Abstract:
A reflectionless termination is provided for an optical fiber that has a core and a cladding. The optical fiber forms part of a device which is mounted in a package and the termination fits inside the package. The termination consists of a short piece of an attenuating fiber connected at the end of the optical fiber and having a doped core which absorbs the light emanating from the core of the optical fiber.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical fiber terminations. In particular it relates to terminations which produce essentially no light reflection into the fiber core, thus avoiding the optical return loss (ORL), and which are sufficiently short to be included within the packaging of the fiber optic device in which the termination is made. 
     BACKGROUND OF THE INVENTION 
     Many fiber optic devices include optical fibers that are not used. One such device is a 2×2 coupler in which light emanating from two ports is coupled and exits via one port. It is desirable in such cases to provide the end of the unused fiber with a termination which will minimize or even eliminate reflection of light back into the fiber core and thereby avoid the ORL that is produced by such reflection and which may disturb the operation of the laser. 
     It is known that reflections can be essentially eliminated if the fiber end is cleaved at an angle such that light is reflected into the cladding of the fiber and none of the light goes back into the core of the fiber. However, the surface of such cleavage must be extremely well polished to achieve a satisfactory performance. The presence of even the slightest discontinuity, such as specks of dust, will severely disturb the proper reflection of the light and will usually lead to some of the light being reflected into the core, thus producing an optical return loss (ORL). This requirement of very high polish of the angular end face is a significant disadvantage as has, for instance, been recognized in U.S. Pat. No. 5,263,103. To obviate this disadvantage, U.S. Pat. No. 5,263,103 provides for an attachment to the end of the fiber of a terminator that has a substantially constant refractive index, such as a core-less silica fiber. It is, however, indicated in this patent that the length of such terminator must be 3 cm or more in order to achieve a back reflection of approximately −70 dB. Such a length normally puts the termination outside of the package of the device, which in itself is a disadvantage, since it requires some kind of external protection for the termination or an extension of the package for the device. 
     There is, therefore, a need for a termination that would be short enough to be included within the package of the fiber optic device, which is herein called an internal termination for optical fibers. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an internal (short) termination for an optical fiber having a core and a cladding. 
     Another object is to provide a simple and effective termination which essentially eliminates the ORL. 
     Other objects and advantages of the invention will be apparent from the following description thereof. 
     In essence, the present invention comprises a termination for an optical fiber having a core and a cladding and being a part of an optical device mounted in a package, which termination comprises a short length of an attenuating fiber with a doped core connected to the end of the optical fiber which is being terminated, the attenuation produced by the doped core and the length of the attenuating fiber being selected so as to fit within the package of the device, while essentially eliminating light reflection into the optical fiber. 
     The length of the attenuating fiber should normally not exceed 1 cm and it can be connected to the end of the optical fiber by fusion splicing or a similar technique. 
     To achieve an essentially reflectionless termination (e.g. having −60 dB or less of back reflection or ORL), the core of the attenuating fiber should preferably have an attenuation of at least 20 dB/cm The attenuating fiber should be of such quality and composition as to avoid defects and/or compositional elements that can diffuse light back into the core of the optical fiber. This can be achieved with attenuating fibers available on the market which are doped with transition metals such as cobalt, nickel and copper. Such fibers are disclosed, for example, in U.S. Pat. No. 6,097,874 where they are used as an optical attenuator. There is, however, no hint in this patent that a short length of such attenuating fiber could provide an internal termination for an optical fiber of a fiber optic device, as has been found by the present applicants. 
     The doped core of the attenuating fiber normally has the same diameter as the core of the optical fiber to be terminated. However, the diameters need not be precisely identical, so long as light coming out of the optical fiber to be terminated flows into the doped core of the attenuating fiber and is absorbed therein. Moreover, the modes between the cores of the optical fiber to be terminated and the attenuating fiber connected thereto should be substantially matched so as to minimize or eliminate any back refection at the connection. 
     Part of the light absorption takes place when the light travels forward in the doped core of the attenuating fiber until it reaches the outer end of the fiber where some of the light will escape out of the attenuating fiber and some will be reflected back into the attenuating fiber where it will be re-absorbed by the doping in the fiber core. Thus, depending on the desired length of the termination, an attenuation of 20 dB/cm is sufficient to achieve an essentially reflectionless internal termination. 
     The outer end of the short length of the attenuating fiber need not be of any particular configuration or polish. However, it may be cleaved at an angle so that any light reflected back into the attenuating fiber will be reflected not only into the core but also into the cladding thereby increasing the attenuation. However, unlike in the prior art situations, such angular end need not be extremely smooth, because the doped core provides its own attenuation and there is thus no objection that some of the light be reflected back into the core of the attenuating fiber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will now be described with reference to the appended drawings, in which: 
     FIG. 1 illustrates a prior art configuration of a 2×2 coupler within the confines of a package, having a cleaved termination for one of its exit fibers; 
     FIG. 2 illustrates a prior art configuration of a 2×2 coupler within the confines of a package, having a connection of a core-less fiber projecting outside of the confines of the package; 
     FIG. 3 illustrates a configuration of a 2×2 coupler within the confines of a package having an internal termination provided by a piece of attenuating fiber in accordance with the present invention; and 
     FIG. 4 illustrates the configuration shown in FIG. 3 where the end of the attenuating fiber is cleaved at an angle, 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings in which the same elements are designated by the same reference numbers, FIG. 1 illustrates a configuration which is known in the prior art and in which a 2×2 coupler  10  is mounted within a package  12 . The coupler is made of optical fibers  14  and  16  having cores  18  and  20  respectively and claddings  22  and  24  respectively. Light is projected into the cores  18 ,  20  by means of a laser as shown by arrows  26 ,  28 . At the other end of the coupler, most of the light exits through fiber  14  only as shown by arrow  30 , while fiber  16  is terminated by having its end  32  cleaved at an angle, usually of between 5° and 10°, so that the portion of the light that passes through the core  20  of this fiber is reflected back into the cladding  24  where it is dissipated. This requires that the end  32  be extremely smooth or polished, otherwise some light will go back into the core  20  producing an ORL. 
     FIG. 2 illustrates another known termination arrangement in which cleaved end  32  of fiber  16  is replaced by a piece of core-less fiber  34  where the light emanating from the core  20  of fiber  16  is dissipated as illustrated by arrows  36 ,  38  and  40 ,  42 . The problem with this arrangement is that it requires a fairly long piece of fiber  34  that will usually go beyond the confines of the package  12 . 
     FIG. 3 illustrates an embodiment of an internal termination in accordance with the present invention. Here again, light represented by arrows  26 ,  28  is injected into cores  18 ,  20  of fibers  14 ,  16  of the coupler  10  confined within package  12 . Most of the light exits via exit fiber  14  as shown by arrow  30 . Some of the light will go into exit fiber  16  through core  20  which is terminated in such a way that essentially none of such light would be reflected back into core  20  at this end, while keeping the termination within the confines of the package  12 . This is done by connecting, for example, by fusion splicing, a short length of attenuating fiber  44  to the outer end of fiber  16  which thereby produces an interface  50 . The connected fibers are in intimate contact with each other at the interface  50 . The attenuating fiber  44  has a doped core  46  which m this case has been doped with cobalt so that its attenuation at 1310 nm is 49.0±1.0 dB/23 mm and at 1550 nm is 46.8±1.0 dB/23 mm. Core  46  abuts core  20  so that the light emanating from core  20  propagates through core  46  while being absorbed therein. Any light remaining at the end  48  of fiber  44  will either dissipate outwardly or be reflected back into the core  46  where it will be re-absorbed by the doping. A 1 cm length of fiber  44  has been found sufficient to provide an ORL which is quite satisfactory for most terminations. Such small length of fiber  44  is easily included within the confines of packaging  12 . The end face  48  of the attenuating fiber  44  need not have any special configuration or polish It can be simply broken-off or cut-off and/or coated with adhesive or the like. However, as shown in FIG. 4, it may be preferable to cleave it at a predetermined angle, (e.g. between 5° and 10°) so that any light emanating from the core  46  at the end  48 A be directed at least partly into the cladding of fiber  44 , as shown by arrow  52 , where it would quickly dissipate. This will usually lead to an even shorter termination for the same attenuating fibers. The cleaved angular end  48 A need not be highly polished since there is no harm if some of the light goes back into core  46  where it will be re-absorbed. 
     As mentioned above, the attenuating fiber  44  used for the purposes of this example was a fiber with the core that has been doped with cobalt. Its designation by the manufacturer Institut National D&#39;Optique is HAF-CMS. It had a core diameter of 8.6 μm. However, as already mentioned, other suitable attenuating fibers can be used, provided their attenuation is such as to produce a desired absorption of light within a short length of fiber that can fit within the packaging of the optical fiber device. Preferred attenuation is 20 dB/cm or greater, and the length of the attenuating fiber should normally not exceed about 1 cm, although it could be longer if the length of the package permits it. 
     The coupler fibers  14  and  16  used in this example were SMF-28 fibers produced by Corning; The connection at the interface  50  was done by fusion splicing. 
     It should be noted that the invention is not limited to the specific embodiment described above, but that obvious modifications can be made by a person skilled in the art without departing from the invention and the scope of the following claims.