Patent Publication Number: US-2012033919-A1

Title: Transition for an optical fiber ribbon

Description:
The invention relates to the production of a transition (or pressure crossing) for an optical fiber ribbon for a submarine optical connector. 
     It is known to be very difficult to seal a transition for an optical fiber ribbon cable at the rear part of an optical connector that can withstand the high pressures in a submarine environment, i.e. at pressures of above 300 bar. 
     The optical fiber ribbon is coated with a protective sheath which is not watertight at these high pressures. 
     Moreover, no transition or watertight barrier that can withstand the high pressures in a submarine environment, suitable for optical fiber ribbon connectors is known. 
     There is therefore a need for a watertight transition able to withstand high pressures for an optical fiber ribbon cable. 
     The invention provides a method of producing a transition for an optical fiber ribbon for a submarine optical cable, the method comprising the following steps:
         a cable comprising an optical fiber ribbon surrounded by a plastic sheath is provided;   a through-hole, for the ribbon to pass through a support piece, and a hole transverse to the through-hole, capable of allowing an adhesive in the form of a liquid or viscous fluid to flow into the through-hole, are provided;   the ribbon is mechanically stripped (at the end or in its middle) over a portion of its length corresponding to that of its passage in the support piece, this length being slightly smaller than that of the through-hole;   the remaining, not mechanically stripped, portion of the ribbon is chemically stripped by means of a solvent for the plastic of the ribbon;   the ribbon is passed into the through-hole of the support piece, placing the stripped portion within the through-hole;   an adhesive of the epoxy type, in liquid or even viscous form, is injected into the transverse hole until the adhesive coats at least the stripped portion of the ribbon and at least partly fills the through-hole; and   the transverse hole is plugged at its end.       

     This arrangement has the effect of creating a watertight seal for the stripped optical fibers of the ribbon cable, which sealingly and strongly adheres to the silica body of the fibers and to the walls of the through-hole for passage of the cable into the support piece, and optionally to the edges of the through-hole. Such a fiber seal can withstand pressures up to 1600 bar in a marine environment. 
     The dimensions of the through-hole for the ribbon in the support piece correspond in cross section, preferably to within a small clearance, i.e. to within about 1 mm, to that of the ribbon. This allows the through-hole to be at least partly filled with the adhesive. 
     The ribbon is passed into the through-hole of the support piece by placing the stripped portion, preferably centered or close to the through-hole of the ribbon, for example by marking one of its ends. This ensures that the adhesive adheres simultaneously to the walls of the through-hole and to the stripped portion of the ribbon. 
     The ribbon is held in position, in the centered position or close to the through-hole, preferably by means of a tool for holding the cable in position relative to the support piece, for example with a retaining template for holding the cable in place on the support piece (preferably by means of a HT (high temperature) adhesive tape bonded to the cable on either side of the support piece). This ensures that the ribbon is in a stable position relative to the support piece. 
     Injection of the epoxy adhesive into the transverse hole is preferably carried out, and for safety, until the adhesive protrudes from each side of the through-hole of the support piece. 
     Advantageously, the adhesive injected into the support piece is polymerised, preferably at a polymerisation temperature above the ambient temperature but below the temperature at which the optical fiber ribbon cable is impaired (for example for one hour at 80° C.). In this way, any creep of the injected adhesive is limited and the polymerisation of the adhesive is accelerated. 
     Advantageously, the step consisting in mechanically stripping the ribbon over a portion of its length corresponding to that of its passage in the support piece is carried out over a length slightly smaller than that of the through-hole, being set back by at least 1 mm at each end of the through-hole. 
     The invention also provides a device for producing a transition for an optical fiber ribbon surrounded by a plastic sheath in a support piece of a submarine optical connector, the device comprising:
         means for mechanically stripping at least one optical fiber ribbon over a defined length corresponding to the passage of the ribbon in a through-hole in the support piece;   means for chemically stripping the not mechanically stripped portion of the ribbon by means of a solvent for the plastic of the ribbon; and   means for injecting an adhesive of epoxy type, in liquid or even viscous form, into the through-hole until the adhesive coats at least the stripped portion of the ribbon and at least partly fills the through-hole.       

     The mechanical stripping means may for example comprise:
         a support plate for supporting the optical fiber ribbons that are placed parallel to one another on a grooved part of the support plate, having a calibrated cross section partly corresponding to that of the optical fiber ribbons;   means for fastening the optical fiber ribbons; and   means for cutting into the surface of the sheath of the optical fiber ribbons, these being intended to strip the ribbons over the defined length.       

     Thus, the optical fiber ribbons may be held trapped in the grooves of the support plate. 
     The cutting means may comprise a blade designed to be moved along the optical fiber ribbons, preferably in the grooved part of the support plate, parallel to the latter and over a course corresponding to the defined length, away from the means for fastening the ribbons. Thus, the cutting of the ribbons is carried out with the ribbons in tension relative to the fastening means, thereby making it easier for them to be held in place on the support plate. 
     The blade may be designed to have a cutting section capable of cutting the portion of the sheath placed above the silica optical fiber body, as a thin cut strip. 
     The fastening means may comprise a system of clamps or the like, these being intended to receive, clamped between it and the support plate, the optical fiber ribbons. This fastening system preferably includes an intermediate pad for protecting the optical fiber ribbons. 
     This device may comprise in succession from the upstream end to the downstream end in the direction of advance of the blade over the optical fiber ribbons, a clamping assembly, for clamping the optical fiber ribbons, the blade for stripping the ribbons, which is mounted so as to move relative to the support plate for the ribbons, and a retaining bar for holding the ribbons in place. 
     The invention also provides a transition for a submarine optical connector, defining a through-hole for an optical fiber ribbon surrounded by a plastic sheath, and comprising, in said through-hole:
         at least one stripped portion of the optical fiber ribbon; and   adhesive of epoxy type, coating at least the stripped portion of the ribbon and at least partly filling the through-hole so as to fasten the optical fiber ribbon to the transition.       

     The invention also provides an optical connector having the aforementioned transition. 
    
    
     
       One embodiment of the invention will now be described with reference to the appended drawing in which: 
         FIG. 1  is a block diagram illustrating the various steps of the method for producing a transition for an optical fiber ribbon cable in a support piece for a submarine optical connector according to one embodiment of the invention; 
         FIG. 2  is a partial schematic perspective view illustrating the mechanical stripping of the optical fiber ribbons of the optical cable; and 
         FIG. 3  is a cross-sectional diagram of the support piece showing the through-hole for an optical fiber ribbon of the optical cable and the injection of adhesive onto the stripped portion of the optical fiber ribbon. 
     
    
    
       FIG. 1  shows a method of producing a pressure-resistant watertight transition  1  for an optical fiber ribbon cable  3  in a support piece  5 , this transition being designed to be firmly mounted on a submarine optical connector  7  and to connect the optical fiber  3  to the rear carcass part  9  of the optical connector  7 . 
     The essential steps of the method have been shown schematically in the form of successive blocks. 
     Firstly, the support piece  5  of the optical connector  7  (not shown) is machined so that it can receive the optical cable  3 , therefore providing, in this support piece, a hole  11  of oblong cross section passing through the support piece  5  and intended to receive the cable  3 , and more precisely the ribbon  13  consisting of adjacent optical fibers, for example of the MT ferrule type, of the optical cable  3 . 
     The dimensions in cross section of the hole  11  corresponds to within a small clearance, for example to within 1 mm, to the cross section of the optical fiber ribbon  13 . 
     A second, blind hole  15  is formed, this being transversely connected to the first hole ( 11 ), approximately in the central region of the latter (cf. also  FIG. 3 ) and emerging at the surface of the support piece  5 . This second hole is, designed to allow adhesive  17  in viscous form to pass into the first hole  11 , as will be explained later. The diameter of the second hole is about 1 mm. 
     A first step of the method consists in stripping the cable  3 , more precisely the optical fiber ribbon  13 , by means of a blade device  19  shown schematically and partially in  FIG. 2 . 
     This device  19  comprises, in succession from the upstream end to the downstream end in the direction of advance of the blade  21 , a set of two opposed clamping bars  23 , for clamping the optical fiber ribbons  13 , a blade  21  for stripping the ribbons  13 , which is able to move relative to a fixed support plate  25  for supporting the ribbons  13 , and a retaining bar  27  for holding the ribbons  13  in place while they are being stripped by the blade  21 . 
     One or more optical fiber ribbons  13  are thus placed on the support plate  25 , the surface of the latter being grooved so as to accommodate each of these ribbons in parallel grooves  29 , the width of which corresponds to that of each ribbon  13 . These grooves  29  are parallel to the direction of advance (along the arrow) of the blade  21 . The depth of these grooves  29  is defined so that the optical fibers of the ribbon in contact with the support plate  25  protrude from the grooves  29  by at least the thickness of the protective sheath of the fibers plus that of the ribbon, i.e. about 0.1 mm, for conventional optical fibers with a 120 μm diameter for the silica body with a ribbon thickness of 310 μm. The depth of the grooves  29  may thus be about 200 μm, the blade being moved very close to the grooved surface  29  of the support plate  25  and cutting the protruding section of the fiber ribbon (with a thickness of about 310 μm) at the surface of the plate  25 . 
     The ribbons  13  thus positioned, each in one of the grooves  29 , are held in position laterally on the support plate  25 . These ribbons  13  are also held in position, clamped by means of an intermediate protective pad  31 , between the clamping bars  23 . This prevents them from sliding under the tensile cutting force for stripping the protective sheath via the blade  21 , while the ribbons  13  are being held in position, downstream, by the retaining bar  27  placed against the ribbons  13 , in contact with a small area (side clearance) against the latter. 
     The blade  21  is designed to have a cutting section, for example beveled, capable of cutting that portion of the sheath placed above the silica body of the optical fiber, as a thin cut strip about 100 μm in thickness. 
     This blade  21  is moved over the ribbons  13 , along the arrow, over a length slightly smaller than that of the through-hole  11  of the support piece  5 , corresponding to the desired stripping length of the optical fibers of the ribbon  13 . 
     This length of the stripped portion  33  of the optical fibers may be from 1 to several mm smaller than that of the through-hole  11  so that, in the centered position of the stripped portion  33  in the through-hole  11 , the stripped portion  33  of each of the fibers, all at the same level transversely to the ribbon, is slightly set back from the edge of the through-hole  11 , by half of the aforementioned value. 
     According to this mechanical stripping method using an upper blade  21 , a first strip of the protective sheath of the ribbons  13  is cut without removing it, each of the ribbons  13  treated is turned over and the cut strip rests on the bottom of the groove  29  so as to act as a cutting seat for stripping the portion of the ribbon sheath opposite that already cut. This new cut or second cut strip is opposite the first one (having the same start, the same end and even the same cut depth) and when this second cut has been completed, the first and second cut protective strips are removed. The ribbon  13  is then practically stripped apart from the lateral portions of the optical fibers, which is also easy to remove (by manually cutting it). 
     Next, the operation of chemically stripping the mechanically stripped ribbon portion  33 , shown in the second block diagram at the top towards the right in  FIG. 1 , is carried out so as to remove all traces, in dust form, of the plastic for protecting the fibers that might remain on the stripped portion  33 . The stripped portion  33  may for example be dipped into a solvent  35  for the plastic, for example a paint stripper, for a defined time corresponding to this chemical dissolution of the aforementioned impurities. 
     Once the chemical stripping operation has been carried out, the stripped portion  33  may be cleaned and degreased, for example using an alcohol-based solvent (not shown). This operation improves the quality of adhesion of the adhesive in the bonding operation described later. 
     The cable  3  or fiber ribbon  13  is then inserted into the through-hole  11 , the position of the stripped portion  33  relative to the through-hole  11  is marked and this stripped portion  33  is held in position relative to the support piece  5 . This situation is shown by the third block diagram, toward the right just after the chemical stripping. The centering of the portion  33  in the through-hole  11  may be accomplished by means of a marker reference on the ribbon positioned at a defined distance from the support piece on one side of the latter (not shown). 
     The stripped portion  33  is held in position relative to the through-hole  11  of the support piece  5  by means of a retaining template  37  (cf. also  FIG. 3 ) comprising for example a support plate  39  for the ribbon  13  and a recessed part  41 , which immobilizes the support piece  5  in such a way that the ribbon  13  can be placed in contact with the plate  39 , being fastened to the latter for example by an HT (high temperature) adhesive tape  43  bonded to the ribbon  13  and to the plate  39 , on either side of the support piece  5 , as may be seen in  FIG. 3 . 
     Next (see the fourth successive block diagram), the epoxy adhesive  17 , which may be a one-component adhesive or a two-component adhesive (with a hardener), is injected into the transverse hole  15  at the through-hole  11 . This adhesive  17  is for example injected by a syringe  45  the needle  47  of which (having a length smaller than the depth of the hole  15 ) is inserted into the hole  15  ( FIG. 3 ). Adhesive is injected in this way until it fills the through-hole  11  in the stripped portion  33 , completely coating this stripped portion  33 . 
     For safety, the injection may continue until the adhesive  17  protrudes from each side of the through-hole  11  at  49 , as shown in the present case. 
     The next step (see the fifth successive block diagram) consists in polymerising the adhesive  17  injected into the through-hole  11 , firstly closing off the second hole  15  by means of a plug  51  so as to prevent the adhesive  17  from migrating. This operation is carried out using an oven  53 , for example at 80° C. for one hour, in which the support piece  5  with the retaining template  37  for the ribbon  13  is placed (see the fifth successive block diagram). 
     Once the adhesive has polymerised, the retaining template  37  is removed from the support piece  5  and this support piece  5  or the transition  1  obtained is mounted on the optical fiber  7  as shown in the sixth block diagram of  FIG. 1 , in the rear part  9  of the carcass of the optical connector  7 . 
     Such an optical connector  7  has a transition  1  for the cable  3  forming a watertight barrier for connecting the optical cable  3  to the part  9 , which can withstand high pressures, up to 1600 bar, and is therefore designed for a sea environment, especially at great depths.