Abstract:
A method and device for relieving thermal stresses on the optical fibers in fiber optic splice housings used in hostile thermal environments.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND 
     Fiber-optic sensors are increasingly being used as devices for sensing some quantity, typically temperature or mechanical strain, but sometimes also displacements, vibrations, pressure, acceleration, rotations, or concentrations of chemical species. The general principle of such devices is that light from a laser is sent through an optical fiber and there experiences subtle changes of its parameters either in the fiber or in one or several fiber Bragg gratings and then reaches a detector arrangement which measures these changes. 
     In particular a growing application field is the use of fiber optic sensing system for sensing when deployed in monitoring or production wells. Such sensors are particularly attractive because more conventional sensing systems are expensive to build, and in liquid environments susceptible to failure making them more expensive and difficult to maintain, Additionally, electrical systems are not well suited for in-well installation due to the hostile environment (pressures, temperatures, corrosion). Fiber optic systems do not suffer many of the limitations of electronics and are thus emerging as the technology of choice. 
     As the applications of fiber optic telemetry have expanded there is increased need to install multiple fiber optic sensors into the down-hole environment. Each sensor may require its own FIMT (fiber in metal tubing). The assembly of multiple sensors and the accompanying multiple FIMT&#39;s requires a number of optical fiber splice housings deployed throughout the down hole environment, In the current designs of these at high temperatures the housing can expand in length much greater than the fiber due to differences in the thermal expansion of metal and glass. This creates stress in the fiber that can affect the optical properties of the signal, or in worst case, cause the fiber to break. 
     There is a need to increase the reliability of the splice in these optical fiber splice housings and potentially eliminate the breakage of spliced optical fibers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a proposed fiber optic splice housing. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made that illustrate embodiments of the present disclosure. These embodiments are described in sufficient detail to enable a person of ordinary skill in the art to practice these embodiments without undue experimentation. It should be understood, however, that the embodiments and examples described herein are given by way of illustration only, and not by way of limitation. Various substitutions, modifications, additions, and rearrangements may be made that remain potential applications of the disclosed techniques. Therefore, the description that follows is not to be taken in a limited sense, and the scope of the disclosure is defined only by the appended claims. 
       FIG. 1  illustrates a proposed solution to this need. Shown is a fiber optic splice housing  100  used down hole to connect fiber optic sensors and devices to optical fiber or FIMT (Fiber in metal tube). Typical sensors include pressure sensors, flow sensors, distributed temperature sensors, distributed acoustic sensors, and the like. A splice tray  125 , typically a ˜6.5 mm diameter metal tube with a cutout along its length, contains optical fiber  140  spliced together in the middle at a splice  130  with a splice cover  135  over it to protect it. The tray is contained in a pressure housing  120  that is attached to FIMTs  110  by compression fittings  115  at each end. Thus the housing takes all the pressure and tension in the string. 
     In existing versions the length of fiber in the splice tray is equivalent to the length of the pressure housing. The fiber is fixed at each end, usually with an adhesive like epoxy or RTV. As a result, when the splice housing is lowered in the well bore, it increases in temperature and expands, and so does the fiber. Since the coefficient of expansion of the metal is typically an order of magnitude greater than the fiber, the fiber is stressed in tension, which can affect the optical signals, and can break. 
     To eliminate this problem, the proposed embodiment overstuffs the fiber into the splice housing by forming a wave pattern on each side of the splice. There is typically a 2.5 mm difference between the expansion of the housing and the fiber over 1 m at a change in temperature of 300 degrees C., and so the wave pattern provides at least 1.5 mm extra length on each side of the splice such that the fiber and the housing are of identical length at maximum operating temperature. Drilling holes  145  in the splice tray  125  and passing removable pins through the holes create the wave pattern. The fiber  140  is woven around the pins to create a wave pattern, and then the pins are removed. The fibers maintain the wave shape inside the splice tray by springing against the walls of the tray. The diameter and number of pins determines the additional length of fiber that is needed. 
     The metal splice housing can now expand and contract as temperature fluctuates, without forcing the fiber into stress or shear. 
     The splice housing overstuffs the fiber in a wave pattern to increase its length inside the housing to prevent stress and shear at elevated temperatures. 
     The splice housing prevents stress from affecting the optical signal and the possibility of breaking the fiber. This increases the reliability of the optical system dramatically for high temperature applications. 
     Although certain embodiments and their advantages have been described herein in detail, it should be understood that various changes, substitutions and alterations could be made without departing from the coverage as defined by the appended claims. Moreover, the potential applications of the disclosed techniques is not intended to be limited to the particular embodiments of the processes, machines, manufactures, means, methods and steps described herein. As a person of ordinary skill in the art will readily appreciate from this disclosure, other processes, machines, manufactures, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufactures, means, methods or steps.