Abstract:
A method for splicing fiber optic cables to each other utilizes a splice tube of a glass material having a passage extending through it. A technician inserts ends of each optical fiber into the glass tube and abuts the ends of the fibers against each other. The technician applies a vacuum to the passage and heat to the glass tube. The heat softens the glass tube, and the reduced pressure in the passage draws the side walls of the glass tube tightly around the optical fiber ends.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to methods for splicing fiber optic cables, and in particular to a method employing a glass splice tube that is heated to shrink the tube around the abutted ends of the fibers. 
   BACKGROUND OF THE INVENTION 
   Fiber optic cables are in widespread use. Typically, a fiber optic cable has an optical fiber that is coated with a buffer coating. An elastomeric jacket surrounds the fiber. The jacket may include a strengthening material, such as carbon fibers. 
   Although protected by a jacket, fiber optic cables in some environments become damaged from time to time, particularly in military fighter aircraft. Fibers installed inside the boxes on the aircraft may be in protected environments with temperature control and vibration isolation, but the optical fiber cables installed in the open aircraft environment that connect the boxes and run with electrical wires are exposed to damage. If damaged, it may be necessary or expedient to remove the damaged area and reconnect portions of the original fiber optic cable with a splice. Alternatively, it might be preferred to splice part of the original fiber optic cable to a new section of fiber optic cable. 
   Splicing a fiber optic cable is a difficult task, particularly in the tight confines of military fighter aircraft. Replacing a single fiber optic cable can take days or even weeks under prior art repair processes. Because of the lack of space and a potentially dirty environment, it is difficult to meet the high requirements of an optical fiber splice. 
   One type of splice is a mechanical type that does not employ heat, rather uses a mechanical splice assembly to hold the ends together. The length of the optical fiber protruding from the buffer must be cleaved within about one thousandth of an inch. The end faces of the cleave must be perfect, with no hackles, burrs or angles. The mechanical process is difficult, and any deviation from the required tolerances will result in the splice failing. 
   Fusion splices employing heat are also made to optical fibers. With a fusion splice, the fiber ends are actually melted and fused together. Optical fibers for military aircraft are made of pure silica glass, which does not soften until a high temperature, such as 1,900° C. Generating that high of a temperature in the confines of an aircraft requires an electrical arc, which can be hazardous. Also, the fusion type repair equipment is large and expensive. 
   SUMMARY OF THE INVENTION 
   The method of this invention employs a splice tube of a glass material having a passage through it. A technician inserts an end portion of each optical fiber into the passage of the tube and abuts the ends. A heater element applies heat, and a vacuum pump reduces gas pressure within the tube to cause the tube to collapse or shrink around the abutted end portions of the optical fibers. When cooled, the splice tube rigidly holds the optical fibers ends in alignment. 
   Preferably the tube is transparent, so that the technician can see when the ends abut each other. The glass material for the tube is selected to have a softening temperature much lower than the optical fibers. The heat is controlled so that the temperature will not reach the softening temperature of the optical fibers. 
   Also, preferably the passage in the tube has end portions that are enlarged. The technician preferably strips back part of the buffer coating and also part of the jacket, exposing a section of optical fiber as well as a section of buffer coating. When inserting the optical fiber into the tube, the fiber will enter the central, smaller diameter portion of the passage, but the buffer coating is too large. The buffer coating enters the end portion of the passage, but the jacket is too large to enter. 
   Preferably the heating and evacuating step is performed with an assembly that has manifold portions for each end of the splice tube. The manifold portions form a seal around the outer diameter of the glass tube and around part of the fiber optic cable, such as the buffer coating. These two seals define a chamber, and a port leads from the chamber to the exterior. A vacuum pump connects by a hose to each port. Operating the vacuum pump causes the air or gas pressure in the passage to reduce relative to the pressure on the exterior of the tube. 
   The manifold is preferably split into two parts, and a clamp secures the two parts around the tube. The clamp has a jaw portion that clamps on the manifold and another portion that clamps around the jacket. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view illustrating a splice tube and a portion of two fiber optic cables being inserted into the splice tube. 
       FIG. 2  is a sectional view of the splice tube of  FIG. 1 , shown installed within a manifold that is clamped to the two fiber optic cables. 
       FIG. 3  is a sectional view of the assembly of  FIG. 2 , taken along the line  3 - 3  of  FIG. 2 , and illustrating one of the clamps. 
       FIG. 4  is a view of the splice tube of  FIG. 1 , shown in a completed form. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , two sections of fiber optic cable  11  are shown in the process of being spliced together. The sections of fiber optic cable  11  may have originally been a single length, but a splice is needed after removal of a damaged section. Alternatively, one of the sections of fiber optic cable  11  may be a new section. 
   Each section of fiber optic cable  11  is conventional and has a central optical fiber  13  surrounded by a buffer coating  15 . An elastomeric jacket  17  (shown in  FIG. 1  only on the left side) surrounds buffer coating  15 . Jacket  17  may have longitudinally extending carbon fibers (not shown) for strength. The technician has prepared the ends by stripping back from optical fiber  13  a portion of buffer  15  and stripping back from buffer  15  a portion of jacket  17 . The end of each optical fiber  13  has been cleaved so as to be perpendicular to the axis. The stripping procedure exposes a portion of optical fiber  13  and a portion of buffer  15  of each cable. 
   A glass splice tube  19  forms a permanent part of the splice. The glass material of tube  19  has a much lower softening temperature than the material of optical fiber  13 . For example for military aircraft, the glass material of optical fiber  13  is typically pure silica glass and has a high softening point, such as 1,900° C. The material of tube  19  may be a borosilicate glass or other type of glass, which, for example, may have a softening temperature of only about 725° C. Preferably, tube  19  is transparent or clear enough for a technician to see through it. 
   A passage extends axially through tube  19 , having a central portion  21  and two end portions  23  in this example. Central portion  21  is smaller in diameter than end portions  23  and also longer in length. The intersection between central portion  21  and end portions  23  is a gradual curved transition. Preferably, the inner diameter of central passage portion  21  is slightly larger than the outer diameter of optical fiber  13 , but smaller than the outer diameter of buffer coating  15 . For example, if the diameter of optical fiber  13  is 125 microns, the inner diameter of central passage portion  21  is preferably about 128 microns (micrometers). For a typical military aircraft fiber optic cable having a 125 micron optical fiber  13 , each passage end portions  23  may having an inner diameter of about 1 mm, although that dimension can vary considerably. The lengths of tube  19  and passage portions  21 ,  23  are not critical. In one embodiment, the entire length of tube  19  is about 1½-2 inches, and the minimum length of central passage portion  21  is about one-half inch. 
   As shown in  FIG. 2 , when each fiber optic cable  11  is stripped as described above, a portion of each buffer coating  15  will be in one of the passage end portions  23  when the cleaved ends of optical fibers  13  abut each other. It is not essential that the end portions of optical fibers  13  be identical in length, nor is it essential that buffer coatings  15  insert the full depth of passage end portions  23 . 
   Referring to  FIG. 2 , a fixture assembly  25  is employed for forming the splice. Fixture assembly  25  includes a manifold  27 , which is preferably split into two halves along its longitudinal axis. Each half has a longitudinally extending semi-cylindrical recess. When assembled together, the recesses within each half of manifold  27  form a cylindrical cavity that clamps closely around tube  19 . 
   The cylindrical cavity includes a central cavity portion  29  that has an inner diameter substantially equal to the outer diameter of tube  19  to form a seal with the exterior of tube  19 . The cylindrical cavity of manifold  27  has two end cavity portions  31  that define a smaller inner diameter than central cavity portion  29 . The inner diameter of each end cavity portion  31  is substantially equal to the outer diameter of the exposed portion of buffer coating  15  in this example to form a seal with buffer coating  15 . Alternately, the inner diameter of each end cavity portion  31  could be substantially equal to the outer diameter of jacket  17  for forming a seal on jacket  17 . 
   The engagement of central cavity portion  29  with the outer diameter of tube  19  forms a low pressure seal. The engagement of each end cavity portion  31  with buffer  15  forms another low pressure seal. If desired, high temperature linings or coatings could be employed in cavity portions  29 ,  31  to enhance sealing, but a tight seal is not necessary. 
   Each cavity portion of manifold  27  between end cavity portion  31  and central cavity portion  29  is larger in diameter than central cavity portion  29  and defines a chamber  33 . Each chamber  33  is in fluid communication with tube passage portions  21 ,  23 , but sealed from the outer diameter of tube  19  and from the exterior of manifold  27  by the seal between central interior portion  29  and tube  19  and the seal between each end cavity portion  31  and one of the buffers  15 . 
   A port  35  leads from each chamber  33  to the exterior of manifold  27 . A hose  37  connects to each port  35 . Hose  37  leads to a conventional vacuum pump  39 . When vacuum pump  39  is operated, it will lower the gas pressure within tube passage portions  21 ,  23  relative to the gas pressure on the outer diameter of tube  19 . 
   An electrical resistance heater element  41  is mounted in a thermally insulated section  43  and positioned around the central portion of tube  19 . In this example, heater element  41  is mounted to a central portion of manifold  27 . Insulated section  43  is located half-way between the opposite ends of manifold  27  and extends for a selected length. Heater element  41  is connected to a power source  45 , schematically illustrated. If desired, insulated section  43  could be formed separately from manifold  27 , and manifold  27  be formed into separate end pieces, each having a chamber  33  for engaging one of the ends of tube  19 . 
   A pair of clamps  47  is illustrated for clamping the halves of manifold  27  to each other and to the fiber optic cables  11 . Clamps  47  may be of a variety of types and could be configured as a single clamp that clamps both ends of manifold  27  simultaneously. As illustrated in  FIG. 3 , each clamp  47  has two clamp halves  49  that are biased toward each other by a spring  51 . Each clamp  47  in this example thus resembles a conventional clothes pin. As shown in  FIG. 2 , clamp halves  49  of each clamp  47  have jaws with two diameters. The larger diameter jaw portion  53  is sized for clamping over the exterior of an end portion of manifold  27 . The smaller diameter jaw portion  55  is configured for clamping around jacket  17  of one of the fiber optic cables  11 . Clamps  47  thus not only retain the two halves of manifold  27  to each other, but also retain the fiber optic cables  11  in the position of  FIG. 2 . 
   To perform the splice, the technician preferably slides a heat shrinkable thermoplastic boot  59  ( FIG. 4 ) along each fiber optic cable  11  a short distance from the end. The technician prepares the end of each fiber optic cable  11 , as shown in  FIGS. 1 and 2 , cleaving and stripping an end portion of optical fiber  13  and an end portion of buffer coating  15 . The technician inserts optical fibers  13  into passage central portion  21  and, by observing through the transparent tube  19 , carefully positions them so that the ends of optical fibers  13  abut each other. This position places part of each exposed portion of buffer  15  within passage end portion  21 . The end of jacket  17  will be spaced a short distance from an end of tube  19 . Initially, there will be a small annular clearance between each optical fiber  13  and the inner diameter of passage central portion  21 . 
   The technician places manifold  27  around tube  19 , and places clamps  47  on each jacket  17  and on each end of manifold  27 . Hoses  37  are connected between vacuum pump  39  and ports  35 . The technician turns on pump  39  to reduce the pressure in passage central portion  21  and applies power to heater element  41 . As the glass material of tube  19  softens, the lower pressure in passage central portion  21  causes tube  19  to collapse around in full 360 degree contact with optical fibers  13 . When cooled, tube  19  becomes rigid again but in its collapsed configuration so that it firmly grips and supports optical fibers  13 . The collapsing due to heat and vacuum removes any clearances between the inner diameter of passage central portion  21  and optical fibers  13 . The passage end portions  23  do not collapse around buffer  15  in this example. The amount of heat is not sufficient to soften, melt or fuse the optical fibers  13 . 
   After cooling, the technician removes clamps  47  and manifold  27 . The technician may place a conventional sealant  57  around buffer coating  15  where it protrudes from tube  19 . The technician slides each boot  59  down from each fiber optical cable  11  over the joint between each fiber optic cable  11  and tube  19 . The technician applies heat to boot  59  in a conventional manner to cause it to shrink around the joint. 
   The method is quicker and easier to perform than the high tolerance mechanical splicing of the prior art. The method does not require the high temperatures required to fuse fiber optic ends together. The equipment to perform the method is simple to manufacture and inexpensive. Tolerances for cleaving are much broader than in the prior art. 
   While the invention has been shown in only one of its forms, it should apparent to those skilled in the art that it is not so limited but it is susceptible to various changes without departing from the scope of the invention.