Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-10-12
2001-05-29
Schuberg, Darren (Department: 2872)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000, C385S147000
Reexamination Certificate
active
06240225
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical fiber grating technology. More particularly, the invention relates to a temperature compensated fiber grating and a method to remove or significantly reduce central wavelength variations of a fiber grating used in fiber optical communication systems caused by the grating's temperature variations.
BACKGROUND OF THE INVENTION
Optical fiber refractive index gratings are widely used in fiber optical communication systems. In particular, fiber Bragg gratings are used as spectral filters for wavelength division multiplexing (WDM) of optical signals. In the WDM systems, it is important that the central wavelength of the filter does not change. However, as the temperature of the fiber Bragg grating rises, the central wavelength fluctuates, typically by 10 pm/° C. Such wavelength variation has a detrimental effect on the performance of the WDM systems.
It is well known that the central wavelength of a fiber Bragg grating varies with temperature by the following expression:
(
λ
-
λ
0
)
λ
0
=
K
T
(
T
-
T
0
)
&lgr;
0
is the wavelength at a grating's temperature T
0
, and K
T
is the thermal expansion coefficient of the fiber Bragg grating. The thermal expansion coefficient of a silica-based fiber Bragg grating has a typical value around 6~7×10
−6
/° C.
In the prior art, there exist several mechanisms to produce temperature insensitivity to the central wavelength of the fiber Bragg grating. The actual amount of compensation required for a particular grating depends on the composition and structure of the grating. One way is to adopt an active system, which depends on a feedback from a temperature active element such as a thermal electric cooler. Another way is to use a passive method. The passive method is more desirable because it does not require power consumption and control logics to maintain a constant wavelength. In one approach, wavelength control under the passive method is accomplished by clamping the fiber containing the fiber Bragg grating with a mechanical structure made of multiple materials with different thermal expansion properties, usually positive thermal expansion coefficients. U.S. Pat. No. 5,042,898 and PCT/US97/23415 provide examples of the multiple materials assembled to produce a compressive strain as the temperature of the package increases. However, these mechanical structures are complicated to fabricate and expensive.
Another approach is to use isotropic materials that have negative thermal expansion coefficients that precisely match the wavelength shift of the fiber Bragg grating. Examples of this approach were presented at the 22
nd
European Conference on Optical Communication (ECOC'96) in Oslo, by D. L. Weidman, et al. of Coming Inc. and U.S. Pat. No. 5,694,503. The composition of a glass ceramic is designed to have a thermal expansion coefficient which matches the fiber Bragg grating's thermal expansion coefficient. However, the precise control of the thermal expansion of a glass ceramic to a desire level is difficult to achieve. Moreover, the design and fabrication of these exotic materials increase expense and complexity in the production these structures.
SUMMARY OF INVENTION
In view of the desire for temperature compensated fiber Bragg gratings, the subject invention provides for a temperature compensated optical device and a simple tunable method to compensate for the inherent temperature variations of the fiber Bragg grating. Specifically, the present invention allows for selection of the thermal expansion coefficient by placing the fiber with the Bragg gratings on a crystal compensating member at a prescribed angle with respect to a crystal axis that has a negative thermal expansion coefficient.
A temperature compensated optical device according to the present invention comprises an optical fiber having a grating therein for reflecting light passing therethrough within a range about a central wavelength and a temperature compensating member affixed to the optical fiber proximate the grating. The temperature compensating member comprises an anisotropic material having a negative temperature expansion coefficient along at least one axis thereof. The optical fiber is aligned at a predetermined angle with respect to the axis to compensate the temperature variations.
In a first embodiment of the temperature compensated optical device of the invention, the temperature compensated optical device has a fiber Bragg grating with a central wavelength at 1536.310 nm. The temperature compensating member is an anisotropic crystal, for example calcite. The Calcite single crystal is cut into a bar such that the long dimension of the bar (about 10~30 mm in length) has a negative thermal expansion coefficient and the short dimension has a positive thermal expansion coefficient. The optical fiber with the grating is bonded to the calcite by epoxy with the predetermined angle of 0.3 degrees along the direction in the calcite crystal that has a compensating thermal expansion value for a given fiber Bragg grating. It is found that the central wavelength of the temperature compensated optical device of the invention remains substantially unchanged as the temperature of the Bragg grating varies from 24° C. to 170° C.
In a second embodiment of the temperature compensated optical device of the invention, the compensating member has a groove on one of its surfaces. The groove is oriented along the predetermined angle with respect to the axis that has a negative thermal expansion coefficient, and the optical fiber with the Bragg grating is placed inside the groove. The groove may be in the form of a V-groove, a rectangular groove or a rounded groove.
In a third embodiment of the temperature compensated optical device of the invention, the compensating member has a through hole therein along the predetermined angle with respect to the axis having a negative thermal expansion coefficient. The optical fiber is placed inside the hole.
In a fourth embodiment of the temperature compensated optical device of the invention, the optical fiber is bonded on the compensating member by a metal coating layer and a solder.
In a fifth embodiment of the temperature compensated optical device of the invention, the optical fiber is bonded on the compensating member by a low melting temperature glass frit.
REFERENCES:
patent: 5514360 (1996-05-01), Sleight et al.
patent: 5694503 (1997-12-01), Fleming et al.
patent: 5721802 (1998-02-01), Francis et al.
Assaf Fayez
Lucent Technologies - Inc.
Schuberg Darren
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