Patent Publication Number: US-2002003936-A1

Title: Fine spaced winding pattern for fiber optic coil

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention relates to fiber optic devices such as fiber optic rate sensors.  
       BACKGROUND OF THE INVENTION  
       [0002] A fiber optic rate sensor is frequently used in advanced global positioning and inertial guidance systems to sense rotation. A fiber optic rate sensor ordinarily comprises an interferometer which includes a light source, a beam splitter, a detector, and an optical path which is mounted on a platform. Light from the light source is split by the beam splitter into two light beams which are directed to opposite ends of the optical path. The two light beams counterpropagate around the optical path and, as the light beams exit the optical path, they are recombined. The recombined light beams are applied to a detector.  
       [0003] If the optical path rotates, the distance traveled by one of the light beams is greater than distance traveled by the other light beam, so that there is a phase difference between the two light beams at their optical path exit points. A sensing circuit connected to the detector determines this phase difference as an indication of the extent and direction of rotation.  
       [0004] The optical path of a fiber optic rate sensor is provided by an optical fiber which is typically coiled around a spool or hub to form a winding configuration. The winding configuration usually has multiple layers where each layer contains multiple turns. Although many different winding configurations are known, coils used in fiber optic rotation sensors are typically wound as quadrupoles or as interleaved patterns.  
       [0005] In order to form a quadrupole, a first end of a continuous optical fiber is wound onto a first intermediate spool, and a second end of the continuous optical fiber is wound onto a second intermediate spool. Then, the optical fiber on the first intermediate spool is used to wind a first layer of turns in a clockwise direction around the hub, the optical fiber on the second intermediate spool is used to wind a second layer of turns in a counterclockwise direction over the first layer, the optical fiber on the second intermediate spool is used to wind a third layer of turns oven the second layer of turns, and the optical fiber on the first intermediate spool is used to wind a fourth layer of turns over the third layer of turns.  
       [0006] If “+” and “−” are used to designate the first and second ends of the optical fiber, respectively, the resulting quadrupole winding pattern has a +−−+ winding configuration, where + indicates a layer wound from the first end of the optical fiber and where − indicates a layer wound from the second end of the optical fiber. Ideally, the length of optical fiber in the “+” layers is equal to the length of optical fiber in the “−” layers. This quadrupole winding pattern may be repeated as often as desired for a fiber optic rate sensor. Accordingly, if a second quadrupole is wound with +−−+ layers about the first quadrupole, the resulting two quadrupole arrangement has a +−−++−−+ winding pattern.  
       [0007] It is also known to wind a reverse quadrupole from the “+” and “−” ends of the optical fiber. In this case, the reverse quadrupole has a +−−+−++− winding pattern and is generally referred to as an octupole. This octupole winding pattern may be repeated as often as desired for a fiber optic rotation sensor. Indeed, a reverse octupole may be wound according to the following winding pattern: +−−+−++−−++−+−−+.  
       [0008] In order to form a coil having an interleaved winding pattern, one or more layers of the coil are wound as alternating turns from first and second ends of an optical fiber. Accordingly, in such a layer, odd numbered turns are wound from a first end of the optical fiber, and even numbered turns are wound from a second end of the optical fiber. The result of such winding is that each turn (other than the outer turns) of an interleaved layer is wound from one end of an optical fiber and is sandwiched between two turns wound from the other end of the optical fiber.  
       [0009] Not all layers of a coil having an interleaved winding pattern are required to be wound with the interleaved winding pattern. For example, all of the turns of the innermost layer of the coil can be wound from the same end of the optical fiber, or one or more groups of adjacent turns of the innermost layer of the coil can be wound from the first end of the optical fiber and one or more other groups of adjacent turns of the innermost layer of the coil can be wound from the second end of the optical fiber.  
       [0010] In winding coil patterns, valleys are created between adjacent turns of the first layer. These valleys provide nesting places for the turns wound in the second layer, and the turns of the second layer form valleys providing nesting places for the turns wound in the third layer, and so on. However, substantial force is usually required in order to nest the turns of one layer into the valleys provided by the adjacent turns of the previous layer. Because of this force, it is likely that the fiber in each turn will deform and push other turns that are adjacent in the same layer. Fiber deformation can cause displacement of turns of the fiber optic sensor.  
       [0011] For example, FIG. 1 shows a portion of a fiber optic coil  10  having first and second layers  12  and  14 . The tension that is applied to the optical fiber during the winding process deforms the fiber such as at turns  16 ,  18 ,  20 , and  22  from a circular shape to an oval shape. As a result, there may not be enough space to accommodate all turns with the deformed dimension. Thus, one or more fiber turns, such as the turn  22 , will be misplaced from the valleys created by adjacent turns of the previous layer.  
       [0012] Moreover, it is known that the diameter of the optical fiber along its length can fluctuate from a nominal diameter. As shown by a fiber optic coil  30  in FIG. 2, if the size of the diameter of the optical fiber that is used to wind a first layer  32  increases slightly during the winding of a second layer  34 , a build-up of cumulative fiber placement error can result. As a result, one or more fiber turns, such as a turn  36 , will again be misplaced from the valleys created by adjacent turns of the previous layer.  
       [0013] Furthermore, in winding an interleaved pattern, alternating adjacent turns in a layer are wound from the first and second ends of an optical fiber. A layer  40  having this interleaved winding pattern is shown in FIG. 3 where a first end of an optical fiber is used to wind turns  42 ,  44 ,  46 ,  48 , and so on, and a second end of the optical fiber is used to wind turns  50 ,  52 ,  54 , and so on. As can be seen from FIG. 3, each turn (except for outer turns) wound from one end of the optical fiber is sandwiched between two turns wound from the other end of the optical fiber. However, as shown in FIG. 4, fluctuating buffer diameter and/or tension applied to the optical fiber during winding can also create winding errors with an interleaved winding pattern. These errors include fiber climbing, such as at a turn  60 , turn misplacement such as at a turn  62 , and missing turns.  
       [0014] Accordingly, as described above, turns of a fiber optic coil may not be positioned as intended with the result that thermal transients and vibrations may cause performance of the fiber optic sensor to degrade.  
       [0015] The present invention is directed to a fiber optic device that allows some space between adjacent turns in a layer so as to mitigate or avoid the thermal transient and vibration problems of the prior art.  
       SUMMARY OF THE INVENTION  
       [0016] In accordance with one aspect of the present invention, a fiber optic coil wound from optical fiber comprises first and second layers of turns. The first layer of turns is wound from the optical fiber, and the optical fiber in the first layer of turns has a first diameter. The second layer of turns is wound from the optical fiber, the turns of the second layer of turns are wound around the turns of the first layer of turns, the optical fiber in the second layer of turns has a second diameter, and the second diameter is less than the first diameter.  
       [0017] In accordance with another aspect of the present invention, a fiber optic coil comprises first, second, third, fourth, and fifth layers of turns. The first layer of turns is wound from a first portion of optical fiber, the first portion of optical fiber has a first diameter, and the first layer of turns has valleys. The second layer of turns is wound from a second portion of optical fiber, the second portion of optical fiber has a second diameter, the second layer of turns has valleys, the turns of the second layer of turns occupy the valleys of the first layer of turns, and the second diameter is less than the first diameter. The third layer of turns is wound from the second portion of optical fiber, the third layer of turns has valleys, and the turns of the third layer of turns occupy the valleys of the second layer of turns. The fourth layer of turns is wound from the second portion of optical fiber, the fourth layer of turns has valleys, and the turns of the fourth layer of turns occupy the valleys of the third layer of turns. The fifth layer of turns is wound from the second portion of optical fiber, and the turns of the fifth layer of turns occupy the valleys of the fourth layer of turns.  
       [0018] In accordance with yet another aspect of the present invention, a fiber optic coil comprises first through ninth layers of adjacent turns. The first layer of adjacent turns is wound from an optical fiber having a first diameter. The second through ninth layers of adjacent turns are wound from an optical fiber having a second diameter, the second through ninth layers of adjacent turns are wound in succession over the first layer of adjacent turns, and the second diameter is less than the first diameter.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019] These and other features and advantages of the present invention will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which:  
     [0020]FIG. 1 illustrates winding errors caused by applying tension to an optical fiber during winding of a fiber optic coil;  
     [0021]FIG. 2 illustrates winding errors caused by fluctuating fiber diameter;  
     [0022]FIG. 3 illustrates an interleaved winding pattern;  
     [0023]FIG. 4 illustrates winding errors in an interleaved winding pattern caused by applying tension to an optical fiber during winding of a fiber optic coil and/or by fluctuating fiber diameter;  
     [0024]FIG. 5 illustrates a general winding pattern that incorporates the present invention;  
     [0025]FIG. 6 illustrates a quadrupole winding pattern that incorporates the present invention;  
     [0026]FIG. 7 illustrates an octupole winding pattern that incorporates the present invention; and,  
     [0027]FIG. 8 illustrates an interleaved winding pattern that incorporates the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0028] A fiber optic coil  70  as illustrated in FIG. 5 includes layers  72 ,  74 ,  76 ,  78 , and  80 . However, as discussed below, the fiber optic coil  70  may include any number of layers as desired. Each of the layers  72 ,  74 ,  76 ,  78 , and  80  includes a plurality of turns wound from an optical fiber. However, the portion of optical fiber that is used to wind the turns in the layer  72  has an outer diameter that is larger than the outer diameter of the portion of optical fiber used to wind the layers  74 ,  76 ,  78 , and  80 . The difference between the outer diameter of the portion of optical fiber used to wind the turns in the layer  72  and the outer diameter of the portion of optical fiber used to wind the turns in the layers  74 ,  76 ,  78 ,  80 , although exaggerated in FIG. 5, may be only large enough so that the adjacent turns in each of the layers  74 ,  76 ,  78 , and  80  are non-touching. Accordingly, the layer  72  may be wound using a portion of optical fiber having a outer diameter which is only slightly larger than the outer diameter of the portion of optical fiber that is used to wind the turns in subsequent layers.  
     [0029] As each layer is wound, an adhesive may be applied in order to bond the turns in the layer together and to bond one layer over a previously wound layer.  
     [0030] The turns of the layer  72  may or may not be a functional part of the fiber optic coil  70 . If the turns of the layer  72  are to be a functional part of the fiber optic coil  70 , then there are a number ways of providing the turns of the layer  72  with a larger outer diameter than the turns of the remaining layers. For example, a first portion of larger diameter optical fiber may be spliced onto a second portion of smaller diameter optical fiber so that the layer  72  is wound from the first portion of optical fiber and the remaining layers are wound from the second portion of optical fiber. As another example, a first portion of optical fiber may be pre-coated to enlarge its diameter relative to the diameter of a second portion of the optical fiber so that the layer  72  is wound from the first portion of the optical fiber and the remaining layers are wound from the second portion of the optical fiber. In both examples, the optical fiber at one end of the layer  72  is optically connected to the optical fiber beginning the layer  74 , and the optical fiber at the other end of the layer  72  is optically connected to an end of the optical fiber beginning the layer  78 , assuming that the layers  72 ,  74 ,  76 , and  78  are to form a quadrupole winding configuration. If some other winding configuration is to be provided for the fiber optic coil  70 , then the ends of the layer  72  should be optically connected to appropriate layers of the fiber optic coil  70 .  
     [0031] If the turns of the layer  72  are not to be a functional part of the fiber optic coil  70 , then the optical fiber of the layer  72  is not optically connected to the optical fiber of any other layer.  
     [0032] Because the diameter of the optical fiber that is used to wind the turns of the layer  72  is larger than the diameter of the optical fiber that is used to wind the turns in the succeeding layers of the fiber optic coil  70 , the adjacent turns in each of the layers  74 ,  76 ,  78 ,  80 , etc. of the fiber optic coil  70  are non-touching. Indeed, a small space is provided between the adjacent turns. Accordingly, the turns in the layer  74  do not touch each other, the turns in the layer  76  do not touch each other, and so forth for subsequent layers of the fiber optic coil  70 . Therefore, the coil structure is free from winding defects, and the performance of the fiber optic coil  70  in thermal transient and vibration conditions is substantially enhanced over prior art fiber optic coils. The winding configuration provided by the fiber optic coil  70  permits a high degree of consistency in the coil winding pattern and structural integrity of the fiber optic coil  70 .  
     [0033] The fiber optic coil  70  has substantial benefits. For example, the fiber optic coil  70  need not be supported by a hub and instead may be a homogenous free-standing coil structure consisting only of optical fiber and adhesive. Also, it is known to provide grooves around a hub upon which a fiber optic coil is wound in order to separate the turns in each layer so as to provide a gap between adjacent turns. The present invention, however, eliminates the need for such grooved hubs. Moreover, grooved winding fixtures are also eliminated. Furthermore, the present invention permits the use of non-stick coated winding fixtures.  
     [0034] The optical fiber of the fiber optic coil  70  may be wound in any type of winding configuration. Examples of three such winding configurations are shown in FIGS. 6, 7, and  8 . A winding configuration  90  shown in FIG. 6 has a quadrupole winding arrangement. A winding configuration  110  shown in FIG. 7 has a reverse quadrupole or octupole winding configuration. A winding configuration  130  shown in FIG. 8 has an interleaved winding pattern. It is assumed that the first layer of turns in each of the winding configurations  90  and  110  is functional and that the first layer of turns in the winding configuration  130  is not functional. Thus, as explained above, the first layer of turns of a winding configuration may be either functional or non-functional.  
     [0035] The winding configuration  90  includes layers  92 ,  94 ,  96 ,  98 ,  100 ,  102 ,  104 , and  106 . The turns of the layers  92 ,  98 ,  100 , and  106  are wound from a first end of an optical fiber and the turns of the layers  94 ,  96 ,  102 , and  104  are wound from a second end of the optical fiber. A portion of the first end of the optical fiber that is used to wind the turns of the layer  92  has an outer diameter that is larger than the outer diameter of (i) the second end of the optical fiber which is used to wind the layers  94 ,  96 ,  102 , and  104  and (ii) the remaining portion of the first end of the optical fiber which is used to wind the layers  98 ,  100 , and  106 .  
     [0036] Accordingly, the layer  94  includes turns wound from the second end of the optical fiber, the layer  96  includes turns wound from the second end of the optical fiber, the layer  98  includes turns wound from the first end of the optical fiber, the layer  100  includes turns wound from the first end of the optical fiber, the layer  102  includes turns wound from the second end of the optical fiber, the layer  104  includes turns wound from the second end of the optical fiber, and the layer  106  includes turns wound from the first end of the optical fiber. One end of the optical fiber in the layer  92  is optically connected to an end of the optical fiber in the layer  94 , and the other end of the optical fiber in the layer  92  is optically connected to an end of the optical fiber in the layer  98  so that the layers  92 ,  94 ,  96 , and  98  form a first quadrupole winding configuration. Similarly, the layers  100 ,  102 ,  106 , and  104  may be arranged to form a second quadrupole winding configuration. Additional quadrupoles may also be provided as desired.  
     [0037] It should be noted that, if the layer  92  is not a functional part of the winding configuration  90 , then the layer  94  includes turns wound from a first end of an optical fiber, the layer  96  includes turns wound from a second end of the optical fiber, the layer  98  includes turns wound from the second end of the optical fiber, and the layer  100  includes turns wound from the first end of the optical fiber. The layers  94 ,  96 ,  98 , and  100  thus form a quadrupole. Subsequent layers may be wound in the same quadrupole winding configuration.  
     [0038] The winding configuration  110  includes layers  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 , and  126 . The turns of the layers  112 ,  118 ,  122 , and  124  are wound from a first end of an optical fiber and the turns of the layers  114 ,  116 ,  120 , and  126  are wound from a second end of the optical fiber. A portion of the first end of the optical fiber that is used to wind the turns of the layer  112  has an outer diameter that is larger than the outer diameter of (i) the second end of the optical fiber which is used to wind the layers  114 ,  116 ,  120 , and  126  and (ii) the remaining portion of the first end of the optical fiber which is used to wind the layers  118 ,  122 , and  124 .  
     [0039] Accordingly, the layer  114  includes turns wound from the second end of the optical fiber, the layer  116  includes turns wound from the second end of the optical fiber, the layer  118  includes turns wound from the first end of the optical fiber, the layer  120  includes turns wound from the second end of the optical fiber, the layer  122  includes turns wound from the first end of the optical fiber, the layer  124  includes turns wound from the first end of the optical fiber, and the layer  126  includes turns wound from the second end of the optical fiber. One end of the optical fiber in the layer  112  is optically connected to an end of the optical fiber in the layer  114 , and the other end of the optical fiber in the layer  112  is optically connected to an end of the optical fiber in the layer  118  so that the layers  112 ,  114 ,  116 , and  118  form a first quadrupole winding configuration. Similarly, the layers  120 ,  122 ,  124 , and  126  may be arranged to form a reverse quadrupole winding configuration so that the layers  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 , and  126  form an octupole. Additional octupoles may also be provided as desired. Indeed, the turns in the layers  112 - 126  may be reversed in the next eight layers of a fiber optic coil and so on.  
     [0040] It should be noted that, if the layer  112  is not a functional part of the winding configuration  110 , then the layer  114  includes turns wound from a first end of an optical fiber, the layer  116  includes turns wound from a second end of the optical fiber, the layer  118  includes turns wound from the second end of the optical fiber, and the layer  120  includes turns wound from the first end of the optical fiber. The layers  94 ,  96 ,  98 , and  100  thus form a quadrupole. A subsequent four layers may be wound as a reverse quadrupole to form an octupole with the layers  94 ,  96 ,  98 , and  100 . A next eight layers may be wound as a reversed octupole, and so on.  
     [0041] The winding configuration  130  includes layers  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144 ,  146 , and  148 . The turns of the layer  132  are wound from a first optical fiber, and the turns of the layers  134 ,  136 ,  138 ,  140 ,  142 ,  144 ,  146 , and  148  are wound from a second optical fiber. Accordingly, the turns in the layer  132  are not a functional part of the winding configuration  130  although, as discussed above, the turns in the layer  132  could be functional. The first optical fiber that is used to wind the turns of the layer  132  has an outer diameter that is larger than the outer diameter of the second optical fiber which is used to wind the layers  134 ,  136 ,  138 ,  140 ,  142 ,  144 ,  146 , and  148 .  
     [0042] As shown in FIG. 8, the layers  134 - 148  include alternate turns wound from the first and second ends of the second optical fiber. A specific interleaved winding pattern for the layers  134 - 148  is shown in FIG. 8, although other interleaved winding patterns can be employed. Examples of interleaved winding patterns are taught in U.S. patent application Ser. No. 08/668,485, which was filed on Jun. 21, 1996, and which has been allowed by the U.S. patent and Trademark Office. The disclosure of U.S. patent application Ser. No. 08/668,485 is incorporated by reference herein.  
     [0043] Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present invention. For example, the present invention has been described above in the context of a fiber optic rate sensor. However, the present invention may also be used in connection with other fiber optic devices as well.  
     [0044] Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.