Abstract:
A method of assembling a preform for a bend-insensitive multimode optical fiber (BIMMF), includes providing a multimode core rod, a glass overclad tube, and a trench tube of down-doped quartz glass with a depressed refractive index sufficient to obtain a desired trench depth in a refractive index (RI) profile of a drawn fiber. The core rod is placed inside the trench tube, and the trench tube and the core rod are placed inside the overclad tube to define the preform. A top end of the trench tube is formed to contact an adjacent part of either the core rod or the overclad tube so that the trench tube is suspended to hang from the adjacent part when the preform is vertically oriented, and a bottom end of the trench tube is restrained from sinking into a lower portion of the preform when the preform is heated to collapse.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention concerns the manufacture of optical fibers, particularly bend insensitive multimode fibers. 
         [0003]    2. Discussion of the Known Art 
         [0004]    Patent Application Pub. No. US 2009/0060437 (Mar. 5, 2009) discloses a bend insensitive, single mode fiber having a relatively low bend loss at a bend radius of about 4 to 15 mm. The disclosed fiber has a core and a cladding region for propagating light in a fundamental transverse mode. The cladding region includes (i) an outer cladding having a refractive index less than that of the core region, (ii) an annular pedestal region having a refractive index higher than that of the outer cladding and comparable to that of the core, (iii) an annular inner trench region disposed between the core and the pedestal region, the inner trench region having a refractive index less than that of the outer cladding, and (iv) an annular outer trench region disposed between the pedestal region and the outer cladding, the outer trench region having a refractive index less than that of the outer cladding. All relevant portions of the &#39;437 Publication are incorporated by reference. 
         [0005]    Typical bend insensitive multimode fibers (BIMMF) have a refractive index profile in which the fiber cladding contains a trench region or layer of depressed index glass. Such index profiles are disclosed in, e.g., U.S. Pat. No. 8,073,301 (Dec. 6, 2011)(see FIG. 2 and related text), and U.S. patent application Ser. No. 13/252,964 which was published as US 2012/0183267 on Jul. 19, 2012, all of which are incorporated by reference. 
         [0006]      FIG. 1  is an example of a refractive index difference profile of a BIMMF, relative to a pure fused quartz overclad (or substrate) tube  14  in a preform from which the fiber is drawn. A trench region  12  in  FIG. 1  is typically obtained by depositing a depressed index glass on the inside diameter of the overclad tube  14 , using either a modified chemical vapor deposition (MCVD) or a plasma chemical vapor deposition (PCVD) process. See, U.S. Pat. No. 7,903,918 (Mar. 8, 2011) which is incorporated by reference. 
         [0007]    A glass core rod  16  is then inserted axially inside the overclad tube  14  to make the fiber preform, and the preform is heated vertically inside a furnace until the overclad tube  14  softens and collapses on the core rod  16  to form a drop at the bottom of the preform. The BIMMF is then drawn from the drop. It will be appreciated that among other drawbacks, the trench deposition process is very costly, ties up a lot of deposition capacity, and the resulting fiber is subject to yield loss related to axial trends in the deposited glass. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the invention, a method of assembling a preform for a bend-insensitive multimode optical fiber (BIMMF), includes providing a multimode core rod, a glass over-cladding tube, and a trench tube of down-doped quartz glass with a depressed refractive index sufficient to obtain a desired trench depth in a refractive index (RI) profile of a drawn fiber. The core rod is placed coaxially inside the trench tube, and the trench tube and the core rod are placed coaxially inside the over-cladding tube to define the preform. 
         [0009]    A top end of the trench tube is formed to contact an adjacent part of either the core rod or the over-cladding tube so that the trench tube is suspended to hang from the adjacent part when the preform is vertically oriented, and a bottom end of the trench tube is restrained from sinking into a lower portion of the preform when the preform is heated to collapse. 
         [0010]    For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]    In the drawing: 
           [0012]      FIG. 1  is a refractive index difference profile of a bend insensitive multimode optical fiber (BIMMF) relative to a quartz overclad tube of a typical preform from which the fiber is drawn, according to the prior art; 
           [0013]      FIG. 2  shows sectional views of a BIMMF preform in planes perpendicular and parallel to the axis of the preform, according to the invention; 
           [0014]      FIG. 3  is a sectional view of an upper portion of a BIMMF preform in a plane parallel to the preform axis, according to a first embodiment of the invention; 
           [0015]      FIG. 4  is a sectional view of a preform trench tube, an open end of which is heated by a torch to be reshaped; 
           [0016]      FIG. 5  is a sectional view of an upper portion of a BIMMF preform in a plane parallel to the preform axis, according to a second embodiment of the invention; 
           [0017]      FIG. 6  is a sectional view of a preform trench tube and an inserted core rod, wherein an open end of the tube is heated to be reshaped; 
           [0018]      FIG. 7  is a sectional view similar to  FIG. 6 , including a pedestal for elevating the trench tube relative to the core rod prior to heating the open end of the tube; 
           [0019]      FIG. 8  is a table showing average bend loss of a number of BIMMFs produced using a preform according to the invention; and 
           [0020]      FIG. 9  is a refractive index difference profile of a bend insensitive multimode optical fiber (BIMMF) relative to an overclad tube in a preform from which the fiber was drawn, according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 2  shows sectional views of a cylindrical BIMMF preform  20  made according to the invention. The view at the top of  FIG. 2  is taken in a plane transverse to the axis A of the preform  20 , and the view at the bottom of  FIG. 2  is shown parallel to the preform axis A. A tube  22  of down-doped or low-index glass (e.g., quartz) is supported inside the preform  20  so that the tube  22  is disposed coaxially with an inner core rod  24 , and with a surrounding glass overclad tube  26  made of, e.g., pure fused quartz. The down-doped glass tube  22  is referred to herein as a “trench” tube. 
         [0022]    When the preform  20  is suspended vertically and lowered into a furnace or other heated region, the trench tube  22  and the surrounding overclad tube  26  collapses over the core rod  24 , and a glass drop forms at the bottom of the collapsed preform  20 . A BIMMF is then drawn from the bottom of the preform  20  in a known manner. The refractive index (RI) profile through the cladding of the drawn fiber has a trench region such as, e.g., the trench region  12  in  FIG. 1 , which region is formed by the collapsed trench tube  22  and enables the drawn fiber to be bend insensitive. Once the preform  20  has been collapsed, it can be withdrawn from the furnace in the collapsed state, allowed to cool, and heated again at a later time to draw more fiber. 
         [0023]    Heating and collapsing the down-doped trench tube  22  and the surrounding overclad tube  26  simultaneously over the core rod  24  was found to be preferable to other possible solutions such as first collapsing the trench tube  22  horizontally on the core rod  24 , and then collapsing the overclad tube  26  on the outer circumference of the trench tube  22  in a vertical furnace during the fiber draw process. The trench tube  22  will generally have a lower softening point than either of the core rod  24  or the overclad tube  26 . If the trench tube  22  is simply dropped into the preform assembly to rest on its lower end, it was found to sink into the lower end of the preform during the fiber draw process, thus causing the trench region  12  in the fiber cladding to have an increased width and additional axial variability. To avoid this problem, it has been found that prior to heating, the trench tube  22  should be physically supported at its upper end so as to hang vertically inside the preform  20 . In accordance with the invention, this is accomplished in either one of two ways: 
         [0024]    A. See  FIG. 3 . A top end  30  of the trench tube  22  is heated and flared radially outward or conically, so that the top end  30  of the tube abuts an inner circumferential edge  32  on the top end of the overclad tube  26  in the vicinity of a weld  34  between the overclad tube  26  and an associated tubular handle  36 . Thus, the trench tube  22  is positively supported by the top end of the overclad tube  26  to hang vertically and coaxially inside the overclad tube to assemble the preform  20 . Alternatively, a number of radially outward, circumferentially spaced protuberances can be formed at the top end of the trench tube  22  so that the protuberances abut the inner circumferential edge  32  of the overclad tube  26 , allowing the trench tube  22  to be suspended and to hang vertically inside the overclad tube  26 . 
         [0025]    If a hydrogen-oxygen torch is used to heat the top end  30  of the trench tube  22  when flaring the top end outward, moisture and air particles may accumulate within the trench tube  22 . This condensation can flow inside the trench tube  22  and contaminate the inside surface, causing, among other issues, prooftest breaks and voids in the drawn optical fiber if the trench tube  22  is not washed promptly after following the above steps. It was found that such contamination can be avoided by flowing a clean and filtered gas (e.g., Nitrogen) through the tube  22  during the heating process. 
         [0026]    Specifically, as shown in  FIG. 4 , a stopper or seal such as a cork  40  is inserted in the open end of the trench tube  22  opposite to the end that is being heated to be flared, and clean gas is introduced through an axial passage  42  in the cork via a nozzle. As the gas flow exits the heated end of the trench tube  22 , the flow prevents moisture and contamination from entering inside the tube. 
         [0027]    B. See  FIG. 5 . Alternatively, the top end  30  of the trench tube  22  is formed to have an hourglass or necked-in shape, or with circumferentially spaced radially inward indentations or dimples, so that the top end of the tube abuts and is suspended to hang from a peripheral top edge  50  of the core rod  24 , near a weld  52  between the core rod and a rod handle  54  after the tube passes over the handle  54 . (The outside diameter of the handle  54  is typically less than that of the core rod  24 ). The trench tube  22  is thus firmly supported by the top edge  50  of the core rod  24  so as to hang vertically and coaxially inside the surrounding overclad tube  26 . This process avoids a need to weld the trench tube  22  to any part of the core rod  24  or to the rod handle  54 . And when the core rod  24  is raised by the handle  54 , the trench tube  22  remains suspended from the top edge  50  of the rod. 
         [0028]    If a hydrogen-oxygen torch is used to heat the top end  30  of the trench tube  22  prior to necking in the top end, or to forming the indentations in the top end, contamination of the inside surface of the tube can be avoided by making the trench tube at least 5 cm longer than the core rod  24 . See  FIG. 6 . In this manner, the open top end of the trench tube  22  extends past the region of the tube to be heated, and the torch flame is not in the vicinity of the open top end  30 . While there may be some local contamination on the outside surface of the tube  22  where the torch is applied, no condensation will accumulate on the inside surface of the tube. Method B is preferable to method A since it does not require gas flow apparatus or subsequent tube washing to eliminate concerns over contamination. 
         [0029]    Further, in method B, it was found that if the necked-in or dimpled region of the trench tube  22  pinches directly against an area of the rod handle  54  that has residual stress, then the handle  54  may become weakened to crack either immediately or within hours after forming the dimpled region. If so, the core rod  24  could fall out of the open bottom end of the preform  20 . Since there is typically a stress region in the rod handle  54  close to the weld  52  between the handle and the core rod  24 , such stress can be relieved for example, by the use of a known slow annealing process prior to forming the necked in or dimpled region at the top end  30  of the trench tube. A faster solution that avoids such annealing was devised, wherein the trench tube  22  is temporarily raised relative to the core rod  24  while the necked in or dimpled region is formed in the top end of the tube. 
         [0030]    Specifically, as shown in  FIG. 7 , a pedestal  70  is used to elevate the trench tube  22  axially by a certain offset distance D relative to the core rod  24 . When the tube is heated by a torch to be necked in or dimpled inwardly at its top end  30 , the tube constricts or pinches against a region along the rod handle  54  that does not have stress regions. Thus, the strength of the handle  54  is not compromised. The pedestal  70  is removed and the trench tube  22  is lowered relative to the core rod  24  so that the tube then hangs from the top edge of the core rod  24  as described previously. While the necked in or inwardly dimpled region may be formed at the top end  30  of the tube before the core rod and rod handle are placed inside the tube, the pedestal technique in  FIG. 7  overcomes situations where the rod handle  54  has a large diameter ball at its top end for supporting the handle and the rod  24  inside the overclad tube  26 . In such cases, the ball could prevent the necked in region of the trench tube  22  from being moved downward along the handle to rest atop the core rod  24 , thus requiring the trench tube to be moved upward from the bottom end of the core rod before the necked in region can be formed at the top end  30  of the trench tube. 
         [0031]    Both of the methods A and B require that clearance gaps G shown in  FIGS. 3 &amp; 5  provided between the trench tube  22  and the inner core rod  24 , and between the trench tube and the surrounding overclad tube  26 , be kept as small as possible to minimize or avoid any radial asymmetry during a collapsing process. The gaps G are preferably as small as possible while allowing enough clearance for the core rod  24  to pass axially inside the trench tube  22 . For example, the inside diameter of the trench tube  22  may be 1 mm to 2 mm larger than the outside diameter of the core rod. 
         [0032]    It has also been discovered that both methods A and B work particularly well when the trench portion in the in the refractive index (RI) profile of the drawn fiber (e.g., portion  12  in  FIG. 1 ) is situated relatively far from the RI profile of the fiber core. 
       Example 
       [0033]    Manufacturing a 50 μm bend insensitive multimode fiber 
         [0034]    A preform  20  was assembled via the method of  FIG. 5  (Method B) using the following components:
       Core Rod  24  Diameter=24.5 mm   Core Rod  24  Length=1175 mm   Trench Tube  22  Inner Diameter=26.54 mm   Trench Tube  22  Outer Diameter=29.51 mm   Trench Tube  22  Length=1280 mm   Trench Tube  22  Delta Refractive Index (relative to pure quartz)=−0.0056   Overclad Tube  26  Inner Diameter=31.43 mm   Overclad Tube  26  Outer Diameter=47.77 mm   Overclad Tube  26  Length=1280 mm       
 
         [0044]    The assembled preform  20  was heated in a vertical furnace and a number of 50/125 μm bend insensitive multimode fibers were drawn, each having a length of approximately 8.8 km. Bend loss test results for the fibers are shown in  FIG. 8 . The data reflects additional loss induced by wrapping the fibers twice around a mandrel of radius 7.5 mm. Bend loss was measured at wavelengths of 850 nm and 1300 nm. Both ends of each fiber were tested and the average loss value is given for each test. 
         [0045]      FIG. 8  shows that 100% of the fiber from the preform  20  passed currently specified bend loss test standards for BIMMFs. This is a significant improvement over prior BIMMF preform assembly and fiber drawing processes, wherein the trench in the RI profile of the drawn fiber tapers at one end, and many fibers fail to meet the specified standards.  FIG. 8  also reflects an increased fiber yield with respect to the prior performs and processes, and increased uniformity that results from preventing the trench tube  22  from sinking during fiber draw. 
         [0046]      FIG. 9  is a typical RI difference profile of fibers that were drawn from the preform  20  of the present Example. Note that the profile in  FIG. 9  is substantially identical to that in  FIG. 1 , thus confirming that the inventive preform and method produce a BIMMF having desired properties. 
         [0047]    As disclosed herein, a bend insensitive multimode optical fiber is manufactured by placing a tube of down-doped quartz glass radially between an inner core rod and a surrounding overclad tube in a preform so that a trench region is formed in the index profile of the cladding of a drawn fiber. The preform is heated vertically in a furnace to collapse on the core rod, and the fiber is then drawn from the preform. Alternatively, once the preform collapses on the core rod, the preform can be withdrawn from the furnace and later re-heated for a fiber draw process. The inventive method provides higher productivity, lower cost, and higher fiber yield than the known prior methods. 
         [0048]    While the foregoing represents preferred embodiments of the present invention, it will be understood by persons skilled in the art that various modifications, additions, and changes may be made without departing from the spirit and scope of the invention. For example, the dimensions and the RI of each component of the preform  20  may differ from the corresponding values given in the above Example, so that certain desired properties in the drawn BIMMF are obtained. Accordingly, the invention includes all such modifications, additions, and changes that are within the scope of the appended claims.