Abstract:
A method and system for providing precise alignment of optical fiber cores to prepare for the splicing thereof without requiring specialized splicer optical systems or extensive redesigns of existing splicer optical systems. The optical fibers themselves are used to magnify an image of the cores at the splice point of the optical for precise alignment thereof. That is, in an optical fiber splicer having an optical system, the imaging device utilizes the cladding of optical fibers that are to be spliced together to precisely align the axial cores of the optical fibers.

Description:
[0001]    This application claims the benefit of priority from provisional patent application U.S. Serial No. 60/259,900, filed Jan. 8, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to a method and system for precisely aligning the cores of optical fibers that are to be spliced together, in particular using the cladding of the optical fibers themselves to produce a magnified image of the optical fiber cores for precise alignment thereof.  
         BACKGROUND  
         [0003]    Optical fibers may be constructed with a protective outer coating, called a “cladding”. When fusing the cores of optical fibers by aligning the claddings of the optical fibers, it is often found that the cores of the optical fibers are not well centered within the cladding of the respective optical fibers. That is, any splicing technique that is based upon the alignment of the claddings of optical fibers is highly vulnerable to misalignments of the cores of the respective optical fibers. The consequences of such misalignments of the optical fibers, even a misalignment of 0.1 μm, include loss of signal strength for signals transmitted through the resulting spliced optical fibers.  
           [0004]    Therefore, techniques have been derived to determine the location of the cores within the claddings of the optical fibers before splicing thereof, so that the cores themselves may be aligned prior to splicing of the optical fibers.  
           [0005]    One technique, called the “hot core alignment process”, requires that the optical fiber cores to be spliced together be heated to significant temperatures, e.g., 1600° C., resulting in the cores becoming clearly visible through the cladding of the respective optical fibers. Then the cores may be aligned based on visual observation thereof. However, the performance of various fibers may be adversely affected by such significant heating prior to splicing thereof.  
           [0006]    On the other hand, so-called “cold image spaced alignment” techniques, which do not require that the optical fiber cores be heated, may be accompanied by undesirable drawbacks including requiring an objective lens that is positioned extremely close (e.g., &lt;30 mm) to the optical fibers, requiring an objective lens having an extraordinarily high magnification level (e.g., &gt;200×), and/or requiring a lens having a high image resolution (e.g., &lt;0.5 μm/pixel). However, an objective lens that is placed less than 30 mm from an optical fiber cladding is placed at significant risk of surface damage. Further, the optical systems described above having significant magnification and resolution specifications would require specialized equipment, including significant redesign of existing optical equipment that is used in the field of optical fiber splicing.  
           [0007]    Conversely, an example of a presently available optical fiber splicer may have an achromatic objective lens of 10 mm or less that may be disposed at a distance of at least 40 mm from the splice point of the optical fibers. The image of the splice point of the optical fibers may be projected onto a charge coupled device (CCD) in the splicer. The resolution of the device may be 1.5 μm/pixel. Such a splicer may produce the image shown in FIG. 3, which shows a single optical fiber  300  having a perfectly centered core. The central line  310  is known as the “lens effect line”, which may include a refracted image of the core, and may be produced even if a core is absent from the optical fiber  300 . The lens effect line  310  obscures the core and thus the core is not visible in FIG. 3, which is a CAD (computer-aided designed)-produced negative image of a single optical fiber, shown at a magnification of 400×, to more clearly show the features therein.  
           [0008]    [0008]FIG. 4 is a CAD-produced negative image of a single optical fiber  400 , also shown at a magnification of 400×, that is captured by the optical system described above, having a core that is measured as being 1 μm off-center. However, the eccentricity is hidden by the lens effect line  410 , which is more clearly shown by the negative imagery of the figure. Lens effect line  410  prevents detection of slightly misaligned cores because it obscures the core. This is a significant drawback because even a misalignment of 0.1 μm between spliced fibers may result in significant loss of strength for signals transmitted through the resulting spliced portion of the optical fibers. As a result of the image of FIG. 4, a technician performing or inspecting a splice would not be aware of the extent of the core eccentricity or even the existence of the core eccentricity. FIGS. 5 and 6 show, again using a CAD-produced negative image of a single optical fiber, the focused images of the optical fibers of FIGS. 3 and 4 respectively, at a magnification of 800×. However, because of the lens effect line, there is little observable difference between the perfectly aligned cores of FIGS. 3 and 5 and the misaligned cores of FIGS. 4 and 6.  
         SUMMARY OF THE INVENTION  
         [0009]    Thus, the present invention is directed towards a method and system for providing a substantially precise alignment of optical fiber without requiring specialized splicer optical systems or extensive redesigns of existing splicer optical systems. Further, the present invention enables optical fiber cores to be aligned with a significantly high degree of precision without requiring the heating of the optical fiber cores or significant redesign of currently implemented optical systems that are used in optical fiber splicer systems. In addition, the present invention enables optical fiber cores to be aligned with substantial precision without requiring expensive imaging equipment having to be disposed close to the optical fibers, particularly at distances where the imaging equipment may incur damage due to heat radiating from the optical fibers.  
           [0010]    Rather, the present invention utilizes the optical fibers themselves to magnify an image of the cores at the splice point of the optical for precise alignment thereof. That is, in an optical fiber splicer having an optical system, the imaging device utilizes the cladding of optical fibers that are to be spliced together to produce the images that are used for precisely aligning the axial cores of the optical fibers.  
           [0011]    First, the splicer must hold in place an end portion of the optical fibers along a same axial path, so as to splice together an end portion of each optical fiber. Then, the optical system, which has an objective plane that is perpendicular to the axial direction of the optical fibers, emits light onto the optical fibers in a direction that is orthogonal to the axial path of the optical fibers. The light rays emitted from the optical system onto the optical fibers may be collimated, to thereby eliminate any divergent rays and enter the optical fibers in parallel, by minute lenses that are disposed adjacent to the light emitting diodes (LEDs) of the optical system. As a result, the collimated light rays simulate a light source at infinity and located behind a 3 mm aperture.  
           [0012]    An image of the splice point of the optical fibers is then defocused by orthogonally moving the objective plane of the optical system away from the axial direction of the optical fibers to a predetermined defocusing distance, which may be in the range of 300 to 350 μm. Thus, the light reflecting from the inside portion of the cladding behind the core of the optical fibers may produce multiple parallel line images corresponding to the optical fiber core that are projected by the objective plane onto a charge coupled device (CCD).  
           [0013]    The optical device may then be utilized to capture a series of defocused images of the optical fibers along the axial path of the optical fibers, from more than one orthogonal position relative to the axial path of the optical fibers. Each of the resulting images is then filtered to remove any optical noise therefrom, and the core position of the optical fibers, particularly the end portions thereof, are then empirically determined in anticipation of splicing.  
           [0014]    Accordingly, the present invention circumvents the need to redesign optical system utilized in conjunction with optical fiber splicers, by using the optical fibers themselves to produce the images required for precise alignment thereof.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The foregoing and a better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written disclosure focus on disclosing example embodiments of this invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto. The spirit and scope of the present invention are limited only by the terms of the appended claims.  
         [0016]    The following represents brief descriptions of the drawings, wherein:  
         [0017]    [0017]FIG. 1 shows a flowchart for an example method for implementing the present invention.  
         [0018]    [0018]FIG. 2A shows a schematic block diagram according to an example of the present invention;  
         [0019]    [0019]FIG. 2B is a profile view of the example of FIG. 2A;  
         [0020]    [0020]FIG. 2C is a schematic block diagram according to an example of the present invention showing defraction of light by an optical fiber having a perfectly centered core;  
         [0021]    [0021]FIG. 2D shows a schematic block diagram according to an example of the present invention showing defraction of light by an optical fiber having an off-center core.  
         [0022]    [0022]FIG. 3 shows an example of an image of an optical fiber having a perfectly centered core obtained by a prior art system.  
         [0023]    [0023]FIG. 4 shows an example of an image of an optical fiber having an off-center core obtained by a prior art system.  
         [0024]    [0024]FIG. 5 shows the optical fiber image of FIG. 3, obtained by a prior art system, at an increased magnification.  
         [0025]    [0025]FIG. 6 shows the optical fiber image of FIG. 4, obtained by a prior art system, at an increased magnification.  
         [0026]    [0026]FIG. 7 shows an optical fiber image having a substantially precise alignment that results from processing according to an example of the present invention.  
         [0027]    [0027]FIG. 8 shows an optical fiber image having a misaligned core that results from processing according to an example of the present invention.  
         [0028]    [0028]FIG. 9 shows the optical fiber image of FIG. 7 at an increased magnification.  
         [0029]    [0029]FIG. 10 shows the optical fiber image of FIG. 8 at an increased magnification. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]    Before beginning a detailed description of the invention, it should be noted that, when appropriate, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing figure drawings. Further, in the detailed description to follow, example embodiments and values may be given, although the present invention is not limited thereto.  
         [0031]    According to an example embodiment of the present invention shown in FIGS. 2A and 2B, optical fiber splicer  200  is provided to splice together an end portion of two optical fibers  230  at splice point  250 . The optical fiber splicer  200  may include a clamp  240  that is disposed on a base of the splicer  200 . The clamp  240  may be utilized to hold the optical fibers  230  in place and is preferably adjustable for precise placement of the respective optical fibers  230 . That is, the clamp  240  is rotatable upon axis  260 , which is attached to a base of the splicer  200 , and is therefore fully rotatable in the (x, y, z) directions so that the clamps  240  may be adjusted as necessary to provide substantially precise alignment of the cores  235  of the optical fibers  230  upon implementation of the present invention. The adjustment of the clamp  240  includes being moved in the axial direction of either of the optical fibers  230 .  
         [0032]    Further, the optical fiber splicer  200  may include light emitting diodes (LEDs)  205  that emit light onto the optical fibers  230 . The light emitted onto the optical fibers  230  may be collimated by lenses  215 , or “light pipes”, that may be disposed adjacent to the LEDs  205 , to thereby simulate a point source at infinity and located behind a 3 mm aperture. Thus, divergent rays may be eliminated and the rays may enter the optical fibers  230  in parallel. FIGS. 2A and 2B merely show the direction of light emitted from LEDs  205 , whereby FIGS. 2C and 2D show more complete light paths of the light emitted from LEDs  205  that would be produced by the examples of FIGS. 2A and 2B.  
         [0033]    Focal plane  220  may be disposed orthogonally to the axial direction of the splice point  250  between the two optical fibers  230 , and an optical system  210 , which may be utilized to facilitate visual alignment of the optical fibers  230  that are to be fused together, may also be disposed orthogonally to the axial direction of the splice point  250 , beyond the focal plane  220 . The optical system  210  may include, for example, a digital image camera or a digital video camera. Thus, light rays from the LEDs  205  may follow a path through the lenses  215 , then be subjected to refraction and vignetting by the core  235  and cladding of the optical fibers  230 , and then defocused by focal plane  220  onto the optical system  210 .  
         [0034]    An example embodiment of the method according to the present invention, which may include computer-implemented instructions, or a program, in conjunction with splicer  200 , is shown in FIG. 1, with reference to the system of FIGS. 2A through 2D. FIG. 2A is a schematic block diagram of an example of the present invention, and FIG. 2B is a profile of the same schematic block diagram. FIGS. 2C and 2D show the light paths according to the example of FIGS. 2A and 2B for, respectively, perfectly aligned cores  230  at the splice point  250  and mis-aligned cores  230  at the splice point  250 , whereby the core offset is an exemplary value of 0.1 μm.  
         [0035]    A first step  5  includes holding in place the optical fibers  230  using optical fiber clamps  240  so that the end portions of the two optical fibers  230  to be spliced together at splice point  250  are aligned along the same axial path. Light may then be emitted from the optical system  210 , as described above, in step  10 . The focal plane  220 , which is orthogonal to the axial direction of the splice point  250  between the two optical fibers  230 , may then moved to a certain defocus distance away from the optical fibers and towards the optical system to defocus the image of the fibers.  
         [0036]    For a splicer having the specifications described above, the desirable defocus distance may be 300-350 μm away from the cladding of the optical fibers at the splice point, although the present invention is not so limited. The defocus distance is the distance at which the lens effect line may appear to a viewer in the form of three parallel lines, encompassing approximately, for example, 40% of the width of an optical fiber. Further, the defocus distance may shift, and therefore may be determined either empirically or by optical modeling for different splicer optical system designs. As the image of the splice point of the optical fibers  230  is defocused by orthogonally moving the objective plane  220  of the optical system away from the axial direction of the optical fibers  230  to the predetermined defocusing distance, in the exemplary range of 300 to 350 μm, the light reflecting from the inside portion of the cladding behind the core of the optical fibers  230  may produce multiple parallel line images corresponding to the optical fiber core  235  that are projected by the objective plane  220  onto a charge coupled device (CCD)  210 .  
         [0037]    The optical system  210 , which may include , a digital image camera or a digital video camera as described above, may then proceed to capture multiple images along the axial path of the optical fibers  230  at intervals of, for example, 5 μm from more than one orthogonal view, as in step  20 . As an example, over forty (40) images of the optical fibers  230  may be taken, from both of two orthogonal perspectives. That is, multiples image samples may be taken along the axial path of the optical fibers  230  from different orthogonal vantage points, and image samples that differ excessively from the average may be discarded, and the remaining samples may be summed up.  
         [0038]    A fast Fourier transform (FFT) “brick wall” filter with a passband of 0.04-0.08 Hz (based on 1.5 μm/pixel) may then be used, in step  25 , to remove the effects of optical imperfections from the gathered images of the optical fibers  230  and their cores  235 . Such optical imperfections may include electronic noise on the CCD, debris on the optical fibers, etc. The filtering is also implemented to isolate the data pertaining to the cores  235  of the optical fibers  230 , at the splice position  250 . In the alternative, the filtering may be accomplished using Gaussian filtering or other methods to determine spatial frequency components that correlate well with the position of the cores of the optical fibers  230 . If it is desirable to locate local maxima and minima of the data, a cubic spline method, for example, may be used. The multiple images are captured from multiple orthogonal perspectives along the axial path of the optical fibers to take into account core displacements that are parallel to the line of sight thereof. Lastly, the positions of the cores  235  of the optical fibers  230 , specifically the cores  235  at the splice position, are empirically determined from the filtered data.  
         [0039]    As a result, using the methodology described above, FIGS. 2C and 2D, respectively show how, for a perfectly centered core and an off-centered core (with an exemplary off-set of 0.1 μm), an image of the splice point of the optical fibers, defocused by orthogonally moving the objective plane of the optical system away from the axial direction of the optical fibers to a predetermined defocusing distance in the exemplary range of 300 to 350 μm, results in the light reflecting from the inside portion of the cladding behind the core  235  of the optical fibers  230  producing multiple parallel line images corresponding to the optical fiber core that are projected by the objective plane  220  onto a charge coupled device (CCD)  210 .  
         [0040]    Using a CAD-produced negative image to more clearly show the intended characteristics, FIG. 7 shows a defocused image of an optical fiber having a perfectly centered core, which can be seen by the shiftless lens effect line  710 , and FIG. 8 shows a defocused image of an optical fiber having the lens effect line  810  having a minute shift corresponding to the 1 μm eccentric core. FIGS. 7 and 8 are magnified images of the optical fibers on the order of 400×. In FIG. 8, the misalignment of the optical fiber cores is magnified many times greater than 1 μm, thus removing any limitations that may be imposed by a 1.5 μm/pixel CCD resolution. FIGS. 9 and 10, which are also CAD-produced negative images, show the images of FIGS. 7 and 8 at a magnification of 800×.  
         [0041]    However, if after the positions of the cores  235  of the optical fibers  230  are determined in step  30  to still be mis-aligned in decision  35 , the axes  260  are adjusted so that the clamps  240  re-align the optical fibers  230  as necessary, as in step  40 . Then, the methodology resumes at step  10  as described above, and the iterations of the method beginning at step  10  are repeated until the cores  235  of the optical fibers  230  are aligned within an acceptable tolerance for splicing thereof.  
         [0042]    This concludes the description of the example embodiments. Although the present invention has been described with reference to illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope and spirit of the principles of the invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings and the appended claims without department from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.