Patent Publication Number: US-2022214158-A1

Title: Force sensing in a distal region of an instrument including single-core or multi-core optical fiber

Description:
RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 17/115,694, filed on Dec. 8, 2020, which is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 16/506,997, filed on Jul. 9, 2019, which is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 15/572,135, filed on Nov. 6, 2017, which is a U.S. National Stage Filing under 35 U.S.C. 371 from International Application No. PCT/US2016/032051, filed on May 12, 2016, and published as WO 2016/186951 A1 on Nov. 24, 2016, which claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/162,062, filed on May 15, 2015, each of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The technology relates to instruments that use optical fiber sensing. 
     INTRODUCTION 
     Optical strain sensing is a technology useful for measuring physical deformation of a waveguide caused by, for example, the change in tension, compression, or temperature of is an optical fiber. This can be done with a standard, single core optical fiber or with a multi-core optical fiber. A multi-core optical fiber includes multiple independent waveguides or cores embedded within a single fiber. A continuous measure of strain along the length of a core can be derived by interpreting the optical response of the core using swept wavelength interferometry. With knowledge of the relative positions of the cores along the length of the  20  fiber, these independent strain signals for each of the cores may be combined to gain a measure of the strain profile applied to the multi-core optical fiber. The strain profile of the fiber refers to the measure of applied bend strain, twist strain, and/or axial strain along the length of the fiber at a high (e.g., less than 50 micrometers) sample resolution. In a technique known as optical position and/or shape sensing, detailed for example in commonly-assigned U.S. Pat. No. 8,773,650 to Froggatt et al, which is incorporated herein by reference, this strain profile information can be used to reconstruct the three dimensional position of the fiber. 
     SUMMARY 
     The inventors realized that it would be desirable to be able to determine one or more forces present at the distal region of an instrument and that optical strain sensing technology could be used to determine such forces. 
     Example embodiments include an optical force sensor that includes an optical fiber, a core included in the optical fiber, an instrument including the optical fiber, the instrument having a distal region, and a tubular structure encasing an end of the optical fiber and secured at the distal region of the instrument. The optical fiber is configured for connection to an optical interferometric system which processes reflected light from a portion of the core included within the tubular structure that does not include Bragg gratings to produce a measurement of a force present at the distal region of the instrument. 
     In a non-limiting example implementation, the optical fiber is tapered within the tubular structure and is secured within the tubular structure by an epoxy index-matched to the optical fiber. The epoxy transfers strain from the distal region of the instrument to the optical fiber. 
     The tubular structure may be made, for example, of metal, glass, or polymer. In a non-limiting example implementation, the tubular structure completely surrounds the end of the optical fiber. 
     The tubular structure may be secured at the end of the optical fiber using a mechanical attachment, an adhesive attachment, or a flame spray attachment. 
     In a non-limiting example implementation, the instrument includes a first conduit into which the optical fiber is inserted. The tubular structure is bonded to the first conduit at a bonded region that extends as far as the tubular structure. The core includes Bragg gratings except on a portion of the core defined by the bonded region. 
     In another non-limiting example implementation, the instrument includes a first conduit into which the optical fiber is inserted. The tubular structure is bonded to the first conduit at the end of the optical fiber at a bonded region that extends beyond the tubular structure and secures the optical fiber to the first conduit. The core includes Bragg gratings that extend into the bonded region. 
     In another non-limiting example implementation, the optical fiber includes multiple cores. 
     In another non-limiting example implementation, the instrument includes a first conduit into which the optical fiber is inserted and a second conduit containing another optical fiber encased in a similar tubular structure. The optical interferometric system processes reflected light from portions of the core contained within the tubular structures of the respective optical fibers that do not include Bragg gratings to produce a measurement of forces present on the respective tubular structures. 
     Further example embodiments include an optical processing apparatus having an optical fiber, a core included in the optical fiber, an instrument including the optical fiber, the instrument having a distal region, a tubular structure encasing an end of the optical fiber and secured at the distal region of the instrument, and an optical interferometric system coupled to the optical fiber. The optical interferometric system is configured to process reflected light from a portion of the core included within the tubular structure that does not include Bragg gratings to produce a measurement of a force present at the distal region of the instrument. 
     In a non-limiting example implementation, the optical fiber is tapered within the tubular structure and is secured within the tubular structure by an epoxy index-matched to the optical fiber, the epoxy transferring strain from the instrument to the optical fiber. In an example application, the optical interferometric system is configured to determine strain present on the distal region of the instrument and to determine the force based on the determined strain. Furthermore, the optical interferometric system may be configured to determine the strain present on the distal region of the instrument based on a first light reflection from a distal end of the optical fiber and on a second light reflection from a distal end surface of the epoxy. The first and second reflections form an interference pattern with a sinusoidal oscillation as a function of optical frequency. In this example, the optical interferometric system is configured to measure a change in a period of the sinusoidal oscillation to determine the strain present on the distal region of the instrument. 
     In another non-limiting example implementation, the optical interferometric system is configured to detect a scatter pattern within the tubular structure, compare the detected scatter pattern to a reference scatter pattern to determine a difference, and determine the force based on the difference. 
     In another non-limiting example implementation, the instrument includes a first conduit into which the optical fiber is inserted and a second conduit into which a second optical fiber is inserted. A second tubular structure encases an end of the second optical fiber located at the distal region and is secured within the second tubular structure by an epoxy index-matched to the optical fiber. The epoxy transfers strain from the first and second conduits to the optical fiber. The optical interferometric system is configured to process reflected light from a portion of respective cores in the first and second optical fibers that do not include Bragg gratings to produce a measurement of multiple forces present on the distal region of the instrument. 
     Further example embodiments include an optical processing method using an optical fiber including a core attached to an instrument having a distal region. A tubular structure encases an end of the optical fiber and is secured at the distal region of the instrument. The method includes processing, using by an optical interferometric system, reflected light from a portion of the core included within the tubular structure that does not include Bragg gratings to produce a measurement of a force present at the distal region of the instrument. The method may further include determining strain present on the distal region of the instrument and determining the force based on the determined strain. 
     In a non-limiting example implementation, the optical interferometric system determines the strain present on the distal region of the instrument based on a first light reflection from a distal end of the optical fiber and on a second light reflection from a distal end surface of the epoxy. The first and second light reflections form an interference pattern with a sinusoidal oscillation as a function of optical frequency. A change in a period of the sinusoidal oscillation is measured to determine the strain present on the distal region of the instrument. 
     In a non-limiting example implementation, a scatter pattern within the tubular structure is detected and compared to a reference scatter pattern to determine a difference. The force is determined based on the difference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a non-limiting example medical application of an instrument. 
         FIG. 2  is a diagram illustrating an example force exerted on an instrument; 
         FIG. 3  is a diagram illustrating example strains on an instrument; 
         FIG. 4  shows a non-limiting example embodiment of an optical fiber structure for use in an instrument; 
         FIG. 5  illustrates a non-limiting example embodiment of an end of an instrument including the optical fiber structure shown in  FIG. 4 ; 
         FIGS. 6A-6C  show various views of the end of the instrument shown in  FIG. 5 ; 
         FIG. 7  shows a non-limiting example embodiment using an OFDR-based, single core fiber force sensing system: 
         FIG. 8  is a flowchart illustrating example procedures for interferometrically-based single core fiber force sensing using the example system in  FIG. 7 ; 
         FIG. 9A  shows a non-limiting example graph of detected reflections from the distal region of the instrument; 
         FIG. 9B  shows a non-limiting example graph of detected strain based on the reflections from the distal region of the instrument shown in  FIG. 10A ; 
         FIG. 10  illustrates another non-limiting example embodiment of an end of an instrument including the optical fiber structure shown in  FIG. 4 ; and 
         FIG. 11  shows a non-limiting example embodiment using an OFDR-based, multiple core fiber force sensing system. 
     
    
    
     DETAILED DESCRIPTION 
     The following description sets forth specific details, such as particular embodiments for purposes of explanation and not limitation. But it will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. In some instances, detailed descriptions of well known methods, interfaces, circuits, and devices are omitted so as not to obscure the description with unnecessary detail. Individual blocks are shown in the figures corresponding to various nodes. Those skilled in the art will appreciate that the functions of those blocks may be implemented using individual hardware circuits, using software programs and data in conjunction with a suitably programmed digital microprocessor or general purpose computer, and/or using applications specific integrated circuitry (ASIC), and/or using one or more digital signal processors (DSPs). Software program instructions and data may be stored on a non-transitory, computer-readable storage medium, and when the instructions are executed by a computer or other suitable processor control, the computer or processor performs the functions associated with those instructions. 
     Thus, for example, it will be appreciated by those skilled in the art that diagrams hemin can represent conceptual views of structures and functional units. It will be appreciated that a flow chart represents processes which may be substantially represented in computer-readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     The functions of the various illustrated elements may be provided through the use of hardware such as circuit hardware and/or hardware capable of executing software in the form of coded instructions stored on computer-readable medium. Thus, such functions and illustrated functional blocks are to be understood as being either hardware-implemented and/or computer-implemented, and thus machine-implemented. 
     In terms of hardware implementation, functional data processing blocks may include or encompass, without limitation, a digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions. 
     In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer, processor, and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term “processor” or “controller” also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above. 
     In one example application, multicore optical fiber can be used to sense the shape of robotic medical instruments.  FIG. 1  shows a non-limiting example medical application of an instrument  10  that include an optical fiber-based sensor. In these and other applications, it is often desirable to provide shape sensing as close as possible to the distal region of the instrument. It is also desirable to terminate the fiber at the instrument distal region to suppress any strong back reflection by angle cleaving or tapering the end of the fiber. A strong reflection at the end of a fiber at a cleaved air/glass interface can overwhelm other, smaller reflections in the fiber such as Rayleigh backscatter, fiber Bragg gratings, or other minute reflections used for sensing strain. Therefore, it is often necessary to terminate the fiber in some way that suppresses the natural Fresnel reflection at the air/glass interface. 
       FIG. 2  is a diagram illustrating an example force exerted on an instrument  10 . Referring to the robotic medical example in  FIG. 1 , the force on the instrument  10  may be due to the distal region of pushing on tissue or some other structure which likely causes the instrument distal region to compress. Reference numeral  12  indicates a distal region of the instrument  10 . 
       FIG. 3  is a diagram illustrating example strains on an instrument  10 . The inventors recognized that sensing strain at the distal region of the instrument  10  can be used to determine what loads or forces are applied to the distal region. It is often advantageous to determine the loads or forces at the distal region of an instrument. For example, in a medical application, it may be advantageous to know how much force is being applied to tissue by a catheter or other medical instrument, e.g., to avoid puncturing the tissue and/or to provide feedback to the instrument operator. 
       FIG. 4  shows a non-limiting example embodiment of an optical fiber structure for use in an instrument  10 . An optical fiber  14  includes an optical core  16  surrounded by a cladding, which is protected by a surrounding coating  20 . A tapered end of the fiber  18  is encased in a protective tube  22  or similar structure and secured therein using any suitable securing mechanism. The tube  22  may be made of metal, glass, or polymer and completely surrounds the end of the optical fiber  18 . In an example embodiment, the core is secured using an index-matched epoxy  24 . The epoxy  24  transfers strain from the tube to the optical fiber. Reference numeral  19  indicates a distal end of the epoxy within the tube  22 . The fiber core shown in  FIG. 4  includes Bragg gratings  16 A, but those gratings are not present in a portion of the core  16 B encased in the tube  22 . Reference numeral  13  generally refers to a distal region of the optical fiber  14 . 
       FIG. 5  illustrates a non-limiting example embodiment of a distal region (indicated generally at  12 ) of an instrument  10  including the optical fiber structure shown in  FIG. 4 . The optical fiber  14  and metal tube  22  protecting the end of the fiber  18  is inserted into a conduit  26  in the instrument  10  and secured to at the end of the instrument. For example, the tube  22  may be mechanically bound to the instrument at the end of the instrument  12  via a bonding region  28 . This bond transfers the strain experienced at the end  12  of the instrument  10  to the end  13  of the fiber  14 . Example securing mechanisms other than mechanical attachment include securing the tube  22  to the conduit  26  at the end of the optical fiber using an adhesive attachment or a flame spray attachment. 
       FIGS. 6A-6C  show various views of the end of the instrument shown in  FIG. 5 .  FIG. 6A  shows a cross-section of the instrument having multiple conduits, with conduits  26   a  and  26   b  being examples of conduits in which optical fiber sensors are inserted.  FIG. 6 b    shows a side view following the cross-section line  6 B- 6 B shown in  FIG. 6A .  FIG. 6C  shows an enlarged portion of an end of one conduit  26 A identified by the dashed circle in  FIG. 6B . 
       FIG. 7  shows a non-limiting example embodiment using an OFDR-based, single core fiber force sensing system for use an instrument  10  including an optical fiber  14  having a distal region  13  such as that shown in Figure S. The optical fiber in the instrument is connected to an Optical Frequency Domain Reflectometry (OFDR) system which is an example of an optical interferometric interrogation system. A continuous measure of strain along the length of a core can be derived by interpreting the optical response of the core using swept wavelength interferometry. Optical time domain measurements with high resolution and high sensitivity may be achieved using OFDR. The single channel OFDR system (a single channel is used to interrogate a single optical fiber core) includes a tunable light source  100 , an interferometric interrogator  140 , a laser monitor network  120 , data acquisition electronic circuitry  180 , and a system controller data processor  200 . 
       FIG. 8  is a flowchart illustrating example procedures for interferometrically-based single core fiber force sensing using the example system in  FIG. 7 . The steps describe the operation for one core. For the multicore example embodiment described below, these steps are applied to each of the cores. 
     During an OFDR measurement, a tunable light source  23  is swept through a range of optical frequencies (step S 1 ). This light is split with the use of optical couplers and routed to two separate interferometers  26  and  28 . The first interferometer  26  serves as an interferometric interrogator and is connected via a connector  24  to a length of sensing fiber. Light enters the multicore sensing fiber  10  through the measurement arm of the interferometric interrogator  26  (step S 2 ). Scattered light from the sensing fiber  14  is then interfered with light that has traveled along the reference arm of the interferometric interrogator  26  (step S 3 ). The laser monitor network  28  contains a Hydrogen Cyanide (HCN) gas cell that provides an absolute wavelength reference throughout the measurement scan (step S 4 ). The second interferometer, within the laser monitor network  28 , is used to measure fluctuations in tuning rate as the light source is scanned through a frequency range (step S 5 ). A series of optical detectors (e.g., photodiodes) convert the light signals from the laser monitor network, gas cell, and the interference pattern from the sensing fiber to electrical signals (step S 6 ). A data processor in a data acquisition unit  32  uses the information from the laser monitor  28  interferometer to resample the detected interference pattern of the sensing fiber  14  so that the pattern possesses increments constant in optical frequency (step S 7 ). This step is a mathematical requisite of the Fourier transform operation. Once resampled, a Fourier transform is performed by the system controller  30  to produce a light scatter signal in the temporal domain for an initial orientation of the single core fiber  14  (step S 8 ). In the temporal domain, the amplitudes of the light scattering events are depicted verses delay along the length of the fiber. Using the distance that light travels in a given increment of time, this delay can be converted to a measure of length along the sensing fiber. In other words, the light scatter signal indicates each scattering event as a function of distance along the fiber. The sampling period is referred to as the spatial resolution and is inversely proportional to the frequency range that the tunable light source  100  was swept through during the measurement. 
     As the fiber is strained, the local light scatters shift as the fiber changes in physical length. These distortions are highly repeatable. Hence, an OFDR measurement of detected light scatter for the fiber can be retained in memory that serves as a reference pattern of the fiber in an unstrained state. A subsequently measured scatter signal when the fiber is under strain may then be compared to this reference pattern by the system controller  200  to gain a measure of shift in delay of the local scatters along the length of the sensing fiber (step S 9 ). This shift in delay manifests as a continuous, slowly varying optical phase signal when compared against the reference scatter pattern. The derivative of this optical phase signal is directly proportional to change in physical length of the sensing core (step S 10 ). Change in physical length may be scaled to strain producing a continuous measurement of strain along the sensing fiber. The measured strain(s) are then converted into force(s) experienced at the end of the instrument (step S 11 ). 
     Detected strain at the end of the instrument may be converted to force in accordance with the following. Young&#39;s modulus, E, may be calculated by dividing tensile stress by extensional strain in an elastic (initial, linear) portion of the stress-strain curve: 
     
       
         
           
             E 
             = 
             
               
                 
                   tensile 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   stress 
                 
                 
                   extensional 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   strain 
                 
               
               = 
               
                 
                   σ 
                   ɛ 
                 
                 = 
                 
                   
                     
                       F 
                       / 
                       
                         A 
                         0 
                       
                     
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         L 
                         / 
                         
                           L 
                           0 
                         
                       
                     
                   
                   = 
                   
                     
                       FL 
                       0 
                     
                     
                       
                         A 
                         0 
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       L 
                     
                   
                 
               
             
           
         
       
     
     where: 
     E is the Young&#39;s modulus (modulus of elasticity) 
     F is the force exerted on an object under tension; 
     A 0  is the original cross-sectional area through which the force is applied; 
     ΔL 0  is the amount by which the length of the object changes; and 
     L 0  is the original length of the object. 
     The Young&#39;s modulus of a material can be used to calculate the force exerted on it under specific strain: 
     
       
         
           
             F 
             = 
             
               
                 
                   EA 
                   0 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 L 
               
               
                 L 
                 0 
               
             
           
         
       
     
     where F is the force exerted on the material when contracted or stretched by ΔL. This equation can be simplified to F=E*A 0 ε, where ε may be the Young&#39;s modulus for and A 0  may be the cross sectional area of the distal region of the instrument  12 . 
     In some cases, the structural contribution of the optical fiber to the instrument may be ignored. On the other hand, as the stiffness of the fiber becomes a significant portion of the stiffness of the instrument, the fiber is preferably considered as part of the cross sectional area and modulus. In this latter situation, which may be the case for many medical instruments, A 0  is taken as the entire cross sectional area of the structure including the fiber and an effective E is modeled analytically for the combined components that make up the distal region structure of the instrument. Another alternative is to calibrate the system with a series of known loads. In this approach, a series of known loads is applied to the instrument, and the strain at the end of the fiber is measured for each load. A proportionality constant or curve is then determined relating strain on the fiber to load applied to the instrument. In effect, this proportionality constant or curve is a measure of A 0 E. 
     Ultimately, the system controller  200  calculates the a at the distal region of the instrument as described above, and the force(s) at the distal region of the instrument using the F=E*A 0 *c and the determined values for A 0  and E. 
       FIG. 9A  shows a non-limiting example graph of detected reflections from the distal region of the sensor. Using OFDR, these reflections appear as two distinct peaks in the time domain plot of reflection vs. distance. The first detected reflection is from the distal region of the instrument  12 , and the second detected reflection is from the terminating surface  19  of the epoxy  24  as shown in Figure S. As strain is applied to the distal region of the instrument  12 , the distance between these peaks changes. 
       FIG. 9B  shows a non-limiting example graph of the reflected spectrum at the distal region of the sensor based on the reflections from the distal region of the sensor shown in  FIG. 9A . In the spectral domain, these two reflections form an interference pattern with a sinusoidal oscillation as a function of optical frequency. The period of this oscillation is determined by the separation between the peaks. Therefore, measuring the change in the period of this spectral interference provides a measure of the strain at the end of the instrument  12 . This process applies if there are two or more reflections in the distal region structure. Each strain measure is converted to a force at the distal region of the instrument  12  using the conversion described above. 
     More minute reflections are caused by scattering in the fiber at the distal region. As can be seen in  FIG. 5 , within the tube  22 , light travels through a small section of fiber that includes Bragg gratings, then passes through a length of fiber where the heat from a fiber tapering process has erased the Bragg gratings, and then exits the fiber core altogether. In one example embodiment a measure of the force present on the tube  22  is obtained by measuring the scattering events within the tube and noting the changes that occur. Strain is determined by examining the cross correlation of the spectral amplitude of the scatter pattern obtained for the distal region in an unstrained reference state and the scatter pattern detected for the distal region in a strained state. 
       FIG. 10  illustrates another non-limiting example embodiment of an end of a fiber optic instrument  10  including the optical fiber structure shown in  FIG. 4 . The bonding region is extended such that some Bragg gratings within the fiber core at region  16 A are present within the bonded region  40 . The spectral shifts or phase derivatives of the reflections determined from these gratings may be used to determine the strain at the distal region. 
     For all of these embodiments, either single-core or multi-core fibers may be used. The selection would likely be determined by what is most convenient to the application. If multi-core fiber is already present for shape sensing, for example, then it may be most convenient to use the already present sensor. If not, then a single core fiber could be a less expensive and simpler to use option. 
       FIG. 11  shows a non-limiting example embodiment using a multiple channel OFDR, multiple core fiber force sensing system. This system is similar to that shown in  FIG. 7  except that the fiber includes multiple cores, i.e. a multi-core fiber  170 . Four examples cores are shown A-D. Similar procedures may be carried out for each core as shown in  FIG. 8  described above. In the case of multi-core fiber, bending at the distal region  13  causes different strains in the outer cores of the fiber, whereas axial strain results in common mode strain in all the cores of the fiber. This difference can be used to distinguish between strain due to bending and axial strains due to, for example, pushing on tissue. This eliminates the need for a second sensor to distinguish between these different causes of strain at the distal region  13 . 
     For multi-core fiber with multiple cores terminating into the same tube  22 , there is the possibility that the signals from each core will be mixed together within the tube  22  which may adversely impact accurate reflection signal detection. Therefore, it may be desirable to interrogate the multicore fiber sensor in this case with at least one of the cores being offset in interferometric path length to prevent interaction with the light from the other cores. See, for example, commonly-assigned, U.S. patent application Ser. No. 13/113,761, filed on May 23, 2011, and entitled “Interferometric Measurement with Crosstalk Suppression.” This is done by changing the fiber lengths in the interferometric interrogator (e.g., OFDR) such that they do not match for different cores in the multi-core sensor. For example, in  FIG. 11 , the reference or measurement path for each fiber core is altered so they are not the same as that of another fiber core. This offsets the apparent location in the time domain of the reflections from the distal region of the sensor for each core. One example offset is more than the length over which the reflections from the distal region occur. 
     As described above, bending forces can cause strain at the end of the instrument in addition to strains due to pushing on tissue or some other structure. It may be desirable to distinguish these two sources of strain. One example way to do this uses two (or more) sensors, e.g., each on opposite sides of the instrument. This is illustrated in  FIG. 6B  showing two fibers in two conduits  26 A and  26 B in the instrument. If the instrument is bent in the plane containing the two fiber sensors, one fiber sensor would experience compression and the other tension in equal but opposite magnitudes. The common mode strain would be the remaining strain due to force applied by the instrument distal region to some structure, e.g., tissue for a medical instrument like that shown in  FIG. 1 . This method is directed to single core fiber embodiments. As described above, using a multicore fiber allows for distinguishing bending from pushing with a single sensor. 
     To measure strains in the second sensor, two OFDR systems may be used, or a network and acquisition hardware may be used to measure all the desired cores in the two sensors. Optical splitters in the network, as shown in  FIG. 12 , may be used to accommodate multiple cores and more detection channels can be added to the acquisition hardware to accommodate more signals. 
     Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above description should be read as implying that any particular element, step, range, or function is essential such that it must be included in the claims scope. The scope of patented subject matter is defined only by the claims. The extent of legal protection is defined by the words recited in the allowed claims and their equivalents. All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the technology described, for it to be encompassed by the present claims. No claim is intended to invoke paragraph 6 of 35 USC § 112 unless the words “means for” or “step for” are used. Furthermore, no embodiment, feature, component, or step in this specification is intended to be dedicated to the public regardless of whether the embodiment, feature, component, or step is recited in the claims.