Abstract:
A method and apparatus for manufacturing fiber-optic devices to precise CWL specifications by plasticizing one attachment end of a mounted fiber-optic device and finely adjusting the fiber tension while monitoring the CWL. In one embodiment, the method of manufacturing an optical fiber component to a precise CWL includes the step of affixing the ends of a fiber component under tension to a substrate to approximate a desired CWL. Subsequently, one end of the fiber is gripped with a movable clamp and the adjacent attachment plasticized while the tension on the fiber is adjusted until the CWL is within a desired range. The attachment is rehardened while the tension on the fiber is maintained by the movable clamp.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to relates generally to the manufacturing of optical fiber components and particularly to a method and apparatus for precisely controlling the optical path length of an optical fiber component. 
     2. Technical Background 
     Optical fiber based devices are widely utilized as components for optical communications due to their relatively low insertion loss and low cost. Foremost of optical fiber components are fiber Bragg gratings (FBG) which are typically made by ultraviolet (UV) wavelength energy exposure. Once an FBG is mounted to a substrate and annealed, it is no longer photosensitive and cannot be further tuned. Thus, it is necessary to empirically predict the final frequency of such a grating which can lead to a significant error resulting in gratings which are not within specifications. Due to the uncertainty of the wavelength shift resulting from the attachment process and annealing, the center wavelength (CWL) of a packaged fiber Bragg grating can vary as much as +/−60 picometers from the desired CWL. Such a wavelength error combined with a wavelength drift of, for example, distributed feedback lasers, which may be from +/−50 picometers, and the residual temperature dependence of +/−20 picometers imposes a highly stringent requirement on the design of, for example, 50 GHz fiber Bragg gratings. 
     A typical attachment process for a fiber Bragg grating is to bond one end of the fiber to a substrate, tension the fiber by an empirically determined amount, and bonding the opposite end of the fiber. FIG. 1 shows the distribution of the CWL for samples manufactured by this process. Since the total available margin is only +/−40 picometers, only a fraction (20% to 30%) of the gratings can be employed. 
     Precise control of optically tuned fiber-optic devices with a CWL within less than +/−15 picometers is desired to minimize cross-talk between adjacent communication channels of a system. In order to maintain the CWL of a tuned fiber-optic device, such as a fiber Bragg grating, a β-eucryptite substrate has been employed having a coefficient of temperature expansion of −7.5 ppm/° C. to compensate for the refractive index change of the fiber with temperature variations. With such substrates, the CWL shift due to temperature changes over a range of from  0 ° C. to 70° C. has been reduced to +/−15 pm. Thus, although the substrate selection has improved the stability of the device once manufactured, there remains a need to manufacture devices such as fiber Bragg gratings or other optically tuned components to a CWL that produces a yield rate for precise CWL devices higher than that previously available with existing manufacturing techniques. 
     It has been discovered that the variability of the CWL of fiber-optic devices is not a function of the laser power employed in the manufacturing of the devices nor is it a result of the substrate material. Instead, it appears that the variability is inherent in the attachment process and there remains a need, therefore, for a process and system for manufacturing precisely tuned fiber-optic devices. 
     SUMMARY OF THE INVENTION 
     The method and apparatus of the present invention achieves the manufacturing of fiber-optic devices to precise CWL specifications by plasticizing one attachment end of a mounted fiber-optic device and finely adjusting the fiber tension while monitoring the CWL. 
     A method of manufacturing an optical fiber component to a precise CWL includes the step of affixing the ends of a fiber component under tension to a substrate to approximate a desired CWL. Subsequently, one end of the fiber is gripped with a movable clamp and the adjacent attachment plasticized while the tension on the fiber is adjusted until the CWL is within a desired range. The attachment is rehardened while the tension on the fiber is maintained by the movable clamp. 
     In one embodiment, one end of a fiber is bonded to a negative coefficient of expansion substrate using a glass frit and the opposite end gripped under tension by a movable clamp while the CWL is monitored. The fiber tension is adjusted by moving the clamp until the CWL is about 0.35 nm below the target CWL for 70 mm substrates. Next, a second frit bonds the fiber to the substrate at an opposite end adjacent the movable clamp. The CWL is again checked and, if off more than 10 picometers, the fiber is retensioned by moving the clamp to the previous position, reheating and plasticizing the second frit, and adjusting the clamp while monitoring the CWL until the CWL change equals an amount corresponding to the difference between the first measured CWL and the target CWL. Once cooled, the clamp is released and the final CWL is measured and recorded. 
     The result of this process produces precisely tuned fiber-optic devices, such as a fiber Bragg grating. Although particularly suitable for manufacturing fiber Bragg gratings, other tunable fiber-optic components likewise can be manufactured using such technique, to precisely control the optical path length of the optical fiber device during manufacturing. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings. 
     It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a chart illustrating the variations in CWL of fiber Bragg gratings manufactured by conventional techniques; 
     FIG. 2 is an enlarged schematic, side-elevational view of a fiber Bragg grating, shown partially packaged and manufactured according to the present invention; 
     FIG. 3 is a schematic, side-elevational view of a fiber Bragg grating and an apparatus for manufacturing the same according to the present invention during a first step in the manufacturing process; 
     FIG. 4 is a schematic, side-elevational view of the next step in the manufacturing process of the fiber Bragg grating shown in FIG. 2; 
     FIG. 5 is a schematic, side-elevational view of a successive step in the manufacturing process of the fiber Bragg grating shown in FIG. 2; and 
     FIG. 6 is a chart illustrating the high yield of precise CWL fiber Bragg gratings resulting from the manufacturing process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The chart of FIG. 1 illustrates the CWL shift between predicted CWL and actual CWL for 40 samples made using existing techniques which, as seen, vary between approximately 70 nm below the desired CWL (shown by the dotted line in the chart) to about 75 nm above the desired CWL. The number of samples falling within +/−40 picometers of the desired CWL is relatively small, representing from 20% to 30% of the total number of fiber Bragg gratings made by the conventional empirical process. 
     In order to improve the yield rate to that shown in FIG. 6, the fiber Bragg grating illustrated in FIG. 2 is manufactured according to the process and apparatus described in connection with FIGS. 3-5. As seen in FIG. 6, a sample of approximately 70 fiber Bragg gratings manufactured according to the present invention yields 80% of gratings falling within +/−10 picometers of the desired CWL. This tolerance band of precisely manufactured gratings is shown between the dotted lines in FIG.  6 . Even gratings falling outside the precisely controlled band for the most part fell well within an acceptable range of +/−40 picometers for most applications. Thus, while only very few conventionally manufactured fiber Bragg gratings fall within a +/−10 picometers criteria, more than 80% of the gratings manufactured according to the process of the present invention fell within such tolerance range. The dramatic results achieved by the process and apparatus of the present invention and the resulting fiber-optic component is now described in the context of a fiber Bragg grating with reference to FIGS. 2-5. 
     A partially packaged fiber Bragg grating assembly  10  is shown in FIG.  2  and includes an optical fiber  12  in which there is imprinted a grating  14  in a central area with a precisely controlled CWL of, for example, 1559.25 nm. The optical fiber  12  is supported on a negative coefficient of expansion substrate  16 , such as β-eucryptite, by a pair of spaced glass frits  18  and  20 . between the glass frits  18  and  20 , there exists fiber end zones  22  and  24  spanning the grating  14 . Fiber  12  is mounted to an upper surface  15  of the substrate  16  according to the manufacturing process of the present invention, with grating  14  being conventionally formed utilizing ultraviolet light to selectively change the index of refraction of the core of the fiber  12  in a pattern selected for a wavelength of approximately 1559.25 nm in the example. Other frequency fiber Bragg gratings or other tuned optical devices can be manufactured with the method of the present invention. 
     In FIG. 3, there is shown the apparatus for manufacturing the fiber Bragg grating  10  according to the present invention. The apparatus includes a holder  30  for the substrate, which holder generally comprises an open rectangular frame into which the substrate  16  is positioned for holding it in place without adding any significant lateral forces to the substrate during the manufacturing process. The holder  30  rests upon a reference surface  32  during the manufacturing process. At one end (shown on the left in FIG. 3) is a fixed clamp  34  holding one end  19  of the fiber  12  in position with respect to the substrate  16 . At an opposite end  21  of the fiber  12 , there is positioned a movable clamp  36  comprising a clamp mounted on a moveable stage with micrometer adjustment capabilities. Coupled to the left end of the fiber  12 , as seen in FIG. 3, is a broad band optical source  40  while coupled to the opposite end of the optical fiber  12  is an optical spectral analyzer  42  to allow the CWL of the grating to be monitored during the manufacturing process. 
     In one example for a 0.125 nm fiber during the first step of manufacturing, a 5 gram load is placed on the fiber. Next, a frit paste  18  is applied to the area adjacent end zone  22  of fiber  12  and a beam from a C 0   2  laser  50  is directed to the undersurface  17  of substrate  16  directly under frit paste  18 , heating the undersurface of the substrate to a temperature of between 800° C. to 900° C. and the upper surface  15  onto which the frit past has been applied to approximately 500° C. to 600° C., fusing the frit to the left end of fiber  12  and to the substrate. The substrate is then allowed to cool for approximately five minutes and the CWL is measured utilizing the optical spectral analyzer  42 . 
     Next, the stage for movable clamp  36  is moved to the right in the direction shown by arrow A in FIG. 4 while watching the CWL from analyzer  42  until the CWL is at a predetermined relationship to the desired final CWL. For a negative temperature coefficient 70 mm substrate, such as β-eucryptite employed in the present invention, typically the strain placed upon the fiber  12  by movable clamp  36  places the CWL below the desired CWL to account for substrate expansion during the cooling of the substrate. In this example, the selected CWL is 0.35 nm below the target wavelength. 
     A second bead  20  of glass frit paste is applied to the right end of the fiber  12 , as shown in FIG. 4, and the CO 2 , laser  50  is positioned under the frit  20  to again heat the under surface of substrate  16  to from 800° C. to 900° C. and the top surface to from 500° C. to 600° C. for fusing the glass frit and anchoring end  24  of fiber  12  to the substrate  16 . During the fusing process, the CWL is measured twenty seconds after the top hold segment (the twenty second CWL value) while the clamp  36  is held in position. Substrate  16  is then allowed to cool for approximately five minutes, after which the movable clamp releases the tension on the end  21  of fiber  12 , and, again, the CWL is tested. Typically, the CWL will at this stage be higher or lower than the target CWL by an amount greater than 10 picometers. For precise fiber Bragg gratings of the type which can be commercially manufactured utilizing the present invention, 10 picometers above or below the CWL is the tolerance limit desired for such precise gratings. 
     With clamp  34  continuing to maintain the left end of the fiber anchored together with frit  18 , movable clamp  36  is moved to the same position as before the load was released to retension the fiber while the second frit  20  is reheated by laser  50 . When the heating has reached the top hold segment with the upper surface of the substrate  16  at about 500° C. to 600° C., depending upon the direction of the measured CWL, the movable clamp  36  is either moved in a direction indicated by arrow A to the right in FIG. 5 to increase the tension and thereby increase the CWL or to the left as indicated by arrow B in FIG. 5 to decrease the tension in the fiber  12 , thereby decreasing the CWL. If, for example, the CWL is 40 picometers below the target CWL after the first attachment, upon reheating frit  20 , the movable clamp is moved in a direction indicated by arrow A while monitoring the CWL through analyzer  42  until the twenty second CWL value is increased by 40 picometers. 
     Movement of movable clamp  36  during the fine tuning of the grating, while observing the CWL, allows real time control of the manufacturing of the grating  10 , and, when at the desired changed twenty second CWL, the clamp  36  holds the fiber  12  stationary for a five minute cooling period of the substrate  16 . The final CWL is then recorded and typically falls well within the +/−10 picometers criteria, with over 80% of samples tested, as shown by FIG. 6, falling within such range. A strain relief epoxy is placed over the ends of fiber  12  adjacent the outside edges of frits  18  and  20  on the substrate, and the substrate removed from holder  30 . Subsequently, the partially assembled grating is finally packaged in a conventional process to complete the manufacture of the optical fiber device. 
     Although the preferred embodiment of the invention is employed to manufacture a fiber Bragg grating utilizing glass frits for anchoring the gratings to a negative coefficient substrate, other material for anchoring a glass fiber to other substrates can also be employed. Thus, any anchoring material which can be plasticized after an initial estimated manufacturing CWL has been tested can be employed for manufacturing an optical device, such as a fiber Bragg grating, employing the technique of the present invention. Further, the method and apparatus of the present invention can be employed to manufacture any fiber-optic device requiring precise tuning. 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.