Abstract:
A method and apparatus for reducing the difficulty of controlling the length of a section of optical waveguide wrapped around a mandrel separating Bragg gratings forming an interferometric sensor are provided. The section of optical waveguide may be wrapped on a mandrel having at least two different outer diameters. The mandrel may also include one or more bores for receiving and protecting the Bragg gratings.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Embodiments of the present invention relate to optical waveguide interferometric-based hydrophones and, more specifically, to mandrels used in such hydrophones.  
         [0003]     2. Description of the Related Art  
         [0004]     A Bragg grating is an optical element that is reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Bragg gratings are usually formed by photo-induced periodic modulation of the refractive index of an optical waveguide&#39;s core. A pair of Bragg gratings having a common Bragg wavelength and separated by a length of waveguide (e.g. a coiled fiber or coil) can form an interferometer that may be interrogated by sending light of the same Bragg wavelength through the interferometer. Reflections of light from the (partially-transmissive) Bragg gratings are sent back to optical detection equipment through the waveguide. By assessing the phase shift in light coincidently reflected from the two Bragg ratings, the length of the coil can be determined, as is well known.  
         [0005]     Optical waveguide interferometers can be deployed in various ways to make the length of the coil (and hence, the phase shifts between coincidentally reflected pulses) dependent on physical parameters. For example, Bragg grating interferometers can be deployed in a number of different ways to make acoustic sensors. Reference, “A Fiber Laser Hydrophone Array,” by D. J. Hill, et al., SPIE Vol. 3860. An optical waveguide hydrophone is typically made by winding a section of an optical waveguide (e.g., an optical fiber) separating a pair of Bragg gratings around a compliant cylindrical mandrel. When acoustic pressure impinges on the mandrel, the mandrel deforms slightly, changing the length of the waveguide separating the Bragg gratings. When forming such an acoustic sensor it is beneficial to tightly wind the optical waveguide (optical fiber) around the compliant cylindrical mandrel, which makes the fiber to follow the response of the mandrel that is designed to respond to acoustic pressure wave. The sensitivity of the sensor is proportional to the number of turns (or wraps), as described below.  
         [0006]     One issue with mandrel-based optical waveguide, Bragg grating acoustic sensors is that the Bragg gratings themselves should be protected. Strain on the Bragg gratings can cause an excessive shift in the center frequency of the Bragg wavelength such that the Bragg gratings are no longer highly reflective at the correct wavelength. One way to isolate the Bragg gratings from excessive strain is to locate them within the mandrel itself. This can be accomplished by forming bores through the mandrel, locating a Bragg grating in one bore, wrapping the optical waveguide around the mandrel, and then bringing the optical waveguide through another bore such that the other Bragg grating is located in that bore. By placing the Bragg grating loosely inside the bore will isolate the grating from excessive strain and protect it from physical damages.  
         [0007]     The length L of an optical waveguide wrapped on a cylindrical mandrel is about:
 
 L≈N·π·d 
 
 where N is the number of turns and d is the outer diameter of the mandrel. In order to have optimum interferometer performances in a system utilizing multiple acoustic sensors (e.g., an array), the length between the two gratings should be nearly identical between devices. However, manufacturing tolerances may lead to significant variations in length between the gratings. For example, when a mandrel is turned on a CNC machine, its outside diameter can vary by about +/−0.001 inch. If the optical waveguide is wrapped around the mandrel 70 times, the wrapped length can vary by as much as 0.14 inch. Furthermore, the process used to produce the Bragg gratings can locate the gratings only within a tolerance of about +/−0.1 inch. Thus, it is difficult to tightly wrap an optical waveguide around a cylindrical mandrel while positioning the Bragg gratings inside the mandrel (which may require a precision of +/−0.040 inch to do). 
 
         [0008]     Therefore, a mandrel that reduces the difficulty of accurately controlling the length of an optical waveguide wrapped around the mandrel and allowing Bragg gratings separated by the length of optical waveguide to be accurately positioned would be useful.  
       SUMMARY OF THE INVENTION  
       [0009]     One embodiment that is in accord with the principles of the present invention is a mandrel that reduces the difficulty of wrapping an optical waveguide Bragg grating interferometer such that the Bragg gratings are accurately positioned. Such a mandrel has at least two outer diameters.  
         [0010]     Another embodiment that is in accord with the principles of the present invention is a bored mandrel that reduces the difficulty of wrapping an optical waveguide Bragg grating interferometer on the mandrel such that the Bragg gratings are accurately positioned within bores. Such a mandrel has at least two outer diameters and a bore for receiving a section of an optical waveguide that includes a Bragg grating.  
         [0011]     Another embodiment of the present invention is an interferometric hydrophone having Bragg gratings that are physically protected in a bore or bores of a mandrel having at least two outer diameters. The mandrel enables controlled routing of the optical waveguide to prevent excessive optical loss while protecting the Bragg gratings from physical damages due to shock and vibration.  
         [0012]     Another embodiment of the present invention is a method of controlling a length of an optical waveguide section during manufacture of an acoustic sensor. The method generally includes providing a mandrel having at least a first section with a first outer diameter and a second section with a second outer diameter, wrapping the optical waveguide section a first number of times around the first section and a second number of times around the second section, and controlling the wrapped length of the optical waveguide section by varying the first number and the second number. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0014]      FIG. 1  schematically depicts a Bragg grating interferometric sensing system;  
         [0015]      FIG. 2  illustrates a prior art mandrel;  
         [0016]      FIG. 3  is an isometric view of a mandrel that is in accord with the principles of the present invention;  
         [0017]      FIG. 4  illustrates a cut-away view of a mandrel that is in accord with the present invention; and  
         [0018]      FIG. 5  illustrates a wound mandrel;  
         [0019]      FIG. 6  illustrates a mandrel having three portions and that is in accord with the principles of the present invention; and  
         [0020]      FIG. 7  illustrates a mandrel having three diameters and that is in accord with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     The principles of the present invention provide for optical waveguide interferometric hydrophones having Bragg gratings between optical waveguide sections that are wound on mandrels having at least two different outer diameters. Some embodiments of the present invention include bored mandrels, where the bores physically protect the Bragg gratings from both physical damage and from excessive strain. The different outer diameters enable accurate control of the length between the gratings which, in turn, allows accurate positioning of the Bragg gratings.  
         [0022]     To facilitate understanding, embodiments of the present invention are described below with reference to acoustic sensors (hydrophones) as a specific, but not limiting application example. However, it should be appreciated that the apparatus and techniques described herein may be used to control the length between (and facilitate precise placement of) reflective elements of any type of interferometric sensor device.  
         [0023]      FIG. 1  schematically illustrates a simplified optical waveguide interferometric hydrophone system  100 . The hydrophone system may operate in a similar manner to the hydrophone system described in the commonly owned co-pending application entitled “High Pressure And High Temperature Acoustic Sensor” (Attorney Docket WEAT/0471P1) filed herewith and incorporated by reference in its entirety. The hydrophone system  100  includes a sensing coil  102  comprised of a number of tightly wrapped turns of an optical waveguide  104  (such as an optical fiber) around a mandrel  106 . The mandrel  106  should be understood as generically representing any of the inventive mandrels that are subsequently described. The sensing coil  102  is bounded by a pair of Bragg gratings  110  and  112  that have the same Bragg wavelength (λB). In the illustrated configuration, the sensing coil  102  acts as a sensor. This is because the length of the sensing coil  102  depends on the diameter of the mandrel  106 , which, in turn, depends on the acoustic pressures impingent upon the mandrel  106 .  
         [0024]     Well known interferometric interrogation techniques, such as Fabry-Perot, Michelson, or Mach-Zehnder, can determine the length of the sensing coil  102 . For example, a series of optical pulses from a pulse generator  114  can be applied to the sensing coil  102  through an input optical waveguide  120 . Reflections of optical pulses from the Bragg gratings  110  and  112 , which are partially transmissive, are detected by a detector  116  and analyzed by an analyzer  118 . By assessing the phase shift in the pulses that are reflected from the two Bragg gratings  110  and  112 , the length of the sensing coil  102  can be determined. An output optical waveguide  130  can be connected to other optical components or sensors deployed along with the hydrophone system  100 .  
         [0025]     In some applications, it may not be practical to form the sensing coil  102  and the Bragg gratings  110  and  112  along a continuous section of optical waveguide. In that case, the individual components, such as the input and output optical waveguides  120  and  130 , the sensing coil  102 , and the Bragg gratings  110  and  112  can be individually formed and then spliced together.  FIG. 1  illustrates such splices using slash marks  136 .  
         [0026]     The length L of the sensing coil  102  that is on the mandrel  106  (described in more detail below) is tightly wrapped on the outer surface of the mandrel  106  and, for some embodiments, is such that the Bragg gratings  110  and  112  are located in predetermined and protected positions. Acoustic energy and the compliance of the mandrel  106  cause the length of the mandrel  106  to change, which induces changes in the outer diameter of the mandrel  106 . This causes a change in length ΔL of the length L and a corresponding change in the round trip path of pulses reflected from the second Bragg grating  112 , which causes the phase relationship between the light pulses detected at the detector  116  to vary. The analyzer  118  senses the phase variance and provides an electrical output that corresponds to the acoustic energy. The compliance of the mandrel  106  provides the restoring force.  
         [0027]      FIG. 2  illustrates a prior art mandrel  200 . As shown, that mandrel has a cylindrical shape and an outer surface  202  formed at a diameter d. The mandrel  200  further includes a bore  204  for passing an optical waveguide back through the mandrel  200  after the winding is complete. As noted, positioning the Bragg gratings  112  in the bore  204  is beneficial as that enables sealing the bore  204  to protect the Bragg gratings  112  from physical damage and from external factors such as pressure. A significant problem with the mandrel  200  is wrapping an optical waveguide such that the Bragg gratings were both located within the bore  204 . As noted in the “Background” section, mandrels turned on a CNC machine have diameters that can vary by about +/−0.001 inch and Bragg grating positions can vary by as much as 0.14 inch. Thus, it is very difficult to locate both Bragg gratings within the bore  204 . Furthermore, the end of the optical waveguide that is brought back through the bore  204  after wrapping can be bent at an excessive angle. This can cause excessive optical losses.  
         [0028]     The mandrel  106  of  FIG. 1  generically represents a class of mandrels that can be configured in various ways. For example,  FIG. 3  illustrates a mandrel  300  that is in accord with the principles of the present invention. As shown, the mandrel  300  includes a generally cylindrical body  304  having at least two diameters, d 1  and d 2 .  FIG. 3  shows diameters d 1  and d 2  as being very different. However, in practice, diameters d 1  and d 2  need vary only slightly. The first diameter d 1  is selected to give the best acoustic response without excessive optical power loss produced by bending the optical waveguide  104 . The second diameter d 2  is selected to provide for accurate placement of the Bragg gratings  112 . By varying the number of turns on the surfaces of each diameter, the Bragg gratings on both ends of the optical waveguide  104  can be precisely placed in a bore  306 .  
         [0029]     Thus, the different diameters permit small designed features that will protect and stabilize the optical characteristics of the Bragg gratings, and at the same time keeping the optical waveguide tightly wound on the sensing surface of the mandrel to give a better signal to noise ratio. The length L of the optical waveguide  104  for the mandrel  300  is determined by the following formula:
 
 L≈N   1 · π·d   1 + N   2 ·π ·d   2 
 
 Where N 1  is the number of turns wrapped around diameter d 1 , and N 2  is the number of turns wrapped around diameter d 2 . By providing a relatively small difference between d 1  and d 2 , L 2  can be accurately controlled by varying N 1  and N 2  to accommodate variations due to manufacturing tolerances. For example, if the circumference of the section having diameter d 1  is 0.010 inch smaller than the circumference of the section having diameter d 2 , by winding (N 1 −1) turns on the first section and (N 2 +1) turns on the second section, the total length is increased by 0.010 inch (e.g., L′=L+0.01 inch), while the total number of turns is maintained (N 1 +N 2 ). Thus it can be seen how the number of wraps around each diameter (N 1  and N 2 ) may be varied to precisely control the wrapped length, which may also facilitate locating the Bragg gratings  110  and  112  in bore  306  
 
         [0030]     Depending on the application the mandrel  300  can be comprised of a variety of materials, including Nylon, Teflon, or Peek. A good material for most applications will have a low coefficient of thermal expansion and will operate at high temperature.  
         [0031]     While the mandrel  300  is beneficial, it may not be optimal in all applications. One drawback of the mandrel  300  is that one or more relatively sharp bends in the optical waveguide  104  is required to bring both Bragg gratings  112  into the bore  306 . Sharp bends tend to attenuate optical power in the optical waveguide  104 .  FIG. 4  illustrates another mandrel  400  that is in accord with the principles of the present invention. As shown, the mandrel  400  includes a generally cylindrical body  404  having at least two diameters, d 1  and d 2 . The first diameter d 1  is selected to give the best acoustic response without excessive optical power loss created by bending of the optical waveguide  104 , while the second diameter d 2  is selected to provide for accurate placement of the Bragg gratings  112 . The mandrel  400  includes two bores  408  and  410 . The mandrel  400  further includes a guide slot  412  in the mandrel  400  at the second diameter d 2 , a transition slot  413  that spans across d 1  and d 2 , a guide slot  414  in the mandrel  400  at the first diameter d 1 , and end slots  416  (one end slot on each end).  
         [0032]     To wrap the mandrel  400 , the optical waveguide  104  is inserted into the bore  408  such that an optical lead extends from end  428  and such that a Bragg grating  112  is located within the bore  408 . The optical waveguide  104  is then placed in the slot  416  at end  430  and brought back through the bore  410 . The optical waveguide  104  is then located in the end slot  416  and wrapped so that it enters and follows the guide slot  414 . After the guide slot  414  terminates the optical waveguide  104  is tightly wrapped around the portion of the mandrel  400  having the diameter d 1 . Then, to assist properly locating the other Bragg grating  112  the optical waveguide  104  is placed in the transition slot  413 . As the optical waveguide  104  is wrapped further it exits the transition slot  413  and is tightly wrapped on the mandrel  400  at the portion having the diameter d 2 . Slightly before wrapping the second Bragg grating  112 , the optical waveguide  104  is inserted into guide slot  412 . Further wrapping causes the optical waveguide  104  to follow the guide slot  412  into the end slot  416  on the end  430 . That end slot  416  then guides the optical waveguide  104  into either bore  408  or  410  (depending on how the end slot  416  terminates). The optical waveguide  104  is then passed through that bore such that the Bragg grating  112  is located within the bore. The bores are then sealed to protect the Bragg grating.  
         [0033]     The end result is illustrated in  FIG. 5 . By varying the number of turns on the surfaces having diameters d 1  and d 2 , the Bragg grating  112  can both be precisely located within the mandrel  400 . The diameter d 2  permits small changes in the wrapping length of the optical waveguide  104  so as to accurately control the length between and precisely locate the Bragg gratings, while at the same time permitting tight winding of the optical waveguide on the surfaces of the mandrel, thus improving signal to noise ratios.  
         [0034]     While the foregoing has described inventive mandrels having two sections with different diameters, it should be understood that more than two sections are contemplated. For example,  FIG. 6  illustrates a mandrel  600  having initial and end mandrel portions  602  and  604 , respectively that have diameters d 2 , and a central portion  606  having diameter d 1 . Another contemplated embodiment is the mandrel  700  that is illustrated in  FIG. 7 . That mandrel has a first portion  702  having a diameter d 1 , a second portion  704  having a diameter d 2 , and a third portion  706  having a diameter d 3 . It should also be understood that some applications will use mandrels with bores, while others will not.  
         [0035]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention exist or may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.