Abstract:
A photosensitive optical fiber being scanned by an ultraviolet beam to form an optical filter is aligned with the beam by detecting fluorescence produced in the fiber. The relative position of the fiber and beam is made to oscillate in a direction cutting across the scanning direction. Misalignment is corrected according to the phase relationship between the fluorescence detection signal and the oscillation, permitting the alignment to be controlled accurately regardless of the beam dimensions, scanning distance, scanning rate, and natural fluorescence variations.

Description:
This application is a Continuation-In_Part of application Ser. No.09/149,120, filed Sep. 9, 1998 now U.S. Pat. No. 6,038,358. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and system for aligning an ultraviolet beam and an optical fiber during 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. The manufacturing process is difficult, because the optical characteristics of the filter are extremely sensitive to the degree of index modulation. Unintended variations in the degree of index modulation can cause Fabry-Perot resonance and undesired chirping, leading to such problems as ripple in the reflection band, generally reduced reflectivity in the reflection band, and prominent sidelobes outside the reflection band. 
     The filter characteristics also depend on the length of the in-fiber Bragg grating, and some applications require fairly long gratings. Dispersion compensation on the order of one thousand four hundred picoseconds per nanometer over a bandwidth of seven nanometers, for example, requires an in-fiber Bragg grating with a length of one to two meters. 
     Since the degree of index modulation depends on the amount of ultraviolet illumination received by the fiber core, highly uniform illumination is a key consideration in the manufacturing process. The source of the ultraviolet light is a laser apparatus. Uniform illumination requires the use of a highly stable source, such as an argon ion laser, which emits a comparatively weak and narrow ultraviolet beam by second harmonic generation. (The comparison is with a krypton-fluoride or KrF laser. A KrF laser emits a larger and stronger beam, but the beam intensity and plane of polarization tend to fluctuate over time.) The intensity of an argon ion laser beam can be increased by focusing the beam, but the beam then becomes even narrower. This narrow beam must be scanned along the length of the optical fiber, and for uniform illumination, the center of the beam must be kept accurately aligned on the fiber core throughout the scan. Accurate alignment of a narrow beam on a thin optical fiber is no simple matter, particularly when a long in-fiber grating is being manufactured. 
     A known method of alignment is to monitor visible light produced by fluorescence when the core of the optical fiber is illuminated by the ultraviolet beam. The fluorescence arises from the germanium doping that makes the core photosensitive. The fluorescence is measured by an optical power meter connected to one end of the optical fiber, and the measured value is fed back to a positioning system that positions the fiber under the beam. The feedback control system operates to keep the fiber in the position that produces maximum fluorescence, this being the position in which the beam is centered on the fiber core. 
     This fluorescence monitoring method of alignment leaves room for improvement, however, because while a decrease in the measured optical power may indicate that the beam is drifting away from the fiber axis, the decrease does not indicate the direction of the drift. The positioning system must move the fiber in one direction, and determine whether this movement produces an increase or a further decrease in the measured optical power. If a further decrease is detected, then the positioning system has moved the fiber in the wrong direction, and must now move the fiber in the opposite direction. 
     Natural variations in the amount of fluorescence also make it difficult to determine when the maximum optical power has been obtained. Even when the ultraviolet beam is held stationary on the same spot on the optical fiber, the intensity of the fluorescence varies considerably over time, not reaching a constant level until about thirty seconds have elapsed. One consequence is that the measured optical power depends on the scanning rate, which may be varied intentionally to create an apodized Bragg grating. 
     For these reasons, a feedback control system targeted at maximum fluorescence requires complex control circuitry. Moreover, even with complex control circuitry, the beam alignment has a tendency to wander, due to natural fluorescence variations and wrong-direction control. With a sufficiently wide beam, such wandering could be tolerated, but the optical filter manufacturer would prefer to be able to focus the beam as tightly as possible, thereby making the beam as intense as possible, so that the scanning process can be completed as quickly as possible. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to maintain accurate alignment between a photosensitive optical fiber and an ultraviolet beam, without unnecessary wandering of alignment, while the ultraviolet beam is scanned along the optical fiber to create an optical filter in the optical fiber. 
     Another object of the invention is to maintain accurate alignment between a photosensitive optical fiber and a tightly focused ultraviolet beam. 
     Another object is to provide a simple control circuit for maintaining accurate alignment between a photosensitive optical fiber and an ultraviolet beam. 
     The invented alignment method comprises the steps of: 
     producing an oscillation in relative position between the photosensitive optical fiber and the ultraviolet beam in a direction differing from the scanning direction; 
     detecting light produced by fluorescence in the photosensitive optical fiber; 
     converting the detected light to a first signal having an oscillating component due to the above-mentioned oscillation in relative position; 
     detecting the phase relationship between the first signal and the oscillation in relative position; and 
     adjusting the relative position of the photosensitive optical fiber and ultraviolet beam according to the detected phase relationship. 
     According to a first aspect of the invention, the oscillation in relative position is produced by vibrating the photosensitive optical fiber. According to a second aspect of the invention, the oscillation is produced by tilting a mirror that reflects the ultraviolet beam. 
     The oscillating component of the first signal is preferably compared with a threshold to generate a binary signal, and the phase relationship is preferably detected by comparing the binary signal with a second signal that is used to produce the oscillation in relative position. The relative position of the photosensitive optical fiber and ultraviolet beam is not adjusted when the oscillating component of the first signal remains below the threshold. 
     The invented alignment system comprises a positional transducer for producing the oscillation in relative position, an optical transducer for converting the light produced by fluorescence to the first signal, and a control circuit for comparing the phase of the first signal with the phase of the oscillation in relative position and adjusting the relative position of the photosensitive optical fiber and ultraviolet beam. The positional transducer may be piezoelectrically actuated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
     FIG. 1 is a schematic drawing of a conventional apparatus for manufacturing 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 mechanical components of an alignment system according to a first embodiment of the invention; 
     FIG. 5 is a frontal view illustrating the mechanical components in the first embodiment; 
     FIG. 6 is a block diagram of the control circuit in the first embodiment; 
     FIG. 7 is a graph of the intensity profile of the ultraviolet beam in the first embodiment; 
     FIG. 8 is a plan view of the photosensitive optical fiber, illustrating incorrect beam alignment; 
     FIG. 9 is a plan view of the photosensitive optical fiber, illustrating correct beam alignment; 
     FIG. 10 is a plan view of the photosensitive optical fiber, illustrating incorrect beam alignment; 
     FIG. 11 is a waveform diagram illustrating waveforms produced by the alignments in FIGS. 9,  10 , and  11 ; 
     FIG. 12 is a side view illustrating mechanical components of an alignment system according to a second embodiment of the invention; 
     FIG. 13 is a frontal view illustrating the mechanical components in the second embodiment; and 
     FIG. 14 is a block diagram of the control circuit in the second 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 greatly 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 Λ 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 photosensitive 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 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 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. The width of the ultraviolet beam P 1  is, 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) and increases the beam intensity proportionately. 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 secured to a Y-θ stage assembly  22  comprising a Y-axis translational stage  24 , a θ-axis rotational stage  26 , a Y-axis vibrating stage  28 , a base plate  30 , and a pair of fiber holders  32 . 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 Y-axis vibrating stage  28  is vibrated by a piezoelectric transducer (not visible) in the Y-axis direction. The base plate  30  supports the fiber holders  32 , which hold the phase mask  10  and photosensitive optical fiber  12 . The photosensitive optical fiber  12  is held suspended above the base plate  30 , so that the refractive index of the fiber core is not significantly affected by ultraviolet light reflected from the base plate  30 . 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  34  that converts visible light received from the photosensitive optical fiber  12  to a direct-current or dc voltage signal having a voltage level responsive to the intensity of the visible light. The optical transducer  34  is supported by a separate holder (not visible). A suitable optical transducer is the Model 8235-C optical power meter manufactured by the Newport Corporation of Irvine, Calif. 
     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). The X-axis translational stage  38  moves the vertical column  40  and horizontal arm  42 , thereby producing the scanning motion indicated by the arrow X in FIG.  4 . The scanning rate is in the range from, for example, ten millimeters to one hundred millimeters per second (10 mm/s to 100 mm/s). 
     Referring to FIG. 6, the optical transducer  34  is part of a misalignment detection circuit  44  that also comprises an alternating-current or ac extractor  46 , a pre-amplifier  48 , a threshold comparator  50 , and a phase comparator  52 . The ac extractor  46  extracts the oscillating or ac component of the output of the optical transducer  34  by, for example, subtracting the dc component. The ac component is amplified by the pre-amplifier  48 , then compared with a positive threshold voltage by the threshold comparator  50 . The threshold comparator  50  outputs a binary signal that is high when the instantaneous voltage level of the amplified ac component exceeds the threshold, and low when the instantaneous voltage level of the amplified ac component is less than the threshold. 
     As additional control circuitry, the first embodiment has a reference clock generator  54 , a dither signal generator  56 , an offset signal generator  58 , a summing circuit  60 , and a stage driving circuit  62 . The reference clock generator  54  generates a reference clock signal with a constant frequency. The dither signal generator  56  divides this frequency to create a dither signal, which is an oscillating signal that alternates between the high and low levels. The dither signal is supplied to the phase comparator  52  and summing circuit  60 . 
     The phase comparator  52  outputs a phase-comparison signal that indicates the phase relationship between the binary signal received from the threshold comparator  50  and the dither signal received from the dither signal generator. The phase-comparison signal is, for example, a pulse signal with a duty cycle that depends on the phase relationship. 
     The offset signal generator  58  receives the phase-comparison signal from the phase comparator  52 , and outputs a dc offset voltage signal. When the phase-comparison signal indicates that the binary signal is in phase with the dither signal, the offset signal generator  58  increases the dc offset voltage. When the phase-comparison signal indicates that the binary signal and dither signal have mutually reverse phases, the offset signal generator  58  decreases the dc offset voltage. 
     The summing circuit  60  adds the dc offset voltage output by the offset signal generator  58  to the dither signal output by the dither signal generator  56  to create a compensation signal, which is supplied to the stage driving circuit  62 . The stage driving circuit  62  drives the piezoelectric transducer in the Y-axis vibrating stage  28  according to the value of this compensation signal. The output of the stage driving circuit  62  is a voltage signal with a variable range of, for example, ten volts (10 V), which is converted by the piezoelectric transducer to a positional displacement with a variable range of, for example one hundred micrometers (100 μm). The peak-to-peak oscillation of the output of the stage driving circuit  62  is, for example, a few tenths of a volt, which the piezoelectric transducer converts to a vibration of a few micrometers. 
     The control circuit shown in FIG. 6 is concerned with alignment control during the scanning process. For initial alignment, and for control of the scanning process itself, the X-axis translational stage  38 , Y-axis translational stage  24 , and θ-axis rotational stage  26  are controlled by a conventional computer system, not shown in the drawings. 
     The operation of the first embodiment will now be described with reference to the preceding figures and FIGS. 7 to  11 . 
     To begin the manufacture of an optical filter, the phase mask  10  and photosensitive optical fiber  12  are secured to the fiber holders  32  so that the photosensitive optical fiber  12  is aligned with the long axis of the phase mask  10  (the dashed line in FIG.  3 ). Conventional methods are then used to adjust the Y-axis translational stage  24  and θ-axis rotational stage  26  to initial positions in which the photosensitive optical fiber  12  is aligned with the X-axis or scanning axis in FIG. 4, and is substantially centered under the path of the ultraviolet beam P 1 . Next, the stage driving circuit  62  is switched on, causing the Y-axis vibrating stage  28  to vibrate the base plate  30  in the Y-axis direction. If necessary, the Y-axis translational stage  24  is readjusted so that the vibration is centered under the path of the beam. 
     Next, the X-axis stage assembly  36  begins moving the mirror  6  and cylindrical lens  8  along the X-axis under computer control, causing the ultraviolet beam P 1  to move along the long axis of the phase mask  10 , and the diffracted ultraviolet beam P 2  to scan along the length of the photosensitive optical fiber  12 . During the scan, the Y-axis vibrating stage  28  acts as a positional transducer that produces an oscillation in the relative position of the photosensitive optical fiber  12  and diffracted ultraviolet beam P 2 , in the Y-axis direction. 
     FIG. 7 shows an intensity profile of the ultraviolet beam P 1  incident on the phase mask  10 . Position on the Y-axis is indicated in micrometers on the horizontal axis, and optical power in nanowatts on the vertical axis. The beam has a substantially Gaussian profile, with a full width of about one hundred micrometers (100 μm). The diffracted ultraviolet beam P 2  incident on the photosensitive optical fiber  12  has a similar profile. 
     Ultraviolet illumination causes the core of the photosensitive optical fiber  12  to fluoresce, emitting visible light in the 400-nm wavelength vicinity. Some of this visible light propagates through the photosensitive optical fiber  12  to the optical transducer  34 . 
     Referring to FIGS. 8,  9 , and  10 , the diffracted ultraviolet beam P 2  has an elliptical cross section, the one-hundred-micrometer beam width being the minor axis of the ellipse. The width of the photosensitive optical fiber  12  is slightly greater. 
     In FIG. 8, the photosensitive optical fiber  12  is misaligned in the positive Y-axis direction. In this misaligned state, a displacement of even a few micrometers in the Y-axis direction makes a great difference in the amount of ultraviolet illumination received by the fiber core, as can be seen from the beam profile in FIG.  7 . The vibration of the photosensitive optical fiber  12  by the Y-axis vibrating stage  28  therefore leads to a large oscillation in the intensity of the fluorescence produced in the fiber, and a strong ac component in the electrical signal output by the optical transducer  34 . The amplified ac component exceeds the positive threshold employed in the threshold comparator  50  each time the fiber is moved in the negative Y-axis direction, toward the center of the beam. 
     In FIG. 9, the photosensitive optical fiber  12  is centered directly under the diffracted ultraviolet beam P 2 . The vibration imparted by the Y-axis vibrating stage  28  does not greatly change the amount of ultraviolet illumination received by the photosensitive optical fiber  12 , so the intensity of the fluorescence oscillates only slightly, and the output of the optical transducer  34  has only a weak ac component. The amplified ac component fails to reach the positive threshold employed in the threshold comparator  50 . 
     FIG. 10 is similar to FIG. 8, with the photosensitive optical fiber  12  now misaligned in the negative Y-axis direction. The amplified ac component exceeds the positive threshold in the threshold comparator  50  when the photosensitive optical fiber  12  is moved in the positive Y-axis direction, toward the beam center. 
     FIG. 11 shows waveforms of signals occurring in the control circuit in FIG.  6 . Waveform D is the dither signal output by the dither signal generator  56 . The Y-axis vibrating stage  28  moves in the positive Y-axis direction when the dither signal is high, and in the negative Y-axis direction when the dither signal is low. Waveforms A, B, and C are examples of the binary signal output by the threshold comparator  50  when the photosensitive optical fiber  12  is in positive misalignment, correct alignment, and negative misalignment, respectively. 
     When the photosensitive optical fiber  12  is in negative misalignment, as in FIG. 10, if the misalignment is great enough to send the amplified ac component of the output of the optical transducer  34  over the threshold in the threshold comparator  50 , then the binary signal has a waveform generally similar to waveform C in FIG. 11, with substantially the same phase as the dither signal D. The phase-comparison signal output by the phase comparator  52  causes the offset signal generator  58  to increase the dc offset voltage added to the dither signal by the summing circuit  60 . The stage driving circuit  62  responds by moving the center of vibration of the Y-axis vibrating stage  28  in the positive Y-axis direction, correcting the misalignment. This process continues until the amplified ac component no longer reaches the threshold, at which point the photosensitive optical fiber  12  is close to the correct alignment shown in FIG.  9 . 
     When the photosensitive optical fiber  12  is correctly aligned, or when the alignment is nearly enough correct for the amplified ac component to remain below the threshold in the threshold comparator  50 , the output of the threshold comparator  50  remains low (waveform B in FIG.  11 ). The phase-comparison signal output by the threshold comparator  50  indicates neither that the binary signal and the dither signal have the same phase, nor that they have mutually reverse phases. For example, the phase-comparison signal has a duty cycle of fifty percent. The output of the offset signal generator  58  is accordingly held constant, and the alignment remains unchanged. 
     When the photosensitive optical fiber  12  is in positive misalignment, as in FIG. 8, if the misalignment is great enough to send the amplified ac component over the threshold in the threshold comparator  50 , then the binary signal has a waveform generally similar to waveform A in FIG. 11, opposite in phase to the dither signal D. The phase-comparison signal output by the phase comparator  52  causes the offset signal generator  58  to decrease the dc offset voltage added to the dither signal by the summing circuit  60 . The stage driving circuit  62  responds by moving the center of vibration of the Y-axis vibrating stage  28  in the negative Y-axis direction, correcting the misalignment. This process continues until the amplified ac component no longer exceeds the positive threshold and the alignment is correct, or nearly correct. 
     The positive threshold employed in the threshold comparator  50  sets an alignment tolerance of the phase comparator  52 , a constant low waveform being produced when the alignment is within the tolerance, and a waveform such as waveform A or C in FIG. 11 being produced when the alignment is out of tolerance. The appropriate threshold can be determined from the alignment requirements, which are related to the frequency and magnitude of vibration of the Y-axis vibrating stage  28 , as well as to the beam dimensions. 
     Compared with the prior art, which attempts to maximize the dc level of the electrical signal output by the optical transducer  34 , the first embodiment responds to the phase of the electrical signal. When alignment drifts out of tolerance, the phase of the electrical signal immediately indicates the direction in which the photosensitive optical fiber  12  should be moved to correct the misalignment. The simple control circuit shown in FIG. 6 is accordingly able to respond quickly and appropriately to alignment drift, and to maintain highly accurate alignment without unnecessary wandering. Alignment is moreover maintained despite natural variations in the level of the fluorescence and changes in the scanning speed, as these factors do not affect the phase of the electrical signal. 
     The alignment accuracy is also independent of the beam size and the length of the scan. The first embodiment permits the use of a tightly focused and therefore intense beam during long scans, with resultant gains in manufacturing efficiency. 
     As a variation of the first embodiment, the control system in FIG. 6 can be modified to correct misalignment by controlling the Y-axis translational stage  24  as well as the Y-axis vibrating stage  28 . Arbitrarily large misalignments can be corrected in this way, relaxing the need for exactness in the initial alignment of the θ-axis rotational stage  26 . The variable displacement range of the Y-axis vibrating stage  28  can also be reduced, from one hundred micrometers to about fifteen micrometers (15 μm), for example. 
     Next, a second embodiment will be described. 
     The mechanical components of the second embodiment are shown in side view in FIG.  12  and frontal view in FIG. 13, using the same reference numerals as in FIGS. 4 and 5 for components common to the first embodiment. The differences from the first embodiment are that in the second embodiment, the Y-θ stage assembly  22  has no Y-axis vibrating stage, and the mirror of the first embodiment is replaced by a tilting mirror  64  having a piezoelectrically actuated mounting, such as the PHS 1z NV-102 tilting mirror mount manufactured by Piezosystem Jena of Jena, Germany. 
     FIG. 14 shows the alignment control circuit of the second embodiment, using the same reference numerals as in FIG. 6 for elements common to the first embodiment. The difference from the first embodiment is that the compensation signal output by the summing circuit  60  is supplied to a mirror driving circuit  66  which is configured to drive the tilting mirror  64 , thereby tilting the ultraviolet beam P 1  in the Y-axis direction, as indicated by the arrow in FIG.  13 . The tilting motion is oscillatory, similar to the vibration in the first embodiment. The ultraviolet beam P 1  oscillates around a center angle determined by the dc component of the compensation signal. The variable range of the tilt is sufficient to move the landing point of the ultraviolet beam P 1  on the phase mask  10  by at least, for example, one hundred micrometers (100 μm). The oscillations are smaller, moving the landing point by only a few micrometers. 
     The operation of the second embodiment is similar to the operation of the first embodiment. After initial alignment, the control system in FIG. 14 is turned on and begins vibrating the tilting mirror  64 . Next, the scanning process begins, the ultraviolet beam P 1  moving in the X-axis direction while oscillating in the Y-axis direction as described above. As in the first embodiment, the oscillations produce an ac component in the electrical signal output by the optical transducer  34 , the ac component being weak when the photosensitive optical fiber  12  and diffracted ultraviolet beam P 2  are correctly aligned, and becoming stronger if the beam begins to drift out of alignment. When the ac component reaches a threshold level, the threshold comparator  50  produces a binary signal with a phase that indicates the direction in which the beam needs to be moved to restore correct alignment, and the phase comparator  52  outputs a phase-comparison signal that causes the offset signal generator  58  to alter the dc offset voltage added to the dither signal, so as to move the beam in the necessary direction. The waveforms of the binary signal and dither signal are similar to the waveforms in the first embodiment, illustrated in FIG.  11 . 
     The second embodiment provides the same advantages as the first embodiment, enabling accurate alignment to be maintained regardless of the dimensions of the beam and the length of the scan. A further advantage of the second embodiment is that a small angular displacement of the tilting mirror  64  can produce a large displacement of the beam landing position. Large drifts in alignment can therefore be corrected more easily than in the first embodiment. 
     As in the first embodiment, the alignment control system can also be modified to correct large misalignments by controlling the Y-axis translational stage  24 , in which case the variable tilting range of the tilting mirror  64  can be reduced. 
     To review the operation of the two embodiments, both have an X-axis translation stage  38 , a Y-axis translational stage  24 , and a θ-axis rotational stage  26 , as illustrated in FIGS. 4,  5 ,  12 , and  13 . When the in-fiber Bragg grating is manufactured, the optical fiber  12  is secured in the pair of fiber holders  32 , which are then placed on the base plate  30 . Initial alignment is carried out by use of the X-axis translation stage  38 , Y-axis translational stage  24 , and θ-axis rotational stage  26 . During initial alignment, the Y-axis vibrating stage  28 , in the first embodiment, or the tilting mirror  64 , in the second embodiment, is set at a neutral position. After initial alignment, the X-axis translational stage  38  is driven, the Y-axis vibrating stage  28  or tilting mirror  64  is driven, and the relative positions of the optical fiber  12  and ultraviolet beam are aligned continuously as the fiber grating is formed. 
     In the embodiments, the X-axis motion and Y-axis vibration were described as being mutually perpendicular. In this invention, however, while the X-axis translational stage  38  is driven to form a fiber grating of the desired length, the Y-axis vibrating stage  28  or tilting mirror  64  is vibrated simultaneously. As a result, the trace of the ultraviolet beam does not cross the optical fiber  12  at exactly a right angle to the fiber axis. The actual angle depends on the speed of vibration of the Y-axis vibrating stage  28  or tilting mirror  64 , and the driving rate of the X-axis translational stage. 
     The alignment system measures the intensity of the natural fluorescence produced when the ultraviolet beam illuminates the fiber core, maximum fluorescence being obtained when the peak of the beam intensity profile is centered on the axis of the fiber core. For this purpose it is not necessary for the trace of the ultraviolet beam to be perpendicular to the X-axis. The fiber core will fluoresce if the ultraviolet beam crosses the fiber at any angle. The vibration thus need not be perpendicular to the X-axis. It is only necessary for the fluorescence intensity to yield a binary signal when compared with a predetermined threshold by the threshold comparator  50 . This depends on the power distribution characteristic of the laser light source  2 , the stroke length of the vibration, the gain of the preamplifier  48 , and the threshold value employed in the threshold comparator  50 . 
     Various other modifications can be made in the preceding embodiments. For example, if the base plate  30  is coated so as to prevent reflection of ultraviolet light, the photosensitive optical fiber  12  can be held in contact with the base plate  30  instead of being suspended in midair. 
     The cylindrical lens  8  can be replaced by another type of lens, or by other aperture control means. 
     The laser light source, optical transducer, and tilting mirror mount are not limited to the specific devices mentioned in the preceding description. 
     A bandpass filter that passes the dither signal frequency, but attenuates a frequency twice as high as the dither signal frequency, can be used as the ac extractor  46 . This enables the threshold of the threshold comparator  50  to be reduced, because the frequency of the ac component of the output of the optical transducer  34  is equal to the dither signal frequency when the photosensitive optical fiber and ultraviolet beam are misaligned as in FIG. 8 or  10 , but is equal to twice the dither signal frequency when the alignment is correct as in FIG.  9 . 
     Those skilled in the art will recognize that further variations are possible within the scope claimed below.