Temperature stabilized optical fiber package

Optical waveguides – Accessories – Splice box and surplus fiber storage/trays/organizers/ carriers

Reexamination Certificate

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C385S134000, C385S136000, C385S092000, C385S123000, C385S037000, C398S147000, C398S081000

Reexamination Certificate

active

06668126

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a temperature-stabilized package that improves the performance of a chromatic dispersion compensation Bragg grating by providing a substantially uniform temperature around a fiber optic coil including the Bragg grating.
BACKGROUND OF THE INVENTION
Telecommunication systems using optical fiber networks rely on a succession of short pulses of light to transmit coded information between widely separated locations. Light pulses include a range of wavelengths. It is well known that the velocity of light varies as a function of wavelength as the pulses of light move along an optical fiber. This phenomenon is known as chromatic dispersion. It occurs because longer wavelength light travels more slowly than those of shorter wavelength. As a pulse lengthens, it begins to interfere with the succeeding light pulse making it difficult to distinguish the end of one pulse from the beginning of the next. Typically, signals begin to merge as they travel through long optical fiber cables.
It is known to compensate chromatic dispersion using a Bragg grating to recompress the pulses to their original length. Light of shorter wavelength penetrates further into the grating than longer wavelengths, before reflection. The longer reflection path delays the shorter wavelengths to make them substantially coincident with the longer reflected wavelengths, and to compress all reflected wavelengths into the time interval of the originally transmitted pulse.
The reflection characteristics of a Bragg grating are known to change with temperature. It is preferable, therefore, to locate a dispersion compensating Bragg grating in a uniform temperature environment. The need for temperature uniformity in the vicinity of optical fiber coils has been addressed previously. U.S. Pat. No. 6,226,438, for example, describes a package for containment of an optical fiber that includes a Bragg grating. A housing, having a lower conductivity than a retaining member, provides containment of the optical fiber and the retaining member. The insulating material of the housing provides a primary defense against non-uniform heating of the package. Filler in the form of a gel provides improvement of thermal stability inside the housing. The need to maintain contact between substantially the entire length of the Bragg grating and a surface of the retaining member is a demanding requirement.
Other optical fiber devices subject to temperature-related output-drift include optical gyroscopes. Although different from fiber optic Bragg gratings, optical fiber gyroscopes operate best in a temperature-stabilized environment. For example, U.S. Pat. No. 4,702,599 describes a rotation-rate measuring instrument that uses an optical fiber coil. Fluctuations in ambient temperature induce measurement errors. Embedding the optical fiber in a conductive sealing compound and placing the sealed coil inside a housing constructed from a very good thermal conductor significantly reduces these errors. Thermal radiation striking the outer wall of the housing dissipates rapidly due to thermal conductivity of the housing. Redistribution of the heat provides a means for compensating temperature induced measurement errors. This reference teaches the need to place an optical fiber coil in intimate contact with a thermally conducting compound and thereafter enclose the sealed coil in contact with the inner wall of a double walled housing having an air gap between the walls. Heat bridges link walls of a relatively complex structure that comprises a material of high thermal conductivity.
A thermally stabilizing enclosure may include both thermally conducting and thermally insulating materials, as in U.S. Pat. No. 5,208,652. This describes an optical branching/coupling unit for an optical fiber gyroscope including a thermal buffer box that prevents the influence of temperature on gyroscope output. A heat transmitting case surrounds an optical fiber loop wound on a spool made from a material of high thermal conductivity. The heat transmitting case resides inside a heat insulating case contained inside a heat-transmitting casing that provides the outermost layer of the thermal buffer box. External changes in temperature are moderated during passage of heat through the alternating layers of heat conducting and heat insulating materials. As a result any temperature changes in the vicinity of the optical fiber coil are slight and uniform. The buffer box requires multiple alternating layers of thermally conducting and insulating materials.
Temperature compensation was attempted using only thermally insulating materials. In this case, U.S. Pat. No. 5,245,687 describes an optical fiber coil for a fiber optic gyro wound on a bobbin, contained in an annular case resting on a relatively massive support plate that is essentially a heat sink. The thermal conductivity of the bobbin and the case substantially equals that of the fiber coil. Both the coil and the case respond slowly to abrupt changes in ambient temperature to reduce drift in the output of a fiber optic gyro made from the coil. Such a construction teaches that the entire optical fiber coil is surrounded, essentially encapsulated, with a material of low thermal conductivity. Consequently, use of a low thermal conductivity bobbin suppresses the drift in the gyro output due to the influence of an ambient temperature change.
U.S. Pat. No. 5,416,585 describes a relatively complex approach for correcting fiber optic gyro drift rate error due to changes in temperature. Sensing of temperature differences between an optical fiber coil-carrying spool and a housing for the spool may be used to compensate drift rate error. Temperature differences, measured by sensors in the housing and/or the coil, provide input to associated electronic circuitry, connected to the sensors. The electronic circuit calculates the temperature difference between the gyro housing sensor and the coil spool sensor and produces and applies a correction factor to the output of the fiber optic gyro. The structure surrounding the coil in this case does not appear to provide a uniform temperature since any drift in gyro output, with time, requires detection and compensation by the external monitoring equipment that uses the electronic circuitry. Although not specified, there is indication that the housing is a metallic housing.
The previous discussion suggests the need for a relatively simple device for containment of optical fibers in a uniform temperature environment. Suitable devices should have few parts and be easy to assemble as packages that contain optical fibers, particularly fiber optic Bragg gratings, at a uniform temperature. This would allow a grating to operate, substantially without change, during exposure of a package to temperature gradients such as those present in enclosures that house power supplies and other heat generating components used for telecommunications networks.
SUMMARY OF THE INVENTION
The present invention satisfies the need for a simple, easily-assembled package that maintains a substantially uniform temperature inside a container for an optical fiber that preferably includes a long fiber optic Bragg grating. Construction of a container according to the present invention requires a material, such as copper or aluminum, having high thermal conductivity. Due to its long length the fiber optic Bragg grating may be coiled to fit inside the container. The package includes a housing coupled to the container of the fiber optic Bragg grating. While material selection is not necessarily limiting, preferably the housing according to the present invention comprises a material, such as a plastic resin, that is a poor conductor of heat. Use of the term coupled for attachment of the container to the housing indicates that intervening structures may exist between the two. Coupling means include those that minimize the temperature gradient across the high conductivity container, while placing the fiber optic Bragg grating in a region of substantially uniform temperature inside the cont

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