Abstract:
There is provided an apparatus for fabricating an apodized fiber grating. In the apodized fiber grating fabricating apparatus, a UV (Ultraviolet) laser emits a UV laser beam, a beam splitter splits the UV laser beam emitted from the UV laser into two beams, a plurality of mirrors form light paths to concurrently project the split beams onto an optical fiber from two directions by reflecting the split beams, a phase mask passes the reflected beams therethrough in such a way to form gratings in the optical fiber in a predetermined period, a first blocking device is disposed between the phase mask and one of the mirrors, progressively blocks one of the two beams from being projected toward the optical fiber from one direction, and provides apodization to the formed gratings, and a second blocking device, which is mobile and opposite to the first blocking device with respect to the optical fiber, progressively blocks the other beam from being projected toward the optical fiber from another direction and provides apodization to the formed gratings, so that an average refractive index variation is constant across the whole gratings.

Description:
CLAIM OF PRIORITY 
     This application claims priority to an application entitled An Apparatus for Fabricating Apodized Fiber Grating filed in the Korean Industrial Property Office on Jul. 2, 1999 and assigned Serial No. 99-26671, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fiber gratings, and in particular, to an apparatus for fabricating apodized fiber gratings. 
     2. Description of the Related Art 
     As the data transmission capacity of a WDM (Wavelength Division Multiplexing) system increases, channel spacing gets narrower. Therefore, there is an increasing need for optical filters that have a narrow bandwidth and excellent adjacent channel isolation characteristics. 
     Fiber gratings satisfy the requirements of such optical filters, i.e., low loss, low polarization dependence, and high channel selectivity. Further, the cost effectiveness of the fiber gratings makes them popular as optical filters. 
     When a general fiber grating is fabricated in a conventional method using an excimer laser and a uniform phase mask, the refractive index of the fiber is constant over the length of the grating. In such a fiber, however, a sidelobe occurs and as a result, no apodization is achieved at the fiber grating. This sidelobe can be reduced by apodizing the fiber grating such that the magnitude of a refractive index variation is decreased toward the ends of the fiber grating. 
     An apodized fiber grating refers to a fiber grating of which the refractive index increases or decreases toward the center or both ends. The apodized fiber grating shows minimized sidelobes in both a short wavelength band and a long wavelength band. Although this apodization is effective in reducing a sidelobe in a longer wavelength band, it has limitations in reducing a sidelobe in a shorter wavelength band due to self-induced chirping of a fiber grating. 
     The self-induced chirping is attributed to an inconstant average refractive index of the fiber grating. Accordingly, the average refractive index should be made constant with respect to grating length in order to reduce a sidelobe which arises from the self-induced chirping. 
     Other conventional fiber grating apodizing methods besides the conventional method discussed above include overlap writing, use of a PZT (Piezo Transducer), optical scanning, and use of a spatial filter. The overlap writing method is called an interference method, in which apodization is achieved by writing gratings superimposed on other gratings of different periods and sizes in an optical fiber. 
     As another conventional apodization method, gratings are written on an optical fiber while a tensile force is applied to the optical fiber using a piezo transducer. During writing the gratings, the optical fiber or a phase mask is vibrated for a desired length in the length direction of the optical fiber by the use of the piezo transducer. 
     Thirdly, apodized gratings are written by scanning an optical fiber covered with a phase mask with UV (Ultraviolet) light lengthwise with different light intensities at different scanning rates. 
     A spatial filter operates based on light interference. In this method, the intensity of interference light passed through a diffraction slit exhibits a Gaussian distribution. A spatial filter with a different transmission is disposed before a phase mask along the length direction of an optical fiber and UV light is projected onto the phase mask. 
     Other examples of fiber gratings and manufacturing methods of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 5,712,715, to Erdogan et al., entitled OPTICAL TRANSMISSION SYSTEM WITH SPATIALLY-VARYING BRAGG REFLECTOR, describes a Bragg grating produced using two collimated non-collinear beams to form an interference pattern. 
     U.S. Pat. No. 5,717,799, to Robinson, entitled OPTICAL WAVEGUIDE FILTERS, describes a reflection pass-band filter with particular chirp and apodization profiles. The apodized grating is produced by varying the strength of the writing of the grating elements as a function of distance along the fiber. 
     U.S. Pat. No. 5,912,999, to Brennan III et al., entitled METHOD FOR FABRICATION OF IN-LINE OPTICAL WAVEGUIDE GRATING OF ANY LENGTH, describes a method and apparatus for writing apodized Bragg gratings. In this method, the intensity of the writing beam is varied to control the envelope of the refractive index profile to write an apodized grating. 
     U.S. Pat. No. 5,953,472, to Boschis et al., entitled METHOD OF AND A DEVICE FOR MAKING BRAGG GRATINGS IN OPTICAL FIBERS OR WAVEGUIDES, describes a method of making gratings in which the fiber is illuminated through a phase mask. A diaphragm is used to give the beam a Gaussian intensity distribution. 
     U.S. Pat. No. 6,035,083, to Brennan III et al. entitled METHOD FOR WRITING ARBITRARY INDEX PERTURBATIONS IN A WAVEGUIDING STRUCTURE, describes a method in which a waveguide is translated relative to a writing beam, and the writing beam is modulated as a function of time to write the grating. Apodization can be achieved by controlling the amplitude envelope of the writing beam modulation. 
     U.S. Pat. No. 6,043,497, to Quetel et al., entitled PHOTO-IMPRINTING STAND FOR THE MAKING OF BRAGG GRATINGS, describes a photo-imprinting stand which has a dynamic masking device with a variable surface. The dynamic device may have a rotating shutter with a particular profile, and masks UV rays during the photo-imprinting process. 
     The above conventional apodized fiber grating fabricating methods have the following problems: 
     (1) In overlap writing, a device for accurately controlling a length smaller than a grating period is required for appropriate overlapped writing, thereby making it complicated to fabricate fiber gratings; 
     (2) In the use of a piezo transducer, it is also difficult to control a length smaller than a grating period reliably; 
     (3) In optical scanning, the optical scanning rate and optical intensity must be controlled appropriately to obtain a desired apodized grating; and 
     (4) With use of a spatial filter, vibrations must be prevented since gratings are fabricated using interference patterns, and for this purpose an expensive device is required. 
     Especially, when a fiber grating is fabricated using a phase mask, the phase mask should be fabricated by focused ion beam implantation and wet etching to have an effective profile. A new phase mask is needed at every change in apodization conditions. Therefore, this method is not effective in terms of cost and flexibility. 
     Despite the advantage of production of gratings with various characteristics, the method of scanning an optical fiber lengthwise with UV light at a controlled light intensity has the distinctive shortcomings of long fabrication time and bad reproducibility. 
     Consequently, the method using an apodizing phase mask is not effective in terms of cost and flexibility since a phase mask is difficult to fabricate and a new phase mask is needed at every change in apodization conditions. Moreover, the beam scanning method has the disadvantages of difficult fabrication, long fabrication time, and bad reproducibility. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved method and apparatus for fabricating an apodized fiber grating. 
     A further object of the invention is to provide a method and apparatus for fabricating an apodized fiber grating with a short fabrication time. 
     A yet further object is to provide a method and apparatus for fabricating an apodized grating with high reproducibility. 
     A still further object of the invention is to provide a method and apparatus for fabricating an apodized grating which are less expensive. 
     Another object of the invention is to provide an apparatus which does not require expensive vibration damping equipment. 
     Yet another object of the invention is to provide an apparatus and method which do not require a separate phase mask for different apodization conditions. 
     Still another object of the present invention to provide an apparatus for fabricating an apodized fiber grating readily using a beam splitter and a screen mask. 
     Yet another object of the present invention to provide an apparatus for fabricating an apodized fiber grating with a uniform refractive index distribution in the length direction. 
     To achieve the above objects, in an apodized fiber grating fabricating apparatus according one aspect of the present invention, a UV (Ultraviolet) laser emits a UV laser beam, a beam splitter splits the UV laser beam emitted from the UV laser into two beams, a plurality of mirrors form light paths to concurrently project the split beams onto an optical fiber from two directions by reflecting the split beams, a phase mask passes the reflected beams therethrough in such a way to form gratings in the optical fiber in a predetermined period, a first blocking device is disposed between the phase mask and one of the mirrors, progressively blocks one of the two beams from being projected toward the optical fiber from one direction, and provides apodization to the formed gratings, and a second blocking device, which is mobile and opposite to the first blocking device with respect to the optical fiber, progressively blocks the other beam from being projected toward the optical fiber from another direction and provides apodization to the formed gratings, so that an average refractive index variation is constant across the whole gratings. 
     In an apodized fiber grating fabricating apparatus according to another aspect of the present invention, a first UV laser emits a first UV laser beam toward an optical fiber from one direction and a second UV laser emits a second UV laser beam toward the optical fiber from an opposite direction. A phase mask forms gratings in the optical fiber in a predetermined period by reinforcement and interference of the first UV laser beam. A first blocking device, disposed between the first UV laser and the phase mask, progressively blocks one of the beams from being projected to the optical fiber and thus provides apodization to the formed gratings, and a second blocking device opposite to the first blocking device with respect to the optical fiber, progressively blocks the other beam from being projected to the optical fiber and provides apodization to the formed gratings, so that an average refractive index variation is constant across the whole gratings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a graph showing the variation of a refractive index in the length direction of a general fiber grating fabricated using a uniform phase mask in a conventional method; 
     FIG. 2 is a graph showing the variation of a refractive index in the length direction of an apodized fiber grating fabricated in another conventional method; 
     FIG. 3 is a schematic view of an apodized fiber grating fabricating apparatus according to a preferred embodiment of the present invention; 
     FIG. 4 is a schematic view of an apodized fiber grating fabricating apparatus according to another preferred embodiment of the present invention; 
     FIG. 5A illustrates the operation of a first screen mask when t=0; 
     FIG. 5B illustrates the operation of a second screen mask when t=0; 
     FIG. 6A illustrates the operation of the first screen mask when t=t 1 ; 
     FIG. 6B illustrates the operation of the second screen mask when t=t 1 ; 
     FIG. 7A illustrates the operation of the first screen mask when t=t 2 ; 
     FIG. 7B illustrates the operation of the second screen mask when t=t 2 ; 
     FIG. 8 is a graph showing the variation of a refractive index in the length direction of an apodized fiber grating fabricated using the first screen mask according to the present invention; 
     FIG. 9 is a graph showing the variation of refractive index in the length direction of an apodized fiber grating fabricated using the second screen mask according to the present invention; 
     FIG. 10 is a graph showing the variation of a refractive index in the length direction of an apodized fiber grating fabricated by concurrently shifting the first and the second screen masks according to the present invention; and 
     FIG. 11 illustrates the reflective spectrums of fiber gratings with respect to the refractive index variations shown in FIGS. 1,  2 , and  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, when a general fiber grating is fabricated in a conventional method using an excimer laser and a uniform phase mask, as discussed above, the refractive index varies on the whole as shown in FIG.  1 . As indicated by a one-dot-dashed line ( 1 ) in FIG. 11, a sidelobe occurs and as a result, no apodization is achieved at the fiber grating. 
     This sidelobe can be reduced by apodizing the fiber grating such that the magnitude of a refractive index variation decreases toward the ends of the fiber grating. Another one-dot-dashed line ( 2 ) in FIG. 11 indicates the variation in the refractive index of an apodized fiber grating. The self-induced chirping of such an apodized grating, as discussed above, is attributed to an inconstant average refractive index of the fiber grating as shown in FIG.  2 . 
     Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail where they would obscure the invention in unnecessary detail. 
     FIG. 3 schematically illustrates the configuration of an apodized fiber grating fabricating apparatus according to a preferred embodiment of the present invention. In FIG. 3, the apodized optical fiber grating fabricating apparatus includes a UV laser  2  as a light source, a beam splitter  10  for splitting a beam  8  emitted from the UV laser  2 , a plurality of mirrors  20 ,  22 , and  24  for controlling the direction of beams projected from the beam splitter  10 , first and second mobile screen masks  30  and  50  to be used for apodizing a grating when the beams reflected from the mirrors  20 ,  22 , and  24  are projected onto an optical fiber  60 , and a phase mask  40  for generating the grating by interference and reinforcement of the projected light. 
     The beam splitter  10  splits the beam  8  emitted from the UV laser  2  into two beams  12  and  14 . One  12  of the beams is reflected from the mirrors  20  and  22  and reaches the first screen mask  30  and the other beam  14  is reflected from the mirror  24  and reaches the second screen mask  50 . The two beams  12  and  14  travel mutually orthogonally from the beam splitter  10 . The beam  12  is sequentially reflected from the first and second mirrors  20  and  22  and impinges on the phase mask  40 . The first mask  30  and second mask  50  are disposed face to face at both sides of the optical fiber  60 . The phase mask  40  is interposed between the first screen mask  30  and the optical fiber  60 . 
     The beam  12  is projected onto the phase mask  40  and forms a plurality of gratings in the optical fiber  60  in a predetermined period by reinforcement and interference of light as the beam  12  passes through the phase mask  40 . The beam  14  is reflected from the third mirror  24  and reaches the second screen mask  50 . Consequently, the beam  8  emitted from the UV laser  2  is concurrently projected onto the first and second screen masks  30  and  50  through the beam splitter  10  and the mirrors  20 ,  22 , and  24 . The first and second screen masks  30  and  50  are mobile during the writing process for apodization of the gratings according to the present invention. 
     FIG. 4 is a schematic view of an apodized fiber grating fabricating apparatus according to another preferred embodiment of the present invention. While the apodized fiber grating fabricating apparatus of FIG. 3 projects a beam from a UV laser onto an optical fiber in two directions using a beam splitter and a plurality of mirrors, the one shown in FIG. 4 projects a beam onto an optical fiber in two directions using two UV lasers. 
     Referring to FIG. 4, the apodized fiber grating fabricating apparatus according to the second embodiment of the present invention includes a first UV laser  4  disposed at one side of the optical fiber  60 , a second UV laser  6  disposed at the other side of the optical fiber  60 , the phase mask  40  through which a beam  16  emitted from the first UV laser  4  passes to write a grating in the optical fiber  60  utilizing light reinforcement and interference, the first screen mask  30  over the phase mask  40  to apodize the fiber grating, and the second screen mask  50  between the second UV laser  6  and the optical fiber  60  to apodize the optical fiber grating. The first and second screen masks  30  and  50  face each other a predetermined distance apart with the interposition of the optical fiber  60 . Also, the first and second screen masks  30  and  50  are mobile for apodization of the written grating. The beam  16  emitted from the first UV laser  4  is projected onto the phase mask  40  and forms gratings in the optical fiber  60  in a predetermined period by light reinforcement and interference as it passes through the phase mask  40 . The first and second screen masks  30  and  50  apodize the fabricated gratings as they are approaching each other. 
     The configurations and operations of the first and second screen masks  30  and  50  will be described hereinbelow. It is first to be noted that the first and second screen masks  30  and  50  act to block the light beams and may be formed of any material that is opaque and can block the travel of the beams. 
     According to the present invention, beams  16  and  18  induced through a plurality of mirrors impinge on the optical fiber  60  from two directions. As the beams  16  and  18  pass through the phase mask  40 , they form gratings in the optical fiber  60 . Then, the beams  16  and  18  apodize the gratings through the first and second screen masks  30  and  50  while the first and second screen masks  30  and  50  transfer beams with a Gaussian profile to the phase mask  40 , moving over a predetermined time. 
     Referring to FIGS. 5A to  7 B, the configuration and operation of the mobile first and second screen masks  30  and  50  will be described in detail. FIGS. 5A and 5B illustrate the operations of the first and second screen masks  30  and  50 , respectively when time t=0. FIGS. 6A and 6B illustrate the operations of the first and second screen masks  30  and  50 , respectively when t=t 1 . FIGS. 7A and 7B illustrate the operations of the first and second screen masks  30  and  50 , respectively when t=t 2 . 
     In the drawings, the X-axis represents the movement direction of the first and second screen masks  30  and  50  and the Z-axis is the length direction of the optical fiber  60 . Reference symbol_also indicates the movement direction of the first and second screen masks  30  and  50 . Reference character L indicates the length of the optical fiber  60  for which gratings are written, reference character B indicates the mid point of L, and reference characters A and C indicate both ends of L. 
     The first screen mask  30  includes a concave portion  30   a  at the center and convex portions  30   b  that are protruded gradually from the concave portion  30   a  towards both ends of the first screen mask  30 . The second screen mask  50  includes a convex portion  50   a  at the center and concave portions  50   b  that are sunken gradually from the center towards both ends of the second screen mask  50 . The illustrated shapes of the light-blocking edges of screen masks  30  and  50  are essentially complementary to each other, but they need not be. The configurations of the first and second screen masks  30  and  50  in FIGS. 5A to  7 B are merely exemplary applications, and it is clear that many variations can be made as far as they are used for apodization of written gratings. For example, the first and second screen masks  30  and  50  can be shaped into steps. 
     When t=0, a first laser beam emitted from the first UV laser is all projected onto the phase mask  40  and forms a grating in the optical fiber  60  in a predetermined period through the phase mask  40 , as shown in FIG.  5 A. 
     Referring to FIG. 6A, when t=t 1 , i.e., the first screen mask  30  approaches the optical fiber  60  at a predetermined speed, the first screen mask  30  moves in a direction as indicated by_until it covers portions A and C of the optical fiber  60 . Therefore, the portions A and C are excluded from irradiation of the first UV laser beam emitted from the first UV laser, whereas a portion B of the optical fiber  60  is irradiated with the first UV laser beam for a predetermined time. 
     Referring to FIG. 7A, when t=t 2 , the first screen mask  30  further moves in the direction as indicated by the arrow until it covers all the portions A, B, and C. Hence, the first UV laser beam reaches any of the portions A, B, and C of the optical fiber  60  no longer. 
     As stated above, the intensity of the first laser beam is controlled by progressively covering the phase mask  40  with the first screen mask  30 . As a result, a refractive index varies as shown in FIG.  8 . FIG. 8 is a graph showing a variation in the refractive index with respect to the length direction of an optical fiber when gratings are formed in the optical fiber using the first screen mask. An X axis represents the variation of the refractive index and a Z axis, the length direction of the optical fiber. 
     An apodized fiber grating experiences a greater variation in refractive index as it is nearer to the portion B of the optical fiber. On the contrary, the refractive index is less changed as an apodized grating is formed nearer to the portion A or C. As for the second screen mask  50 , a second UV laser beam is projected onto the optical fiber  60  with a predetermined width when t=0 as shown in FIG.  5 B. 
     Referring to FIG. 6B, when t=t 1 , the second screen mask  50  moves in a direction as indicated by the arrow until the portion B of the optical fiber is gradually covered and excluded from irradiation of the second UV laser beam. Meanwhile, the second UV laser beam is projected onto the portions A and C of the optical fiber  60  for a predetermined time. 
     Referring to FIG. 7B, when t=t 2 , the second screen mask  50  further moves in the direction as indicated by_until it covers all the portions A, B, and C and thus the second UV laser beam no longer reaches any of the portions A, B, and C. 
     As stated above, the intensity of the second laser beam is controlled by progressively covering the phase mask  40  with the second screen mask  50 . As a result, the refractive index varies as shown in FIG.  9 . FIG. 9 is a graph showing a variations in the refractive index with respect to the length direction of an optical fiber when gratings are formed in the optical fiber using the second screen mask. The X-axis represents the variation of the refractive index and the Z-axis represents the length direction of the optical fiber. 
     An apodized fiber grating experiences a greater variation in refractive index as it is nearer to the portion A or C of the optical fiber. On the contrary, the refractive index is less changed as an apodized grating is formed nearer to the portion B. 
     FIG. 10 is a graph showing a variations in refractive index in the length direction of a fiber grating when the grating is written in an optical fiber by projecting a UV laser beam onto the optical fiber from two directions, moving the first and second screen masks  30  and  50  concurrently. 
     Apodization is achieved in the grating by setting an average refractive index to be constant along the length of the fiber grating as shown in FIG. 10, and sidelobes in short and long wavelength bands are minimized as shown in FIG.  11 . That is, the total exposure to the first and second beams is the same at any point along the length of the fiber grating. 
     The characteristics of an apodized fiber grating according to the present invention will be described referring to FIG.  11 . In FIG. 11, the one-dot-dashed line ( 1 ) indicates the reflective spectrum of a general optical fiber fabricated using a uniform mask as shown in FIG.  1 . 
     Another one-dot-dashed line ( 2 ) indicates the reflective spectrum of an apodized fiber grating of which the refractive index varies as shown in FIG.  2 . As noted from (2), it is difficult to minimize a sidelobe in a short wavelength band. 
     A solid line ( 3 ) indicates the reflective spectrum of an apodized fiber grating at wavelengths according to the present invention. Sidelobes in short and long wavelength bands are reduced. 
     As described above, an apodized fiber grating fabricating apparatus of the present invention forms apodized fiber gratings readily using a beam splitter and screen masks in addition to an existing optical equipment and phase mask, instead of separately procuring a new apodizing phase mask at every change in apodization conditions, or the beam scanning method having the disadvantages of difficult fabrication, long fabrication time, and bad reproducibility. Further, an apodized fiber grating can be written easily by the use of two UV lasers. 
     The present invention has been described in the context with an apodized fiber grating fabricating apparatus having two screen masks as beam blocking means, but the number of the screen masks is not limited so long as an average refractive index variation is constant over the entire apodized fiber gratings. 
     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
     As described above, an apodized fiber grating fabricating apparatus of the present invention forms apodized fiber gratings readily using a beam splitter and screen masks in addition to an existing optical equipment and phase mask, instead of separately procuring a new apodizing phase mask at every change in apodization conditions, or the beam scanning method having the disadvantages of difficult fabrication, long fabrication time, and bad reproducibility.