Abstract:
A mechanism for reducing radial offset in a collimnated optical beam associated with an angled end fiber and minimizing an angular offset caused by collimator lens centration errors is presented. An angled end fiber is oriented in a supporting collimator so that the angled surface of the fiber is parallel to the collimator axis, thereby compensating for the radial offset. The angled end fiber is again so oriented after the angular-offset producing collimator lens rotation (used to determine the optical center of the lens) and the collimator lens is moved in a direction parallel to the collimator axis to provide a desired output focal position for the lens.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from the following U.S. Provisional Patent Applications, the disclosures of which are incorporated by reference in their entirety for all purposes: 
     U.S. Provisional Patent Application Ser. No. 60/176,941, entitled “Precision Fiber Optic Collimator,” filed Jan. 18, 2000; U.S. Provisional Patent Application Ser. No. 60/165,863, entitled “Improvements for an Optical N×N Switch,” filed Nov. 16, 1999; and U.S. Provisional Patent Application Ser. No. 60/191,411, entitled “Collimator Plate for an N×N Switch,” filed Mar. 23, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to optical devices known as collimators. 
     A collimator includes a lens positioned in a output beam that emanates from a single mode optical fiber. The lens transforms the beam into an approximately parallel optical beam over an extended distance. Minimization of back-reflection from the fiber end necessitates an angle of approximately 8 degrees on the fiber output surface. This angled fiber-end, in turn, produces a radial offset of the parallel optical beam from the collimator axis. Centration errors in lenses produce angular offsets between the parallel optical beam and the collimator axis. Both of these offsets can prevent the collimated optical beam from reaching an intended target. 
     Fiber-optic networks use arrangements of collimators with micromachined mirrors to provide optical interconnect switching between optical fibers. These optical networks require that the collimators point input light beams towards the mirrors with great precision. More particularly, each of these collimators has a one to one direct correspondence to the mirror in front of it and needs to aim its beam directly towards the center of the corresponding mirror. As switches increase in size, the distance between the collimator and the corresponding mirror increases also, and the pointing becomes more of a challenge. 
     To achieve accurate pointing, collimators provide optical beams with small radial offset over various distances to the micromachined mirrors. In addition, the centration of the lens focus and the external cylindrical axis of the lens body must be better than 5 micron, a figure which is extremely difficult to achieve in the fabrication of lenses. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, collimating an optical beam includes: providing a collimator body having a ferrule coupled to a concentric bore which is coupled to a lens holder for supporting a lens, the concentric bore being centered within the body around a first axis, the ferrule having an angled end fiber centered therein and being positioned within the body about a second axis that is inclined relative to the first axis, the axes forming a plane therebetween; and adjusting the position of the ferrule to position an optical beam exiting the angled end fiber within the plane along a path that is parallel to the first axis within the plane. 
     Embodiments of the invention may include one or more of the following features. 
     Collimating an optical beam can further include rotating the lens holder about the first axis to set an optical center of the lens in the plane, again adjusting the position of the ferrule and moving the lens holder in a direction parallel to the first axis to position the optical center at a desired distance from the angled end fiber. 
     Among the advantages of the present invention are the following. The collimating scheme of the invention produces a degree of pointing accuracy that is required by demanding applications like fiber-optic switching. 
     Other features and advantages of the invention will be apparent from the following detailed description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1D are schematic illustrations of offset and offset minimization for a collimator lens being used with an angled fiber. 
     FIG. 2 is a detailed perspective side view of a collimator operated to achieve offset minimization as shown in FIG.  1 . 
     FIGS. 3A-3B are diagrammatic illustrations of a collimator being rotated about its axis and the effect of that rotation on pointing error at a mirror target. 
     FIG. 4 is a perspective view of a spherical-shaped collimator holder having holes into which collimators are inserted or from which collimators are retracted. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A-1D illustrate a scheme for improved pointing accuracy  10  in an optical path having an optical fiber  12  being received at a collimator lens  14 , which produces a collimated output optical beam  16  that is directed towards a target (not shown). The optical fiber has an angled end for reduction of back reflection, as is known in the art. 
     Referring to FIG. 1A, the lens  14  is an ideal lens and the optical beam  16  produced by the lens has an offset  18 . Referring to FIG. 1B, it can be seen that inclining the optical fiber  12  at an inclined axis to align with the axis of the ideal lens  14  produces the collimated optical beam  16  with no offset  20 . Referring to FIG. 1C, the lens  14  is a real (non-ideal) lens having an associated centration error. Thus, the collimated optical beam  16  is produced with an angular tilt and thus results in larger offset  22  at a target. Referring to FIG. 1D, movement of the optical fiber  12  along the inclined fiber axis to align it with the decentered optical axis of the lens  14  and moving the lens  14  along the geometrical collimator axis to correct the focal position of the lens produces as the collimated optical beam  16  an optical beam having a minimum offset  22 , which is equal to the decentration of the non-perfect lens. 
     Referring to FIG. 2, collimator  30  has a collimator body  32 , which includes a concentric cylindrical bore  34  with concentric bore axis  36  that supports a lens holder  38 . The lens holder is coupled to and supports a lens  40 . The body  32  further includes an inclined bore  42  with an inclined bore axis  44 . The inclined bore  42  is connected to the concentric bore  34  and supports a ferrule  46 . A fiber  48  is bonded in an axial bore  50  centered in the ferrule  46 . An anti reflection coated end of ferrule  46  and the fiber  48  is polished at a fiber angle  52 . The ferrule  46  is rotationally aligned around the inclined bore axis  44  to position a normal to fiber end  54  of the fiber  48  in a plane defined by inclined bore axis  44  and concentric bore axis  36 , a plane  56 , at an inclination angle  58 , which is the angle between the axes  44  and  36 . The inclination angle  58  is chosen such that, together with the fiber angle  52 , a refracted optical beam exits the fiber  48  into a direction that is parallel to the concentric bore axis  36 . Adjusting the position of the ferrule  46  along the inclined bore  42  and the inclined bore axis  44  thus positions the direction of the refracted optical beam exactly along the concentric bore axis  36 . If the lens  40  is a perfectly centered lens  40 , there is no offset associated with the collimated beam (as was illustrated in FIG.  1 B). 
     If the lens  40  is not perfect and has a centration error (as was illustrated in FIG.  1 C), additional steps can be taken to ensure offset minimization. Typically, the lens holder  38  is rotated in a bearing provided by the concentric bore  34  to set the optical center of the lens in the plane defined by the inclined bore axis  44  and concentric bore axis  36 . Such rotation produces the condition shown in FIG.  1 C. Thus, the position of the ferrule  46  is again adjusted along the inclined bore axis  44 . This adjustment results in the optimum condition illustrated in FIG.  1 D. In addition, to position the focal point of the lens  40  at a desired distance (in the Raleigh range) from the end fiber  48 , the lens holder  38  is moved parallel with itself along the concentric bore axis  36  to position the waist of the exit beam at a desired maximum throw condition. The maximum throw of the Gaussian beam waist out of the collimator is measured with beam diameter measuring equipment. This cycle of movements is repeated as many times as necessary to produce convergence at the target (not shown). 
     Alternatively, a lens may be translated perpendicular to the axis  36  to set an optical center of the lens in the plane  56 , and a ferrule (like ferrule  46 ) and the lens then adjusted along the axes  44  and  36 , respectively, to achieve the results of FIG.  1 D. 
     Thus, a systematic combination of ferrule and fiber movement along the inclined bore axis  44 , and the rotation and movement of the lens holder  38  along the concentric bore axis  36  yields the result shown in FIG.  1 D. The resulting offset is equal to the lens de-centration all along the optical beam and is the best achievable with a decentered lens. Thus, a very highly accurately positioned beam (relative to the bore axis  36  of the collimator  30 ) is obtained. Preferably, the collimator is conical in shape, but it may be cylindrical if so desired. 
     The above-described technique for improving collimator pointing accuracy can be implemented to use image detectors such as quad arrays, image tubes or CCD arrays placed at two distances from the collimator. Again referring to FIG. 2, image detectors (not shown) are aligned accurately on the axis of a support (not shown) for holding the collimator body  32 . The ferrule  46  is rotated about the inclined bore axis  44  until the beam is centered in the direction perpendicular to the plane  56  at a detector close to the lens. This produces the condition of FIG. C, but, because the detector is close to the lens, the measurement will be very close to the same as in FIG.  1 A. The lens holder  38  with the lens  40  is then rotated about the concentric bore axis  36  until the optical beam is centered in the direction perpendicular to the plane  56  at a detector a large distance from the lens  40 , thus producing the condition of FIG.  1 D. The ferrule  46  is then moved along the inclined bore axis  44  and the lens holder  38  along the concentric bore axis  36  until the beam is centered in the plane  56  at both detectors and is of the correct beam size as measured with a beam diameter scanning device (not shown). 
     Turning now to FIGS. 3A and 3B, a collimator rotation scheme  60  for further enhancing pointing accuracy is shown. A conical-shaped collimator  62  causing an optical beam  64  to arrive at an intended target  66  on a targeted mirror  68  is shown. 
     Referring to FIG. 3A, if the collimator  62  is oriented exactly at the middle of the target  66  (an intended direction  70  indicated in dashed lines), then any deviation α  72  in the optics causes the optical beam  64  to hit the mirror  68  at a distance from the target  66  (i.e., the mirror center). In this instance, rotation  67  of the collimator in its mating surface (not shown) does not produce any improvement. 
     Referring to FIG. 3B, if the centerline of the collimator  62  is instead directed a small distance away from the center of the target  66  (that is, an intended direction  74 ), then the rotation  67  of the collimator  62  produces at least one location where the beam arrives much closer to the center of the mirror. Hence, it is desirable that an optical beam be directed away from the center  66  by a random amount equal to the average expected deviation α  76  of the beam from its intended direction. In this manner, therefore, the conical shape of the collimator  62  may be advantageously used to remove some of the alignment tolerances through rotation of the collimator  62 . 
     Referring to FIG. 4, a collimator holder  80  having a curved portion, with a rectangular or near rectangular shaped plate  82  and holes  84   a ,  84   b  in the curved portion, is shown. A plurality of holes not shown exist between  84   a  and  84   b . Since the holes  84   a  are located further from the corresponding mirrors, the collimator fabrication is made easier, as an optical beam has more time to contract before it hits a targeted mirror. The increased separation of the holes  84   a ,  84   b  also makes the insertion or retraction of the collimators easier to achieve. 
     Other embodiments are within the scope of the following claims.