Abstract:
A reflective element for directing an optical signal into a fiber optic sensor having an optical fiber includes a plane containing a sharply defined straight line that separates between a first area of low reflectivity and a second area of high reflectivity. The plane is disposed parallel to a free end surface of the optical fiber so that the free end surface intersects the line of the reflective element, whereby relative movement between the free end surface of the optical fiber and the line in response to a physical change sensed by the fiber optic sensor induces variations in an optical signal reflected by the reflective element through the optical fiber, which variations allow measurement of the physical change.

Description:
[0001]    The present application is a continuation of U.S. patent application Ser. No. 14/287,361, filed May 27, 2014, and entitled REFLECTIVE ELEMENT FOR FIBER OPTIC SENSOR, the entirety of which is incorporated herein by reference. 
     
    
     THE FIELD OF THE INVENTION 
       [0002]    The present invention relates to fiber optic sensors, particularly to sensors substantially not affected by very strong electromagnetic fields able to work in high temperature conditions. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fiber optic sensors are known that use light energy and optical fibers to sense different physical parameters such as pressure, temperature, acceleration etc. Most of them consist of light source, photo detector, one or few optical fibers, reflective target and a sensitive to a certain physical effects element someway attached to the optical fibers or reflective target. Via a transmitting optical fiber light from a light source is dispatched to reflective target that partly reflects it back through a receiving optical fiber to a photo detector. Under a certain physical effects a sensitive element changes the relative position of the optical fiber and reflective target thereby changing the intensity of light reflected by the target into receiving optical fiber and transformed by the photo detector into electrical signal. Some of the fiber optic sensors include only one optical fiber combining transmitting and receiving fibers in one. Examples of such sensors are disclosed in U.S. 2009/0123112, U.S. 2007/0247613 and U.S. Pat. No. 5,771,091. 
         [0004]    The reflective target is the most exacting and thus most expensive part of these sensors. Even small distortions of its shape or degradation of its reflective surfaces caused by temperature variations can dramatically deteriorate the sensor characteristics. 
         [0005]    U.S. Pat. No. 4,915,882 discloses a method for manufacturing uniformly smooth monocrystal reflectors of copper, silver or gold using a crucible polished to optical quality on the surface in contact with the reflecting surface of the monocrystal. It is noted that monocrystal reflectors withstand much better the extreme thermal loads caused by laser beams, but that the advantages inherent in the monocrystalline structure of the reflecting metal body are partially lost again during forming and/or machining as these operations modify the homogeneous crystalline texture. Reflection produced by monocrystal reflectors was found to be better for etched surfaces than in polished surfaces. Etched surfaces, however, are nonhomogeneous so that while of interest as protective shields against laser beams they do not lend themselves to optical or similar purposes in which a specific optical path requires a precisely defined reflecting surface. 
         [0006]    U.S. Pat. No. 4,414,471 discloses sensing of acoustic waves achieved by providing spaced apart stationary and cantilevered optic fibers whereby inertial forces created by acoustic signals modulate an optical signal carried by the fibers through vibration of the cantilevered fiber. In one embodiment, the sensor includes a cantilevered beam mounted at the far end to a rigid structure and having a reflective member such as a concave minor at the free end thereof. The end of optical fiber is disposed at the center of the sphere of which the minor surface is a section. Light fed into the fiber is reflected from the minor, received by the fiber and applied to a detector at. When acoustic waves are incident on the transducer they will cause vibration of the cantilevered beam due to inertial forces. The minor attached to the beam also vibrates and amplitude modulates the light received by the mirror and returned to the fiber. 
         [0007]    Our co-pending U.S. Ser. No. 13/935,955, whose contents are wholly incorporated herein by reference, discloses a fiber optic accelerometer comprising a cantilever section which moves upon vibration or acceleration of the accelerometer such that its position relative to a reflective target changes thereby reducing the instantaneous intensity of light reflected by the target into the second end of the optical fiber and measured by the photo detector. The reflective target is formed of an optical fiber stub having a first end proximate the free second end of the optical fiber and a second end remote therefrom. 
         [0008]    In one embodiment, the first end of the optical fiber stub has a slanted surface formed at an angle to an optical axis of the optical fiber stub and the second end of the optical fiber stub is cut perpendicularly to the optical axis and is coated with a highly polished efficient light reflecting material. 
         [0009]    In another embodiment, the first end of the optical fiber stub has a stepped cut so as to present a first surface portion closer to the end of the optical fiber and a more distant second surface section and the second end of the optical fiber stub is cut perpendicularly to said optical axis and is coated with a highly polished efficient light reflecting material. 
         [0010]    In both cases, there is no change in reflectivity of the optical fiber stub, the variation in signal injected into the optical fiber being caused solely by the off-axis reflection of light from the optical fiber stub owing to the deflection of the cantilever such that movement of the free end of the optical fiber causes a lessor or greater amount of the reflected light to be captured by the free end of the optical fiber. The same is true in U.S. Pat. No. 4,414,471 where the concave mirror reflects light into the cantilever regardless of its deflection, the vibration of the minor serving to modulate the light prior to its reflection into the free end of the fiber. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    It is an object of the present invention to provide a reflective element for fiber optic sensors possessing in some cases sensitivity to a certain physical effects and having a simpler construction, and being low cost for its production. 
         [0012]    This object is realized in accordance with the invention by a reflective element for a fiber optic sensor having the features of the respective independent claims. 
         [0013]    The invention provides a reflective element for a fiber optic sensor based on a single optical fiber, said reflective element comprising a plane containing a sharply defined straight line that separates between a first area of low reflectivity and a second area of high reflectivity, said plane being disposed parallel to a free end surface of the optical fiber so that said free end surface intersects said line, whereby relative movement between the free end surface of the optical fiber and the line in response to a physical change sensed by the fiber optic sensor induces variations in an optical signal reflected by the reflective element through the optical fiber, said variations allowing measurement of the physical change. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1   a  shows schematically the construction of a reflective element for a digital fiber optic sensor made of mono-crystal material by technology of anisotropic etching and vapor deposition; 
           [0016]      FIGS. 1   b  and  1   c  are graphical representations showing intensity of a series of optical pulses directed to the free end of the fiber optic sensor of  FIG. 1   a;    
           [0017]      FIGS. 2   a  and  2   b  show schematically a respective cross-section and end elevation of a reflective element for a single axis fiber optic sensor based on a single fiber; 
           [0018]      FIG. 3  is a schematic view of a dual axis fiber optic sensor having two independent fibers and a single reflective element similar to that depicted in  FIG. 2 ; 
           [0019]      FIGS. 4   a  and  4   b  show schematically a reflective element for a fiber optic pressure sensor in an initial and subsequent deflected state, respectively; 
           [0020]      FIG. 5  shows a schematic view of a reflective element for a fiber dynamometer made of mono crystal material; and 
           [0021]      FIG. 6  shows a schematic view of a reflective element for a fiber optic temperature sensor made of mono crystal material. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0022]    In the following description of some embodiments, identical components that appear in more than one figure or that share similar functionality will be referenced by identical reference symbols. 
         [0023]      FIG. 1   a  is a schematic cross-section showing construction of a reflective element for a fiber optic sensor. The reflective element may be a plate made of mono-crystal material  1  with multiple parallel reflective stripes  2  disposed on the side faced to the free end  3  of an optical fiber  4  and separated by v-grooves. The v-grooves define lines separating the areas of high reflectivity from the areas of low reflectivity. The optical fiber  4  emits light  5  toward the reflective element and collects the back reflected light  6 . Each reflective stripe  2  comprises an uppermost reflective surface  7  deposited on a substrate layer  8  by vapor deposition or sputtering. The reflective surface  7  may be formed of a material having high reflectivity such as gold while the substrate layer  8  may be formed of a material having medium reflectivity such as platinum or nickel such that the respective reflectivity of the two layers is different. The v-grooves are made by wet anisotropic etching. The angle  8  between the opposing faces of the v-grooves depends of the selected mono-crystal material and should be no greater than 70°. In this case the intensity of light reflected in the direction of the free end of the optical fiber will be close to zero. Thus when, under a given physical load, the optical fiber  4  is displaced in the direction  10  relative to the reflective element and its free end  3  intersects the lines between adjacent areas of high and low reflectivity, the intensity of the reflected light collected by the free end  3  of the optical fiber  4  appears in the time domain  12  as a series of pulses having an asymmetrical profile  13  as shown in  FIG. 1   b.  When the optical fiber  4  is displaced in the reverse direction  11 , the profile of the pulses in the time domain will be of inverse shape as shown in  FIG. 1   c.  So the number of pulses and the shape of their profile define the magnitude and direction of optical fiber displacement. Such a reflective element may be used for direct digitization of a fiber optic sensor output signal. 
         [0024]      FIG. 2   a  shows schematically a partial cross-sectional view of a reflective element  14  for a single axis fiber optic sensor based on one optical fiber  4  wherein the reflective element has the shape of flat hollow frame made of mono-crystalline material  15  with one face  16  coated with a highly polished, efficient light reflecting material such as gold so as to form a good reflecting surface  7 . The internal edges of the aperture are beveled to form a truncated square pyramidal shaped aperture  17  with the base of the pyramid remote from the reflective face. The aperture is preferably formed by means of anisotropic wet etching, whereby the internal edges  18  and  19  are formed absolutely straight and strictly orthogonal to each other. The classic approach is by etching a hole in &lt;100&gt; silicon wafer using a chemical such as potassium hydroxide. The result is a square pyramidal shape aperture. The selected reflective material may be deposited by vapor deposition technology. Both of these technologies are well-suited to mass production enabling the manufacturing price of so delicate and precise a component to be dramatically reduced. The free end  3  of the optical fiber  4  emits light  5  in the direction of the reflective element and collects the reflected light  6 . Only movement of the free fiber end  3  in a direction perpendicular to the edge  19  shown in  FIG. 2   b  can produce variation of intensity of the reflected light and thus the valuable signal while movement in the parallel direction to the edge  19  cannot. Thus the reflective element produces a spatial filtration of a fiber movement making the fiber optic sensor sensitive to a given physical effect such as acceleration, transverse force, deformation, etc. in only one direction. 
         [0025]      FIG. 3  is a schematic view of a dual axis fiber optic sensor employing the reflective element  14  depicted in  FIG. 2  in combination with two independent optical fibers  21  and  27  capable of displacement in random directions under a given physical load. The free ends of both fibers are respectively mounted proximate the internal orthogonal edges  18  and  19  of the reflective element  14 . Thus the reflective element produces a spatial filtration of movement of the two fibers simultaneously making the fiber optic sensor sensitive to physical effects such as acceleration, transverse force, deformation, etc. in two strictly orthogonal directions as shown by the arrows  28 ,  29 . 
         [0026]    In the embodiments described so far, the sensor signal is obtained upon movement of the optical fiber in a direction that is perpendicular to a fixed edge of the reflective element. Only the optical fiber moves with the surface of its free end being substantially parallel to the highly reflective surface of the reflective element, which does not move. However, the equivalent effect can be achieved using other configurations wherein the reflective element itself moves in response to an applied force. In some embodiments movement of the reflective element induces movement of the free end of the optical fiber, while still retaining some relative movement with an edge of the reflective element. In other embodiments, the free end of the optical fiber remains fixed in space so that the required relative movement with an edge of the reflective element is induced by motion of the reflective element only. Non-limiting examples of these embodiments will now be described. 
         [0027]      FIG. 4   a  shows a schematic cross section of a fiber optic pressure sensor  30  having a sensor housing  31  supporting the optical fiber  4  and wherein a reflective element is formed as a generally L-shaped diaphragm  32  sealing a channel  33  in the sensor housing  31 . The diaphragm  32  is formed by wet processing of mono crystal material and has a generally elongate body portion that spans the width of the channel  33  terminating at an end of the channel proximate the free end of the optical fiber  4  in a stepped portion  34  whose height is about half the diameter of the optical fiber  4 . The internal face of the stepped portion  34  is coated with a highly polished efficient light reflecting material so as to form a reflective surface  7 . The optical fiber represents a cantilever beam that passes beneath a membrane parallel to its plane and under its center on the minimal distance from its inner surface. The optical fiber  4  conveys the light from a source of light (not shown) to the reflective surface  7  and conveys the reflected light back to a photodetector (not shown). Under applied force, P, the diaphragm is deflected by a deflection  37  as a function of applied force thereby applying a bending force to the optical fiber  4  and changing the position of its free end relative to the reflective stepped portion  34  ( FIG. 4   b ). Consequently, the intensity of light conveyed to the photodetector will also change proportional to the applied force, P. 
         [0028]      FIG. 5  shows a schematic cross section of a reflective element in the shape of cantilever beam  38  for a fiber optic dynamometer wherein the beam  38  is made of mono crystal material by wet etching with a stepped portion  34  on its free end coated by a highly reflective material to form a reflecting surface  7 . Under applied force  39  the beam  38  bends and thus changes the position of the reflective stepped portion  34  relative to the free end  3  of the optical fiber  4 . Consequently, the intensity of light conveyed to photodetector will also change proportional to the applied force. The cantilever beam defines an elongated surface that is perpendicular to an internal face of the stepped portion  34  and to which an applied force induces deflection of the stepped portion  34  relative to the free end  3  of the optical fiber  4 . 
         [0029]      FIG. 6  shows a schematic cross section of reflective element in the shape of cantilever beam  40  for a fiber optic thermometer wherein the beam  40  is made of mono crystal material by wet etching with a stepped portion  34  on its free end coated by a highly reflective material to form a reflecting surface  7 . One side of the beam  40  is coated with a layer of material  41  characterized by a coefficient of thermal expansion (CTE) that is very different from that of the mono crystal material of which the beam  40  is formed. At ambient temperature variations, the beam  40  bends and thus changes the position of the reflective stepped portion  34  relative to the free end  3  of the optical fiber  4 . Consequently, the intensity of light conveyed to photodetector will also change proportional to ambient temperature change. 
         [0030]    In the embodiment of  FIG. 6 , the cantilever beam defines an elongated surface that is perpendicular to an internal face of the stepped portion  34  and that supports the layer of material  41 . Typically, the layer of material  41  is coated on the elongated surface of the beam. But it could equally well be riveted or attached thereto using adhesive as in known per se.