Optical waveguides – Accessories – Bushing structure
Reexamination Certificate
2000-07-28
2002-09-03
Sircus, Brian (Department: 2839)
Optical waveguides
Accessories
Bushing structure
Reexamination Certificate
active
06445868
ABSTRACT:
TECHNICAL FIELD
The present invention relates to feedthroughs for optical waveguides, and more particularly, to hermetically sealed feedthroughs suitable for use in high pressure, high temperature, and/or other harsh environments.
BACKGROUND ART
In many industries and applications, there is a need to have small diameter wires or optical waveguides penetrate a wall, bulkhead, or other feedthrough member wherein a relatively high fluid or gas differential pressure exists across the feedthrough member. In addition, one or both sides of the feedthrough member may be subjected to relatively high temperatures and other harsh environmental conditions, such as corrosive or volatile gas, fluids and other materials. In the case of electrical wires, these devices, called feedthroughs or penetrators, typically are constructed by using electrically conductive metal ‘pins’ having a low thermal coefficient of expansion. The pins are concentrically located within a hole in a housing, and the resulting annular space is filled with a suitable sealing glass. Critical to the success of such seals is the selection and approximate matching of the thermal expansion rates of the various materials, i.e., the metal housing, sealing glass, and electrical pin. As the temperature range over which the feedthrough is exposed increases, the matching of thermal expansion rates becomes increasingly important in order to avoid failure of the feedthrough by excessive thermal stress at the interface layers between the various materials. This technology is relatively mature for electrical feedthroughs, and commercial devices are readily available that meet service temperatures in excess of 200° C.
More recently, with the introduction of optical sensors, particularly sensors for use in oil and gas exploration and production, a need has emerged for a bulkhead feedthrough that can seal an optical fiber at high pressures of 20,000 psi and above, and high temperatures of 150° C. to 250° C., with desired service lives of 5 to 10 years. The sensing assembly of
FIG. 3
is of the type disclosed in co-pending U.S. patent application Ser. No. 09/440,555 filed Nov. 15, 1999, entitled “Pressure Sensor Packaging For Harsh Environments”, which is assigned to the Assignee of the present invention and is hereby expressly incorporated by reference as part of the present disclosure.
There are several problems associated with constructing such an optical fiber feedthrough. One of these problems is the susceptibility of the glass fiber to damage and breakage. This is due to the small size of the fiber, the brittle nature of the glass material, the susceptibility of the glass to stress corrosion cracking due to moisture exposure, and the typical presence of a significant stress concentration at the point at which the fiber enters and exits the feedthrough. Attempts to use a hard sealing glass, such as used with electrical feedthroughs, have had problems of this nature due to the high stress concentration at the fiber-to-sealing glass interface.
Another problem with sealing an optical fiber, as opposed to sealing an electrically-conductive metal ‘pin’ in an electrical feedthrough, is that the fused silica material of which the optical fiber is made, has an extremely low thermal expansion rate. Compared to most engineering materials, including metals, sealing glasses, and even the metal ‘pins’ typically used in electrical feedthroughs, the coefficient of thermal expansion of the optical fiber is essentially zero. This greatly increases the thermal stress problem at the glass-to-sealing material interface, particularly as the application temperatures rise.
One technique used to produce optical fiber feedthroughs is the use of a sealed window with a lensing system. In this technique, the optical fiber must be terminated on each side of a pressure-sealed window, thus allowing the light to pass from the fiber into a lens, through the window, into another lens, and finally into the second fiber. The disadvantages associated with this system include the non-continuous fiber path, the need to provide two fiber terminations thus increasing manufacturing complexity, and the light attenuation associated with these features.
Another approach to producing optical fiber feedthroughs involves passing the fiber through a bulkhead without termination, while providing a seal around the fiber to prevent leakage across the bulkhead. One such seal has been implemented by means of a sapphire compression fitting to take advantage of the pressure differential typically present across a bulkhead in a harsh environment. One disadvantage associated with this type of seal, however, is that it has been found to suffer from creep of material across the bulkhead in the direction of the decreasing pressure gradient, which can, in turn, compromise both the optical fiber and seal.
It is often desirable to mount fiber optic based sensors in harsh environments that are environmentally separated from other environments by physical bulkheads. An exemplary such fiber optic based sensor is disclosed in co-pending U.S. patent application Ser. No. 09/205,944 entitled “Tube-Encased Fiber Grating Pressure Sensor” to T. J. Bailey et al., which is assigned to the Assignee of the present invention and is hereby expressly incorporated by reference as part of the present disclosure. This exemplary optical sensor is encased within a tube and certain embodiments are disclosed wherein the sensor is suspended within a fluid. Some such fiber optic sensors have sensors and tubes that are comprised of glass, which tends to be relatively fragile, brittle and sensitive to cracking. Thus, the use of such a sensor in a harsh environment, such as where the sensor would be subjected to substantial levels of pressure, temperature, shock and/or vibration, presents a significant threat of damage to the sensor. In certain environments, such sensors are subjected to continuous temperatures in the range of 150° C. to 250° C., shock levels in excess of 100 Gs, and vibration levels of 5G RMS at typical frequencies between about 10 Hz and 200 Hz and pressures of about 15 kpsi or higher.
However, as discussed above, the harsh environments where the sensors are located generally must be isolated by sealed physical barriers from other proximate environments through which the optical fiber communication link of the sensor must pass. It is important to seal the bulkhead around the optical fiber to prevent adjacent environments from contamination, as well as to protect the optical fiber as it passes through adjacent environments. If the optical fiber is compromised by contamination from an adjacent harsh environment, the optical fiber and all sensors to which it is connected are likely to become useless.
Accordingly, it is an object of the present invention to provide an optical waveguide feedthrough assembly, and a method of making such an assembly, which overcomes one or more of the above-described drawbacks and disadvantages of the prior art, and is capable of relatively long-lasting operation at relatively high pressures and/or temperatures.
SUMMARY OF THE INVENTION
The present invention is directed to an optical waveguide feedthrough assembly for passing at least one optical waveguide, such as an optical fiber, through a sensor wall, bulkhead, or other feedthrough member. The feedthrough assembly of the present invention comprises a tubular member or like support defining an axially elongated, annular surface, wherein the annular surface forms an axially elongated optical feedthrough cavity. The optical fiber or like waveguide is received through the axially-elongated optical feedthrough cavity, and is spaced radially inwardly relative to the annular surface to thereby define an axially-elongated annular cavity between the fiber and annular surface. A sealant, such as an epoxy adhesive, is received within and substantially fills the annular cavity. The sealant exhibits adhesive properties at the interface of the sealant and optical fiber, and at the interface of the sealant and the annular surface, to adh
Chipman Christopher
Currier Bradley A.
Grunbeck John
Maron Robert J.
Cummings & Lockwood
Sircus Brian
Weatherford / Lamb, Inc.
Webb Brian S.
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