Abstract:
A multi-core optical fiber pressure sensor is described, which sensor includes an optical fiber having at least two cores, wherein the cores have collocated measurement portions, for example in-fiber interferometers or Bragg Grating portions. In an exemplary embodiment, the fiber is arranged such that a pressure induced force will act on the multi-core fiber at said collocated position, affecting the light guiding cores in a different manner. In another exemplary embodiment, the optical fiber is configured to bend in response to pressure changes.

Description:
BACKGROUND 
       [0001]    Optical fiber pressure sensors, particularly those utilized in harsh environments, such as in downhole environments, are predominantly plagued by temperature changes and drift sources. Thus, where measurement is attempted, additional sensors have been required to attempt to compensate for such temperature changes, and drift of the measurement. For example, two pressure sensors might be employed near each other having different sensor characteristics (i.e., different responses to the undesired parameter), and calculations may be made in an attempt to eliminate the effect of the parameter on the measurement (effectively in an attempt to isolate the parameter of interest, e.g., temperature effects at the point of interest). 
         [0002]    While this may appear to be a good solution, conditions at the two sensors must be exact to accurately eliminate the influences of the undesired parameter. Also, the need to set up and run multiple sensors at every measurement point of interest can be tedious and costly. 
         [0003]    What is needed in the art is a simple, low cost solution to elimination of temperature changes and drift sources in optical fiber pressure sensors. 
       SUMMARY 
       [0004]    The above-described and other problems and deficiencies of the prior art are overcome and alleviated by the presently described multi-core optical fiber pressure sensor, which includes an optical fiber having at least two cores, wherein the cores have collocated measurement portions, for example, in-fiber interferometers or Bragg grating portions. In an exemplary embodiment, the fiber is arranged such that a pressure induced force will act on the multi-core fiber affecting the collocated measurement portions in a different manner. In another exemplary embodiment, such arrangement causes one grating to be in compression and another to be in tension. In another exemplary embodiment, the fiber is actuated by a pressure sensitive bellows or diaphragm. In another exemplary embodiment, the fiber is actuated by a force normal to the axis of the fiber. In another exemplary embodiment, the fiber is asymmetrically actuated along a longitudinal axis of the fiber. 
         [0005]    In other exemplary embodiments, different portions of the multi-core fiber are engineered to react differently to pressure, and light guiding cores in the collocated measurement portions are configured to sense pressure. In an exemplary embodiment, the fiber contains a lower modulus core near a first light guiding core and a higher modulus core near a second light guiding core. The provision of the multi-core fiber and the differential reaction of the pressure to the fiber portions containing the lower and higher modulus cores, respectively, at the measurement portions of the multiple cores, eliminate temperature changes or drift sources that might otherwise affect the measurements. In another exemplary embodiment, the multi-core fiber comprises at least two cores that have the same doping. In another embodiment, at least two cores are reflective to the same wavelength. 
         [0006]    In other exemplary embodiments, a reference pressure acts on a multi-core fiber in addition to a well bore (or other application) pressure. In such embodiment, the multi-core fiber contains at least two light guiding cores provided in different spatial relationship relative to a hollow core. The hollow core acts as a port causing different pressure induced reactions with regard to the light guiding cores. 
         [0007]    The above-discussed and other features and advantages of the presently described multi-core optical fiber pressure sensor will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES: 
           [0009]      FIG. 1  is a cross-sectional plan view of an exemplary multi-core fiber utilizing Bragg Gratings at a same distance along the fiber; 
           [0010]      FIG. 2  is a cross-sectional plan view of an exemplary multi-core fiber actuated by a push rod and bellows; 
           [0011]      FIG. 3  is a cross-sectional plan view of an exemplary multi-core fiber actuated by a push rod and diaphragm; 
           [0012]      FIG. 4  is a cross-sectional plan view of an exemplary multi-core fiber asymmetrically actuated by a push rod and diaphragm; 
           [0013]      FIG. 5  is a cross-sectional plan view of an exemplary multi-core fiber actuated by well pressure; 
           [0014]      FIG. 6  is a cross-sectional view of an exemplary multi-core fiber having different modulus cores and light guiding cores; 
           [0015]      FIG. 7  is a cross-sectional plan view of an exemplary multi-core fiber actuated by well and reference pressures; and 
           [0016]      FIG. 8  is a cross-sectional view of an exemplary multi-core fiber having a hollow, port core and light guiding cores. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0017]    Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. 
         [0018]    Referring now to  FIG. 1 , a cross-sectional plan view of an exemplary multi-core fiber is illustrated generally at  10 . A first core  12  and a second core  14  include Bragg grating portions  16 ,  18  at a same measurement portion, shown generally at  20 , relative to a longitudinal axis, illustrated by line  22 , of the fiber  10 . 
         [0019]    The grating portions  16  and  18  may be written in the cores by any fashion and at any time. However, in an exemplary embodiment, the grating portions  16  and  18  are photo etched in cores  12  and  14  during fiberization. More specifically, the grating portions are written during the drawing process and prior to the application of a protective coating. In such exemplary embodiment, the collocated sensors are particularly insensitive to drift factors since all collocated grating portions will drift together. 
         [0020]    Also, while the above exemplary embodiment describes use of Bragg gratings, it should be recognized that other structures useful for reading out such fibers may be used, such as in-fiber interferometers, Rayleigh scatter and random photo etched structures, among others, as long as collocated measurement portions are provided in the fiber. 
         [0021]    Referring now to  FIG. 2 , a cross-sectional plan view of an exemplary multi-core fiber  10  is illustrated in a system designed to actuate the fiber by a push rod  24  and bellows  26 . The push rod  24  extends from the bellows normally against the multi-core fiber, which is provided in a media isolated housing  28 . The bellows  26  is responsive to a pressure change to cause the push rod  24  to bend the fiber  10 . 
         [0022]    Referring back to  FIG. 1 , it is noted that the push rod  24  and bellows  26  is an exemplary mechanism to provide the pressure-induced force on the fiber illustrated by arrow  30 . Such force  30  bends the fiber  10 , placing exemplary grating  16  in tension and exemplary grating  18  in compression. Differential measurements in core  12  and  14  may then be taken to sense the pressure change. It is noted that not only are drift factors eliminated due to the collocated nature of the core measurement portions (e.g., gratings written during fiberization in multiple cores of a multi-core fiber), but temperature effects are also eliminated due to the nature of the multi-core system. In another exemplary embodiment, the multi-core fiber comprises at least two cores that have the same doping to minimize differential reactions to pressure. In another embodiment, at least two cores are reflective to the same wavelength. 
         [0023]    Referring now to  FIG. 3 , a cross-sectional plan view of an exemplary multi-core fiber  10  is illustrated as being actuated by a push rod  24  and diaphragm  32 . Other than use of the diaphragm  32  instead of the bellows  26 , operation of the collocated sensor system is identical to that described above with regard to  FIG. 2 . It should be noted that any mechanism effective to transmit a force representative of pressure against the fiber is contemplated herein, the bellows and push rod and diaphragm and push rod embodiments being merely exemplary. 
         [0024]    Referring now to  FIG. 4 , a cross-sectional plan view of an exemplary multi-core fiber  10  is illustrated as being asymmetrically actuated by a push rod  24  and diaphragm. It should be recognized that any kind of actuation on the fiber may be performed, as long as the core measurement portions ( 20  in  FIG. 1 ) of cores  12  and  14  are differentially affected by a force representative of a pressure change. 
         [0025]    Referring now to  FIG. 5 , a cross-sectional plan view of an exemplary multi-core fiber  34  is illustrated as being actuated by well pressure, illustrated generally at  36  as acting on the multi-core fiber  34  within the media isolated housing  28 . Referring now to  FIG. 6 , in this exemplary embodiment, the multi-core fiber  34  includes light guiding cores  12  and  14 , as well as a low modulus core  38  and a high modulus core  40 . As the well pressure  36  acts on the fiber  34 , the low modulus core  38  and the high modulus core  40  react differently, causing the fiber  34  to bend. This bend accordingly affects the light guiding cores  12  and  14  differently (note that cores  12  and  14  should be arranged within the fiber such that they bend differently relative to the effects of the low and high modulus core reactions to pressure), and pressure may be calculated independent of temperature effects and drift factors. Also, while provision of low modulus and high modulus cores have been described with regard to this exemplary embodiment, any fiber construction that causes the fiber to deform under pressure is contemplated, including for example, a single core (provided at least partially along the core measurement portion) having a different modulus than the light guiding cores and having a different spacing with regard to cores  12  and  14 . Also, the terms “low modulus” and “high modulus” are merely indicative of a difference in the modulus of the two cores, and are not meant to necessarily imply a great difference in modulus properties between the two cores  38  and  40 . 
         [0026]    Referring now to  FIG. 7 , a cross-sectional plan view of an exemplary multi-core fiber  42  is illustrated as being actuated by well and reference pressures, illustrated generally at  36  and  44 , respectively. A media isolated housing  46  is provided over the fiber  42  and includes a pressure seal  48 , separating the well and reference pressure zones. Referring now to  FIG. 8 , the fiber  34  includes light guiding cores  12  and  14 , which are differentially spaced relative to a hollow core  50 . Hollow core  50  extends from the well pressure zone  36  to the reference pressure zone  38 , and causes deformation of the fiber  34  due to the difference in pressure between the reference pressure zone and the well pressure zone. Due to the differential spacing of the cores  12  and  14  relative to the hollow core  50 , the bending will affect the light guiding cores  12  and  14  differently, and the change in pressure in the well pressure zone  36  can be measured. 
         [0027]    It will be apparent to those skilled in the art that, while exemplary embodiments have been shown and described, various modifications and variations can be made to the embodiments disclosed herein without departing from the spirit or scope of the invention. Accordingly, it is to be understood that the various embodiments have been described by way of illustration and not limitation.