Patent Publication Number: US-2002009271-A1

Title: Method and apparatus for splicing optical fibers

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001] This application claims the benefit of European Application No. 00402079.8, filed Jul. 21, 2000.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a method of precisely aligning and splicing optical fibers and particularly multiple optical fibers and apparatus employed in such method.  
       [0004] 2. Technical Background  
       [0005] The manufacture of complex optical components requires the splicing of optical fibers, typical multiple splices, where numerous components are placed on a single substrate commonly referred to as a cassette. In the past, electric arc splicing has been commonly used in which fiber ends are placed in an alignment fixture in which an approximate 6 mm gap exists to allow the electric arc to fuse the ends of the fibers. This results in a cantilevered distance of 3 mm for each of the fibers, which allows the intrinsic curl of the fibers as well as the cantilevered bending of the fibers to cause misalignment of the fiber ends resulting in losses as high as 1 dB or more in signals propagating through the spliced area. In order to overcome the difficulties of the splices resulting from electric arc splicing, CO 2  lasers have recently been employed and their power controlled to provide precise splicing with less of a gap between the holding fixtures. In order to manufacture complex optical components which include multiple optical elements, such as filters, switches, amplifiers and the like, all positioned on a substrate subsequently packaged for use in the field, it is desirable to provide a fiber alignment and splicing apparatus which efficiently allows multiple splices for coupling fibers to be made utilizing a CO 2  laser.  
       SUMMARY OF THE INVENTION  
       [0006] The system of the present invention accommodates this need by providing a method and apparatus for aligning fibers to be spliced utilizing a grooved block assembly and positioning arms which engage fiber ends, bringing them precisely together in alignment for splicing utilizing a CO 2  laser. In a preferred embodiment of the invention, the splicing apparatus includes a primary clamp including a grooved holder with aligned grooves having a channel therebetween for aligning ends of optical fibers to be spliced. A pair of secondary clamps may be employed to provide an initial holding structure. The groove holder cooperates with positioning arms having resilient pads to provide the final alignment. The groove holder is formed either by precise machining of a suitable material or alternately etching silicon to form a plurality of rows of parallel grooves for holding optical fibers therein, such that multiple splices can be made utilizing the fixture so-formed. The precisely formed grooves provide X and Y alignment of the fibers under the compression of resilient pads attached to positioning arms to precisely position the fibers in orthogonal planes in horizontal and vertical directions, while the resilient pads attached to the positioning arms provide precise coupling of fiber ends along the Z direction of the fiber axis.  
       [0007] Methods embodying the present invention include the steps of positioning fiber ends in aligned grooves, holding the fibers in the grooves with outer clamps allowing movement of the fibers thereunder, engaging fiber ends with positioning arms having resilient pads, bringing the fiber ends together utilizing the positioning arms, and fusing the fiber ends employing a CO 2  laser. In a preferred embodiment of the invention, the method includes clamping a plurality of aligned pairs of optical fibers in a plurality of grooves formed in a holder, such that successive splices of optical fibers can be made.  
       [0008] Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.  
       [0009] It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010]FIG. 1 is a perspective, schematic, and block diagram of the apparatus for splicing multiple optical fibers utilizing the method and apparatus of the present invention;  
     [0011]FIG. 2 is an enlarged cross-sectional view, taken along section line II-II of FIG. 1, of the splicing apparatus;  
     [0012]FIG. 3 is a greatly enlarged cross-sectional view, taken along section line III-III of FIG. 1, showing one of the positioning arms and its relationship to the associated groove of the apparatus; and  
     [0013]FIG. 4 is a plan schematic view of a complex optical component including multiple optical elements and a plurality of splices formed utilizing the method and apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0014] Referring initially to FIG. 1, there is shown a splicing apparatus  10  for splicing optical fibers, such as fiber  11  to fiber  12 , fiber  13  to fiber  14 , and fiber  15  to fiber  16 . Although only three pairs of fibers to be spliced are shown in FIG. 1, it is to be understood that numerous splices can be made utilizing the apparatus and method of the present invention and, for example, in FIG. 4 ten such splices are illustrated.  
     [0015] The fibers  11 - 16  can be conventional optical fibers, such as SMF 28™, available from Coming Incorporated of Corning, N.Y., having a core diameter of approximately 7 μm, a standard cladding diameter of 125 μm covered with a polymeric covering. The ends of the fibers are stripped, as shown at  17  and  18 , for example, for fibers  11  and  12 , exposing about 10 mm of the fiber cladding at fiber ends  17  and  18  for each of the pairs of fibers to be spliced. The splicing apparatus comprises a primary clamp  20  and secondary clamps  40  and  50  for accurately positioning the pairs of fibers in alignment with one another for subsequent splicing by a CO 2  laser  60  whose beam is focused on the splicing area by lens  65  as described in greater detail below.  
     [0016] Each of the clamp assemblies  20 ,  40  and  50  are mounted on a stage  80  which provides X, Y, and rotational alignment of the fiber holding clamps with respect to the focused laser beam and with respect to a camera  70 , which is directed to the splicing area and employed to control micro-manipulators utilized for bringing the fiber ends together as described below. Thus, in a preferred embodiment of the invention, the laser  60  is stationary and the stage  80  moves the clamped optical fibers in alignment under the laser beam. The camera is tilted at an angle of approximately 45° to the laser beam to provide a signal input indicating the alignment of optical fiber ends. Each of the clamps  20 ,  40  and  50  include a base and a clamping member for holding pairs of optical fibers in precise alignment with one another for splicing. Having described the overall structure, a detailed description of the clamping assembly is now presented in connection with FIGS.  1 - 3 .  
     [0017] The primary clamp  20  comprises a monolithic holder  22  having a plurality of precisely formed, parallel spaced grooves  24 ,  26 , and  28  (for the three grooves illustrated in the figure). It is to be understood, however, that multiple grooves can be formed in the monolithic holder  22  to allow mass production of multiple splices in a component such as that shown in FIG. 4. The holder  22  can be precisely machined of a material, such as stainless steel or silica, or can be photolithographicly etched utilizing, for example, silicon to precisely form the grooves which each, as seen in FIG. 3, have converging side walls  21  and  23  at an angle α of from between 45° and 90° but preferably about 70°. The vertical depth of each of the grooves is about 120 μm, while the opening at the top is approximately 150 μm wide, such that an optical fiber end, such as end  18  illustrated in FIG. 3, will project above the top surface  25  of holder  22  a distance to allow the positioning arm assembly  30  to engage the outer surface of the cladding of each of the optical fibers.  
     [0018] The grooves  24 ,  26 , and  28  are formed continuously along the transverse direction of holder  22  having a pitch of about 1 mm between the apices of each of the grooves to allow sufficient spacing between optical fibers for their splicing. Holder  22  is formed by first forming the spaced, parallel grooves. Subsequently, a generally rectangular channel  27  is formed transverse to the grooves to divide each of the grooves and provide a splicing area between side walls  29  and  29 ′ of channel  27 . Channel  27  has a width of from about 2 mm to about 3 mm and a similar depth and provides an open work area for the laser splicing to occur. By providing the grooves through holder  22  first and subsequently forming channel  27 , the grooves  24 ,  26 , and  28  and the mating separate grooves  24 ′,  26 ′, and  28 ′ (FIG. 1), are precisely aligned for aligning the opposite ends of pairs of fibers.  
     [0019] Alternatively, the channel  27  of the holder  22  may be replaced by a depression over which the opposite ends of fiber pairs may be extended.  
     [0020] The primary clamp  20  further includes a pair of resilient positioning arms  30  and  30 ′ (FIGS. 1 and 2), which comprise a micro-manipulator holder  32  gripping a mounting block  34  to which there is integrally molded a block of resilient polymeric material  36 , such as silicon, having a length along the axis of the optical fiber of about 7 mm, a width transverse to the axis of the optical fiber of about 200 μm and a thickness of about 200 μm, such that the resilient material  36  grips the outer cladding of the fiber (such as fiber end  18  shown in FIG. 3) for moving the fiber ends together in directions indicated by arrows A and B in FIG. 2 from their initial position, shown in FIG. 1, to a precisely aligned position with a gap of approximately 1μ between the fiber ends for splicing. The micro-manipulator arms  32 ,  32 ′ are controlled under the influence of the signal detected by camera  70  through control circuit  72  and a micro-stage actuator  74  utilizing conventional vision control systems utilized, for example, the optical alignment systems found in Ericsson fusion splicers.  
     [0021] Before the final alignment of the ends of pairs of optical fibers, the fibers are initially held by the secondary clamps  40 ,  50 , each of which include a holder  42  and  52 , respectively, similarly including a plurality of grooves  44  and  54  which are aligned with the corresponding grooves  24 ′,  26 ′and  28 ′ and are of the same dimensions. A clamping arm  49  and  59  having a polymeric coating surface  49 ′ and  59 ′ clamps to the covering of the pairs of optical fibers loosely holding the fibers in an initially aligned position but allowing the optical fibers to be moved utilizing resilient clamps  30 ,  30 ′ toward one another centered within channel  27 . In some embodiments, it may be desired to manufacture holders  22 ,  42  and  52  as an integral component mounted to the adjustable stage  80  as opposed to separate elements mounted to the stage. Thus, in some embodiments, continuous grooves can be formed through a single block of material and channel  27  and open spaces  62  and  64  (FIG. 2) formed partially downwardly through the upper surface of the holder to provide outer holding areas for each of the pairs of optical fibers and the primary clamping and moving structure of holder  22 .  
     [0022] The critical alignment, however, takes place in the primary clamp  20 , which precisely aligns the ends and, by virtue of the shape of the grooves  24 ,  26 ,  28 ,  24 ′,  26 ′, and  28 ′, holds the pairs of fiber ends, such as  17  and  18 , in precise alignment in vertical and horizontal directions (or the X and Y axis, as illustrated in FIG. 3), while the resilient positioning arms  30  and  30 ′ move the fibers along their longitudinal or Z axis, as shown in FIG. 2 by arrows A and B. Although a V-shaped groove is illustrated as the preferred embodiment of the fiber end holding element, other shaped grooves having converging walls which accurately position a fiber in the X and Y planes may also be employed. Thus, partial cylindrical grooves, U-shaped grooves, or other grooves may be employed as long as they are accurately machined to precisely position the ends of opposed pairs of fibers to be spliced in alignment in the X and Y axes.  
     [0023] The method of splicing the fibers employing the present invention comprises the steps of initially placing the fibers in the secondary holders  40 ,  50  with their facing ends approximately 1 mm apart and actuating clamps  49 ,  59 , which are solenoid-actuated clamps of conventional construction, to hold the fibers generally in alignment with the ends  17 ,  18  of each pair of fibers laying within the corresponding grooves  24 ,  24 ′,  26 ,  26 ′,  28  and  28 ′. The initial gap, shown in FIG. 1, between fiber ends is centered within channel  27 , which can be confirmed by viewing the structure through camera  20 , while controlling the movable stage  80  which can rotate and translate in orthogonal directions for positioning the fiber to be spliced precisely under the laser beam.  
     [0024] Next, the resilient clamps  30 ,  30 ′ are brought downwardly into engagement with the top of fiber, such as fiber end  18  as shown in FIG. 3, deflecting the resilient pad  36  about 10 μm to 30 μm to provide enough frictional engagement to grip the cladding and seat the ends of optical fibers  17  and  18  within the groove without binding the fiber in the groove such that they can be slid along the grooves in a direction indicated by arrows A and B in FIG. 2. The micro-manipulator arms  30 ,  30 ′ can move approximately 1 mm and, initially, the fiber ends are brought within one another a distance of about 10 μm to 20 μm which is determined by viewing the fiber ends through camera and controlling the micro-manipulators through control circuit  72  and actuator  74 .  
     [0025] Once the ends are aligned precisely by holding the fiber ends within the corresponding grooves and brought together within a gap of about 10 μm to about 20 μm, as illustrated in FIG. 2, laser  60  is actuated to perform the splicing. During the splicing, which takes approximately 2 seconds for SMF-28™ optical waveguide fiber available from Coming Incorporated of Corning, N.Y., micro-manipulators  30 ,  30 ′ are moved again together in a direction indicated by arrows A and B about 15 μm to about 25 μm for each fiber to complete the splicing by fusion process. The beam from CO 2  laser  60  is focused by lens  65  to a beam diameter of about three to four times the fiber diameter or about from 100 to 400μ, with the laser providing a power of about 4 watts for 2 seconds to fuse the fibers and achieve splicing. Details of the control of laser  60  is provided in the parent patent application identified above. Upon completion of the first splice of a series of splices for a component, stage  80  is moved in a direction corresponding to the longitudinal axis of channel  27  to position the next pair of fiber ends to be spliced under the laser beam for repeating the bringing together of the fiber ends and fusion of the aligned ends.  
     [0026] A number of splices can be made for an optical component, such as optical component  100  shown in FIG. 4, which includes a substrate or cassette  110  on which multiple optical elements, such as components  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 , and  126 , are mounted and intercoupled by multiple optical fibers  130 . The primary holder  22  of the present invention may be mounted directly to the substrate  110  with the cassette  100  being placed on the stage  80  for manipulation of the splicing area  150  (FIG.  4 ) for the interconnection of optical fibers coupling each of the components  112  through  126 . In the embodiment shown, ten such splices  160  through  169  of fibers  170 ,  170 ′ through  179 ,  179 ′ are shown. In such an embodiment, the secondary clamps  40  and  50  may be integrated with holder  22  as described above.  
     [0027] Thus, as seen with the apparatus and method of the present invention, numerous splices for coupling a number of optical components can be made to provide an overall complex optical component having multiple interconnected elements. With the splicing of the present invention, a splice length of about less than 2.5 mm is provided, which results in a loss of only about 0.04 dB per splice at 1310 nm as opposed to a 6 mm splice for electric arc splices, which can result in as much as a 1 dB loss per splice.  
     [0028] It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.