Abstract:
The present invention relates to a method for enabling reliable, consistent alignment of a beam of light eating the end of an optical fiber, which has been cut at a non-perpendicular angle, with a collimating lens. The method involves utilizing a spherical shape to generate two bores in the object which intersect the center at a predetermined angle. The predetermined angle is equivalent to the angle of the light beam exiting the end of the optical fiber.

Description:
RELATED APPLICATION 
     This application claims the benefit of the filing date of copending U.S. Provisional Application, Ser. No. 60/189,395 filed Mar. 15, 2000, entitled “High Precision Optical Collimator for Optical Waveguide,” the entirety of which provisional application is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to optical communication systems and, more specifically, to optical collimators used in optical communication systems. 
     BACKGROUND OF THE INVENTION 
     Optical communication systems use collimating lenses to transfer light from optical waveguides or fibers to other optical elements. Typically, light exiting an optical fiber will rapidly diverge. In order to efficiently transmit the optical signal, it is desirable to capture as much of the diverging light as possible. Collimating the exiting light beam is one method to accomplish this. Collimating the light beam involves positioning a collimating lens to receive the optical signal in such a way that substantially parallel light exits the collimating lens. Ideally the end of the fiber should be cut at right angles to its axis so that the light exiting the fiber will be parallel to its axis. However, one major difficulty with this technique is that portions of the optical signal will be reflected by the end surface of the cut fiber and propagated in a reverse direction through the optical fiber. This is an undesirable condition known as back reflection. One way to minimize back reflections is to cut the end of the optical fiber at an angle so that reflected light is not guided in the fiber. In this way, much of the reflected light will be lost and not returned through the optical fiber. 
     One way to minimize back reflection is shown in FIG. 1, where the end of an optical fiber  10  contained within ferrule  20  is polished at the standard Angular Physical Contact (APC) angle of 8°. As a result, a principle light beam coming out of the end of the fiber deviates from the fiber axis. The resultant angle of the light beam exiting the fiber can be shown, using Snell&#39;s law, to be approximately 3.62° (shown as angle  26 ) for a typical communication fiber. To properly align fiber  10  to a collimating lens  12 , a ferrule  20  containing the fiber  10  is inserted into a collimator housing  22  and the angle  26  of the ferrule  20  to the axis  24  should be 3.62° to ensure that the light exiting the fiber along the axis  24  is coincident with the axis  13  of the collimating lens  12 . 
     Although the axis of the fiber  10  is aligned to the axis of the ferrule  20 , it is difficult to reliably and consistently align the ferrule  20  to the housing  22  at the correct angle and in the correct location. Any misalignment will result in degraded collimation. In the event that the end of the fiber  10  is misaligned, for example if it is located at point  25  instead of point  23 , or if its actual angle  26  does not accurately compensate for the angle of the light beam exiting the fiber  10 , the light exiting collimator lens  12  will deviate from the desired axis  24 . In order to construct a transverse spatial mode transformer, such as the one described in pending U.S. patent applications Ser. No. 09/249,830, 09/248,969 and 09/249,920 all filed on Feb. 12, 1999 whose contents are incorporated by reference and which are assigned to the assignee of this application, the collimation must be sufficiently accurate. Prior art methods generally are either cost prohibitive or not accurate enough to be used in precision applications. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a multi-element assembly and a method for aligning a first component and a second component inside the assembly. The multi-element assembly includes an object having spherical surface, a first component, and a second component. The object includes a first bore and a second bore. The first and second bores have longitudinal axes which intersect at a predetermined angle. In one embodiment the object has a center and the longitudinal axes of the first and second bores intersect the center. In another embodiment the first component is an optical fiber. In another embodiment the second component is a lens. In a further embodiment, the lens is a collimating lens. 
     The method includes the steps of providing an object having a spherical surface and generating a first bore and a second bore in the object. The first and second bores have longitudinal axes which intersect at a predetermined angle. The method also includes the steps of positioning a first component in the first bore at a first position and positioning a second component in the second bore at a second position. In one embodiment the first and second components are separated by a predetermined distance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a typical apparatus known to the prior art, which is used for aligning a fiber and a collimating lens. 
     FIG. 2 illustrates a cross-sectional diagram of an embodiment of an optical collimator constructed in accordance with the present invention. 
     FIG. 3 a  illustrates a cross-sectional diagram of an embodiment of an optical collimator housing with a single bore. 
     FIG. 3 b  illustrates an embodiment of an alignment jig constructed according to the present invention. 
     FIG. 4 illustrates an embodiment of an assembly for fabricating the collimator of the present invention using the alignment jig of FIG.  3 . 
     FIG. 5 a  illustrates another embodiment of the invention using a two-piece assembly to fabricate a collimator housing. 
     FIG. 5 b  illustrates another embodiment of the invention using a sphere and a ferrule. 
     FIG. 6 illustrates a complete collimator assembly according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In brief overview, the present invention relates to a method for fabricating a high precision optical collimator for use in conjunction with an optical waveguide and the collimator made by that method. In one embodiment of the invention and referring to FIG. 2, a collimator housing  30  includes a portion of a sphere  32  (generally referred to as a sphere) having a protruding tube  34 . In one embodiment the sphere  32  is accurately machined so that the deviation from a perfect sphere is approximately 2 μm. The tube  34  includes a first bore  25  concentric with the tube  34  and having a longitudinal axis  36 . The sphere  32  includes a second bore  37  whose longitudinal axis  39  is offset from the longitudinal axis  36  of the first bore  25  by a predetermined angle  26 . In one embodiment, the predetermined angle is equal to 3.62°. 
     The first bore  25  extends through the center  38  of the sphere  32 . A first component  10 , such as an optical fiber, is inserted into a ferrule  20 . The first component  10  is secured into the ferrule  20  with an adhesive. This assembly (ferrule  20  and first component  10 ) is polished at the desired APC, which in one embodiment is 8°, and is inserted in the first bore  25 . In one embodiment, the ferrule  20  is secured into the first bore  25  with an adhesive. In another embodiment the ferrule  20  is secured into the first bore  25  with a set screw. The diameter of the first bore  25  is between +5 and +10 μm greater than the nominal outer diameter of the ferrule  20 . The optical fiber may be a single mode fiber, a multi-mode fiber, a high order mode fiber, or a few mode fiber. 
     The second bore  37 , whose longitudinal axis  39  is offset by the predetermined angle  26  from the longitudinal axis  36  of the first bore  25 , intersects the center  38  of the sphere  32 . A second component  12 , which in one embodiment is a lens  12 , is inserted into the second bore  37 . In one embodiment, the lens  12  is secured into the second bore  37  with an adhesive. In another embodiment, the lens  12  is secured into the second bore  37  with a locking O-ring. The lens  12  can be a collimating lens, a GRIN lens, a compound lens, or a singlet lens. The diameter of the second bore  37  is dependent upon the diameter of the lens. The lens  12  is positioned inside the second bore  37  so that the focal point of the lens  12  is positioned at the center  38  of the sphere  32 . 
     In more detail, the method of fabricating the collimator housing  30  (referring to FIG. 3 a ) begins with the first bore  25  having a longitudinal axis  36  machined into the tube  34 . Tube  34  includes an alignment notch  35  to ensure the later proper registration of the ferrule  20  with respect to the first bore  25 . The first bore  25  extends through the center  38  of the sphere  32  and extends beyond the center  38  by a distance dependent on the size of the sphere  32  and the components  10 ,  12 . Then the center  38  of the sphere  32  is found by rotating the sphere  32  on a lathe and using standard techniques known to those of ordinary skill. 
     FIG. 3 b  illustrates an alignment jig  40 , which is used to offset the longitudinal axis  39  of the second bore  37  in the sphere  32  by the predetermined angle  26  with respect to the longitudinal axis  36  of the first bore  25 . The alignment jig  40  is constructed to include a top surface  43 , a bottom surface  44 , fasteners  45  and bore  46 . Plane  42  is parallel to bottom surface  44  and intersects top surface  43 . Top surface  43  is polished or machined to be offset with respect to plane  42  by the predetermined angle  26  of 3.62°. Next, a bore  46  is drilled perpendicular to the new top surface  43 . The bore  46  has a center axis shown as dashed line  47 . Line  41 , which is drawn perpendicular to the bottom surface  44 , is shown for comparison. Alignment extension  35 ′ is inserted to engage the alignment notch  35  of tube  34 . 
     Referring to FIG. 4, the tube  34  of the collimator housing  30  is inserted into the bore  46  of the alignment jig  40 . The collimator housing  30  is then fastened into the jig  40  by a threaded cap  65  that secures the collimator housing  30  in place. The assembly  50  is mounted on jig  66  using the fasteners  45  (not shown) inserted through the holes  47  and jig  66  is secured in the center of the rotatable table  52 . The location of jig  40  is adjusted to ensure that axis of rotation  54  of the rotatable table  52  is coincident with the center  38  of the sphere  32 . The jig  40  is then secured by tightening the fasteners  45  (not shown). In an alternative embodiment (not shown) assembly  50  is directly secured to rotatable table  52 . The assembly  50  is then rotated on the rotatable table  52  about the axis of rotation  54  of the table  52 . The axis of rotation  54  is offset by the predetermined angle  26  from the longitudinal axis  36  of the first bore  25 . Although the longitudinal axis  36  of the first bore  25  is not aligned with the axis of rotation  54  of the table  52 , the center  38  (not shown) of sphere  32  can still be found as long as a sufficient portion of the surface of the sphere  32  exists. In one embodiment, the predetermined angle  26  is not greater than the maximum angle subtended by the portion of the surface of the sphere  32 . 
     As the assembly  50  is rotated, typically measurement devices (e.g. calipers or probes) are used to contact the surface of the sphere  32  to determine the center  38  and the second bore  37  is drilled to intersect the center  38 . The second bore  37  extends beyond the center  38  by a distance to accommodate the optical path of the light beam. The diameter of the second bore  37  is dependent on the clear aperture required for lens  12 . 
     A larger bore  58  concentric with the second bore  37  is then drilled to create a seat for the lens  12  at a predetermined distance from the center  38  of the sphere  32 . In one embodiment, a slightly larger bore  59  is cut to prevent breakage of the edges of the lens  12 . Note that the top of the sphere  32  is shown to be a flat surface  60  and can have additional mechanical indents such as indent  61 . However, in other embodiments, the sphere  32  is not machined and includes the spherical top  64 . In another embodiment, a large bore  62  is drilled to allow for the propagation of the light collimated by lens  12 . 
     FIG. 5 a  illustrates an embodiment of the invention using a two piece assembly  70  of a sphere  32  and a tube  34 . A bore  72  is drilled through the center  38  of the sphere  32  to accept the tube  34 . Tube  34  is designed to hold ferrule  20 . 
     FIG. 5 b  illustrates an embodiment of the invention using ferrule  20  and sphere  32 . A bore  72  is precisely drilled through the center  38  of the sphere  32  to accept the ferrule  20 . 
     A complete multi-element collimator assembly is shown in FIG.  6 . The diagram shows the sphere  32  with the lens  12 , bore  62 , bore  58 , bore  37 , and bore  25  as well as the ferrule  20  containing the first component  10 . Lens  12  is first inserted and secured in bore  58 , following which ferrule  20  containing fiber  10  is inserted into the first bore  25  to a predetermined distance from lens  12 . In one embodiment the predetermined distance is the focal length of the lens  12 . The position of ferrule  20  is finely adjusted to achieve precise collimation. To prevent rotation of the ferrule  20 , which if allowed would affect the position of the light beam exiting from the first component  10 , the ferrule  20  is secured once the final position is achieved. In one embodiment the ferrule  20  is secured inside the tube  34  using a locking screw (not shown). In another embodiment the ferrule  20  is secured using an adhesive. 
     Using the method and apparatus of the invention, the predetermined angle  26  of 3.62° can be precisely manufactured. In another embodiment of the invention, any desired angle between the longitudinal axes  36 ,  39  of the bores  25 ,  37  can be realized by using the method and apparatus of the invention. 
     It will be appreciated that the embodiments described above are merely examples of the invention and that other embodiments incorporating variations therein are considered to fall within the scope of the invention.