Abstract:
Various techniques are disclosed to align an optical fiber with a light source or a photo-detector using a low-force contact.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority from U.S. Provisional Application Serial No. 60/358,840, filed on Feb. 20, 2002. 

   TECHNICAL FIELD 
   This disclosure relates to alignment of optical fibers with light sources or photo-detectors. 
   BACKGROUND 
   Optical fibers may be used to carry light emitted from a source of light or to carry light to a photo-detector. As part of this transmission, the optical axis of the optical fiber may be aligned with a preferred location in relation to the light source or the photo-detector. 
   The preferred location may be found by moving the fiber with respect to the light source or the photo-detector and measuring a response of the photo-detector or, alternatively, the quality or quantity of the light coupled to the fiber. 
   In a “force-free alignment” the light source/fiber or photo-detector/fiber pairs are aligned without being in physical contact. A small gap is maintained between the fiber and the light source or the photo-detector. The fiber is scanned across the surface of the light source or the photo-detector until a preferred location is determined. The fiber is then secured in that location by, for example, bonding. 
   Greater structural integrity may be obtained when the fiber is in contact with the light source or photo-detector than when there is a gap between them. Also, when the parts are in contact there is less opportunity for misalignment and/or optical losses than when the light must traverse a gap between the parts. 
   However, frictional forces between the fiber and the light source or photo-detector may interfere with locating the fiber when the fiber is in contact with the light source or the photo-detector. One source of interference is introduced by system elastic deformation and metal surface finish resulting in a stick-slip motion that may hinder precise alignment. System elastic deformation includes the frictional forces introduced by contact between the fiber and the light source or the photo-detector. 
   BRIEF SUMMARY 
   A first implementation includes a method of aligning an optical fiber with a light source or a photo-detector by locating the optical fiber to a first position on the light source or photo-detector. The fiber is moved toward the light source or the photo-detector until they contact each other. An alignment value is measured. The fiber is separated from the light source or photo-detector and re-located to another position on the light source or photo-detector. The movement towards and contacting then measuring, separating and relocating continues until a predetermined number of measurements are taken. A preferred alignment location is determined from an analysis of the measurements. The fiber is moved to the preferred location, a preferred contact pressure is applied, and the fiber is secured in place. 
   A second implementation includes a method of aligning an optical fiber with a light source or a photo-detector by first determining a stick-slip force associated with a predetermined contact force between the optical fiber and the light source or photo-detector. The fiber is then moved to a first position on the light source or photo-detector. The predetermined contact force is applied between the fiber and the light source or photodetector and an alignment valued is measured. The fiber is re-located to another position on the light source or photo-detector at a distance remote enough from the first position to at least overcome the stick-slip force. A preferred alignment location is determined from an analysis of the measurements. The fiber is moved to the preferred location, a preferred contact pressure is applied, and the fiber is secured in place. 
   In a third implementation, the fiber and light source or photo-detector are optically aligned at a first distance between the parts. The fiber is moved to second distance from the light source or photo-detector and again optically aligned. From the distance and the orientation, the intersection point of the fiber optical axis with the light source or the photodetector optical axis is determined. The fiber is moved to the calculated intersection point. The fiber is moved in the Z-axis to achieve a predetermined contact pressure and secured in place. 
   A system for implementing the methods is also disclosed. 
   It is an advantage of some of the implementations that measurement of an alignment value is taken while the fiber is in contact with the light source or photo-detector. Some implementations have the advantage that the fiber is separated from the light source pr photo-detector while the fiber is moved. It is also an advantage of some implementations that the fiber is in contact with the light source or photo-detector when the fiber is secured. Other features and advantages will be readily apparent from the detailed description, figures and claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is an illustration of the concept of stick-slip motion; 
       FIG. 2  is an illustration of an implementation of a low force alignment method; 
       FIG. 3  is an illustration of the motion of the fiber with respect to the light source or the photo-detector of  FIG. 2 ; and 
       FIG. 4  is an illustration of an alternative implementation of the low force alignment method. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates the effect of stick-slip on the motion of a fiber  10  across the surface of a light source or photo-detector  12 . The fiber may also include any associated ferrules or other holding device. The light source or photo-detector may also include any associated lens and/or metal housing. “Stick-slip” is a phenomena in which the fiber  10  at a location “A” sticks to the light source or the photo-detector  12  because of friction and sudden release when the fiber is moved to another location “B.” The tension in the fiber builds because the fiber end in contact with the light source or the photo-detector at point  14  may not move immediately with the movement of the rest of the fiber. After sufficient motion of the fiber  10  to a location “C,” for example, the tension reaches a critical level where the tension due to the bending of the fiber  10  exceeds the frictional forces between the fiber and the light source or photo-detector. The fiber then suddenly releases from the light source or the photo-detector and re-contacts at a point  16 . “Stick-slip” motion is characterized by jittery movement of the fiber. The result is that locations such as  18  on the light source or the photo-detector between the initial and final positions can not be reached and measured for a preferred alignment position. Thus, locating the preferred location is limited by the stick-slip which may result in the fiber being located in a position other than the preferred location. 
     FIG. 1  illustrates an implementation of a low force alignment of an optical fiber with a light source or photo-detector. A XY-stage  204  for two-dimensional horizontal movement and a Z-stage  202  for vertical movement are associated with a fiber  200 . The stages are capable of moving the fiber in a XYZ orthogonal coordinate system. A load cell  208  associated with the fiber is situated to detect and measure a force with which an end  212  of the fiber contacts another surface such as the surface of a light source or photo-detector  12 . A current meter, oscilloscope or other measurement device  210  is coupled to the fiber and the light source or photo-detector and measures the output of the fiber  200  or photo-detector  206 . The measuring device  210  may also be coupled to a memory system (not shown) for recording and/or manipulating the output results. 
   Movement of a fiber  10  can be achieved by moving the fiber a sufficient distance to overcome the stick-slip force described above. The Z-stage  202  and the load cell  208  move the fiber  10  to contact the light source or the photo-detector at a desired force at a location  14 . An output measurement is taken and associated with the location. The XY-stage moves the fiber to a location “C.” Distance “c” is chosen as that distance sufficient to overcome the stick-slip phenomena due to system elastic deformation under constant load and metal interface friction effects as described above. The XY movement step size is chosen so that the stick-slip effect is overcome and the XY-stage moves the fiber a substantially equal amount to location  16  on the surface of the light source or the photo-detector. The locations and associated measurements are recorded and the preferred location selected. The XY-stage then moves the fiber to the preferred location where the fiber may be affixed in position. Thus, the fiber is in contact with the light source or the photo-detector at the preferred location and at the desired force. 
     FIG. 2  illustrates an alternative implementation of a low force alignment method for aligning an optical fiber at a preferred location with a light source or the photo-detector. A XY-stage  204  and a Z-stage  202  are associated with a fiber  200 . The stages are capable of moving the fiber in a XYZ orthogonal coordinate system. A load cell  208  associated with the fiber is situated to detect and measure a force with which an end  212  of the fiber contacts another surface such as the surface of a light source or photo-detector  206 . A current meter, oscilloscope or other measurement device  210  is coupled to the fiber and the light source or photo-detector and measures the output of the fiber  200  or photo-detector  206 . The measuring device  210  may also be coupled to a memory system (not shown) for recording and/or manipulating the output results. 
     FIG. 3  illustrates the movement of the optical fiber  200  of  FIG. 2  to locate the preferred alignment location. Fiber  200  starts at position “A” separated from the light source or the photo-detector  206 . The Z-stage (not shown in  FIG. 3 ) moves the fiber towards the light source or the photo-detector until the load cell (not shown in  FIG. 3 ) detects contact with the light source or the photo-detector at a location “B”. The Z-stage in coordination with the load cell moves the fiber so that there is a predetermined force between the fiber and the light source or the photo-detector. A measurement of the output of the photo-detector or fiber then is taken by the measurement device (not shown in  FIG. 3 ) and associated with the location. Thus, the measurement is taken with the fiber in contact with the light source or the photo-detector at a desired contact force. The Z-stage then moves the fiber away from the light source or the photo-detector to location “A,” for example. 
   Next, the XY-stage (not shown in  FIG. 3 ) moves the fiber to a location “C.” Since the fiber is not in contact with the light source or the photo-detector there is no stick-slip motion as was described above. The fiber may be moved to any location with respect to the light source or the photo-detector. As before, the Z-stage again moves the fiber to contact the light source or the photo-detector at a location “D.” Another measurement is taken and associated with this second location. The Z-stage then moves the fiber away from the light source or the photo-detector to location “C,” for example. 
   Measurements are associated with as many locations as desired. From an analysis of the measurements, a preferred location is selected. The XY-stage moves the fiber to that location. The Z-stage then moves the fiber to contact the light source or the photo-detector until the predetermined force detected by the load cell. The fiber may then be affixed in the preferred location by bonding, for example. 
   The foregoing description has the fiber moving and the light source or the photodetector stationary. However, the motion is relative and a similar result may be achieved by having a stationary fiber and moving the light source or photo-detector. 
     FIG. 4  illustrates another implementation of a low force alignment of an optical fiber with a light source or photo-detector. A light source or photo-detector  206  is held in a fixture  400 . The light source or photo-detector  206  has an optical axis  404   b . Fixture  400  establishes a frame of reference in the X, Y and Z orthogonal axes  406  for the fixture. Optical axis  404   b  need not be parallel to the frame of reference axis, the Z-axis, for example, in FIG.  4 . An optical fiber  200  is located, initially, at a distance “E” from the light source or the photo-detector and has an optical axis  404   a . A XY-stage (not shown in  FIG. 4 ) moves fiber  200  so that fiber optical axis  404   a  is aligned with light source or photodetector optical axis  404   b . A Z-stage (not shown in  FIG. 4 ) moves the fiber  200  to a distance “F” from the surface of the light source or photo-detector. The XY-stage again moves fiber  200  so that the fiber optical axis  404   a  is aligned with the optical axis  404   b . The angular orientation of the optical axis  404   a  with respect to the coordinate system can then be determined from the change in the X-Y coordinates and the change in distance from “E” to “F” in any manner known to those of ordinary skill in the art. From the orientation determined and the distance “F,” an intersection point  402  of optical axis  404   a  and the surface of the light source or the photo-detector  206  can be determined through ordinary geometry. 
   As an example, assume the fiber optical axis  404   a  is aligned with the light source or photo-detector optical axis  404   b  at a distance “E” of 3 microns. The Z-stage then moves the fiber to a distance “F” of 2 microns from the light source or photo-detector. The XY-stage again aligns the optical axes  404   a  and  404   b.    
   The XY-stage and the Z-stage may be used to move fiber  200  along optical axis  404   a  in a direction indicated by arrow  408  until a load cell (not shown in  FIG. 4 ) detects contact, at the desired force, with the light source or the photo-detector at the intersection point  402 . The Z-stage in coordination with the load cell maintains the desired contact force. The fiber can be aligned and in contact with the light source or the photo-detector at a preferred contact force avoiding the stick-slip motion accompanying movement of the fiber while in contact with the light source or the photo-detector. 
   Other implementations are within the scope of the following claims.