Abstract:
The present invention provides the capability of ascertaining, through a quick and simple measurement, locations on a structure that may have experienced damage that could result in reduced structure lifetime, strength, or reliability. The sensing element is a connectorized section of polarization maintaining (“PM”) optical fiber, where a length of PM fiber represents a fully distributed sensor array. Stress-induced changes to the sensor are measured through white-light Polarimetric interferometry. The output of the measurement is a data array representing the stress concentration magnitude at an array of locations along the length of the sensor. In an application, the knowledge of the optical fiber position on the structure, coupled with the measurement of stress locations along the fiber length, allows the user to determine locations on the structure with large stress concentrations. These locations may signify structural damage. This knowledge would allow the user to employ a more sophisticated system, albeit a larger and slower one, to fully characterize and evaluate that area of potential damage and take appropriate action.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Provisional Patent Application U.S. Ser. No. 60/619,419, entitled “Embeddable Polarimetric Fiber Optic Sensor and Method for Monitoring of Structures” and filed on Oct. 15, 2004, which is fully incorporated herein by reference. 
     
    
     GOVERNMENT LICENSE RIGHTS  
       [0002]     This invention was made with Government support under contract number F33615-02-C-5013, awarded by the Missile Defense Agency. The Government has certain rights in the invention. 
     
    
     BACKGROUND  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates generally to a stress sensor. More particularly, the present invention relates to a fiber optic sensor that monitors stress on a structure.  
         [0005]     2. Background of the Invention  
         [0006]     Embedding miniature sensors in structures, systems, storage and shipping containers, and other items allows the monitoring of these items to determine health, maintenance needs, lifetime, and other item characteristics. Information from miniature embedded stress sensors in a composite or other structure can tell a user information including whether or not the item has been dropped sufficiently to cause damage, stressed to a point that makes the integrity of the structure suspect, undergone more cyclic variations than it can handle, or has other health or performance issues.  
         [0007]     Most current structures do not employ embedded sensors to aid in the determination of structural health. Instead, structures are assigned an operational lifetime that is significantly shorter than what could be achieved. In addition, structures are given a maintenance schedule and maintenance or disposal guidelines that are more frequent and strict than may be necessary. These procedures result in excess cost to an application and the user as structures are replaced, repaired, and maintained more often than necessary, while also using maintenance techniques and products that are more sophisticated and costly than they need to be. By using embedded sensors within structures, users can measure the actual health of the structure at a given point in time, regardless of the previous conditions seen by the structure, and then perform maintenance and replacement activities as necessary based on diagnosis of the structures&#39; health rather than on a predetermined schedule.  
         [0008]     To maintain reliability and integrity of the materials, sensors are often needed to monitor structure degradation while in storage, transport, or before use. A common method for detection of strain in a structure is to embed fiber optic sensors within the material. The two methods that are often used to detect damage utilize Fiber Bragg Gratings (“FBGs”) and etalons.  
         [0009]     FBGs and the associated Bragg phenomenon have long been studied and implemented in numerous commercial applications, some of which have been related to structural health monitoring (“SHM”). A Fiber Bragg Grating is a periodic index of refraction gradient created in the optical fiber core. The grating allows only certain wavelengths to pass through the grating and others to be reflected back from the grating. Strains in the structure, and hence the optical fiber, alter the periodicity of the grating, thereby altering its transmission and reflection properties. These changes can be measured and stress concentrations can be calculated from the data.  
         [0010]     In particular to SHM, FBGs have long been studied at a research level while some have been carried to the commercial availability stage. Regardless of the commercial readiness of a particular FBG, it can be described as a localized fiber optic sensor. In other words an FBG is a sensor that has a sensitive area that is dependent on the length of the grating that has been written on the optical fiber. A FBG cannot, for example, detect an applied strain that is not coincident or directly applied to the area of the optical fiber that contains the Bragg Grating. In many applications this is an acceptable operational procedure, while others may require a more distributed sensing technique.  
         [0011]     One procedure to allow for multiple FBG sensing sites along a single optical fiber is to multiplex the FBGs together using more sophisticated instrumentation. Three problems often exist with this approach to achieve a partial distributed sensor: multiple sites along the fiber must be prepared to write the FBGs, sites of interest on the structure must be predetermined and matched to the location of the gratings, and more sophisticated and expensive instrumentation is required to interrogate multiple FBGs.  
         [0012]     One concern that often accompanies the procedure to embed FBGs into a structure is that in many cases the protective acrylate coating of the optical fiber has to be removed in order to write the FBG. The removal of the protective coating requires special procedures to minimize potential damage due to handling and or environmental conditions. These procedures can often restrict the type of environment that can implement a FBG or require a protective device that is not allowable in the structure to be monitored.  
         [0013]     Similar in sensor arrangement to FBGs are Fabry-Perot Etalons (“FPE”). Etalons are a point source detection scheme that can measure stress applied to the structure that the FPE is embedded in. Etalons have been shown to have exceptional characteristics for detecting stress in a matrix and indicating relative levels of that stress. To be useful and to ensure optimum sensitivity, however, the potential locations for damage must be identified before the etalons are placed in the structure. Also, as is the case with FBGs, the protective coating on the fiber near the etalon is most often removed to improve sensitivity of the sensor. This removal of material drastically decreases the robustness of the sensor, which could limit its end application for use in hostile environments.  
         [0014]     It would therefore be desirable to have a fiber optic stress sensing system that overcomes the limitations of FBGs and FPEs and offers a more robust and cost effective sensor that can be installed in a distributed fashion over a structure without requiring pre-designation of areas of potential stress.  
       SUMMARY OF THE INVENTION  
       [0015]     It is therefore an object of the present invention to provide a stress sensing system using optical fiber distributed throughout a structure without requiring predetermined areas of interest on the structure.  
         [0016]     It is a further object of the invention to provide a fiber optic stress sensing system that does not require the removal of the protective coating from the optical fiber.  
         [0017]     It is a further object of the invention to provide a fiber optic stress sensing system that utilizes commercially-available instrumentation and processes to monitor strain on a structure.  
         [0018]     In contrast to the prior art techniques, which have limited robustness and which require pre-identification of the potential locations of damage, the present invention uses simple white-light polarimetric interferometry and commercial-of-the-shelf (“COTS”) polarization-maintaining optical fibers capable of sensing strain applied directly to the fiber anywhere along its length, without the need to create separate discrete sensor sites or the need to multiplex multiple sensors to cover an entire region of the structure. This fully distributed sensing capability makes selective sensor placement less difficult and increases the overall sensitive area on a structure in which it is embedded.  
         [0019]     The present invention consists of a method and apparatus for the monitoring of structures using embeddable polarimetric fiber optic sensors.  FIG. 1  is a simplified block diagram of the invention, which is comprised of polarization-maintaining optical fiber  1  of sufficient length to cover the region of the structure to be monitored  6 , a broadband white light optical source  2  with a short coherence length, an optical interferometer  3  with an optical path-scanning length sufficient to cover the length of the fiber sensor  1 , a signal processing unit  4 , and a computer or other microcontroller  5 . The optical fiber sensor  1  is first embedded or attached to the structure  6  in a fashion that allows the stresses in the structure  6  to be transferred to the fiber  1 . Then, a broadband optical source  2  launches a polarized short coherence length optical pulse into one end of the optical fiber  1 . Localized stresses within the structure  6  delay the transmission of that pulse as it travels down the fiber. At the other end of the fiber, a polarizer  11  (illustrated in  FIG. 2 ) projects the polarized output light onto a common polarization axis. The light then enters the interferometer  3  where light from one leg of the interferometer  3  is superimposed onto light from the other leg at varying amounts of delay. When the delay caused by changing the distance of one leg of the interferometer equals the pulse delay caused by localized stresses in the fiber sensor  1 , large constructive interference occurs, and the distance of the stress concentration from the end of the fiber sensor  1  can be calculated.  
         [0020]     For purposes of summarizing the invention, certain aspects, advantages, and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any one particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.  
         [0021]     These and other embodiments of the present invention will also become readily apparent to those skilled in the art from the following detailed description of the embodiments having reference to the attached figures, the invention not being limited to any particular embodiment(s) disclosed. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a simplified block diagram of an embodiment of the invention and its operation.  
         [0023]      FIG. 2  is a schematic diagram of the one embodiment of the sensor and its readout system.  
         [0024]      FIG. 3  illustrates an example of the data collected from one embodiment of the sensor.  
         [0025]      FIG. 4  is a graphical representation of the sensing operation.  
         [0026]      FIG. 5  is a graphical representation of the readout operation.  
         [0027]      FIG. 6  is a simplified illustration of the process of embedding the fiber optic sensor in a composite wound structure.  
         [0028]      FIG. 7  shows an example of a technique for embedding the fiber optic sensor in a composite structure.  
         [0029]      FIG. 8  is an illustrative scan from sensitivity testing of a fiber optic sensor. 
     
    
       [0030]     Repeat use of reference characters throughout the present specification and appended drawings is intended to represent the same or analogous features or elements of the invention  
       DETAILED DESCRIPTION  
       [0031]     The present invention and its advantages are best understood by referring to the drawings. The elements of the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention.  
         [0032]     One embodiment of the invention is shown in  FIG. 2 . This embodiment uses a 30 mW 1565 nm super-luminescent light emitting diode (“SLED”) with a 3 dB bandwidth of 38 nm, resulting in a coherence length of 41 μm as the broadband optical source  2 , although other optical sources could be used without departing from the scope of the invention. The output of the optical source  2  is polarized and aligned to one axis of the embedded fiber sensor  1 . The fiber sensor  1  itself is a continuous length of polarization-maintaining single mode optical fiber wound around and embedded into the device under test  6 . The fiber sensor  1  output is first transmitted through a polarizer  11  aligned at 45 degrees to the polarization axis of the fiber sensor  1 . The output from the polarizer  11  is launched into the input of an optical interferometer  3  configured as a standard Michelson interferometer. One leg of the interferometer  3  is scanned by an optical stage and stepper motor (within the scanning mirror  13 ), with the scanning distance recorded using an optical encoder (within the scanning mirror  13 ). A signal processor  16  controls the movement of the scanning mirror  13  and converts analog data from the interferometer into digital form for recording on the PC  17 . The amplitude of the interferogram that results from at each step of the scan is collected on a photodiode detector  12  and recorded by the PC  17 . As shown in  FIG. 3 , large amplitudes within the interferogram represent points of localized stress  18 .  
         [0033]     Although the embodiment discussed above utilizes a Michelson interferometer, the system and method disclosed herein would also work with other types of optical interferometric devices (such as a Sagnac interferometer) in which the optical path distance of one beam of light can be varied significantly and then superimposed upon the original beam of light.  
         [0034]     The readout technique for the sensor is a white light interferometric technique that operates as shown in  FIG. 4 . The polarized light pulse  20  of the optical source is launched into the input end  27  of the optical fiber sensor  1 . Due to the birefringence of that fiber, the input light pulse  20  will have a tendency to align itself with one of the two stable polarization axes (the primary polarization axis  28  and the orthogonal axis  29 ) within the fiber. However, local stress gradients  21  along the length of the fiber sensor  1  alter the local birefringence of the fiber sensor  1 . These stress-induced local changes in birefringence act as stress sensors and couple light from one of the polarization axes into the other, resulting in a division of the light pulse  20  into a “remaining” pulse  22  that remains in the primary polarization axis  28  and a “coupled” pulse  23  in the orthogonal axis  29 . Larger stresses in the fiber sensor  1  produce a larger amplitude of the coupled pulse  23 . The birefringence also yields a difference of light propagation velocities for the two polarization axes. Therefore, as the original launched light travels farther away from the point of local stress, the light pulses will separate in space, but will remain in their respective polarization states, creating a “pulse delay” between the remaining pulse  22  and the coupled pulse  23 . At the output end  25  of the fiber sensor  1 , a 45° polarizer  11  projects the light pulses  22  and  23  from their separate polarization axes onto a common axis  26 .  
         [0035]     Referring to  FIG. 2 , after the light exits the fiber sensor  1  and polarizer  11 , it is launched into an interferometer  3 . The interferometer  3  uses a beamsplitter  15  to divide the light into two optical paths. A photodiode detector  12  within the interferometer  3  then detects intensity changes resulting from the interference of the two waves with different phases traveling in the two optical paths. The purpose of the interferometer  3  is to measure the stress-induced delay in the light pulses  22  and  23  ( FIG. 4 ) by altering the optical path difference of two beams of that light. The amount of optical path difference required translates to the amount of delay and the distance of the output end of the optical fiber sensor  1  back to the local stress gradient. This process is illustrated in  FIG. 5 .  
         [0036]      FIG. 5A  illustrates the condition of the interferometer  3  at the output of the beamsplitter  15  before the scanning mirror  13  is adjusted. At this stage of the process, the beamsplitter  15  has split the beam of light containing the light pulses  22  and  23  into two beams ( 30  and  31 ). At this stage, because the stationary mirror  14  and the scanning mirror  13  are equidistant from the beamsplitter  15 , the output monitor would show no path length difference between the two beams, so the pulse interference and detector output would be high (i.e., there is large constructive interference). This first high output level does not represent a stress concentration, but rather represents the position of the optical source in the system. The readout for this condition would appear as that of sensor length zero (“0”) on  FIG. 3 .  
         [0037]     In  FIG. 5B , the scanning mirror  13  has moved some distance from the beamsplitter  15 , but the relative delay between the beams  30  and  31  from the two mirrors is smaller than the pulse delay created in the optical fiber sensor, and the output monitor would show low pulse interference (i.e., no stress concentration). In  FIG. 5C , the scanning mirror  13  has been adjusted until the delay induced by the interferometer  3  equals that from the fiber-induced pulse, producing large constructive interference, which is displayed on the output monitor and which represents a stress concentration, as illustrated in  FIG. 3  by stress concentrations  18 .  
         [0038]     When a stress concentration is placed on the optical fiber sensor a distance, L ( FIG. 4 ), from the output end of the fiber, a first light pulse is coupled from one axis of the optical fiber into the orthogonal axis, creating a second light pulse. Due to the fiber&#39;s birefringence, one light pulse travels down the fiber faster than the other. The speeds, v 1  and v 2  ( FIG. 4 ), with which each pulse travels down the optical fiber depends on the index of refraction for that axis of the fiber.  
           v   1     =     c     n   1         ,       and   ⁢           ⁢     v   2       =     c     n   2             
 
 , where c is the speed of light, and n 1  and n 2  are the indices of refraction for the two axes of the optical fiber sensor. 
 
         [0039]     When the first pulse reaches the end of the fiber, after having traveled a distance, L, from the stress concentration, the separation of the two pulses is given by: 
 
Δ x   f   =L *( n 1 −n 2)= L*Δn  
 
 , where Δn can be calculated from the specified beat length of the polarization maintaining fiber used in the fiber sensor (L B ) and the wavelength at which that beat length was measured (λ):  
         Δ   ⁢           ⁢   n     =     λ     L   B           
 
         [0040]     After the first pulse leaves the end of the optical fiber, its speed changes due to entering the air inside the interferometer, which has a different index of refraction than the optical fiber. During the time that the first pulse is outside of the fiber, but the second pulse is still in the fiber, the delay between the first pulse and the second pulse, Δx T , will widen, and is given by: 
 
Δ x   T   =L*n   fiber   Δn  
 
 , where n fiber  is the average index of refraction for the optical fiber. 
 
         [0041]     In the readout interferometer, the legs of the interferometer need to create an optical path length difference that removes the delay, Δx T . In a Michelson configuration, only one leg of the interferometer moves, and must travel half of that required distance. Therefore, the required scanning distance for the mirror to see a stress concentration is given by  
         Δ   ⁢           ⁢     x   T       =         1   2     ⁢   L   *     n   fiber     ⁢           ⁢   Δ   ⁢           ⁢   n     =       L   ⁢           ⁢     n   fiber     ⁢   λ       2   ⁢     L   B               
 
 , where L is the distance from the output of the fiber sensor to the stress concentration, λ is the wavelength of light used in the measurement, L B  is the beat length for the polarization maintaining fiber in the sensor, and n fiber  is the average index of refraction for the optical fiber in the sensor. 
 
         [0042]     The readout interferometer tracks the scanning distance, allowing the system to determine the distance from the output end of the fiber to the stress concentration through the relation:  
       L   =       2   ⁢     L   B     ⁢   Δ   ⁢           ⁢     x   T           n   fiber     ⁢   λ           
 
         [0043]     For the illustrated embodiment of the sensor, the following parameters are used: 
 
L B =2 mm 
 
Δx T =3.2 mm 
 
n Fiber =1.65 
 
λ=1.55 μm 
 
 , which yield a total of 10 meters of sensor length that can be scanned in the illustrated embodiment. The fiber length that can be scanned by the system is dependent on the mirror scan length achievable in the interferometer, and the precision of the optical components and their alignment in the interferometer. 
 
         [0044]      FIG. 6  illustrates in simplified form one technique for embedding the fiber sensor into a structure utilizing a filament winding system that is well known in the art of composite fabrication. In this technique, the fiber sensor  1  is embedded into a composite wound structure  40 . The winding system for the composite structure  40  has a fiber placement spool  41  that scans back and forth as the structure rotates to place the fiber sensor  1  spirally about the circumference of the structure  40 . After sensor winding is completed, additional layers of composite material are wound on the structure  40  on top of the fiber sensor  1 , thereby embedding it into the structure  40 . After the winding operation is complete, the fiber sensor  1  is connectorized for connection to the readout system (not illustrated).  
         [0045]      FIG. 7  illustrates an alternative method of embedding the fiber sensor  1  in a composite panel by sandwiching the fiber sensor  1  between a top composite panel  45  and a bottom composite panel  46 .  
         [0046]     Although  FIGS. 6 and 7  illustrate two methods of embedding the fiber sensor into a structure, any method whereby an optical fiber can be rigidly adhered to a structure could also be employed without departing from the scope of the invention. This rigid attachment can be achieved by the composite embedding processes discussed above, but also could be achieved with a variety of adhesives. Additionally, the fiber sensor can be installed on virtually any rigid material or structure, e.g., metals, composites, hard plastics, etc.  
         [0047]     One object of this invention is to provide the user with a means of readily identifying potential areas of damage on a structure. This quick determination that there is potential damage would then be followed by a more sophisticated analysis of the potential damage area.  
         [0048]     An alternative embodiment of the invention would use a pulsed laser source rather than a broadband optical source such as a SLED. The pulsed source, after traveling through the sensor, could be launched into an interferometer and a similar readout performed.  
         [0049]     Tests have been performed to evaluate the sensitivity of one embodiment of the fiber optic sensor. A finite element model was developed in preparation for a test of a ten gram round cylinder loading a bare optical fiber. The model estimated 450με of strain for a 10 g applied weight. The test was performed and resulted in the scan of  FIG. 8 , showing a peak amplitude of approximately 0.5 volts, yielding a sensitivity of the sensor to mechanical strain of 1.11 mV/με. The noise floor of the sensor is approximately 10 mV, yielding a strain resolution of approximately 9με.