Abstract:
An alignment system finds the coordinates of a first point on a photosensitive optical fiber by scanning an ultraviolet beam across the fiber several times at successively decreasing speeds and detecting light returned from the fiber. The coordinates of a second point on the fiber are found similarly. On the basis of the coordinates of the first and second points, the alignment system automatically aligns the fiber in a direction suitable for the formation of an optical filter in the fiber.

Description:
This application is a division of Ser. No. 09/113,152 filed Jul. 10, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and system for aligning an ultraviolet beam and an optical fiber preparatory to the manufacture of an optical filter in the optical fiber. 
     Optical filters comprising Bragg gratings formed in optical fibers are useful for dispersion compensation in optical communication systems, and for various other applications. A known method of manufacturing such filters illuminates a photosensitive optical fiber with ultraviolet light through a phase mask, thereby imprinting a modulation pattern in the refractive index of the fiber core. To obtain uniform modulation, the manufacturing process preferably uses a highly stable ultraviolet light source, such as a continuous-wave argon ion laser apparatus emitting an ultraviolet beam by second harmonic generation. 
     The beam emitted by this type of light source is relatively weak. To increase the beam intensity, the beam is focused to a width comparable to the diameter of the optical fiber, and scanned lengthwise along the fiber. The fiber diameter is only about one hundred twenty-five micrometers (125 μm), so successful scanning requires careful alignment between the beam and the fiber. In particular, the fiber must be accurately aligned at the beginning of the scan, so that the fiber is centered under the beam and the fiber axis is aligned in the scanning direction. 
     During the scanning process, alignment can be maintained by the known method of monitoring visible light produced by fluorescence when the core of the optical fiber is illuminated by the ultraviolet beam. Some of this visible light propagates through the fiber core, and can be measured by an optical power meter connected to one end of the optical fiber. The measured value can be fed back to an automatic positioning system that keeps the beam centered on the fiber core. 
     It would be advantageous if the initial alignment of the fiber could also be carried out by automatic control. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to align a photosensitive optical fiber automatically, in an initial position suitable for scanning with an ultraviolet beam to form an optical filter. 
     Another object of the invention is to align the photosensitive optical fiber accurately. 
     The invented alignment method comprises the steps of: 
     (a) holding the photosensitive optical fiber in a plane having an X-axis and a Y-axis; 
     (b) scanning the ultraviolet beam repeatedly across the photosensitive optical fiber, parallel to the Y-axis, at successively decreasing speeds; 
     (c) detecting light returned from the photosensitive optical fiber; 
     (d) thereby determining first X- and Y-coordinates at which the photosensitive optical fiber is centered under the ultraviolet beam; 
     (e) repeating steps (b) and (c) at a different location on the photosensitive optical fiber, thereby determining second X- and Y-coordinates at which the photosensitive optical fiber is centered under the ultraviolet beam; and 
     (f) rotating the photosensitive optical fiber through an angle determined from the first and second X- and Y-coordinates. 
     The invented method preferably comprises the additional steps of: 
     (g) repeating steps (b) and (c) again after step (f), thereby determining a third Y-coordinate at which the photosensitive optical fiber is centered under the ultraviolet beam; and 
     (h) moving the photosensitive optical fiber parallel to the Y-axis according to the third Y-coordinate, thereby aligning the photosensitive optical fiber with the ultraviolet beam. 
     When step (b) is carried out, the scans can be performed in identical directions, or at least two of the scans can be performed in mutually opposite directions. In either case, the scans can be performed at a fixed X-coordinate, or at different X-coordinates. 
     The Y-coordinate in step (d) can be determined from the last scan in step (b), or from the last two scans in step (b). In either case, the Y-coordinate can determined by finding a coordinate at which peak light power is detected during a scan, or by finding a coordinate centered between a pair of half-power coordinates. 
     The invented alignment system comprises at least one fiber holder for holding the photosensitive optical fiber, a first scanning stage for scanning the ultraviolet beam parallel to the X-axis, a second scanning stage for scanning the ultraviolet beam parallel to the Y-axis, a third scanning stage for rotating the photosensitive optical fiber about an axis perpendicular to the X- and Y-axes, an optical transducer for detecting light returned from the photosensitive optical fiber when illuminated by the ultraviolet beam, and a computing device controlling the first, second, and third scanning stages to carry out the steps described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
     FIG. 1 is a schematic drawing of a conventional apparatus for imprinting an in-fiber Bragg grating; 
     FIG. 2 is an enlarged sectional view of the phase mask and photosensitive optical fiber in FIG. 1; 
     FIG. 3 is an enlarged plan view of the phase mask in FIG. 1; 
     FIG. 4 is a side view illustrating an initial alignment system according to a first embodiment of the invention; 
     FIG. 5 is a frontal view illustrating the first embodiment; 
     FIG. 6 is a graph illustrating the intensity profile of the ultraviolet beam; 
     FIG. 7 is a graph illustrating changes in fluorescent emission over time; 
     FIG. 8 is a flowchart illustrating the overall operation of the first embodiment; 
     FIG. 9 is a flowchart illustrating the procedure for finding a Y-coordinate in the first embodiment; 
     FIG. 10 is a plan view illustrating the first two series of scans performed in the first embodiment; 
     FIG. 11 is a plan view illustrating rotational alignment of the photosensitive optical fiber; 
     FIG. 12 is a plan view illustrating the third series of scans performed in the first embodiment; 
     FIG. 13 is a graph illustrating data obtained during one series of scans in the first embodiment; 
     FIG. 14 is a flowchart illustrating the procedure for finding a Y-coordinate in a second embodiment; 
     FIG. 15 is a plan view illustrating a series of scans performed in the second embodiment; 
     FIG. 16 is a graph illustrating data obtained during the last two scans in FIG. 15; 
     FIG. 17 is a flowchart illustrating the procedure for finding a Y-coordinate in a third embodiment; 
     FIG. 18 is a plan view illustrating a series of scans performed in the third embodiment; 
     FIG. 19 is a graph illustrating data obtained during the last two scans in FIG. 18; 
     FIG. 20 is a flowchart illustrating the procedure for finding a Y-coordinate in a fourth embodiment; and 
     FIG. 21 is a graph illustrating the determination of a Y-coordinate in the fourth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will be described with reference to the attached illustrative drawings, following a general description of the conventional method of using a phase mask to create an in-fiber Bragg grating. 
     FIG. 1 shows a conventional apparatus comprising a laser light source  2 , an optical attenuator  4 , a mirror  6 , and a cylindrical lens  8  that focuses an ultraviolet beam P 1  through a phase mask  10  onto a photosensitive optical fiber  12 . The mirror  6  and cylindrical lens  8  are movable in the direction of arrow X, this motion causing the ultraviolet beam to scan the photosensitive optical fiber  12  in the lengthwise direction. The optical attenuator  4  is necessary if a high-intensity laser light source is used, but may be omitted if a lower-intensity source such as an argon ion laser is used. 
     Referring to FIG. 2, the photosensitive optical fiber  12  comprises a clad  14  and a photosensitive core  16 . The core  16  is doped with germanium, so that its refractive index is alterable by exposure to ultraviolet light. The core may also be doped with hydrogen. The SMF-28 fiber manufactured by Corning, Incorporated, of Corning, N.Y., is an example of a suitable photosensitive optical fiber. 
     The lower surface of the phase mask  10  has a pattern of parallel corrugations forming a diffraction grating. The phase mask  10  can be fabricated by reactive ion etching of a silica glass substrate, using a thin-film chromium mask patterned by electron-beam photolithography. 
     Diffraction of the ultraviolet beam P 1  by the phase mask  10  produces a diffracted ultraviolet beam P 2  that illuminates the photosensitive optical fiber  12 . If the grating pitch-of the phase mask  10  is 2Λ, then the diffracted ultraviolet beam P 2  alternates between high and low intensity with a period of Λ. The refractive index of the fiber core  16  is most altered in regions  18  exposed to the high-intensity parts of the diffracted ultraviolet beam P 2 . A Bragg grating with a grating pitch of A is thus created in the fiber core  16 . 
     The Bragg grating selectively reflects light with a wavelength λ b  given by the following formula, in which n eff  is the effective refractive index of the fiber core  16 . 
     
       
         λ b =2· n   eff ·Λ 
       
     
     If light P 3  with a plurality of wavelengths, including λ b , is directed into the photosensitive optical fiber  12  from one end, the light P 4  of wavelength λ b  is reflected back to the same end. The optical fiber  12  thus becomes a filter that extracts a particular wavelength λ b . 
     Referring to FIG. 3, the phase mask  10  may have a chirped grating  20  with a grating pitch that varies, either continuously or in discrete steps. The grating  20  can be divided into zones, for example, and a different grating pitch can be employed in each zone, to create a filter that extracts a plurality of wavelengths. 
     FIG. 4 illustrates an apparatus generally similar to the above, but employing a first embodiment of the invention for initial alignment of the ultraviolet beam and the optical fiber. The laser light source  2  is a continuous-wave argon ion laser such as the INNOVA 300 FRED laser manufactured by Coherent, Incorporated, of Santa Clara, Calif. An optical slit, omitted to simplify the drawing, extracts an ultraviolet beam P 1  with a width of, for example, six-tenths of a millimeter (0.6 mm) . The mirror  6  and cylindrical lens  8  move in the direction indicated by arrow X. By focusing the ultraviolet beam, the cylindrical lens  8  reduces the beam width to substantially one hundred micrometers (100 μm) . The phase mask  10  may have either a constant or a chirped grating pitch; a constant grating pitch will be assumed in the following description. The above-mentioned Corning SMF-28 fiber may be employed as the photosensitive optical fiber  12 , or any other type of photosensitive optical fiber may be used, provided the fiber core fluoresces when illuminated by ultraviolet light. 
     The phase mask  10  and photosensitive optical fiber  12  are held by a Y-θ stage assembly  22  comprising a Y-axis translational stage  24 , a θ-axis rotational stage  26 , a base plate  28 , and a pair of fiber holders  30 . The Y-axis translational stage  24  is driven by a stepping motor (not visible) in a Y-axis direction perpendicular to the drawing sheet. The θ-axis rotational stage  26  is driven by another stepping motor (not visible), the axis of rotation being vertical in the drawing. The base plate  28  supports the fiber holders  30 , which hold the phase mask  10  and photosensitive optical fiber  12 . The photosensitive optical fiber  12  is held suspended above the base plate  28 , so that the refractive index of the fiber core is not significantly affected by ultraviolet light reflected from the base plate  28 . The phase mask  10  is held parallel to the photosensitive optical fiber  12 . 
     One end of the photosensitive optical fiber  12  is coupled to an optical transducer  32  such as an optical power meter that converts visible light received from the photosensitive optical fiber  12  to a data signal indicating the received optical power level. The received light, having a wavelength of about four hundred nanometers (400 nm), is generated by fluorescence in the core of the optical fiber  12  when the core is illuminated by the ultraviolet beam. The optical transducer  32  is coupled by a cable such as a general-purpose interface bus cable to a computer  34  that controls the Y-axis translational stage  24  and θ-axis rotational stage  26 . 
     Referring to FIG. 5, the apparatus also has an X-axis stage assembly  36  comprising an X-axis translational stage  38 , a vertical column  40 , and a horizontal arm  42  attached to the vertical column  40 . The horizontal arm  42  supports the mirror  6  and cylindrical lens  8  (the physical attachment between the cylindrical lens  8  and horizontal arm  42  is omitted to simplify the drawing) . Under control of the computer  34 , the X-axis translational stage  38  moves the vertical column  40  and the horizontal arm  42 , thereby producing the scanning motion indicated by the arrow X in FIG.  4 . The scanning rate is controllable in the range from, for example, ten micrometers to one hundred millimeters per second (10 μm/s to 100 mm/s). 
     FIG. 6 shows an intensity profile of the ultraviolet beam incident on the phase mask  10  and photosensitive optical fiber  12 . Position on the Y-axis is indicated in micrometers on the horizontal axis; optical power is indicated in nanowatts on the vertical axis. The beam has a substantially Gaussian profile with a full width of about one hundred micrometers (100 μm). 
     FIG. 7 indicates how the intensity of the fluorescence varies over time when the photosensitive optical fiber  12  is illuminated at a single location by the ultraviolet beam. Illumination time is shown in seconds on the horizontal axis, and fluorescence as measured by the optical transducer  32  on the vertical axis. The optical power of the fluorescence rises to a peak level in about one second, then declines toward a stable level over about thirty seconds. The present invention is able to align the photosensitive optical fiber  12  accurately despite this varying fluorescence level. 
     The initial alignment procedure carried out by the first embodiment will next be described with reference to the flowcharts in FIGS. 8 and 9. In this procedure the computer  34  controls the X-axis translational stage  38  and Y-axis translational stage  24  according to X- and Y-coordinates denoting the point at which the ultraviolet beam P 1  is aimed on the base plate  28  of the Y-θ stage assembly  22 . 
     Referring to FIG. 8, the initial alignment procedure starts with the mounting of the photosensitive optical fiber  12  in the fiber holders  30  on the base plate  28  in step S 401 . In this step the phase mask  10  is also secured to the fiber holders  30 , so that the photosensitive optical fiber  12  is aligned with the long axis of the phase mask  10 , along the dashed line marked X in FIG. 3, which is roughly aligned with the X-axis of the X-axis stage assembly  36 . 
     In step S 402 , the computer  34  is given an alignment command. The succeeding steps are carried out under control of the computer  34 . In the following description, the notation Nxy will denote a variable used by the computer  34  to keep track of the current stage of the operation. The initial value of Nxy is zero. 
     In step S 403 , Nxy is incremented by one, and tested to determine the next step. The first time step S 403  is executed, the value of Nxy is incremented from zero to one, and the next step S 404  is to move the X-axis translational stage  38  and Y-axis translational stage  24  to X-Y coordinates (X 11 , Y 11 ) such that the ultraviolet beam is aimed at a point disposed in the negative Y-axis direction from the fiber, in the area between one end of the grating  20  of the phase mask  10  and one of the fiber holders  30 . In this area, the ultraviolet beam P 1  passes through the phase mask  10  without being diffracted. 
     In the next step S 405 , the laser light source  2  is turned on, and the Y-axis translational stage  24  is driven to find the Y-coordinate at which the ultraviolet beam P 1  best illuminates the photosensitive optical fiber  12 . This step is carried out by a procedure that will now be described in more detail, with reference to FIG.  9 . 
     The procedure in FIG. 9 is controlled by a variable Ny that is set to zero each time the procedure begins. 
     In the first step S 501 , Ny is incremented and tested to determine the next step. The first time step S 501  is executed, Ny is incremented from zero to one, and in the next step S 502 , the scanning speed of the Y-axis translational stage  24  is set to one hundred micrometers per second (100 μm/s). 
     In the next step S 503 , the Y-axis translational stage  24  is driven so that the ultraviolet beam P 1  scans in the positive Y-axis direction from Y-coordinate Y 11  toward a Y-coordinate Y 12  on the other side of the photosensitive optical fiber  12 . As this first scan is carried out, the computer  34  monitors the optical power measured by the optical transducer  32 . At the end of the scan, in step S 504 , the computer  34  finds the Y-coordinate Y 13  at which the peak optical power value was detected. Since Y 13  is the Y-coordinate of peak fluorescence, Y 13  is approximately equal to the Y-coordinate at which the ultraviolet beam P 1  was centered on the photosensitive optical fiber  12 . 
     The procedure then returns to step S 501 . The variable Ny is incremented and tested again, now producing a value of two. This sends the procedure to step S 505 , in which the Y-axis translational stage  24  is driven to a Y-coordinate one millimeter less than the Y-coordinate Y 13  at which peak fluorescence was detected. The ultraviolet beam P 1  is thus positioned at (X 11 , Y 13 −1 mm). 
     In the next step S 506 , the scanning speed of the Y-axis translational stage  24  is set to ten micrometers per second (10 μm/s) . Then in step S 507 , the Y-axis translational stage  24  is driven at this speed to a Y-coordinate one millimeter greater than Y 13 ; that is, to (Y 13 +1 mm). As this second scan is carried out, the computer  34  again monitors the optical power measured by the optical transducer  32 . At the end of the scan, in step S 508 , the computer  34  finds a new Y-coordinate Y 14  at which peak optical power was detected. 
     The procedure then returns once more to step S 501 . The variable Ny is incremented and tested again, now producing a value of three. This sends the procedure to step S 509 , in which the Y-axis translational stage  24  is driven to a Y-coordinate one-tenth of one millimeter less than the Y-coordinate Y 14  at which the peak fluorescence was detected in the second scan. The ultraviolet beam P 1  is thus positioned at (X 11 , Y 14 −0.1 mm). 
     In the next step S 510 , the scanning speed of the Y-axis translational stage  24  is set to one micrometer per second (1 μm/s). Then in step S 511 , the Y-axis translational stage  24  is driven at this speed to a Y-coordinate one-tenth of one millimeter greater than the Y-coordinate Y 14  where the peak fluorescence was detected in the second scan; that is, to (Y 14 +0.1 mm). As this third scan is carried out, the computer  34  once again monitors the optical power measured by the optical transducer  32 . At the end of the scan, in step S 512 , the computer  34  stores the new Y-coordinate Y 15  at which the peak optical power was measured. 
     Because of the successively decreasing speeds of the three scans, the optical power measurements become increasingly accurate. The coordinates (X 11 , Y 15 ) found in the last scan in the series are the X- and Y-coordinates at which a first point on the photosensitive optical fiber  12  is closely centered under the ultraviolet beam P 1 . 
     Referring again to FIG. 8, step S 403  is now executed once more, incrementing the variable Nxy and obtaining a value of two. This causes the execution of step S 406 , in which the X-axis translational stage  38  and Y-axis translational stage  24  are moved to coordinates (X 21 , Y 21 ) These coordinates are also selected so that the ultraviolet beam P 1  lands on the phase mask  10  at a point outside the grating  20 , slightly to one side of the photosensitive optical fiber  12 . X 21  must be different from X 11 , and a large difference is preferable. 
     In the next step S 407 , the procedure in FIG. 9 is carried out again, using X-coordinate X 21  instead of X 11 , so that the ultraviolet beam P 1  crosses the photosensitive optical fiber  12  at a different location. Coordinates (X 21 , Y 25 ) at which the ultraviolet beam P 1  is closely centered on a second point on the photosensitive optical fiber  12  are thereby obtained. 
     FIG. 10 illustrates the two series of scans performed in step S 405  (Nxy=1) and step S 407  (Nxy=2). Each series comprises three scans (Ny=1, 2, 3). 
     In the next step S 408  in FIG. 8, from the coordinates (X 11 , Y 15 ) and (X 21 , Y 25 ) of the first and second points, the computer  34  calculates the angle θ between the X-axis of the X-axis stage assembly  36  and the axis of the photosensitive optical fiber  12 , and controls the rotating stage  26  so as to reduce this angle to zero, as shown in FIG.  11 . The photosensitive optical fiber  12  is now aligned parallel to the X-axis. The angle θ can be calculated by use of the arctangent function, as follows. 
     
       
         θ=arctan{( Y   25   −Y   15 )/( X   21   −X   11 )} 
       
     
     Step S 403  in FIG. 8 is now executed for a third time, incrementing the variable Nxy and obtaining a value of three. This causes the execution of step S 409 , in which the X-axis translational stage  38  and Y-axis translational stage  24  are positioned again at coordinates (X 21 , Y 21 ), causing the ultraviolet beam P 1  to land again at a point outside the grating  20  and slightly to one side of the photosensitive optical fiber  12 . 
     In the next step S 410 , the procedure in FIG. 9 is carried out once more to find a third pair of coordinates (X 21 , Y 35 ) at which the photosensitive optical fiber  12  is centered under the ultraviolet beam P 1 . FIG. 12 illustrates the three Y-axis scans performed in this step. In step S 411  in FIG. 8, the computer  34  causes the Y-axis translational stage  24  to move to Y-coordinate Y 35 . The photosensitive optical fiber  12  is now aligned parallel to the X-axis and centered under the ultraviolet beam P 1 , in the correct position to begin the X-axis scan that will form the Bragg grating. 
     The alterations to the refractive index of the core of the photosensitive optical fiber  12  made during the initial alignment procedure described above are comparatively small, because the ultraviolet beam is scanned across the photosensitive optical fiber  12  instead of lengthwise along the fiber core. Moreover, the ultraviolet beam is not diffracted, so no Bragg grating is formed during the initial alignment. Such alterations of the refractive index as occur during the initial alignment have substantially no effect on the final optical characteristics of the filter. 
     By performing angular alignment according to the coordinates (X 11 , Y 15 ) and (X 21 , Y 25 ), followed by Y-axis alignment according to (X 21 , Y 35 ), the first embodiment is able to align the photosensitive optical fiber  12  accurately and automatically. By determining each pair of coordinates in a series of three successively shorter and slower scans, the first embodiment is able to obtain accurate coordinates in a reasonably short time. 
     FIG. 13 illustrates fluorescence measurements obtained during one series of three scans. The horizontal axis represents time. The vertical axis represents the measured optical power of the fluorescence. Each of the three scans takes about the same length of time. Since the scanning speed decreases by a factor of ten from the first scan (Ny=1) to the second scan (Ny=2), and by a further factor of ten in the third scan (Ny=3), the spatial resolution of the coordinate data increases by a factor of ten in each scan. The third scan (Ny=3) is slow enough for the hat-top shape of the peak to be fully resolved. 
     Next, a second embodiment will be described. The second embodiment follows the same general initial alignment procedure as the first embodiment, shown in FIG. 8, but modifies the scanning procedure used to find coordinates in steps S 405 , S 407 , and S 410 . The modified procedure for finding the coordinates of the first point (step S 405 ) will be described below with reference to FIG.  14 . 
     Steps S 801 , S 802 , S 803 , and S 804  are similar to steps S 501 , S 502 , S 503 , and S 504  in the first embodiment. The variable Ny is incremented, the ultraviolet beam P 1  is scanned across the photosensitive optical fiber  12  at a speed of one hundred micrometers per second in the positive Y-axis direction, and the computer  34  finds the Y-coordinate Y 13  at which the peak fluorescence is measured. 
     Steps S 805 , S 806 , S 807 , and S 808  are similar to steps S 505 , S 506 , S 507 , and S 508  in the first embodiment. The Y-axis translational stage  24  is controlled to position the ultraviolet beam P 1  at a Y-coordinate one millimeter less than Y 13 , at (X 11 , Y 13 −1 mm); then the ultraviolet beam P 1  is scanned across the photosensitive optical fiber  12  at a speed of ten micrometers per second in the positive Y-axis direction, to the point with coordinates (X 11 , Y 13 +1 mm). The computer  34  finds and stores the Y-coordinate Y 14  at which the peak fluorescence is measured. 
     In step S 809 , the ultraviolet beam P 1  is positioned at a Y-coordinate one-tenth of one millimeter greater than Y 14 , at (X 11 , Y 14 +0.1 mm). In step S 810 , the scanning speed is set to one micrometer per second (1 μm/s) . In step S 811 , the ultraviolet beam P 1  is scanned at this speed in the negative Y-axis direction, to (X 11 , Y 14 −0.1 mm). In step S 812  the computer  34  finds the Y-coordinate Y 15  at which peak fluorescence is measured during this scan. In step S 813 , the computer  34  computes the mean value Y 16  of the peak coordinate Y 14  obtained in the second scan (Ny=2) and the peak coordinate Y 15  obtained in the third scan (Ny=3). This mean value Y 16  is regarded as giving the true location of the fiber. 
     FIG. 15 illustrates the three scans (Ny=1, 2, 3) in the procedure in FIG. 14, showing that the second scan (Ny=2) and third scan (Ny=3) are performed in opposite directions. FIG. 16 shows examples of data obtained during the second and third scans. The vertical axis indicates the measured optical power of the fluorescence. The horizontal axis in FIG. 16 is a Y-coordinate axis; differing from FIG. 13, both scans are shown in terms of position coordinates, at the same spatial resolution. 
     The other series of scans, (in steps S 407  and S 410  in FIG. 8) are carried out in the same way, by scanning the ultraviolet beam P 1  twice in one direction, then once in the opposite direction, and taking the mean value of the peak Y-coordinates found in the second and third scans. These mean values are used in the position-adjusting steps (steps S 408  and S 411  in FIG.  8 ). 
     By taking the mean of two peak values obtained from scans in opposite directions, the second embodiment is able to reduce the effect of the time dependency of the fluorescence illustrated in FIG. 7, thereby improving the accuracy of the positional data. 
     Next, a third embodiment will be described. The third embodiment is generally similar to the second embodiment, but makes a further modification to the scanning procedure used in steps S 405 , S 407 , and S 410  in FIG.  8 . The modified procedure for step S 405  is shown in FIG.  17 . 
     Steps S 1101 , S 1102 , S 1103 , and S 1104  are similar to steps S 801 , S 802 , S 803 , and S 804  in the second embodiment. The variable Ny is incremented, the ultraviolet beam P 1  is scanned across the photosensitive optical fiber  12  at a speed of one hundred micrometers per second in the positive Y-axis direction, and the computer  34  finds the Y-coordinate Y 13  at which peak fluorescence is measured. 
     In step S 1105 , the X-axis translational stage  38  and Y-axis translational stage  24  are driven to coordinates differing from the peak fluorescence coordinates (X 11 , Y 13 ) by one millimeter in the negative Y-axis direction and by an amount ΔX in the X-axis direction. ΔX is, for example, one half of one millimeter (+0.5 m). The ultraviolet beam P 1  is thus positioned at coordinates (X 11 +ΔX, Y 13 −1 mm). 
     Steps S 1106 , S 1107 , and S 1108  are similar to steps S 806 , S 807 , and S 808  in the second embodiment. The ultraviolet beam P 1  is scanned from (X 11 +ΔX, Y 13 −1 mm) to (X 11 +ΔX, Y 13 +1 mm) at a speed of ten micrometers per second. The computer  34  stores the Y-coordinate (Y 14 ) at which peak fluorescence is measured. 
     In step S 1109 , the X-axis translational stage  38  and Y-axis translational stage  24  are driven to coordinates differing from the peak fluorescence coordinates (X 11 +ΔX, Y 14 ) of the second scan by one-tenth of one millimeter in the positive Y-axis direction and by the above amount ΔX in the X-axis direction. The ultraviolet beam P 1  is thus positioned at coordinates (X 11 +2ΔX, Y 14 +0.1 mm) 
     Steps S 1110 , S 1111 , S 1112 , and S 1113  are similar to steps S 810 , S 811 , S 812 , and S 813  in the second embodiment. The ultraviolet beam P 1  is scanned from (X 11 +2ΔX, Y 14 +0.1 mm) to (X 11 +2ΔX, Y 14 −0.1 mm) at a speed of one micrometer per second. The computer  34  finds the Y-coordinate (Y 15 ) at which peak fluorescence is measured, then calculates the mean value Y 16  of this peak Y-coordinate (Y 15 ) and the peak Y-coordinate (Y 14 ) found in the second scan. The mean value (X+1.5 ΔX) of the corresponding X-coordinates (X+ΔX and X+2ΔX) is also calculated. The fiber is regarded as centered under the ultraviolet beam P 1  at these mean coordinates (X+1.5 ΔX, Y 16 ). 
     Steps S 407  and S 410  in FIG. 8 are modified similarly, by moving the X-axis translational stage  38  through a distance ΔX between scans. The mean-value coordinates obtained from the second and third scans in each step are used in the adjustment steps (steps S 408  and S 411 ). 
     FIG. 18 illustrates the three scans (Ny=1, 2, 3) in the procedure in FIG. 17, showing that the photosensitive optical fiber  12  is crossed at points with three different X-coordinates. FIG. 16 shows examples of the data obtained during the second and third scans. The vertical axis indicates the measured optical power of the fluorescence, the horizontal axis is the Y-coordinate axis, and both scans are shown at the same spatial resolution. 
     The third embodiment eliminates memory effects, so that the amount of fluorescence measured in one scan is not affected by the amount of fluorescence already emitted from the same point in a previous scan. The positional data obtained in the third embodiment are accordingly even more accurate than the data in the second embodiment. 
     Next, a fourth embodiment will be described. The fourth embodiment also follows the general initial alignment procedure shown in FIG. 8, but modifies the scanning procedure used in steps S 405 , S 407 , and S 410 . The modified procedure for finding the first pair of coordinates (step S 405 ) will be described below with reference to FIG.  20 . 
     Steps S 1401 , S 1402 , and S 1403  are similar to steps S 501 , S 502 , and S 503  in the first embodiment. The ultraviolet beam P 1  is scanned across the photosensitive optical fiber  12  in the positive Y-axis direction at a speed of one hundred micrometers per second, and the optical power of the fluorescence is measured. The computer  34  stores all of the measured optical power data. 
     Step S 1404  is illustrated in FIG.  21 . From the data measured in step S 1403 , the computer  34  determines the half-power coordinates Y a1  and Y b1  of the fluorescence. The fiber is regarded as being centered halfway between these coordinates, at a Y-coordinate Y 13  equal to the mean value of Y a1  and Y b1 . 
     
       
           Y   13 =0.5 Y   a1 +0.5 Y   b1    
       
     
     Steps S 1405 , S 1406 , and S 1407  in FIG. 20 are similar to steps S 505 , S 506 , and S 507  in the first embodiment. The ultraviolet beam P 1  is scanned from (X 11 , Y 13 −1 mm) to (X 11 , Y 13 +1 mm) at a speed of ten micrometers per second, and the optical power values of the fluorescence are stored. Step S 1408  is similar to step S 1404 : the computer  34  calculates and stores a center Y-coordinate Y 14  equal to the mean value of the two half-power coordinates in the data obtained in step S 1407 . 
     Steps S 1409 , S 1410 , and S 1411  are similar to steps S 509 , S 510 , and S 511  in the first embodiment. The ultraviolet beam P 1  is scanned from (X 11 , Y 14 −0.1 mm) to (X 11 , Y 14 +0.1 mm) at a speed of one micrometer per second, and the optical power data are stored. In step S 1412 , the computer  34  calculates another center Y-coordinate Y 15  equal to the mean value of the two half-power coordinates of the data obtained in step S 1411 . In step S 1413 , the computer  34  calculates a final Y-coordinate Y 16  by taking the mean value of the center coordinates Y 14  and Y 15  obtained in the second and third scans. 
     Similar procedures are used in steps S 407  and S 410  in FIG. 8, to obtain the coordinates used in adjusting the Y-θ stage assembly  22  in steps S 408  and S 411 . 
     By taking the center coordinate between two half-power coordinates, the fourth embodiment can obtain accurate positional data despite the presence of noise in the optical power measurements. The fourth embodiment is particularly effective in extracting accurate positional information from optical power data having a noisy hat-top profile, as in the example shown in FIG.  21 . 
     The fourth embodiment can be modified by omitting step S 1413  and using the center coordinate Y 15  obtained in the third scan as the Y-coordinate of the fiber. 
     The second and third embodiments can be modified by taking the center coordinate between the two half-power coordinates, as in the fourth embodiment, instead of simply finding the peak coordinate in each scan. 
     The second, third, and fourth embodiments can be modified by taking a weighted average of the Y-coordinates Y 14  and Y 15  obtained in the second and third scans as the final Y-coordinate Y 16  instead of simply taking the mean value. 
     The number of scans in each series is not limited to three, but may be any number greater than one. 
     If the base plate  28  is coated so as to prevent reflection of ultraviolet light, the photosensitive optical fiber  12  can be held in contact with the base plate  28 , instead of being suspended in midair. 
     Those skilled in the art will recognize that further variations are possible within the scope claimed below.