Abstract:
An athermal package for fiber photonic devices includes a ferrule to attach the optical fiber to the package. The ferrule has an opening to receive the optical fiber. The ferule is collapsed to attach the optical fiber to the athermal package. Alternatively, the athermal package uses adhesive bonds disposed in pockets of the package. The pockets have a narrow end and a wide end, with the narrow ends facing each other. The adhesive bonds are disposed in the pockets in contact the narrow ends of the pockets but not with the wide ends. The narrow ends physically confine the adhesive bonds so that if the bonds expand or contract due to environmental conditions (or the curing process), the adhesive either expands or contracts near the wide ends of the pockets. This allows the strain on the optical fiber segment between the bonds to remain substantially constant.

Description:
FIELD OF THE INVENTION 
     Embodiments of invention relate generally to fiber photonic devices and, more specifically but not exclusively relate to packages for fiber photonic devices. 
     BACKGROUND INFORMATION 
     An optical transmission system transmits information from one place to another by way of a carrier whose frequency is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal. Such optical signals are commonly propagated in optical fibers. 
     In some systems, photonic devices are built into or include a segment of optical fiber. For example, a Bragg grating can be implemented in a segment of optical fiber by forming regions of periodically alternating refractive indices in the fiber segment through which an optical signal is propagated. This type of Bragg grating is commonly referred to as a fiber Bragg grating (FBG) and is typically used as a wavelength selective filter in fiber optic communication systems. For example, the FBG can be used to filter out a particular wavelength (known as the Bragg wavelength). The Bragg wavelength depends on the average or effective refractive index of the optical fiber segment and on distance between gratings of the alternating regions (i.e., the period). As is well known, the Bragg wavelength of a FBG is dependent on the temperature and the strain on the fiber segment containing the FBG. 
     Typically, the optical fiber segment containing the FBG is attached under strain to a package that can be mounted on a board or otherwise incorporated into a unit or assembly. As schematically illustrated in  FIG. 1  (PRIOR ART), the optical fiber segment is attached to a conventional package at two attachment points, with the FBG section being between the attachment points. 
     In this example of a conventional package, an optical fiber  100  is attached to a package having portions  101  and  102 . Optical fiber  100  is attached under strain to portions  101  and  102  using bonds  103  and  104 . Bonds  103  and  104  are commonly solder, epoxy or other adhesive. A FBG (or other photonic device) can be implemented in the portion of optical fiber  100  between bonds  103  and  104 . A screw  105  can be adjusted to vary the length between bonds  103  and  104 , thereby adjusting the strain on the portion of optical fiber  100  between bonds  103  and  104 . Precise control of the dimensions of the package and the length of the optical fiber between bonds  103  and  104  are needed to achieve the desired temperature compensation of the package. 
     As seen in  FIG. 1 , bonds  103  and  104  are relatively large so that optical fiber  100  is reliably attached to the athermal package. However, this relatively large size makes it difficult to determine the exact attachment point of optical fiber  100  to bond  103  (and bond  104 ) and tends to cause the exact attachment point to vary from bond to bond. As previously described, precise control of the length of the portion of optical fiber  100  between bonds  103  and  104  is needed to achieve the desired temperature compensation. Thus, the relatively large size of bonds  103  and  104  can undesirably cause variations in the performance of the athermal package. 
     Bonds  103  and  104  are typically implemented using adhesives (e.g. organic adhesive such as epoxy) or by soldering (e.g., glass solder or metal solder). However, organic adhesives can have reliability issues caused by ageing, temperature cycling, humidity, etc. Soldering tends to be complex (e.g., requiring the deposition of a metal on the optical fiber), which are generally undesirable in a large-scale manufacturing environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG. 1  is a schematic diagram illustrating a conventional athermal package for a FBG. 
         FIG. 2  is a schematic diagram illustrating an athermal package with collapsible ferrules, according to one embodiment of the present invention. 
         FIGS. 3A and 3B  are cross-sectional diagrams illustrating attachment of an optical fiber to a collapsible ferrule, according to an embodiment of the present invention. 
         FIG. 4  is a top view illustrating the attachment of a ferrule within a recessed portion of the athermal package, according to an embodiment of the present invention. 
         FIG. 5  is a top view illustrating a bonding pocket formed in a portion of an athermal package, according to an embodiment of the present invention. 
         FIGS. 6A ,  6 B and  6 C illustrate dimensional changes of the adhesive in the bonding pocket caused by environmental changes. 
         FIG. 7  is a block diagram illustrating an optical system using an althermal package according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are directed to an athermal package using a collapsible ferrule at each end of an optical fiber segment. The optical fiber segment can include or contain a photonic device such as, for example, a FBG. In one embodiment, the ferrules have a tip that can be collapsed to attach the ferrule to the fiber. This mechanical attachment process is relatively simple and reliable under varying environmental conditions. Further, the collapsed tip forms a relatively short bond region on the optical fiber, thereby advantageously reducing variations in the effective length of the optical fiber segment between the ferrules. 
     In further refinement of this embodiment of the invention, first and second ferrules are disposed in recesses in first and second portions of the athermal package. The first and second portions are separated by a third portion. In some embodiments, the length of the third portion can be adjusted to adjust the strain on the optical fiber connected between the first and second ferrules. 
     Other embodiments of the present invention are directed to an athermal package with bonding pockets for containing an adhesive to bond an optical fiber to the athermal package. The bonding packets advantageously allow the adhesive to experience dimension changes (e.g., due to curing and/or environmental conditions) while minimizing changes in the strain on the optical fiber segment between the bonding pockets. 
     Although the following detailed description includes references to a FBG embodiment with two bonding regions at each end of an optical fiber segment, other embodiments can include other types of photonic devices and/or more than two bonding regions. 
       FIG. 2  schematically illustrates an athermal package for use with an optical fiber  200 , according to one embodiment of the present invention. In one embodiment, optical fiber  200  includes a FBG. This embodiment of the athermal package includes ferrule portions  201  and  202 , ferrules  203  and  204 , and a connecting portion  205 . In some embodiments, the length of portion  205  can be adjusted, for example, by a screw mechanism (not shown). Portions  201  and  202 , in some embodiments, are made of one material, while portion  205  is made of another material. These materials are selected so that they have different thermal coefficients of expansion, so that temperature dependent changes in the lengths of portions  201  and  202  are offset or compensated for by an opposite change in length of portion  205 . In some applications, the materials may be selected to achieve a partial compensation or an overcompensation. In other embodiments, different combinations of materials may be used to implement portions  201 ,  202  and  205  to achieve athermal performance. 
     The elements of this embodiment of the athermal package are interconnected as follows. Portion  201  has one end that is attached at or near one end of portion  205 . Similarly, portion  202  has an end that is attached to the other end of portion  205 . Ferrules  203  and  204  are attached to portions  201  and  202 , respectively. Optical fiber  200  is disposed to run through ferrules  203  and  204 , which are then collapsed to attach optical fiber  200  (under a selected strain) to the athermal package. In an FBG application, the FBG is disposed in the segment of optical fiber  200  that is located between ferrules  203  and  204 . 
     In this embodiment, portions  201  and  202  respectively include raised portions  206  and  207  used to attach ferrules  203  and  204  that extend to just below the level of optical fiber  200 . For example, portions  201  and  202  can be machined to create raised portions  206  and  207 . In other embodiments, raised portions  206  and  207  may extend higher than optical fiber  200 , with a hole or slot formed in the raised portions pass the optical fiber. For example, to form the raised portions, portions  201  and  202  can include recesses sized to accept ferrules  203  and  204 , with a slot running longitudinally along portions  201  and  202  and through the recesses to pass optical fiber  200 . For example, see  FIG. 4  in which portion  201  has a recess  401  for receiving ferrule  203  and slot  403  for passing optical fiber  200 . The raised portions can advantageously avoid slippage due to asymmetric forces and torque experienced by the ferrules when the optical fiber is connected to ferrules under strain. 
       FIGS. 3A and 3B  illustrate attachment of an optical fiber  302  to a collapsible ferrule  301 , according to an embodiment of the present invention. In one embodiment, ferrules  203  and  204  are substantially the same as ferrule  301 . 
     Optical fiber  302  is fitted through ferrule  301  and another substantially identical ferrule (not shown) so that the ferrules can slide along the optical fiber to desired positions. The openings in ferrules are slightly larger in diameter than that of optical fiber  302 . Thus, as shown in  FIG. 3A , a small gap  303  exists between the outer surface of optical fiber  302  and the inner surface at the end of ferrule  301 . 
     Ferrule  301  is then positioned at the desired location on optical fiber  302 , and then collapsed. In this embodiment, ferrule  301  is collapsed using a tool (not shown) to mechanically fix ferrule  301  to optical fiber  302  at the desired location. The collapsed ferrule  301 , as illustrated in  FIG. 3B , forms a tightly fitted interface  304  with optical fiber  302 , eliminating gap  303  (FIG.  3 A). Suitable collapsible ferrules and tools are commercially available. 
     In an alternative embodiment, the temperature of the ferrules is changed (significantly beyond the expected operating range) to increase the size of the opening to allow the ferrule to be fitted to the optical fiber. The ferrule opening then returns to its normal size (selected to be slightly smaller than the diameter of the optical fiber) when the temperature returns to the expected operating range, thereby securely attaching the ferrule to the optical fiber. In other embodiments, different attachment mechanisms can be used. 
     The second ferrule (not shown) is positioned along optical fiber  302  and fixedly attached by collapsing the ferrule as described above for ferrule  301 . The ferrules (and thus optical fiber  302 ) are attached to portions of an athermal package (similar to portions  201  and  202  of FIG.  2 ). In this embodiment, the athermal package can adjust the distance between the ferrules to achieve a desired strain on the segment of optical fiber  301  between the ferrules. In one exemplary embodiment, optical fiber  302  is loaded to experience a strain of about 800-1000 microstrains when the temperature is at the lowest end of the expected or specified operating temperature. 
     These embodiments are advantageous over conventional adhesive and soldering bonding in that the collapsible ferrules can provide a relatively simple (i.e., in the attachment operation) and reliable optical fiber attachment mechanism. For example, similar collapsible ferrules have been used in field replaceable connector applications, meeting various specifications (including military standards) for reliability. In addition, the attachment process can be faster in that once the ferrules are collapsed, there is no “curing” period as in conventional adhesive bonds. 
     In addition, the physical attachment region of the ferrule on the optical fiber can be significantly less than conventional adhesive and solder bonds. As previously described, precise control of the length of the portion of optical fiber between the bonding mechanisms is needed to achieve the desired temperature compensation. Thus, the relatively small effective attachment regions of the ferrules can advantageously reduce variations in the performance of the athermal package compared to conventional adhesive and solder bonds. 
     In another embodiment of the present invention, an athermal package  500  that is similar to the athermal package of  FIG. 2  except that adhesive disposed in pockets are used to attach optical fiber  200  to the athermal package instead of ferrules. As will be described in more detail below, this embodiment advantageously reduces changes in strain on the segment of optical fiber  200  between the adhesive bonds that may be caused by environmental changes. It is believed that heretofore that the effect of environmental changes on the size and physical shape of adhesives used in adhesive bonds have neither been appreciated nor accounted for in designing athermal packages. For example, increases in temperature and/or humidity tend to cause adhesives to swell or expand. Conversely, the curing process and decreases in temperature and/or humidity tend to cause adhesives to shrink or contract. If the dimensions of the adhesive bonds are unconstrained, the volume of the bonds can change with these environmental conditions (and the curing process), thereby changing the strain on the segment of optical fiber between the bonds. In many applications, this effect is undesirable. This effect is addressed and compensated for in athermal package  500 . 
     In the embodiment of  FIG. 5 , athermal package  500  includes portions  501  and  502  with adhesive bonds  503  and  504 , respectively. Portions  501  and  502  have pockets  506  and  507 , respectively, in which adhesive bonds  503  and  504  are formed. In some embodiments, pockets  506  and  507  are conical (with a truncated tip) in shape, with optical fiber  200  to be co-axial with the longitudinal axis of the pocket. In other embodiments, the pockets may have different shapes or geometries. For example, the pockets can be relatively narrow at one end and relatively wide at an opposite end (e.g., a wedge-shape). The geometry of the pockets can be designed to match the expansion/contraction properties of the adhesive. 
     In this embodiment, adhesive bonds  503  and  504  do not completely fill pockets  506  and  507 , respectively. Rather, the adhesive bonds are disposed in contact the “narrow” end of the pockets without touching the wide end. As can be seen in  FIG. 5 , the adhesive material is constrained by pockets  506  and  507  so that any changes in dimension in adhesive bonds  503  and  504  can only occur toward the wide ends of pockets  506  and  507 , respectively. In this embodiment, the segment of optical fiber  200  to have a controlled strain is between the narrow ends of pockets  506  and  507 . Thus, in this particular embodiment, the wide ends in effect define the starting points of “unstrained” segments of optical fiber  200 . 
     In other embodiments (not shown), four adhesive bonds may be used (a pair at each end), so that the fiber segment between a pair of adhesive bonds may also be under strain, while the segment between the pairs has a controlled strain as described above. 
     In operation, changes in dimension of adhesive bonds  503  and  504  are generally along the longitudinal axis of pockets  506  and  507 . This is especially true at the narrow ends of the pockets (i.e., near the controlled strain segment of optical fiber  200 ). Thus, dimensional changes of the adhesive material at the critical narrow end are in effect eliminated due to the geometry of the pocket. Rather, the adhesive material generally changes dimension along the longitudinal axis at the wide ends of pockets  506  and  507 . The forces that would otherwise be exerted on optical fiber  200  tend to be dissipated at the exposed surface of adhesive bonds (i.e., at the wide end of the pockets). Thus, the strain on the segment of optical fiber  200  between adhesive bonds  503  and  504  is substantially (and advantageously) unchanged in response to dimensional changes in the adhesive material caused by the curing process and environmental changes (e.g., temperature, humidity, pressure, etc.). 
       FIGS. 6A-6C  illustrate how athermal package  500  responds to changes in dimension (i.e., volume) of the adhesive material. For example,  FIG. 6A  illustrates the size and shape of a “baseline” adhesive bond  504   BASE  in pocket  507  (FIG.  5 ).  FIG. 6B  illustrates how the adhesive bond shrinks (e.g., due to curing or to a decrease in temperature or humidity), as shown by “shrunken” adhesive bond  504   SHRINK .  FIG. 6C  illustrates how the adhesive bond expands (e.g., due to an increase in temperature or humidity), as shown by “expanded” adhesive bond  504   EXPAND . As previously described, the physical confinement at the narrow end of pocket  507  prevents the adhesive from changing its size and shape at the narrow end of the bond. The constant geometry of the adhesive at the narrow end helps keep the strain on the segment of optical fiber between bonds  506  and  507  substantially constant in response to changes in the dimensions of the adhesive (e.g., due to the environment or curing). 
       FIG. 7  is included to illustrate an exemplary optical system  700  that can use an athermal package according to an embodiment of the present invention. In this embodiment, optical system  700  includes an optical signal source  702 , an optical fiber  704 , a photonic device  706  coupled to one end of optical fiber  704 , and an athermal package  708  attached to photonic device  706 . Photonic device  706  can also be coupled to one or more other elements (not shown) of optical system  700 . 
     Optical system  700 , in this embodiment, is a wavelength division multiplexed (WDM) system and photonic device  706  is a fiber Bragg grating. In one embodiment, photonic device  706  is implemented in a section of optical fiber  704 . In other embodiments, photonic device  706  can be spliced onto an end of optical fiber  704 . Athermal package  708  is substantially similar to the embodiments described above in conjunction with  FIG. 2  or  5 . 
     In operation, optical signal source  702  can provide an optical signal to photonic device  706  via optical fiber  704 . Photonic device  706  operates on the optical signal (e.g., filters out a wavelength component of the optical signal) and provides the operated signal to one or more other elements of system  700  as in known WDM systems. As previously described, athermal package  708  (e.g., according to  FIG. 2  or  5 ) can be more easily/accurately fabricated with the desired temperature compensation and high reliability, compared to conventional athermal packages. 
     Embodiments of method and apparatus for an athermal package for photonic devices are described herein. In the above description, numerous specific details are set forth (such as the sizes, shapes and materials of various portions of the athermal package) to provide a thorough understanding of embodiments of the invention. One of ordinary skill in the relevant art will recognize, however, that embodiments of the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring the description. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible, as those skilled in the relevant art will recognize. These modifications can be made to embodiments of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.