Abstract:
A fiber optic sensor for simultaneously and independently measuring temperature and axial stress. The fiber sensor includes a pair of polarization-maintaining fibers that have known strain and temperature response curves. Each fiber has a plurality of fiber segments in which the elliptical cores are rotated 45° relative to the preceding core segment. Thus, the phase shift induced by temperature or stress in each of the fibers is detected, and the strain and temperature are derived from the detected phase shift. The fiber optic sensor is capable of dual operation. As both a temperature sensor and an axial stress sensor.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/112,726, filed Dec. 18, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fiber optic sensors, and more particularly to a method and apparatus for independently measuring the temperature and axial strain of an optical fiber. 
     2. Technical Background 
     Fiber optic sensors, and in particular distributed fiber sensors, are of interest for smart structures and other monitoring applications. Smart structures are often composite structures that may incorporate electrical communication devices for monitoring or actively controlling the operation of the structure. A sensor is required to monitor the conditions the smart structure is subjected to. A fiber sensor, for example, can be embedded within the layers of the composite structure to determine strain and temperature. There are other industrial applications that require knowledge of the environment in order to control both the quality and productivity of the process. Interest has peaked recently with the encouraging results obtained using Bragg gratings distributed along the length of the sensing fiber. One issue that arises with fiber optic sensors relates to their sensitivity to both temperature and strain. In one approach that has been considered, a combined strain and temperature sensor using polarization-maintaining fibers was developed. Unfortunately, it was determined that the temperature and the strain values obtained by the sensor were dependent upon one another. Thus, the values measured by the sensor were inherently skewed. 
     A sensor that can measure temperature without being adversely affected by a strain component, or conversely, a sensor that is able to measure strain without a temperature component is therefore desired. 
     In another approach, a first polarization-maintaining fiber having an elliptical core is fused to a second polarization-maintaining fiber having an elliptical core. The major axis of the second fiber is rotated 90° with respect to the first fiber. When a polarized light signal is transmitted through the fibers, the temperature and strain affect the phase of the light signal differently. This relationship is characterized by the following equations: 
     
       
         Δφ 1   =A   1   L   1   ΔT+B   1   ΔL   1 ,  (1) 
       
     
     
       
         Δφ 2   =A   2   L   2   ΔT+B   2   ΔL   2 .  (2) 
       
     
     wherein Δφ 1  is the change in phase difference in the first fiber, A 1  is the temperature coefficient for the change in temperature of the first fiber, L 1  is the length of the first fiber, ΔT is the change in temperature, B 1  is the strain coefficient for the change in strain of the first fiber, ΔL 1  is the change in the length of the first fiber due to strain, Δφ 2  is the change in phase difference in the second fiber, A 2  is the temperature coefficient for the change in temperature of the first fiber, L 2  is the length of the second fiber, ΔT is the change in temperature, B 2  is the strain coefficient for the change in strain of the second fiber, ΔL 2  is the change in the length of the second fiber due to strain. 
     In order to “de-couple” temperature and strain, the two fibers must be selected such that either their strain coefficients are equal, or that their temperature coefficients are equal, such that: 
     
       
           B   1   ΔL   1   =B   2   ΔL   2 , or  (3) 
       
     
     
       
           A   1   L   1   ΔT=A   2   L   2   ΔT.   (4) 
       
     
     Thus, when the phase differences of the two fibers are subtracted, 
     
       
         Δφ=Δφ1−Δφ2  (5) 
       
     
     The variable having equal coefficients is eliminated. Thus, a single variable is obtained. However, there are disadvantages to this approach. First, the two have fibers must be precisely selected to equalize the phase difference between the first and second fibers caused by either strain or temperature. Secondly, it is understood from equations 3, 4, and 5 that the sensor is limited to detecting either temperature or strain. It cannot detect both simultaneously. 
     Thus, a need exists for a fiber optic sensor that has the ability to accurately measure strain on a fiber without that measurement being affected by the temperature, while simultaneously being able to accurately measure the temperature of the fiber&#39;s environment without the temperature measurement being affected by the applied strain. 
     SUMMARY OF THE INVENTION 
     The existing problems discussed above are solved with the present invention. The present invention includes a pair of fibers each having a plurality of polarization-maintaining fiber segments and a phase shifter disposed therein. Because the strain and temperature response curves of the phase shifters deployed in each fiber are so different, temperature and strain can be measured independently and simultaneously. 
     One aspect of the invention relates to an optical fiber that propagates a light signal characterized by a center wavelength. The optical fiber is disposed in an environment and used for measuring a plurality of environmental parameters. The optical fiber includes a plurality of polarization-maintaining fiber segments, each of which has a cladding and an elliptical core. The major axis of each of the plurality of polarization-maintaining fiber segments is rotated 45° with respect to a preceding fiber segment, and optically connected to that preceding fiber segment. The optical fiber also includes a sensing element disposed within the plurality of polarization-maintaining fiber segments. The sensing element shifts the center wavelength of the light signal at a predetermined rate in response to the plurality of environmental parameters. 
     Another aspect of the invention relates to a Mach-Zehnder device that couples a light signal characterized by a center wavelength. The Mach-Zehnder device is disposed in an environment and used to measure a plurality of environmental parameters. The Mach-Zehnder device includes a first polarization-maintaining fiber for propagating the light signal. The first polarization maintaining fiber includes a first elliptical core, a first cladding, and a plurality of first fiber segments, wherein each of the plurality of first fiber segments is rotated 45° with respect to a preceding first fiber segment and optically connected to the preceding first fiber segment. It also includes a second polarization maintaining fiber disposed adjacent to the first polarization-maintaining fiber. The second polarization maintaining fiber includes a second elliptical core, a second cladding, and a plurality of second fiber segments, wherein each of the plurality of second fiber segments is rotated 45° with respect to a preceding second fiber segment and optically connected to the preceding second fiber segment. A coupling region is disposed between the first polarization-maintaining fiber and the second polarization-maintaining fiber for coupling the light signal between the first and second polarization-maintaining fibers. A sensing element is disposed in the first and second polarization maintaining fibers. The sensing element shifts the center wavelength of the light signal at a first predetermined rate in the first polarization-maintaining fiber and by a second predetermined rate in the second polarization-maintaining fiber, in response to one or more of the plurality of environmental parameters. 
     Another aspect of the invention relates to a fiber optic sensor disposed in an environment and used for measuring a plurality of environmental parameters. The fiber optic sensor includes a polarized light source for transmitting a light signal having a center wavelength and a first polarization-maintaining fiber connected to the polarized light source. The first polarization-maintaining fiber includes a first elliptical core, a first cladding, and a plurality of first fiber segments. The fiber optic sensor also includes a second polarization-maintaining fiber disposed adjacent the first polarization-maintaining fiber. The second polarization-maintaining fiber includes a second elliptical core, a second cladding, and a plurality of second fiber segments. A coupling region is disposed between the first polarization-maintaining fiber and the second polarization-maintaining fiber, such that the light signal is coupled between the first and second polarization maintaining fibers. A sensing element is disposed in the first and second polarization maintaining fibers. The sensing element shifts the center wavelength of the light signal at a first predetermined rate in the first polarization-maintaining fiber and by a second predetermined rate in the second polarization-maintaining fiber, in response to the plurality of environmental parameters. 
     The fiber optic sensor of the present invention results in a number of advantages over sensors disclosed in the related art. First, the present invention accurately measures strain on a fiber without that measurement being affected by the temperature. Secondly, it accurately measures the temperature of the fiber&#39;s environment, without that measurement being affected by the strain. Further, the present invention has dual functionality, in that the sensor can be used to simultaneously measure both temperature and strain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of one embodiment of the fiber optic sensor according to the present invention; 
     FIG. 2 is a sectional view of a first polarization-maintaining optical fiber of the sensor of FIG. 1 showing the relationship between the three fiber segments that make up the polarization maintaining optical fiber; 
     FIG. 3 is detail view of the first and second polarization-maintaining fibers; 
     FIG. 4 is a graph comparing the relative wavelength shift with respect to temperature of the various polarization maintaining fibers used in the sensor of the present invention; 
     FIG. 5 is a graph comparing the relative wavelength shift with respect to applied strain of various polarization maintaining fibers used in the sensor of the present invention; and 
     FIG. 6 is a block diagram showing the detector assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the fiber optic sensor is shown in FIG. 1, and is designated generally throughout by reference numeral  10 . 
     In accordance with the invention, the fiber optic sensor  10  includes a pair of fibers  30 ,  40  that can be deployed in a ribbon cable and used to independently measure both longitudinal stress and temperature. Fiber optic sensor  10  has the utility of being able to accurately measure strain on a fiber without the measurement being dependent upon the temperature. Likewise, it is able to accurately measure the temperature of the fiber&#39;s environment without the temperature measurement being dependent upon the strain. 
     As embodied herein and depicted in FIG. 1, fiber optic sensor  10  includes a polarized light source  80  connected to a Mach-Zehnder device  20 . The Mach-Zehnder device  20  is connected to detector assembly  90 . The Mach-Zehnder device  20  includes a first polarization-maintaining fiber  30  and a second polarization-maintaining fiber  40 . A coupling region  50  is formed between the first polarization-maintaining fiber  30  and the second polarization-maintaining fiber  40 . The polarized light signal that is injected into the first polarization-maintaining fiber  30  is evanescently coupled into the second polarization-maintaining fiber  40  in the coupling region  50 . The light signals propagating in both of the fibers  30 ,  40  terminate in detector assembly  90 . The first fiber  30  includes segments  32 ,  34 , and  36 . The second fiber  40  includes segments  42 ,  44 , and  46 . Segments  34  and  44  are disposed in the environment  100  that is being measured. 
     FIG. 2 is a sectional view of the first polarization-maintaining fiber  30  shown in FIG.  1 . The first polarization-maintaining fiber  30  includes three fiber segments that are spliced or fused together in a back-to-back arrangement. The first segment  32  has cladding  320  and an elliptical core  322 . The second fiber segment  34  also has a cladding  340  and an elliptical core  342 . Note that the second fiber segment  34  is rotated around its longitudinal axis 45° with respect to the first segment  32 . The third fiber segment  36  also has cladding  360  and an elliptical core  362 . The third fiber segment  36  is rotated around its longitudinal axis 45° with respect to the second segment  34  and 90° with respect to the first segment  32 . Each polarization-maintaining fiber includes a sensing element  38 . The sensing element  38  is implemented by using either a doped core  380 , doped cladding  382 , or a grating  384 . 
     FIG. 3 is a detail view of the first polarization-maintaining fiber  30  and the second polarization-maintaining fiber  40 . In one embodiment of the present invention, the first polarization-maintaining fiber  30  and the second polarization-maintaining fiber  40  are deployed in a ribbon cable  110 . As briefly mentioned above, the first polarization-maintaining fiber  30  consists of three first fiber segments  32 ,  34 , and  36  that are fused together. The second polarization-maintaining fiber  40  consists of three second fiber segments  42 ,  44 , and  46  which are likewise fused together to form a single polarization-maintaining fiber  40 . The portion of ribbon cable  110  that is disposed in environment  100  includes segments  34  and  44 . 
     FIG. 4 shows the change in wavelength with respect to temperature of the polarization-maintaining fibers  22  and  24  using different sensing elements  38 . A sensing element  38  represented by temperature response curve  60  is implemented by using a 7% boron doped core. This fiber has a Δ=1% and provides a phase shift at the rate of −0.632 nm/C.° in response to temperature changes, wherein Δ signifies the fractional refractive index difference between the core and cladding. A sensing element  38  represented by temperature response curve  62  is implemented by using a 7% boron-doped cladding. This fiber has a Δ=2% and provides a phase shift at the rate of −0.222 nm/C.° in response to temperature changes. A sensing element  38 , is implemented by using a parabolic germania doped core that has 40% germania doping in the center of the core, 0% at the edge of the core, and has a Δ=2%. This fiber is represented by temperature response curve  64 . It provides a phase shift at the rate of −0.033 nm/C.° in response to changes in temperature. 
     FIG. 5 shows the change in wavelength with respect to axial stress of the three fibers discussed in FIG.  4 . The first sensing element implemented by using a 7% boron-doped core is represented by axial strain response curve  70 . It provides a phase shift at a rate of +9.67 nm/mε in response to axial strain, wherein mε is. The second sensing element is implemented by using a 7% boron doped cladding is represented by axial strain response curve  72 . It provides a phase shift at a rate of −23.37 nm/mε in response to axial strain. The third sensing element is implemented by using a parabolic germania doped core that has 40% germania doping in the center of the core, 0% at the edge of the core, and has a Δ=2%. The germania doped core is represented by axial strain response curve  74 . It provides a phase shift at the rate of −0.033 nm/mε in response to axial strain on the fiber. In one embodiment of fiber optic sensor  10  depicted in FIGS. 1-3, the polarization-maintaining fiber pair includes a first fiber  30  having the 7% boron-doped core and a second fiber  40  having the 7% boron-doped cladding. This fiber pair is appropriate because the response curves shown in FIGS. 3 and 4 are very different. In an alternate embodiment, the germania-doped fiber with a Δ=2% can be used with any of the other two fibers previously discussed, e.g., the boron-doped core fiber or boron-doped clad fiber. It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made in the selection of sensing element  38 . For example, any two of the three fibers discussed above with respect to FIGS. 3 or  4  can be selected and used to implement fiber sensor  10 . 
     FIG. 6 is a detail view of the detector assembly  90 . The detector assembly  90  consists of a polarizer  92 , a detector  94  and a processor  96 . The detector  94  receives an output signal from the first polarization-maintaining fiber  30  and a second output signal from second polarization fiber  40  after they are polarized by polarizer  92 . The presence of the sensing element  38 , not shown, in the first fiber  30  causes its output signal to be phase shifted by an amount φ 1  with respect to the input light signal from light source  80 , see FIG.  1 . The presence of the sensing element  38  in the second fiber  40  also causes its output signal to be phase shifted by an amount φ 2  with respect to the input light signal from light source  80 . The values of φ 1  and φ 2  are used by processor  96  to calculate a plurality of environmental parameters. Using any two of the fibers discussed above, the values for stress and temperature can be de-coupled using the following equations: 
     
       
         φ 1   =C   1   ΔT+K   1   ΔS   (6) 
       
     
     
       
         φ 2   =C   2   ΔT+K   2   ΔS   (7) 
       
     
     In equation (6) and (7), ΔT is the change in temperature, ΔS is the change in axial strain, C 1  and C 2  are the slopes of the temperature response curves (FIG. 4) of the first and second polarization-maintaining fibers  30 ,  40 , and K 1  and K 2  are the slopes of the axial stress response curves (FIG. 5) of the first and second polarization-maintaining fibers  30  and  40 . By subtracting equation (6) from equation (7), the following equation is obtained: 
     
       
         φ=φ 1 −φ 2 =( C   1   +C   2 )Δ T +( K   1   +K   2 )Δ S   (8) 
       
     
     By knowing the physical properties of each of the polarization maintaining fibers, an appropriate multiple can be chosen to eliminate a variable in equation 8: 
     
       
         φ=χφ 1 −φ 2 =( C   1   +C   2 )Δ T   (9) 
       
     
      φ=φ 1 −γφ 2 =( K   1   +K   2 )Δ S   (10) 
     Equation (9) can then be easily solved to find temperature, whereas equation (10) can be solved to find the axial stress on the fibers. 
     The fiber sensor shown in FIGS. 1 and 6 operates as follows. Light source  80  directs a polarized light signal into fiber  30 . The light signal is coupled into fiber  40  in coupling region  50 . Segments  34  and  44  are disposed in the environment  100  being measured. The temperature and the strain in the environment  100  change the path length of segments  34  and  44  and thereby shift the phase of the light signal. By way of example, the sensing element  38  in fiber  30  is a 7% boron doped core and the sensing element  38  in fiber  40  is a 40% parabolic germania doped core. Thus, as the path length changes in fiber  30  and fiber  40  due to the temperature and strain changes in the environment, the phase shift φ 1  and φ 2  sensed by each fiber is different because of the different type of sensing element present in each fiber. These values will be in accordance with the Temperature and Strain Response Curves shown in FIGS. 4 and 5. The phase shifts are detected by the detector  94  after the light signal is polarized by polarizer  92 . The processor  96  is programmed to know which sensing element  38  is present in each fiber. Thus, after receiving φ 1  and φ 2  from the detector  94 , it supplies the appropriate constants for equations (6)-(10) and calculates temperature and strain. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations provided they come within the scope of the appended claims and their equivalents.