Abstract:
The disclosure relates to encapsulated optical fiber end-coupled devices. An optical assembly may include a fiber holder and a cap with an optical component mounted to its underside. The optical component may be aligned with a fiber passing between a substrate and a second section of the fiber holder to allow optical signals to be coupled between the optical component and the fiber. Techniques for assembling the fiber holder and the cap may facilitate the formation of hermetic seals to provide a protective environment for the optical component.

Description:
BACKGROUND  
         [0001]    The present disclosure relates to encapsulated optical fiber end-coupled devices.  
           [0002]    Equipment used in the field of optical communications includes assemblies of optical components that may be used in optical fiber networks. The optical components in the assembly may include, for example, an active device, such as a semiconductor laser, and an optical fiber to receive output signals from the laser. To maintain the reliability of such an assembly, the semiconductor laser and optical fiber need to be aligned and firmly fixed in position. Electrical contacts may need to be provided to the components within the assembly. Additionally, it often is desirable to protect the assembly from environmental elements that may deteriorate performance of the optical devices. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]    [0003]FIG. 1 illustrates a sealed assembly for a single fiber implementation.  
         [0004]    [0004]FIG. 2 illustrates a substrate of a fiber holder for the assembly of FIG. 1.  
         [0005]    [0005]FIG. 3 illustrates a second section of the fiber holder for the assembly of FIG. 1.  
         [0006]    FIGS.  4 - 6  illustrate steps in the process for assembling the fiber holder.  
         [0007]    [0007]FIG. 7 illustrates an example of cap for the assembly of FIG. 1.  
         [0008]    [0008]FIG. 8 is a cross-section of FIG. 7 taken along lines X-X.  
         [0009]    [0009]FIG. 9 illustrates optical components mounted to the cap of FIG. 7.  
         [0010]    [0010]FIGS. 10 and 11 illustrate steps in the process for assembling the cap to the fiber holder.  
         [0011]    FIGS.  12 - 14  illustrate alternative implementations for the cap and for mounting optical components to the cap.  
         [0012]    [0012]FIG. 15 illustrates another implementation of a fiber holder.  
         [0013]    [0013]FIG. 16 illustrates a sealed assembly for a multi-fiber implementation.  
         [0014]    [0014]FIG. 17 illustrates a substrate of a fiber holder for the assembly of FIG. 16.  
         [0015]    [0015]FIG. 18 illustrates a second section of the fiber holder for the assembly of FIG. 16.  
         [0016]    [0016]FIG. 19 illustrates a cap and mounted optical component for the assembly of FIG. 16.  
         [0017]    FIGS.  20 - 23  illustrate steps in a process for assembling the fiber holder and cap for the assembly of FIG. 16. 
     
    
     SUMMARY  
       [0018]    The disclosure relates to encapsulated optical fiber end-coupled devices.  
         [0019]    An optical assembly includes a fiber holder and a cap with an optical component mounted to its underside. The optical component may be aligned with a fiber passing between a substrate and a second section of the fiber holder to allow optical signals to be coupled between the optical component and the fiber. In some implementations, techniques for assembling the fiber holder and the cap may facilitate the formation of hermetic seals to provide a protective environment for the optical component.  
         [0020]    According to one aspect, an apparatus comprises an optical fiber holder and a cap. The fiber holder includes a substrate and a second section attached to the substrate. The cap is attached to the second section of the optical fiber holder such that the second section of the fiber holder is disposed between the cap and the substrate.  
         [0021]    A recess in the substrate, an inner surface of an opening in the second section and the underside of the cap define an interior region. An optical fiber, mounted to the substrate, passes between the substrate and the second section to the interior region. The cap may include a conductive through-hole to provide an electrical connection from outside the interior region to the underside of the cap. An optical component, mounted to the underside of the cap and in electrical contact with the conductive through-hole, is positioned with respect to the optical fiber to allow an optical signal to be coupled between the optical fiber and the optical component.  
         [0022]    In some implementations, one or more of the following features may be present. For example, the optical component may be hermetically encapsulated within the interior region. A hermetic seal may be provided at a location where the fiber passes between the substrate and the second section. A heretic seal also may be provided at the conductive through-hole in the cap.  
         [0023]    The optical component may include one or more opto-electronic devices. Examples of such devices include a light emitting device, a semiconductor laser chip, a light detector, an optical modulator or optical amplifier.  
         [0024]    Some implementations may include multiple optical fibers aligned with an opto-electronic device mounted to the cap and located within the interior region of the assembly.  
         [0025]    According to another aspect, a method includes positioning an optical fiber on a substrate with the an end of the optical fiber extending to an area defined by a recess in a first surface of the substrate. A second section is fixed to the First surface of the substrate so that a portion of the fiber passes between the substrate and the second section. A material, for example, a solder, an adhesive or a polymer, may be provided through a hole in the second section to fix the fiber in place. The hole is located adjacent the portion of the fiber passing between the substrate and the second section.  
         [0026]    The method includes positioning a cap on the second section so that the second section is disposed between the substrate and the cap. The cap includes an optical component mounted to its underside and, when the cap is positioned on the second section, the optical component is encapsulated within an interior region defined by the recess in the substrate, an inner surface of an opening in the second section, and the underside of the cap. The cap may be fixed to the second section with the optical component aligned with the optical fiber to allow an optical signal to be coupled between the fiber and the optical component.  
         [0027]    Various implementations may include one or more of the following advantages. The assembly may provide efficient coupling between one or more fibers and an encapsulated optical device. The assembly also may provide long-term stability by encapsulating the active devices in a controlled, hermetic environment. Some implementations may be particularly advantageous for high frequency or high current applications. The assembly also may facilitate the testing of optical devices.  
         [0028]    Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims.  
       DETAILED DESCRIPTION  
       [0029]    As shown in FIG. 1, an optical assembly  20  includes a fiber holder comprising first and second sections  22 ,  24  to hold an optical fiber  30  in place. The first section  22  serves as a substrate to mount the optical fiber. One or more optical components (not shown in FIG. 1) may be housed within an interior region of the assembly and may be optically coupled to the end of the fiber extending to the assembly&#39;s interior region. The optical components may include pure optical, opto-electronic or opto-mechanical devices. The optical component(s) may be hermetically sealed within the housing. As discussed below, the optical component(s) (not shown in FIG. 1) may be mounted to the underside of a cap  26 , or lid, which may provide an airtight, hermetic seal. The second section  24  of the fiber-holder is sandwiched between the fiber mounting substrate  22  and the cap  26 .  
         [0030]    If the optical fiber includes a polymer or similar coating, the coating along the portion  32  of the fiber near the interior of the assembly may be removed. A groove  36  may be provided near the edge of the fiber mounting substrate  22  to accommodate the fiber coating  34  so that the fiber lies substantially flat on the substrate. The second section  24  of the fiber holder may include a V-shaped or other groove  38  to accommodate the section of the fiber  30  that passes between the substrate and second section of the fiber holder and that extends to the interior of the assembly. As explained below, a hermetic seal may be provided in the area where the fiber passes between the substrate and the second section of the fiber holder.  
         [0031]    Electrical contacts  28  may be provided on the outer surface of the cap  26 . The contacts  28  may be electrically coupled to optical components within the hermetically sealed interior region using conductive through-hole connections that extend from the outer surface of the cap  26  to its underside. The through-holes may form hermetical seals as well.  
         [0032]    Further details of the cap  26 , the fiber holder substrate  22  and the second section  24  of the fiber holder, as well as the assembly process, are described below.  
         [0033]    Referring to FIG. 2, the substrate  22  may be formed, for example, of a transparent material, such as glass. The glass substrate is a poor heat conductor and, therefore, can help reduce overheating of the assembly. Alternatively, other materials, such as silicon, may be used for the substrate. The fiber mounting substrate  22  may have a generally flat, rectangular upper surface. In addition to the groove  36 , the substrate  22  includes an indented recess  40  that partially defines the interior region. The recess  40 , which may be formed, for example, by etching, is sized so that when the cap is positioned in place, as shown in FIG. 1, the optical components mounted to the underside of the cap fit within the interior region. Preferably, the recess area  40  is sufficiently deep that the optical components do not contact the sides of the recess. The substrate  22  also may include a groove or other recess  44  to receive solder paste to facilitate the formation of a hermetic seal around the circumference of the fiber as discussed below.  
         [0034]    Referring to FIG. 3, the second section  24  of the fiber holder may be formed, for example, of single crystal silicon. The second section  24  also may have a generally flat surface. In addition to the groove  38 , the second section  24  includes an opening  42  whose inner surface partially defines the interior region of the assembly. A rectangular hole  46  may be formed through the second section  24  substantially perpendicular to the groove  38 . The hole  46  extends through the thickness of the second section  24  and serves as an access hole to facilitate the application of solder paste to hold the fiber in place and form a hermetic seal.  
         [0035]    The two sections  22 ,  24  of the fiber holder may be held together, for example, by a glass solder paste  48  applied to the substrate  22  as illustrated in FTG.  4 . The paste may comprise, for example, a dispersion of ground glass solder combined with a binder in a solvent. The glass solder paste may be applied around the recess area  40  by screen printing or precision dispensing. After drying, the glass solder paste may be heated to drive out organics from the binder and to allow the glass particles to fuse together.  
         [0036]    If a metal solder is used instead of glass solder, it may be necessary to metallize the surfaces prior to soldering. Thin layers of metals may be evaporated successively prior to applying the metal solder. For example, a layer of titanium may be provided for adhesion, and a layer of platinum on gold may serve as a diffusion barrier. Selective deposition may be achieved using photolithographic techniques. The metal solder may be applied, for example, by evaporation or by electro-deposit techniques. Alternatively, the metal solder may be dispensed as a paste or metal solder pre-form may be used.  
         [0037]    Before fusing the two sections of the fiber holder together, the optical fiber  30  may be positioned in place as shown in FIG. 5. The fiber extends across the groove  44  with one end of the fiber extending to the recess  40  in the surface of the substrate  22 .  
         [0038]    Once the fiber  30  is positioned in place, the second section  24  of the fiber holder may be placed over the substrate  42  as illustrated in FIG. 6. When properly positioned, the opening  46  in the second section  24  is located above the opening  44  in the substrate  22  (FIG. 5). The sections of the fiber holder may be heated while pressing the two sections to fuse them together. The sealing around the circumference of the fiber can be improved by providing additional solder through the opening  46 . Sufficient solder may be added so that, upon further heating, it flows around the circumference of the fiber to at least partially or substantially fill the openings  44 ,  46  so as to fix the fiber in place and provide a hermetic seal at a location where the fiber passes between the substrate and second section of the fiber holder. This technique can allow the functions of fixing the fiber in place and forming a hermetic seal to be performed at the same time. That may reduce the number of steps required for assembly and may reduce the overall size of the package.  
         [0039]    In some cases, such as where the fiber is to be used as part of a Bragg grating, heating at temperatures required to melt the solder may cause the fiber to deteriorate. In that case, adhesives or polymers may be used instead of the solder  48 . The adhesive may be cured by ultra-violet (UV) light through the transparent glass section  22 . The use of adhesives, however, may result in a non-hermetic seal.  
         [0040]    After the two sections  22 ,  24  of the fiber holder are assembled, glue or some other adhesive may be applied in the area between the groove  36  and the section  34  of the fiber  30  to help hold the fiber in place and to provide strain relief for the uncoated brittle section of the fiber  32 . If desired, that section of the fiber also may be covered by glue.  
         [0041]    As shown in FIG. 6, a metallization ring  50 , formed by photoresist masking and the evaporation of suitable metals as previously described, may be provided on the second section  24  of the fiber holder to solder the cap to the fiber holder sub-assembly during subsequent assembly.  
         [0042]    [0042]FIG. 7 illustrates additional details of the underside of the cap  26  according to one implementation. The underside may include an island  54 . When the cap  26 , which may be formed, for example, from a monolithic piece of silicon, is positioned over the upper section  24  of the fiber holder (as in FIG. 1), the island  54  extends into the sealed interior region of the assembly. One or more electronic or optical devices may be mounted to the underside of the cap. Various conductive contacts  66  and lines  60  may be provided on the underside of the cap. The conductive contacts  66  may be connected to the through-hole connections  68 , which, in turn, are connected to the electrical contacts  28  on the opposite (external) surface of the cap.  
         [0043]    Various techniques may be used to form the hermetically sealed through-hole connections  68 . One such technique uses a multilayer structure that includes a substantially etch-resistant layer sandwiched between first and second semiconductor layers. The first and second semiconductor layers may include, for example, silicon, and the etch-resistant layer may include, for example, silicon nitride, silicon oxy-nitride, silicon dioxide. The through-holes may be formed using a double-sided etching process in which the first and second layers are etched until the etch-resistant layer is exposed to define the locations of the through-holes. The semiconductor layer that is intended to be on the underside of the cap  26  may be etched over an area that corresponds to the positions of all or a large number of the through-holes. The through-holes then may be formed by removing part of the etch-resistant layer. Such etching may result in the opening  58  in FIG. 7.  
         [0044]    The through-holes may be hermetically sealed, for example, using all electro-plated feed-through metallization process as the base for the through-hole connections. The feed-through metallization also may include a diffusion barrier, and the sealing material may include, for example, a non-noble metal.  
         [0045]    [0045]FIG. 8 illustrates a cross-section of the opening  58  and one of the throug-hole connections  68  taken along the lines X-X in FIG. 7. The etch-resistance layer  74  is located between the first and second semiconductor layers  70 ,  72 . a dielectric layer  76 A,  76 B, such as a thermal silicon dioxide layer, may be present over the surfaces of the First and second semiconductor layers  70 ,  72 . An adhesion layer  78 A,  78 B may serve as an electroplating base on which the feed-through metallization layer  80  is formed. The metallization layer  80  also may form the contacts  60  and  28 . A combined diffusion and wetting layers  82 A,  82 B may be present over the surface of the metallization  80 . An anti-oxidation layer may be formed over the layers  82 A,  82 B. Other techniques may be used to form the through-hole connections  68  as well.  
         [0046]    In the particular implementation illustrated in FIG. 7, the contacts  66  are positioned for mounting a semiconductor laser, and another recess  56  is provided on the underside of the cap to hold a monitor diode. Conductive contacts  62 ,  64  may be provided along walls of the recess  56  so that electrical signals are provided to and from the monitor diode. Other optical, electronic and optoelectronic components, Such as lenses, optical isolators, integrated circuits, capacitors, and inductors, among others, may be mounted to the underside of the cap as well.  
         [0047]    [0047]FIG. 9 illustrates a semiconductor laser chip  90  and monitor diode  92  mounted to the underside of the cap  26 . In this example, the laser chip is mounted with its active side on the cap and its electrical contacts (not shown) on the active side. Such an arrangement may be particularly useful for lasers or other components using high frequency signals which may not be well-suited for transmission through wires. The arrangement also may facilitate accurate vertical positioning of the light beam from the laser chip relative to the surface of the cap. The monitor diode  92  may be mounted in the recess  56  and can monitor light emitted through the backside of the semiconductor laser chip  90 .  
         [0048]    As previously mentioned, additional or other active and passive devices may be mounted to the cap as well. For example, as illustrated in FIG. 12, an optical lens  94  may be positioned in another recess  96  provided in the island  54  on the underside of the cap  26 . In other implementations, as illustrated for example in FIG. 13, the laser chip  90 A may be mounted to the island  54  with its active side up. Electrical contact may be provided through wires and leads to the top and underside of the laser chip, and a diode  92 A may be provided to monitor the light from the backside of the laser chip. The laser or other devices need not be mounted directly to a monolithic silicon cap. For example, as shown in FIG. 14, the laser chip  90 B and monitor diode  92 B may be mounted to a diamond submount  100 , which in turn is mounted to the silicon cap  26 . The diamond submount can provide good heat conduction, which may be particularly useful for high power laser chips.  
         [0049]    In other implementations, a light detector, such as a p-i-n diode, may be mounted to the cap to receive optical signals from the fiber  30 .  
         [0050]    To facilitate attaching the cap  26  to the fiber holder sub-assembly, a metal solder ring  52  may be provided near the periphery of the cap&#39;s underside (see FIGS. 7 and 9). A UV-curable adhesive having a small expansion coefficient and low shrinkage during curing also may be applied, for example, near the outer perimeter of the cap. The use of metal solder allows the cap to be fused to the fiber holder sub-assembly at a lower temperature that does not cause the glass solder ring  48  (see FIG. 5) to re-flow. To attach the cap to the fiber-holder sub-assembly, the cap  26  may be rotated as shown in FIGS.  10  and  11  so that the optical components  90 ,  92  mounted on the island  54  are disposed within the interior region of the assembly defined by the surfaces of the recess area  40  on the substrate  22 , the inner sidewalls of the second section  24  and the underside of the cap  26 .  
         [0051]    Before fixing the cap  26  in place with respect to the fiber holder sub-assembly, tests may be performed to ensure that the output from the front end of the laser chip  90  is properly aligned with the optical fiber  30 . For example, electrical signals from the backside of the cap may be provided to the laser chip. The optimal position of the laser may be determined, for example, using an active alignment technique in which the laser is turned on and the optical output from the far end of the fiber is measured. The point at which maximum optical coupling occurs between the fiber and laser chip may be used as the optimal position for the laser chip. Once the optimal position is determined, the cap  26  may be fixed in place, for example, using a two-step process. The position of the cap  26  may be fixed initially by hardening the adhesive such as an UV-curable material. Next, a hermetic seal may be formed between the cap  26  and the upper section  24  of the Fiber holder by re-flowing the metal solder.  
         [0052]    In some cases, after determining the optimal position of the laser chip, the cap may be moved back by a predetermined distance. Next, a solder ring may be placed on the second section and melted. The cap then is moved forward to the previously determined optimal position so that metallization on the cap contacts the molten solder. As the solder cools, a seal is formed between the second section of the fiber holder and the cap.  
         [0053]    Alternatively, the cap may be fixed in place in a single step by heating the assembly to the soldering temperature while the cap is held in place by a mounting tool. Other techniques may be used to align the optical component with the fiber and to attach the cap to the second section of the fiber holder.  
         [0054]    After the cap  26  is attached to the fiber holder sub-assembly, the assembly appears as shown in FIG. 1.  
         [0055]    Particular details of the assembly may differ in various implementations, For example, as shown in FIG. 15, the dimensions of the sections  22 A,  24 A of the fiber holder may differ from those shown in the earlier figures and may be tailored to the particular application. To facilitate positioning of the fiber, a V-shaped or other groove  102  may be provided in the substrate  22  underneath the portion of the fiber from which the coating is removed. Similarly, one or both sections  22 ,  24  of the fiber holder may include indentations  104  along their periphery into which overflow solder may flow.  
         [0056]    [0056]FIG. 16 illustrates a sealed assembly  120  that includes multiple fibers  130  that may be optically coupled to one or more devices (not shown in FIG. 16) mounted to the underside of the cap  126 . Details of a substrate  122  and second section  124  of a fiber holder are shown in FIGS.  17 - 18  and may be similar to those for the single fiber implementation(see FIGS. 2 and 3). In this example, the fiber holder is designed to accommodate four fibers—two fibers to direct incoming light to an optical device in the sealed interior region of the assembly, and two fibers to receive light from the device.  
         [0057]    The substrate  122  includes a recess  140  that corresponds to the recess  40  in FIG. 2. The function of the recesses  144  and grooves  136  correspond generally to the recess  44  and groove  136  in FIG. 2. Similarly, the second section  124  of the fiber holder includes an opening  142  that corresponds to the opening  42  in FIG. 3. The function of the grooves  138  and openings  146  correspond generally to the groove  38  and opening  46  in FIG. 3.  
         [0058]    Additional details of one implementation of the underside of the cap  126  are illustrated in FIG. 19. The cap may include an island  154  onto which the optical device  190  is mounted. The optical device may be, for example, an optical modulator such as a semiconductor optical amplifier. When the cap is assembled to the fiber holder, the device  190  is aligned with the fibers so that the optical signals to and from the fibers can be optically coupled with the device. As previously discussed, in some implementations the cap may be formed as a monolithic piece. In other implementations, the optical device may be mounted to a diamond or other submount which in turn is mounted to the underside of the cap.  
         [0059]    FIGS.  20 - 23  illustrate additional steps in the assembly process for the multi-fiber implementation. FIG. 20, for example, illustrates initial positioning of the optical fibers  130  and application of glass solder  148  to the substrate  122  to hold together the sections of the fiber holder (see FIG. 21). A solder paste ring  150  may be provided on the second section  124  of the fiber holder to fuse the cap  126  to the fiber holder sub-assembly. As shown in FIG. 22, the openings ( 146 ) may be filled with solder  147  to hold the fibers in place and to form hermetic seals where the fibers pass between the substrate  122  and the second section  124  of the fiber holder. The cap  126  then may be assembled to the fiber holder (see FIG. 23) so that the optical device is located within the interior region of the assembly and is aligned with the fibers  130  as illustrated in FIG. 16.  
         [0060]    Other implementations are within the scope of the claims.