Patent Publication Number: US-8976365-B2

Title: Interferometric material sensing apparatus including adjustable coupling and associated methods

Description:
GOVERNMENT CONTRACT 
     This invention was made with Government support under Government Contract 03-180 awarded by the FBI. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of interferometers and, more particularly, to optical waveguide interferometers and related methods. 
     BACKGROUND OF THE INVENTION 
     Ultrasonic waves may be used to probe a variety of materials, particularly for thickness gauging and flaw detection. The ultrasonic waves are typically generated with a piezoelectric transducer. The ultrasonic waves propagate through the material, reflecting from interfaces (in thickness gauging applications) or internal features (in flaw detection applications). The scattered ultrasonic waves propagate back to the surface of the material, causing the surface to vibrate at the ultrasound frequency. This vibration may be detected with a piezoelectric transducer similar to the one used to generate the ultrasonic waves, and then analyzed to generate data about the material. 
     Optical detection techniques can be used in place of the piezoelectric transducers to remotely detect the ultrasonic waves. Generally, a laser probe beam is directed onto the material. When the surface vibrates it imparts a phase shift onto the reflected beam. This phase shift is detected with a photodetector after mixing the reflected probe beam with a stable reference beam and measuring the amplitude and frequency or phase of the photodetector output intensity fluctuations. The reference beam originates from the same laser source as the reflected probe beam, and the output signal from the photodetector corresponds to the surface motion. 
     One problem with laser detection systems is low sensitivity. Typically, the material surface that is being probed has a diffusely reflecting or scattering quality. Consequently, the reflected beam is highly aberrated and its wavefront is mismatched with respect to the reference beam. The resulting signal produced by the photodetector is therefore weak and lacks precision. 
     In U.S. Pat. No. 6,075,603 to O&#39;Meara, a contactless system for imaging an acoustic source within a workpiece is disclosed. In this system, an array of discrete optical detectors is arranged in a pattern. A probe beam is directed onto a vibrating surface in a pattern that corresponds to the detector array. The probe beam is reflected onto the detector array and a reference beam is also directed onto the detector array at an angle to the probe beam to produce fringe patterns on the detectors that correspond to the surface vibration pattern. A readout system utilizes the discrete detector outputs to produce an array output signal indicative of at least a size and two dimensional location for the acoustic source relative to the vibrating surface. This system, however, may not provide the desired accuracy, and may be sensitive to fluctuations in the length of the paths between the probe beam and the surface, and the reference beam and the surface. 
     U.S. Pat. No. 7,262,861 to Pepper discloses a laser ultrasonic inspection apparatus which enables remote sensing of thickness, hardness, temperature and/or internal defect detection. A laser generator impinges on a workpiece with light for generating a thermo-elastic acoustic reaction in a workpiece. A probe laser impinges on the workpiece with an annularly-shaped probe light for interaction with the acoustic signal in the workpiece resulting in a modulated return beam. A photodetector having a sensitive region is used for detecting an annularly-shaped fringe pattern generated by an interaction of a reference signal with the modulated return beam at the sensitive region. 
     This system, however, may not provide the desired accuracy, and may be sensitive to fluctuations in the length of the path between the probe beam and the surface, or fluctuations in the path lengths of the reference and measurement arms of the interferometer. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of the present invention to provide a biological material sensing apparatus. This and other objects, features, and advantages in accordance with the present invention are provided by a biological material sensing apparatus permitting adjustment of a gap between an interferometer probe and a workpiece that includes an excitation source configured to induce waves in a workpiece, and an optical waveguide interferometer configured to sense the induced waves in the workpiece. The optical waveguide interferometer comprises a probe segment having a probe segment end, and an adjustable coupler configured to permit setting a gap between the probe segment end and the workpiece. A controller is coupled to the adjustable coupler and configured to set the gap between the probe segment and the workpiece. 
     The optical waveguide interferometer may also include a laser source, and a photodetector coupled to the controller. An optical coupler operatively connects the laser source, the photodetector, and the probe segment. The controller is further configured to generate workpiece data based upon the sensed induced waves. 
     Setting the gap between the probe segment and the workpiece advantageously allows the optical waveguide interferometer to be tuned such that the distance between the probe segment end and the workpiece is a desired fraction or multiple of the wavelength of light emitted from the probe segment end. This helps to minimize distortions in the generated workpiece data. 
     In some applications, the adjustable coupler may comprise a piezoelectric body. Alternatively, the adjustable coupler may comprise a sleeve surrounding the probe segment end and at least one of a heating source and a cooling source associated therewith. In some applications, the heating source may be a laser. 
     The adjustable coupler comprises a sleeve surrounding the probe segment end. In addition, the adjustable coupler may further comprise a biasing member configured to urge the sleeve in contact with the workpiece. The probe end comprises an optical fiber with an angled endface, and the excitation source may comprise at least one pulsed laser. 
     A method aspect is directed to a method of sensing a workpiece comprising inducing waves in the workpiece using an excitation source, and sensing the induced waves in the workpiece using an optical waveguide interferometer comprising a probe segment end by at least setting a gap between the probe segment end and the workpiece. 
     According to another aspect, a biological material sensing apparatus comprises an excitation source configured to induce waves in a workpiece, and an optical waveguide interferometer configured to sense the induced waves in the workpiece. The optical waveguide interferometer comprises a laser source, and a probe segment having a probe segment end to be positioned adjacent the workpiece and defining a gap therebetween. An optical coupler is operatively connecting the laser source and the probe segment. In addition, a controller is coupled to the laser source and configured to adjust the wavelength of the laser source so that a desired multiple of the wavelength equals the gap between the probe segment end and the workpiece. 
     According to a further aspect, a biological material sensing apparatus includes an excitation source configured to induce waves in a workpiece, and an optical waveguide interferometer configured to sense the induced waves in the workpiece. The optical waveguide interferometer comprises a laser source, and a probe segment having a probe segment end to be positioned adjacent the workpiece and defining a gap therebetween. An optical coupler operatively connects the laser source and the probe segment, and a controller is coupled to the laser source and configured to adjust the wavelength of the laser source so that a desired multiple of the wavelength equals the gap between the probe segment end and the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a sensing apparatus, according to the present invention. 
         FIG. 2  is a schematic sectional view of an adjustable coupler, as used with the sensing apparatus of  FIG. 1 . 
         FIG. 3  is a schematic sectional view of another adjustable coupler, such as may be used with the sensing apparatus of  FIG. 1 . 
         FIG. 4  is a schematic sectional view of yet another adjustable coupler, such as may be used with the sensing apparatus of  FIG. 1 . 
         FIG. 5  is a schematic cross sectional view of an additional adjustable coupler, such as may be used with the sensing apparatus of  FIG. 1 . 
         FIG. 6  is a schematic block diagram of another embodiment of a sensing apparatus, according to the present invention. 
         FIG. 7  is a flowchart of a method of sensing a target in accordance with the present invention. 
         FIG. 8  is a partial schematic sectional view of an adjustable coupler of a biological sensing apparatus sensing an arterial wall in accordance with the present invention. 
         FIG. 9  is a partial schematic sectional view of an adjustable coupler of a material inspection apparatus sensing a weld in accordance with the present invention. 
         FIG. 10  is a schematic block diagram of another sensing apparatus in accordance with the present invention. 
         FIG. 11  is a schematic block diagram of yet another sensing apparatus in accordance with the present invention. 
         FIG. 12  is a flowchart of another method of sensing a target in accordance with the present invention. 
         FIG. 13  is a partial schematic sectional view of a biological sensing apparatus sensing an arterial wall in accordance with the present invention. 
         FIG. 14  is a partial schematic sectional view of a material inspection apparatus sensing a weld in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime and multiple prime notations are used to indicate similar elements in alternative embodiments. 
     Referring initially to  FIG. 1 , a sensing apparatus  10  in accordance with the present invention is now described. The sensing apparatus  10  is used to sense, or determine, a variety of properties of a target  12 , including, for example, the dimensions of the target, the material composition of the target, and the thickness of the target. 
     The sensing apparatus  10  includes an excitation source  14 , illustratively a pulsed laser, configured to induce ultrasonic waves in the target  12 . The excitation source  14  is an optical source and is illustratively coupled to the target  12  via an optical fiber  15  having an end portion  16  in physical contact with the target, although it should be appreciated that in some embodiments the excitation source is not coupled to the target via an optical fiber but rather radiates the target via free space. The excitation source  14  induces the ultrasonic waves in the target  12  by rapidly heating it. It should be appreciated that in some applications, the excitation source  14  may be a broadband optical source, or a doped fiber amplifier. 
     An optical waveguide interferometer  17  senses the induced waves and generates target data based thereupon. In particular, the optical waveguide interferometer  17  comprises a probe segment  30  having a probe segment end  31  coupled to the target  12 . The interferometer laser source  18  is connected to an adjustable coupler  14  via optical fibers  21  and through an optical isolator  19  and an optical coupler  22 . Also coupled to the optical coupler  22  is a photo-detector  20  via optical fiber  23 . 
     The adjustable coupler  24  is in physical contact with the target  12 , and permits setting a gap between the probe segment end  31  and the target. A controller  26  is coupled to the adjustable coupler  24  and is configured to control the adjustable coupler to thereby set the gap between the probe segment end  31  and the target  12 . 
     Operation of the optical waveguide interferometer  17  is now described. The interferometer laser source  18  radiates the target  12  via the probe segment end  31 . A portion of the light radiating within the probe segment  30  is reflected back as it hits the probe segment end  31 , through the optical coupler  22 , and into the photodetector  20 . Similarly, a portion of the light radiating within the probe segment  30  is radiated from the probe segment end  31  onto the target  12 . This light is then reflected from the target  12  back into the probe segment  30  via the probe segment end  31 , through the optical coupler  22 , and into the photodetector  20 . Consequently, the light reflected from the probe segment end  31  and the light reflected from the target  12  will combine, and the superposition thereof is detected by the photodetector  20 . The light reflected by the target will typically undergo a phase change due to the ultrasonic waves and resulting vibrations in the target  12 , and therefore will have a different phase than the light reflected by the probe segment end  31 , causing constructive and destructive interference to occur therebetween. This interference therefore reflects a detection of the sensed induced waves and can be analyzed in order to determine various properties of the target, as will be appreciated by those skilled in the art. 
     The controller  26  generates target data based upon the sensed induced waves. To do so, a laser pulse from the pulsed laser  14  triggers the start of a measurement cycle, performed by the controller  26 , in the time domain. Signal peaks observed by the controller  26  correspond to the transit time of surface waves from the point of excitation (that is, the point of the target  12  on which the pulse from the pulsed laser  14  radiates) to the probe segment end  31 . Since the distance between the excitation point and the probe segment end  31  is known, the acoustic velocity of the ultrasonic waves in the target  12  can be calculated. By comparing this acoustic velocity to a table of acoustic velocity for different materials, the material composition of the target can be determined. It should be understood that the adjustable coupler  24  and excitation source probe  16  can be scanned to different locations on the target  12 , so as to gather information about many points of the target. 
     The controller  26  may include a processor and a memory cooperating therewith. The memory may be volatile or non-volatile, and the processor may be an integrated circuit, in some applications. 
     In addition, the controller  26  performs typical interferometric calculations as known to those of skill in the art on the superposition of the light reflected by the probe segment end  31  and the light reflected by the target  12  to potentially determine the dimensions and/or the thickness of the target. Since a difference in the length of the path traveled by the light reflected by the probe segment end  31  and the light reflected by the target  12  will result in an additional phase difference therebetween, it is desirable for the difference in the length of that path to remain the same. That is, it desirable for the gap between the probe segment end  31  and the target  12  to remain constant, such that the gap is a desired multiple of the wavelength of the light radiated by, and reflected into, the probe segment end  31 ′. The multiple used need not be an integer in some embodiments, and need not be greater than one in some embodiments. 
     As stated above, the controller  26  controls the adjustable coupler  24  to adjust the gap. As shown in  FIG. 2 , the adjustable coupler  24 , in some embodiments, may comprise a sleeve  29  surrounding probe segment end  31 , and a biasing member  28  to urge the sleeve  29  in physical contact with the target  12 . The biasing member  28  comprises a cylinder configured to receive the sleeve  29 , and a spring arranged so as to urge the sleeve in contact with the target  12 . A ferrule  35  slidably holds the probe segment end  31  inside the sleeve  29 . The purpose of the biasing member  28  urging the sleeve  29  in contact with the target  12  is to help coarsely adjust the gap between the probe segment end  31  and the target  12  even though the target may be vibrating. 
     Thermal drifting, however, may cause the sleeve  29 , the probe  30 , and the probe segment end  31  to expand and contract at different rates, which leads to the gap changing. Since this is not desirable, the adjustable coupler  24  may include additional components to fine tune the gap. 
     For example, as shown in  FIG. 3 , the adjustable coupler  24 ′ may include a piezoelectric sleeve  32 ′ surrounding the probe segment end  32 ′, which is in turn surrounded by the sleeve  29 ′. The controller  26 ′ applies a voltage to the piezoelectric sleeve  32 ′, causing the piezoelectric sleeve to expand or contract, thereby altering the length of the probe segment end  31 ′. This therefore allows fine tuning of the gap between the probe segment end  31 ′ and the target  12 ′. The controller  26 ′ may be coupled to the piezoelectric sleeve  32 ′ via any suitable method, such as suitable electrical contacts between the sleeve  29 ′ and the piezoelectric sleeve  32 ′. 
     Another embodiment of the adjustable coupler  24  is shown in  FIG. 4 , and includes a temperature control unit  33 ″ surrounding the sleeve  29 ″. The temperature control unit  33 ″ is illustratively a Peltier effect unit, and is controlled by the controller  26 ″. The controller  26 ″ uses the Peltier effect unit  33 ″ to heat or cool the sleeve  29 ″ and probe segment end  31 ″ to thereby cause the sleeve  29 ″ and probe segment end  31 ″ to expand or contract, which in turn allows fine tuning of the gap between the probe segment end and the target  12 ″. 
     A further embodiment of the adjustable coupler  24 ′″ is shown in  FIG. 5 , and includes a laser heating source  34 ′″ configured to radiate the sleeve  29 ′″, and thereby heat the sleeve  29 ′″ and probe segment end  31 ″ to cause the sleeve and probe segment end to expand or contract, which in turn allows fine tuning of the gap between the probe segment end and the target  12 ″. 
     Referring once again to  FIG. 1 , in the above examples, it should be understood the controller  26  controls the adjustable coupler  24  based upon an error signal. This error signal may be the DC component of the light detected by the photodetector  20 , for example. 
     A further embodiment of the sensing apparatus  110  is shown in  FIG. 6 . Here, there is no mechanically adjustable coupler, although the excitation source  114 , optical fiber  115  having an end portion  116 , optical isolator  119 , optical coupler  122 , photodetector  120 , optical fiber  123 , probe segment  130 , probe segment end  131 , and optical fibers  115 ,  121  are similar to those described above with reference to  FIG. 1 . Rather than adjusting the gap between the probe segment end  131  and the target  112  such that the gap is a desired multiple of the wavelength of the light radiated by, and reflected into, the probe segment end, the wavelength of the interferometer laser source  118  is adjusted by the controller  126  such that a desired multiple of the wavelength equals the gap. 
     It should be understood that the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ′″,  110  disclosed above may include an array of excitation sources  14 ,  14 ′,  14 ″,  14 ′″,  114 , and an array of optical waveguide interferometers  18 ,  18 ′,  18 ″,  18 ′″,  118 . 
     With additional reference to the flowchart  40  of  FIG. 7 , a method of sensing a target is now described. After the start (Block  41 ), waves are induced in a target by an excitation source (Block  42 ). Next, the induced waves are sensed by an optical waveguide interferometer (Block  43 ). The optical waveguide interferometer comprises a probe segment having a probe segment end, an adjustable coupler configured to permit setting a gap between the probe segment end and the target. 
     Next, the method includes setting the gap between the probe segment end and the target using a controller coupled to the adjustable coupler (Block  44 ). Then, target data is generated based upon the sensed induces waves, using the controller (Block  45 ). Block  46  indicates the end of the method. 
     It should be understood that the sensing apparatuses  10 ,  10 ′,  110  disclosed above offer numerous advantages. For example, the use of a pulsed laser  14 ,  14 ′,  114  as an excitation source allows a wide bandwidth of ultrasonic waves to be induced in the target  12 ,  12 ′,  112  as opposed to conventional piezoelectric excitation sources which typically produce more narrow bandwidths. For example, the pulsed laser  14 ,  14 ′,  114  can produce ultrasonic waves with a bandwidth above 1 MHz, which is difficult to achieve with conventional piezoelectric excitation sources. In addition, with a piezoelectric excitation source, a physical matching layer of often required to achieve a proper acoustic impedance match between the excitation source and the target. The sensing apparatuses  10 ,  10 ′,  110  disclosed above do not suffer this drawback and are adaptable to a wide range of target materials by adjusting the interferometer spacing either through tuning of the interferometer laser  18 ,  18 ′,  118  wavelength, or tuning of the adjustable coupler  24 ,  24 ′,  124 ′, as opposed to using a variety of matching layers. 
     In addition, the ability of the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ′″ to either adjust the gap between the probe segment end  31 ,  31 ′,  31 ″,  31 ′″ and the target  12 ,  12 ′,  12 ″,  12 ′″ or the wavelength of the interferometer laser source  118 , on the fly and based upon a feedback error signal provides for precise results, as effects that negatively impact the results can be adjusted for and mitigated. Furthermore, the use of a pulsed laser  14 ,  14 ′,  14 ″,  14 ′″,  114  as the excitation source, coupled with the use of the optical waveguide interferometer  17 ,  17 ′,  17 ″,  17 ′″,  117  allows the sensing apparatus  10 ,  10 ′,  10 ″,  10 ′″,  110  to be compact and portable. Moreover, the use of optical fibers to couple the pulsed laser  14 ,  14 ′,  14 ″,  14 ′″,  114  and interferometer laser source  18 ,  18 ′,  18 ″,  18 ′″,  118  to the target  12 ,  12 ′,  12 ″,  12 ′″,  112  allows the sensing of hard to reach targets, since the optical fibers may be inserted into small spaces. 
     The sensing apparatuses  10 ,  10 ′,  10 ″,  10 ′″,  110  disclosed herein are useful in a wide variety of applications. For example, they may be useful in medical imaging systems, for sensing and imaging body parts. For example, the optical fibers of the pulsed laser  14 ,  14 ′,  14 ″,  14 ′″,  114  and interferometer laser source  18 ,  18 ′,  18 ″,  18 ′″,  118  may be inserted into arteries, in order to image those arteries or measure the thickness thereof, or may be inserted into a trachea in order to image various components of the digestive system of a patient. Shown in  FIG. 8  is an embodiment where the sensing apparatus  200  (similar to the sensing apparatuses disclosed above) is a biological sensing device, and the target  212  is an artery having an arterial wall  250 . Here, the controller will generate anatomical data about the arterial wall  250 , such as a thickness or density of the arterial wall. Those skilled in the art will appreciate that any biological sample or body part may be sensed using this sensing apparatus  200 . The illustrated reference numbers in  FIG. 8  have been increased by 200 with respect to  FIGS. 1 and 2  to indicate similar elements in alternative embodiments. Some of the reference numbers in  FIG. 8  will not be discussed to simplify the discussion herein, as readily understood by those skilled in the art. 
     In addition, the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ′″,  110  may be used for materials inspection. For example, small welds, or welds in inaccessible places, may be inspected using the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ″,  110 . Wire bonds in electronic devices may be inspected using the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ″,  110 . Hydraulic lines, such as those used in avionics systems of aircraft, or brake lines of a motor vehicle, may be inspected using the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ′″,  110 . Shown in  FIG. 9  is an embodiment where the sensing apparatus  300  (similar to the sensing apparatuses disclosed above) is a material inspection device, and the target  312  is a workpiece having a weld  350  to be inspected. The illustrated reference numbers in  FIG. 9  have been increased by 300 with respect to  FIGS. 1 and 2  to indicate similar elements in alternative embodiments. Some of the reference numbers in  FIG. 9  will not be discussed to simplify the discussion herein, as readily understood by those skilled in the art. Here, the controller will generate material data about the material weld  350 , such as a thickness, density, or composition of the weld  350 . Of course, this material inspection device  300  need not be limited to weld inspection and may be used to sense or inspect any sort of workpiece. 
     It should be understood that the specific use examples given above are by no means limiting, and that those of skill in the art will appreciate that the sensing apparatuses  10 ,  10 ′,  10 ″,  10 ′″,  110  may be useful in an unlimited number of fields. 
     Referring to  FIG. 10 , another embodiment of a sensing apparatus  400  in accordance with the present invention is now described. The sensing apparatus  400  is used to sense, or determine, a variety of properties of a target  402 , including, for example, the dimensions of the target, the material composition of the target, and the thickness of the target. 
     The sensing apparatus  400  includes an excitation source  404 , illustratively a broadband optical source, configured to induce ultrasonic waves in the target  402 . The excitation source  404  is an optical source and is illustratively coupled to the target  402  via an optical fiber  406  having an end portion  408  in physical contact with the target, although it should be appreciated that in some embodiments the excitation source is not coupled to the target via an optical fiber but rather radiates the target via free space. The excitation source  404  induces the ultrasonic waves in the target  402  by rapidly heating it. It should be appreciated that in some applications, the excitation source  404  may be a coherent optical source (e.g. a pulsed laser), or a doped fiber amplifier. In fact, in some applications, the excitation source  404  may be a pulsed laser having a spectral width that is inversely proportional to the pulse duration. 
     An optical waveguide interferometer  409  senses the induced waves and generates target data based thereupon. In particular, the optical waveguide interferometer  409  comprises a plurality of optical couplers  416 ,  414 ,  422  and interconnecting optical fibers  430   a - 430   e ,  432   a - 432   c  arranged to define a reference arm ( 430   a - 430   e ) and a measurement arm ( 432   a - 432   c ). A probe segment  417  is coupled to portions of the reference arm  430   c ,  430   d  and portions of the measurement arm  432   a . As will be discussed in greater detail below, the optical fibers  430   a - 430   e  making up the reference arm allow reference light as well as input light and measurement light to travel. The probe segment  417  has a probe segment end  418  to be positioned adjacent the target  402   b . An optical path length adjustor  420  is coupled to portions of the reference arm  430   d ,  430   e . The optical path length adjustor  420  is illustratively a piezoelectric body, although it should be understood that any suitable optical path length adjustor or fiber stretcher may also be used. 
     A reference light source  410  is coupled to the reference arm  430   a - 430   e  and is configured to radiate light into the reference arm, and onto the target  402  via the probe segment end  418 . The reference light source can be a laser source or doped fiber amplifier, as will be appreciated by those skilled in the art. For example, the reference light source may be a high gain erbium doped fiber amplifier with a 40 nm bandwidth, centered around a wavelength of 1550 nm. 
     An optical power detector  412  is coupled to the reference arm  430   a - 430   e  and is configured to receive light from the reference light source  410  reflected by the target  402  into the probe segment end  418 . 
     The plurality of optical couplers  416 ,  414 ,  422  includes a first optical coupler  416  coupling portions of the reference arm  430   c ,  430   d  to portions of the measurement arm  432   a  and the probe segment  417 . A second optical coupler  414  couples the first optical coupler  416  to the reference light source  410  and optical power detector  412 . A third optical coupler  422  couples portions of the reference arm  430   e  to portions of the measurement arm  432   a - 432   c , which thereby provides a differential output to the photodetector  424 . 
     A controller  426  is coupled to the optical path length adjustor  420  and is configured to adjust an optical path length of the reference arm  430   a - 430   e  to maintain a constant relationship with respect to an optical path length of the measurement arm  432   a - 432   c . The controller  426  may adjust the optical path length of the reference arm  430   a - 430   e  based upon the optical power detector  412  and/or the differential output provided to the photodetector  424 . 
     Thermal drifting may cause the length of the optical fibers within the reference arm  430   a - 430   e  and the measurement arm  432   a - 432   c  to expand and contract at different rates, which leads to the change of their respective lengths. This is undesirable because it negatively affects the accuracy of the sensing apparatus  400 . The controller  426  helps rectify this undesirable condition by adjusting the path length of the reference arm  430   a - 430   e  using the optical path length adjustor  420 . The matching of the path length of the reference arm  430   a - 430   e  and the measurement arm  432   a - 432   c  by the controller  426  using the optical path length adjustor  420  to within 0.0025 in allows particularly accurate results. 
     Operation of the optical waveguide interferometer  409  is now described. A portion of the light radiated by the reference light source  410  is radiated from the probe segment end  418  onto the target  402 . This light is then reflected from the target  402  back into the probe segment  417  via the probe segment end  418 , through the first optical coupler  416 , through the second optical coupler  414 , and into the optical power detector  412 . The optical power detector  412  measures the optical power reflected from the target  402 , and due to the arrangement of the optical couplers  416 ,  414 ,  422 , only the optical power reflected from the target. That is, the optical couplers  416 ,  414 ,  422  are arranged such that the light directly emitted by the reference light source  410  does not reach the optical power detector  412 , and only the light reflected from the target  402  reaches the optical power detector. 
     A portion of the light radiating from the reference light source  410  is conducted through the reference arm  430   a - 430   e  by the arrangement of optical couplers  416 ,  414 ,  422  and to the photodetector. Consequently, the light reflected from the target  402  and a portion of the light radiated by the reference light source  410  and conducted through the reference arm  430   a - 430   e  will combine, and the superposition thereof is detected by the photodetector  424 . 
     The light reflected by the target  402  will typically undergo a phase change due to the ultrasonic waves and resulting vibrations in the target, and therefore will have a different phase than the light radiated by the reference light source  410  and conducted through the reference arm  430   a - 430   e , causing constructive and destructive interference to occur therebetween. This interference therefore reflects a detection of the sensed induced waves and can be analyzed in order to determine various properties of the target, as will be appreciated by those skilled in the art. 
     The controller  426  generates target data based upon the sensed induced waves. To do so, a pulse from the excitation source  404  triggers the start of a measurement cycle, performed by the controller  426 , in the time domain. Signal peaks observed by the controller  426  correspond to the transmit time of surface waves from the point of excitation (that is, the point of the target  402  on which the pulse from the excitation source  404  radiates) to the probe segment end  418 . Since the distance between the excitation point and the probe segment end  418  is known, the acoustic velocity of the ultrasonic waves in the target  402  can be calculated. By comparing this acoustic velocity to a table of acoustic velocity for different materials, the material composition of the target can be determined. It should be understood that the excitation source probe  408  and probe segment end  418  can be scanned to different locations on the target  402 , so as to gather information about many points of the target. 
     The controller  426  may include a processor and a memory cooperating therewith. The memory may be volatile or non-volatile, and the processor may be an integrated circuit, in some applications. 
     In addition, the controller  426  performs typical interferometric calculations as known to those of skill in the art on the superposition of the light radiated from the reference light source  410  and directed through the reference arm  430   a - 430   e  and the light reflected by the target  402  to potentially determine the dimensions and/or the thickness of the target. 
     Since a difference in the length of the path traveled by the light reflected by the probe segment end  418  and the light radiated from the reference light source  410  and directed through the reference arm  430   a - 430   e  will result in an additional phase difference therebetween, it is desirable for the length of the reference arm  430   a - 430   e  and the length of the measurement arm  432   a - 432   c  to remain the same, or at least for a constant relationship between the length of the reference arm and measurement arm to be maintained. If a constant relationship between the length of the reference arm  430   a - 430   e  and the measurement arm  432   a - 432   c  is to be maintained, it is desirable for the difference in length to be a desired multiple of the wavelength of the light radiated by reference light source  410 . The multiple used need not be an integer in some embodiments, and need not be greater than one in some embodiments. 
     It should be appreciated that the optical path length adjustor  420  need not operate by physically changing a length of an optical fiber in all embodiments. For example, the optical path length adjustor  420  may be an adjustable delay line or phase modulator which can maintain a constant phase relationship between the light reflected from the target  402  and the light radiated by the reference light source  410  and through the reference arm  430   a - 430   e . The maintenance of a constant phase relationship between the light in the reference arm  430   a - 430   e  and the measurement arm  432   a - 432   c  also helps to provide accurate results. 
     In some applications, the reference arm  430   a - 430   e  may even include a free space element. One such embodiment is now described with reference to  FIG. 11 . The illustrated reference numbers in  FIG. 11  have been increased by 100 with respect to  FIG. 10  to indicate similar elements in alternative embodiments. Some of the reference numbers in  FIG. 11  will not be discussed to simplify the discussion herein, as readily understood by those skilled in the art. Here, the sensing apparatus  500  remains the same as the sensing apparatus  400  of  FIG. 10 , except that the reference arm  530   a - 530   e  includes a free space element. Here, the free space element is contained within an adjustable lens arrangement  521 . The reference optical fiber  530   d  terminates at a coupler on the first side of the adjustable lens arrangement  521 , and radiates reference light via free space and through a first lens  523 . The reference light then passed through a second lens  525 , which focuses the light back into the reference optical fiber  530   e  via another coupler. The distance between the first lens  523  and second lens  525  is adjustable based upon input received from the controller  526 . This thereby allows adjustment of the length of the path of the reference arm  530   a - 530   e.    
     A method of operating a sensing apparatus is now described with reference to the flowchart  550  of  FIG. 12 . The sensing apparatus includes an optical waveguide interferometer comprising a plurality of optical couplers and interconnecting optical fibers arranged to define a reference arm, a measurement arm, a probe segment coupled to the reference arm and the measurement arm and having a probe segment end, and an optical path length adjustor coupled to the reference arm. 
     After the start of the method (Block  551 ), the waves are induced in a target using an excitation source (Block  552 ). Then, a probe segment end is positioned adjacent the target (Block  553 ). 
     An optical path length of the reference arm is then adjusted via the optical path length adjustor to maintain a constant relationship with respect to an optical path length of the measurement arm, using a controller (Block  554 ). The induced waves are then sensed using a photodetector coupled to the controller (Block  555 ). Target data is then generated based upon the sensed induced waves, using the controller (Block  556 ). Block  557  indicates the end of the method. 
     It should be understood that the sensing apparatuses  400 ,  500  disclosed above may include an array of excitation sources  404 ,  504 , and an array of optical waveguide interferometers  409 ,  509 . 
     The sensing apparatuses  400 ,  500  disclosed herein are useful in a wide variety of applications. For example, they may be useful in medical imaging systems, for sensing and imaging body parts. For example, the optical fibers  406 ,  408 ,  506 ,  508  of the excitation source and reference light source  410 ,  510  may be inserted into arteries, in order to image those arteries or measure the thickness thereof, or may be inserted into a trachea in order to image various components of the digestive system of a patient. Shown in  FIG. 13  is an embodiment where the sensing apparatus  600  includes optical fibers  608  and  618  (similar to the sensing apparatuses  400  and  500  disclosed above) is a biological sensing device, and the target is an artery having an arterial wall  650 . Here, the controller will generate anatomical data about the arterial wall  650 , such as a thickness or density of the arterial wall. Those skilled in the art will appreciate that any biological sample or body part may be sensed using this sensing apparatus  600 . 
     In addition, the sensing apparatuses  400 ,  500  may be used for materials inspection. For example, small welds, or welds in inaccessible places, may be inspected using the sensing apparatuses  400 ,  500 . Wire bonds in electronic devices may be inspected using the sensing apparatuses  400 ,  500 . Hydraulic lines, such as those used in avionics systems of aircraft, or brake lines of a motor vehicle, may be inspected using the sensing apparatuses  400 ,  500 . Shown in  FIG. 14  is an embodiment where the sensing apparatus  700  includes optical fibers  708  and  718  (similar to the sensing apparatuses disclosed above) is a material inspection device, and the target  702  is a workpiece having a weld  750  to be inspected. Here, the controller will generate material data about the material weld  750 , such as a thickness, density, or composition of the weld  750 . Of course, this material inspection device  700  need not be limited to weld inspection and may be used to sense or inspect any sort of workpiece. 
     It should be understood that the specific use examples given above are by no means limiting, and that those of skill in the art will appreciate that the sensing apparatuses  400 ,  500 ,  600 ,  700  may be useful in an unlimited number of fields. 
     Other details of such sensing apparatuses  10  may be found in co-pending applications INTERFEROMETRIC SENSING APPARATUS INCLUDING ADJUSTABLE COUPLING AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,619 filed May 6, 2011, now U.S. Patent Application Publication No. 2012/0281227 published Nov. 8, 2012; INTERFEROMETRIC BIOMETRIC SENSING APPARATUS INCLUDING ADJUSTABLE COUPLING AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,654 filed May 6, 2011, now U.S. Patent Application Publication No. 2012/0281228 published Nov. 8, 2012; INTERFEROMETRIC SENSING APPARATUS INCLUDING ADJUSTABLE REFERENCE ARM AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,712 filed May 6, 2011, now U.S. Patent Application Publication No. 2012/0281230 published Nov. 8, 2012; INTERFEROMETRIC BIOLOGICAL SENSING APPARATUS INCLUDING ADJUSTABLE REFERENCE ARM AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,732 filed May 6, 2011, now U.S. Patent Application Publication No. 2012/0281231 published Nov. 8, 2012; and INTERFEROMETRIC MATERIAL SENSING APPARATUS INCLUDING ADJUSTABLE REFERENCE ARM AND ASSOCIATED METHODS, U.S. Ser. No. 13/102,755 filed May 6, 2011, now U.S. Patent Application Publication No. 2012/0281232 published Nov. 8, 2012, the entire disclosures of which are hereby incorporated by reference. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.