Abstract:
A method and apparatus for reducing the thermal induced errors in an IFOG system. The apparatus including a highly thermally conductive material configured to encapsulate a waveguide of an interferometric fiber optic gyroscope (IFOG). The highly thermally conductive material more evenly distributes thermal changes encountered by a sensing coil of the IFOG thereby substantially reducing errors in the IFOG system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments described herein generally relate to an apparatus and method for improving the performance of one or more fiber optic sensors. More particularly, embodiments described herein relate to a coating on a fiber configured to improve the performance of the sensor. More particularly still, embodiments described herein relate to a highly thermally conductive material which encapsulates the fiber in order to reduce thermal errors in an interferometric fiber optic gyroscope (IFOG). 
         [0003]    2. Description of the Related Art 
         [0004]    Optical sensor systems operate by exposing a portion of an optical waveguide to an environmental condition that modulates a light signal transmitted within the optical waveguide. This modulation alters one or more parameters of the light transmitted within the optical waveguide, such as amplitude, power distribution versus frequency/wavelength, phase, or polarization. Analyzing modulated light emerging from the waveguide enables determining values indicative of the environmental condition. Such systems utilize sensors based on, for example, Bragg gratings or interferometers to measure a wide variety of parameters, such as strain, displacement, velocity, acceleration, flow, corrosion, chemical composition, temperature, or pressure. In one example of an optical sensor system, an interferometric fiber optic gyroscope (IFOG) enables measuring angular rotation as it alters the path length of counter-propagating waves of light traveling through a sensing coil of an optical fiber, thereby producing phase changes from which measurements can be made. 
         [0005]    Typical IFOG systems use a beam splitter, or coupler, to split light from a light source into counter propagating waves traveling in the sensing coil. A detector having associated electronics measures the phase relationship between the two interfering counter-propagating beams of light that emerge from the opposite ends of the sensing coil. The difference between the phase shifts experienced by the two beams is proportional to the rate of rotation of the platform to which the instrument is fixed, due to the Sagnac effect. 
         [0006]    Typical IFOG systems are highly sensitive to changes in the thermal condition around the IFOG. Changes in the temperature surrounding the coiled sensor produces thermal gradients acting across the IFOG sensing coil that result in variant localized thermal expansion of the fiber that produces non-reciprocal phase errors. The result is the Shupe effect which causes sensor drift over time that is both time and temperature dependent. The accuracy of the IFOG is then limited by the Shupe effect. Thermally induced phase errors occur if there is a time-dependent temperature gradient along the fiber. Non-reciprocity phase errors arise when clockwise and counter clockwise counter rotating beams traverse the same region of the fiber at different times. If the fiber&#39;s propagation velocity varies at different points along the fiber, the two beams traverse slightly different effective path lengths. The resulting phase shift is indistinguishable from the phase shift caused by rotation. It is very difficult to maintain temperature uniformity of the sensing coil required to eliminate these effects and maintain IFOG accuracy—even under steady thermal operating conditions. 
         [0007]    Currently, one method for reducing the Shupe effect is through complex winding patterns. The fiber in the sensing coil is wound so that the sections of the fiber that are at equal distance from the coil center are beside each other, such as in the quadra, hexa, or octapolar wind. The complex winding allows the local thermal effects for each section of the fiber to be experienced at the same moment and at the same magnitude for each of the counter rotating beams. These complex winding patterns are difficult to assemble. Further, even with extreme care in winding, the sensing coil exhibits residual drift. The residual drift is due to an incomplete cancellation of the different contributions on a complex and nonlinear temperature model based upon temperature and time derivatives. Although the temperature model is well understood, precise thermal monitoring of the sensing coil required to compensate for Shupe effect errors, is difficult to implement due to the low thermal mass, insulating properties of typical polymer-coated glass optical fibers. The complex winding patterns minimize gross thermally induced Shupe effect errors. However, inherent winding imperfections and thermal transients lead to residual drift over time and become a function of a complex nonlinear thermal model. 
         [0008]    Therefore, a need exists for a method and apparatus for improved thermal performance in Sagnac fiber optic sensors. Moreover, a need exists for an optical fiber coating system and monitoring system to improve the thermal performance in Sagnac fiber optic sensors. 
       SUMMARY OF THE INVENTION 
       [0009]    This application relates to an IFOG system. The IFOG system comprises a light source and one or more waveguides configured to be interrogated by one or more signals sent from the light source. The IFOG system has a sensing coil which comprises a portion of the one or more waveguides wound around a spool. An encapsulant surrounds at least a portion of each of the one or more waveguides. The encapsulant is constructed of a highly thermally conductive material configured to distribute thermal changes surrounding the sensing coil to the one or more waveguides. The IFOG further comprises a coupler optically coupling the light source to the sensing coil and the sensing coil to the detector. 
         [0010]    This application relates to a method of reducing thermal induced errors in an IFOG system. The method comprises providing an optical fiber and a material having a high thermal conductivity. The method further comprises encapsulating the optical fiber with the material and winding the optical fiber with the encapsulant around a spool thereby forming a sensing coil. The method further comprises substantially distributing thermal changes surrounding the sensing coil to the optical fiber through the material and detecting angular rotation via interrogation of the sensing coil. 
         [0011]    This application relates an apparatus for reducing thermally induced errors in an IFOG system. The apparatus comprises an optical fiber and an encapsulant surrounding the optical fiber. The encapsulant has a outer diameter greater than an outer diameter of the optical fiber, and wherein the optical fiber with the encapsulant is wound around a spool to form a sensing coil and wherein the encapsulant is made from a material having a thermal conductivity of greater than 1 W/m-K. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0013]      FIG. 1  is a schematic view of an interferometric fiber optic gyroscope (IFOG) according to one embodiment described herein. 
           [0014]      FIG. 2  is a cross sectional view of a waveguide according to one embodiment described herein. 
           [0015]      FIG. 3  is a cross sectional view of a sensing coil according to one embodiment described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a schematic view of an interferometric fiber optic gyroscope (IFOG)  100  according to one embodiment described herein. The IFOG  100  includes a light source  102 , a first coupler  104 , a second coupler  106 , a sensing coil  108 , and a detector  110 . The first coupler  104  may also connect to a photodiode and associated pre-amp (not shown). The pre-amp is used to detect the Sagnac effect caused by rotation of sensing coil  108 . The second coupler  106  may include an integrated optics chip (IOC). An optical fiber  114 , or waveguide, used in the sensing coil  108  includes a thermally conductive encapsulant as will be described in more detail below. 
         [0017]    The light source  102  may be any fiber light source. The light source  102  is configured to interrogate the optical fibers  114 . Any light source  102  may be used so long as it is capable of interrogating the optical fiber  114 . 
         [0018]    The optical fiber  114  is used in the IFOG and the fiber optic sensing coil  108 . The optical fiber  114  is typically made of either a polarization maintaining (PM) fiber or a low birefringence (standard telecommunications) fiber. The sensing coil  108  comprises the optical fiber  114  wound upon a supportive spool  300 , shown in  FIG. 3 . The sensing coil  108  serves as an optical guide for receiving a counter-propagating beam pair emitted from the light source  102 . 
         [0019]    The detector  110  detects light split by the couplers  104  and  106  into counter-propagating waves traveling in the sensing coil  108 . The associated electronics measure the phase relationship between the two interfering counter-propagating beams of light that emerge from opposite ends of the sensing coil  108 . The difference between the phase shifts experienced by the two beams is proportional to the rate of rotation of the platform to which the instrument is fixed. 
         [0020]      FIG. 2  is a cross-sectional view of the optical fiber  114  used in the sensing coil  108 . The optical fiber  114  includes a waveguide  200 , an optional waveguide coating  201 , and an encapsulant  202  surrounding the waveguide  200 . The waveguide  200  may be any waveguide described herein or known in the art. The waveguide coating  201  may be any coating used to surround and protect the waveguide  200  from damage. In one embodiment, the waveguide coating  201  is made from the same material as the encapsulant  202 . The waveguide coating  201  may be a separate item from the encapsulant  202  or an integral part of the encapsulant  202 . The encapsulant  202  is constructed with a thermally conductive material. In one embodiment, the thermal conductivity of the encapsulant  202  is greater than 1 watts per meter Kelvin (W/m-K). The encapsulant  202 , according to one embodiment, is a silver-filled silicone, which has a room temperature thermal conductivity of about 1.4 W/m-K. The high thermal conductivity results in a corresponding reduction in Shupe effect or sensitivity to changes in heating rate. Although described as using a silver-filled silicone, it should be appreciated that any highly thermally conductive material may used including, but not limited to, metals, thermal or radiation-cured elastomers including silicone, acrylates, vinyl ethers, epoxies. Further any of the encapsulant can be filled with highly thermal conductive fillers including, but not limited to, aluminum, silver, gold, copper, aluminum oxide, zinc oxide, silicia, graphite and boron nitride. 
         [0021]    The thermal conductivity of the encapsulant  202  and/or the coating  201  allows the waveguide  200  to be rapidly effected by thermal changes surrounding the sensing coil  108  during use. The thermal conductivity of the encapsulant  202  serves to distribute thermal gradients more efficiently over the sensing coil  108  thereby reducing differential thermal gradients on the sensing coil  108  that cause errors. Further, the time constants for any subsequent thermal gradients shorten. These factors reduce the Shupe effect phase errors in Sagnac sensors. This enhances reciprocity and lowers residual drift performance of the quadric-/hex-/octapolar coil winding. 
         [0022]    The use of identical material for the coating  201  and the encapsulant  202  may reduce any differential thermal expansion between the coating  201  and the encapsulant  202 . Therefore, the stress between the coating  201  and the encapsulant  202  reduce, thereby minimizing the stress acting on the optical fiber  114  of the sensing coil  108 . The use of the coating  201  and/or the encapsulant  202  enables practical sensing and monitoring. 
         [0023]    In one embodiment, multiple fast, low mass temperature sensors, such as thermistors or platinum resistant thermometers, strategically placed on or within the sensing coil  108  can provide a true representation of the sensing coil  108  thermal profile in real time to allow computational extraction of the Shupe effect phase error. 
         [0024]    The encapsulant  202  may be coupled directly to the entire fiber or the coating  201  during the optical fiber  114  manufacturing process. Therefore, the optical fiber  114  used to construct the IFOG  100  is predisposed to reduce thermally induced errors in the sensing coil  108  prior to being formed into the sensing coil  108 . Although the encapsulant  202  may be placed on the entire optical fiber  114 , it is contemplated that the encapsulant  202  is located only in the portion of the optical fiber  114  that makes up the sensing coil  108 . 
         [0025]      FIG. 3  is a cross sectional view of the sensing coil  108 . The sensing coil  108  includes the optical fiber  114  wound around the spool  300 . Further, the sensing coil  108  may include a potting material  302  in addition to the encapsulant  202  around the optical fiber  114 . The potting material is constructed of a highly thermally conductive material. The potting material may be constructed of the same material described above for the encapsulant  202 . The potting material is placed around the wound optical fibers  114  of the spool. The potting material serves the dual purpose of reducing thermal errors in the sensing coil  108  and securing the wound optical fibers  114  of the sensing coil  108 . 
         [0026]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.