Abstract:
Passive temperature compensated packages for short-period fiber gratings and other optical components and techniques for forming the packages are described. In one aspect, a hollow tube having a negative coefficient of thermal expansion (CTE) encased in a cylindrical body is employed to form an athermalized cylindrical package. The hollow tube may also include slots for writing a grating onto an optical fiber disposed within the tube. In another aspect, end caps may be disposed on opposite ends of the cylindrical body.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to packaging of fiber optic components. More specifically, the present invention relates to methods and apparatus for packaging fiber gratings, filters, and other fiber optic components to provide a variety of improved features, such as athermalization, support, or protection. 
     BACKGROUND OF THE INVENTION 
     A periodic variation in refractive index of the waveguide along the long axis of the waveguide is commonly known as an optical waveguide grating. A fiber Bragg grating is an optical waveguide grating in a waveguide fiber which will selectively filter propagated light having a wavelength which is twice the period of the grating. Such a fiber Bragg grating is useful as a wavelength filter. 
     Fiber Bragg gratings are particularly sensitive to temperature changes which, through thermal expansion of the waveguide fiber, cause changes in the refractive index of the waveguide fiber. Changes in grating spacing and changes in the refractive index with temperature variations cause wavelength shifts in the device. 
     For many applications, fiber gratings must operate over large temperature ranges with minimal change in spectral properties. While the peak loss of the grating will change with temperature, the primary effect of a temperature change is a shift in peak wavelength. This temperature dependence can be compensated for by attaching the fiber grating to a substrate with a negative coefficient of thermal expansion. In one approach, fiber gratings are athermalized, or temperature compensated, by attaching them to a small bar of β eucryptite, a ceramic substrate with a negative coefficient of thermal expansion (CTE). A frit of at least two compositions attaches the optical fiber to the substrate and an epoxy deposit provides strain relief. The fiber grating attached to the substrate is then typically embedded in a protective fluorogel coating and enclosed in an hermetically sealed metal box to provide protection from the effects of humidity. This design depends upon integral bonding of the frit to a flat surface with a mismatched CTE. Stresses are created at the interface between the flat surface and the frit. This design is asymmetrical, leading to asymmetric forces acting on the optical fiber during thermal cycling. The manufacture of this package involves a large number of process steps and involves a labor intensive process. 
     Accordingly, it would be highly advantageous to provide a passive temperature compensating package assembly for fiber gratings which provides symmetrical packaging, ease of manufacturing, increased reliability, or a single frit composition. 
     SUMMARY OF THE INVENTION 
     The present invention provides advantageous methods and apparatus for packaging fiber gratings and other fiber optic components to provide a variety of improved features, such as athermalization, support, or protection. According to one aspect of the invention, a hollow tube having a negative CTE is employed to form an athermalized hollow tube package. The hollow tube surrounds an optical fiber containing a fiber grating, and is contained within a cylindrical body with end caps. 
     According to another aspect of the invention, a slotted hollow tube having a negative CTE with a longitudinal slot is employed to form an athermalized slotted hollow tube package. The slotted hollow tube surrounds an optical fiber containing a fiber grating, and is contained within a cylindrical body with end caps. The slotted hollow tube allows the fiber grating to be written in the optical fiber after the optical fiber is placed within the slotted hollow tube. 
     According to another aspect of the invention, the end caps fit within the ends of the cylindrical body. 
     A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a cylindrical substrate in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of an athermalized cylindrical package in accordance with the present invention; 
     FIG. 3 is an isometric view of the athermalized hollow tube package of FIG. 2; 
     FIG. 4 is a flowchart of a method of forming the athermalized hollow tube package of FIG. 2 in accordance with the present invention; 
     FIG. 5 is an exploded isometric view of the athermalized hollow tube package of FIG. 2 with foam positioning plugs; 
     FIGS. 6A and 6B show an end view and a side view, respectively, of a slotted substrate in accordance with the present invention; 
     FIG. 7 is a cross-sectional view of an end cap with a plug in accordance with the present invention; and 
     FIG. 8 is a cross-section view of an athermalized swaged cylindrical package in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention now will be described more fully with reference to the accompanying drawings, in which several currently preferred embodiments of the invention are shown. However, this invention may be embodied in various forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these representative embodiments are described in detail so that this disclosure will be thorough and complete, and will fully convey the scope, structure, operation, functionality, and potential of applicability of the invention to those skilled in the art. 
     As described in detail below, the present invention provides advantageous methods and apparatus for the cylindrical packaging of fiber gratings written into an optical fiber. Improved athermalization, support, or protection are provided as described below. In the embodiments of present invention described below, one or more seals are utilized with the packages of the present invention. According to one aspect of the present invention, the seals may include SnO—ZnO—P 2 O 5  or PbO—SnO—P 2 O 5  glass with pyrophosphate filler as described in U.S. Pat. No. 5,721,802 entitled “Optical Device and Fusion Seal” which is incorporated by reference herein in its entirety. According to another aspect of the present invention, the seals may be frits such as copper glass. According to another aspect of the present invention, the seals may be formed of amalgam or epoxy. 
     Additionally, in the embodiments of the present invention described below, one or more plugs are utilized with the packages of the present invention. According to one aspect of the present invention, the plugs may be fabricated of lead-tin solder or indium-tin solder. According to another aspect of the present invention, the plugs may include epoxy. Suitable epoxies are described in greater detail in U.S. Pat. No. 5,552,092 entitled “Waveguide Coupler” which is incorporated by reference herein in its entirety. According to another aspect of the invention, the plugs are composed of a low water-permeability polymer or amalgum. 
     Referring to the drawings, FIG. 1 shows a cross-sectional view of a cylindrical substrate  10  in accordance with the present invention. The substrate  10  is composed of β eucryptite, zirconium tungstate, phosphotungstate or some other suitable material with a negative coefficient of thermal expansion (CTE). Other suitable materials are disclosed in U.S. patent application Ser. No. 09/305,763 filed May 5, 1999 entitled “Negative Thermal Expansion Materials Including Methods of Preparation and Uses Therefor” which is incorporated by reference herein in its entirety. The substrate  10  includes a central bore  12  and a pair of seal sockets  14 ,  15  disposed at the ends of the central bore  12 . The seal sockets  14 ,  15  may be conical, cylindrical or other suitable shape. A mid-span perimeter groove  16  encircles the substrate  10 . 
     FIG. 2 shows an athermalized cylindrical package  20  in accordance with the present invention. The cylindrical substrate  10  partially encloses an optical fiber  22  which includes a coating  24  which has been stripped from portions of the optical fiber  22 . The optical fiber  22  has written into it a short-period grating  26  along a portion of a center length of the optical fiber which has been stripped of the coating  24 . Two seals  28 ,  29  disposed one in each seal socket  14 ,  15  of the cylindrical substrate  10  tensionally maintain and support the region of the optical fiber  22  containing the fiber grating  26 . The seals  28 ,  29  may include SnO—ZnO—P 2 O 5  glass with pyrophosphate filler or other suitable material, as described above. The substrate  10  is enclosed by a cylindrical body  30  and two end caps  32 ,  33 . The body  30  and end caps  32 ,  33  may include metal (such as nickel, stainless steel or Kovar®) glass, ceramic, polymer, or some other suitable material. The optical fiber  22  extends through each end cap  32 ,  33  and is held and sealed by plugs  34 ,  35 . The plugs  34 ,  35  are composed of solder or other suitable material as described above. The cylindrical body  30  has been crimped along the mid-span perimeter groove  16  of the substrate  10 . By allowing for central attachment to the cylindrical body  30 , the mid-span perimeter groove  16  allows for symmetrical substrate shrinkage and growth in both directions with changes in temperature. 
     As best seen in FIG. 3, end caps  32 ,  33  include plug access holes  36 ,  37  to facilitate placement of the plugs  34 ,  35 . To provide for attachment to a mounting surface, a clip base  38  is attached to the body  30 . 
     In order to compensate for the temperatures changes that the cylindrical package  20  undergoes during testing and product life, the cylindrical substrate  10  has a negative CTE, such as −80×10 −7  per ° C. Thus, while the optical fiber  22  has a CTE of approximately 7×10 −7  per ° C., the negative CTE of the cylindrical substrate athermalizes the fiber grating  26 , providing passive temperature compensation. Additionally, the cylindrical substrate  10  protects the fiber grating  26  from external perturbations (such as mechanical stress) and environmental conditions (such as moisture). While presently preferred materials are disclosed herein, one skilled in the art would appreciate that the cylindrical package  20  of the present invention may include a variety of materials and sizes, and should not be construed as limited to the embodiments shown and described herein which are exemplary. 
     FIG. 4 shows a method  50  of forming a cylindrical package (such as the cylindrical package  20 ). In a first step  52 , seal sockets (such as the seal sockets  14 ) are formed in a cylindrical substrate (such as the cylindrical substrate  10 ). In a preferred embodiment, the seal sockets are formed by machining or grinding. In an alternative embodiment, the cylindrical substrate is mounted in a vertical orientation and nitrogen triflouride (NF 3 ) gas is forced through a center bore of the cylindrical substrate. The cylindrical substrate is then rotated, and an angled oxygen and gas torch burns the NF 3 , forming the seal socket. The oxygen and hydrogen gas torch is mounted at a 45° angle with respect to an outer surface of the cylindrical substrate. 
     In a placement step  54 , a waveguide such as, for example, an optical fiber, containing a fiber grating is placed within the cylindrical substrate. During insertion, the coating of the optical fiber acts as a guide for the uncoated section of optical fiber containing the grating, preventing the uncoated section from contacting the inner wall of the cylindrical substrate. Additionally, the conical seal socket guides the coated fiber into the substrate. 
     Next, in a tensioning step  56 , the optical fiber is tensioned by a 5 gram weight. A vacuum is applied to the enter bore of the cylindrical substrate in step  58  with maximum vacuum of about 25 inches of H 2 O. Alternatively, a dry inert gas, such as N 2 , is applied to the center bore. In a next fusing step  60 , a seal (such as the seal  20  described above) is fused to each seal socket by a laser system, ring burner or other focused heating system. Due to the heat sensitivity of the grating, the tube should be of sufficient length to assure that the grating is not affected by heat from the heating system. In step  62 , the vacuum and tension are removed. 
     In a step  64 , the mid-span perimeter groove is filled with epoxy. Next, in an insertion step  66 , the cylindrical substrate is inserted into a body (such as the body  30 ). In a crimping step  68 , the body is crimped into the epoxy along the mid-span perimeter groove. The epoxy is then cured. 
     In an assembly step  70 , end caps (such as the end caps  32 ,  33 ) are placed on the ends of the body. The end caps are then welded or mechanically crimped to the package body. In a n alternative embodiment, the end caps are attached to the body by epoxy, solder, or another suitable material. Alternatively, the end caps may be shrink fit attached to the body by heating the end caps, placing the heated end caps on the body, and allowing the end caps to cool and contract. Alternatively, the end caps may be formed by swaging the ends of the body. In a step  71 , a vacuum or a dry inert gas, such as N 2 , is applied to the body. In a tacking step  72  the ends of the end cap are tacked with plugs (such as the epoxy plugs  34 ,  35  described above) to provide stress relief and hermiticity for the optical fiber. Each epoxy plug is applied manually into a plug access hole (such as the plug access hole  36 ,  37 ) with a small syringe and is then thermally cured. Nominal post cure time is approximately 1.5 hr. at 125° C., or approximately 16 hr. at 90° C. In step  74 , a clip base (such as clip base  38 ) is clipped to the body. In an alternative embodiment, the base can be attached to the body by welding. 
     According to another aspect of the present invention, as shown in FIG. 5, foam positioning plugs  80  may be utilized to provide support for the optical fiber  22  within the end caps  32 , 33 . 
     Another embodiment of the present invention is shown in FIGS. 6A and 6B which depict a slotted cylindrical substrate  90  for use with the athermalized cylindrical package  20  described above. The slotted cylindrical substrate  90  includes two slots  92  and otherwise conforms to the description of cylindrical substrate  10 . The slots  92  are positioned on opposing sides of the substrate  10  and allow the optical fiber  22  to be positioned inside the substrate  10  prior to the grating  26  being written into the optical fiber  22 . 
     FIG. 7 shows an athermalized cylindrical package  120  in accordance with another embodiment of the present invention. In the package  120 , a cylindrical substrate  110  partially encloses an optical fiber  122  which includes a coating  124  which has been stripped from portions of the optical fiber  122 . The optical fiber  122  has written into it a short-period grating  126  along a portion of a center length of the optical fiber which has been stripped of the coating  124 . Two seals  128 ,  129  disposed in each seal socket  114  of the cylindrical substrate  110  tensionally maintain and support the region of the optical fiber  122  containing the fiber grating  126 . The seals  128 ,  129  may include SnO—ZnO—P 2 O 5  glass with pyrophosphate filler or another suitable material, as described above. The substrate  110  is enclosed by a cylindrical body  130  and two end caps  132 ,  133  disposed substantially within the ends of the cylindrical body. The body  130  and end caps  132 ,  133  may include metal (such as nickel, stainless steel or Kovar®) glass, ceramic, polymer, or some other suitable material. The optical fiber  122  extends through each end cap  132 ,  133  and is held by plugs  134 ,  135 . The plugs  134 ,  135  include solder or other suitable material as described above. The cylindrical body  130  has been crimped along the mid-span perimeter groove  116  of the substrate  110 . According to another aspect of the present invention, a foam plug  180  is positioned between each end cap  132 ,  133  and the cylindrical body  130 . 
     In order to compensate for the temperatures changes that the cylindrical package  120  undergoes during testing and product life, the cylindrical substrate  110  has a negative CTE, such as −80×10 −7  per ° C. Thus, while the optical fiber  122  has a CTE of approximately 7×10 −7  per ° C., the negative CTE of the cylindrical substrate athermalizes the fiber grating  126 , providing passive temperature compensation. Additionally, the cylindrical substrate  110  protects the fiber grating  126  from external perturbations (such as mechanical stress) and environmental conditions (such as moisture). While presently preferred materials are disclosed herein, one skilled in the art would appreciate that the cylindrical package  120  of the present invention may include a variety of materials and sizes, and should not be construed as limited to the embodiments shown and described herein which are exemplary. 
     FIG. 8 shows an athermalized swaged cylindrical package  220  in accordance with another embodiment of the present invention. In the package  220 , a cylindrical substrate  210  partially encloses an optical fiber  222  which includes a coating  224  which has been stripped from portions of the optical fiber  222 . The optical fiber  222  has written into it a short-period grating  226  along a portion of a center length of the optical fiber which has been stripped of the coating  224 . Two seals  228 ,  229  disposed in each seal socket  214 ,  215  of the cylindrical substrate  210  tensionally maintain and support the region of the optical fiber  222  containing the fiber grating  226 . The seals  228 ,  229  may include SnO—ZnO—P 2 O 5  glass with pyrophosphate filler or another suitable material, as described above. The substrate  210  is enclosed by a cylindrical body  230 . The cylindrical body  230  has been swaged at each end to form integral end caps  232 ,  233 . The body  230 , including the integral end caps  232 ,  233 , may include metal (such as nickel, stainless steel or Kovar®) polymer, or some other suitable material. The optical fiber  222  extends through each integral end cap  232 ,  233  and is held by plugs  234 ,  235 . The plugs  234 ,  235 , of solder or another suitable material as described above, secure the optical fiber  222  to the end caps  232 . The cylindrical body  230  has been crimped along the mid-span perimeter groove  216  of the substrate  210 . According to another aspect of the present invention, a foam plug  280  is positioned between each integral end cap  232 ,  233  and the cylindrical body  230 . 
     In order to compensate for the temperatures changes that the cylindrical package  220  undergoes during testing and product life, the cylindrical substrate  210  has a negative CTE, such as −80×10 −7  per ° C. Thus, while the optical fiber  222  has a CTE of approximately 7×10 −7  per ° C., the negative CTE of the cylindrical substrate athermalizes the fiber grating  226 , providing passive temperature compensation. Additionally, the cylindrical substrate  110  protects the fiber grating  126  from external perturbations (such as mechanical stress) and environmental conditions (such as moisture). While presently preferred materials are disclosed herein, one skilled in the art would appreciate that the cylindrical package  220  of the present invention may include a variety of materials and sizes, and should not be construed as limited to the embodiments shown and described herein which are exemplary. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the present invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.