired coupling wavelength and a desired extinction ratio, the radiation of the ultraviolet laser should be carried out for an appropriate period of time by accurately adjusting the mask period.
The coupling wavelength of the long period optical fiber grating manufactured as mentioned above is also influenced by temperature. A shift of coupling wavelength depending on a variation in temperature is based on a variation in refractive index depending on the temperature variation and a thermal expansion in length depending on the temperature variation. This can be expressed by the following Expression 2:
ⅆ
λ
m
ⅆ
T
=
ⅆ
λ
m
ⅆ
n
ⅆ
n
ⅆ
T
+
ⅆ
λ
m
ⅆ
Λ
ⅆ
Λ
ⅆ
T
[
EXPRESSION
2
]
where, T represents a temperature.
Where a long period optical fiber grating is applied to general optical fibers for communication or dispersion shifted optical fibers, the second term of the right side in Expression 2 is not taken into consideration because the value defined by the first term of the right side in Expression 2 is greater than the value defined by the second term by about several ten times. For instance, the Flexcor 1060 manufactured by Corning Glass Corporation exhibits a coupling wavelength of about 5 nm per 100 C. Typical dispersion shifted optical fibers exhibit a coupling wavelength shift of about 0.3 nm per 100 C due to a variation in refractive index occurs while exhibiting a coupling wavelength shift of about 5 mn per 100 C due to a length expansion. In the case of a gain flattening filter, which is an example of a practical application of long period optical fiber gratings, however, a temperature stability of about 0.3 nm per 100 C is required.
In order to obtain a temperature compensation meeting the above requirement, a method has been used in which the refractive index of the filter is adjusted in such a fashion that the term d&lgr;/d&Lgr; in Expression 2 has a negative value. There is another conventional method in which the period of the long period optical fiber grating is shortened in such a fashion that a higher-order cladding mode is selected. Another method is also known in which an addition of B
2
O
3
is made to allow the term “dn/dT” in Expression 2 to have a value of 0.
However, all the above mentioned conventional methods use complicated processes because they involve a refractive index adjustment in the filter or an addition of a material serving to avoid a variation in refractive index caused by a variation in temperature. U.S. Pat. No. 5,757,540 for
Long
-
Period Fiber Grating Devices Packaged For Temperature Stability
to Judkins et al discloses a material package surrounding the cladding about the long-period grating for temperature stability. However, Judkins et al '540 does not disclose coating the region of the optical fiber cladding absent from a long period grating. What is needed is an optical fiber with two separate coatings, one for the region containing the long period grating and the other for the region absent the long period grating. This would result in an optical fiber with a uniform diameter in all regions which is easier to handle and use.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a long period optical fiber grating filter which is coated with a material serving to shift a coupling wavelength of the filter in a direction opposite to that of a coupling wavelength shift caused by a variation in temperature.
It is another object to apply one type of coating to the cladding surrounding a long period grating in the optical fiber and to apply a separate and different coating to the cladding surrounding portions of the optical fiber absent of the long period grating.
It is yet another object to have the two different coatings adjacent to each other and have equal inner and outer radiuses so that the fiber construction is smooth and has a uniform diameter throughout the length of the optical fiber.
In accordance with the present invention, this object is accomplished by providing a long period optical fiber grating filter comprising: a core formed with a long period grating; a cladding surrounding the core; a coating coated over a portion of the cladding not surrounding the long period grating; and a re-coating coated over a portion of the cladding surrounding the long period and made of a material exhibiting an increase in refractive index in accordance with an increase in temperature, the re-coating serving to allow a coupling wavelength shift caused by the increase in refractive index to be carried out in a direction opposite to that of a coupling wavelength shift caused by a refractive index difference between the core and the cladding.
REFERENCES:
patent: 5042898 (1991-08-01), Morey et al.
patent: 5077816 (1991-12-01), Glomb et al.
patent: 5694503 (1997-12-01), Fleming et al.
patent: 5703978 (1997-12-01), DiGiovanni et al.
patent: 5757540 (1998-05-01), Judkins et al.
patent: 5999671 (1999-12-01), Jin et al.
patent: 6011886 (2000-01-01), Abramov et al.
patent: 6058226 (2000-05-01), Dmitry
Bushnell , Esq. Robert E.
Samsung Electronics Co,. Ltd.
Ullah Akm E.
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