Patent Publication Number: US-8979394-B2

Title: Self-contained total internal reflection sub-assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/806,166 filed on Mar. 28, 2013, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The disclosure relates generally to electric-optical systems, and more particularly to total internal reflection sub-assemblies used in fiber optic sub-assemblies for active optical cable assemblies or the like. 
     Short-distance data links used for consumer electronics are reaching increasingly higher data rates, especially those used for video and data storage applications. Examples include the USB 3.0 protocol at 5 Gb/s, HDMI at 10 Gb/s and Thunderbolt™ at 10 Gb/s over two channels. At such high data rates, traditional copper cables have limited transmission distance and cable flexibility. For at least these reasons, optical fiber is emerging as an alternative to copper wire for accommodating the high data rates for the next generations of electronic devices such as consumer devices. 
     Unlike telecommunication applications that employ expensive, high-power edge-emitting lasers along with modulators, short-distance optical fiber links are based on low-cost, low-power, directly-modulated light sources such as vertical-cavity surface-emitting lasers (VCSELs). In general, optical fiber links include fiber optic assembles that are used to couple light from the light source into an optical fiber in one direction (i.e., transmit). The fiber optic assemblies are also used to couple light traveling in another optical fiber onto a photodiode in the other direction (i.e., receive). To be viable for consumer electronics and the like, the fiber optic assemblies need to be low-cost. This requirement drives the need for designs that are simple to manufacture yet have suitable performance. 
     SUMMARY 
     Embodiments of a total internal relection (TIR) sub-assembly are disclosed herein. The TIR sub-assembly may be part of a fiber optic sub-assembly, which in turn may be part of an active optical cable assembly (and specifically connectors of such active optical cable assemblies, examples of which are also disclosed). 
     According to one embodiment, a TIR sub-assembly includes a body defining at least a portion of an optical path, a lens supported by the body and positioned in the optical path, and an optical turning member supported by the body and configured to change the direction of the optical path. The TIR sub-assembly also includes a carrier having a first surface coupled to the body and a second surface opposite the first surface. An active device is supported on the first surface of the carrier, which is coupled to the body on opposite sides of the active device. The body and carrier are shaped so that a space is maintained between the active device and an underside surface of the body. The lens is positioned on the underside surface and aligned with the active device. 
     One of the benefits of such a TIR sub-assembly is that it is a self-contained sub-assembly including the active device and lens. This not only allows the active device and lens to be pre-aligned (i.e., aligned before supporting the TIR sub-assembly on a printed circuit board), but also allows the optical system to be tested independently of any printed circuit board on which the TIR sub-assembly is to be placed. 
     Corresponding methods of manufacturing are also disclosed. To this end, one method for manufacturing a TIR sub-assembly involves providing a body that defines at least a portion of an optical path. The body supports a lens that is positioned in the optical path and an optical turning member that is configured to change the direction of the optical path. The method also involves supporting an active device on a first surface of a carrier, and coupling the first surface of the carrier to the body on opposite sides of the active device. Consistent with the embodiment mentioned above, the body and carrier are shaped so that a space is maintained between the active device and an underside surface of the body. Additionally, the lens is positioned on the underside surface of the body and aligned with the active device. 
     Some methods may involve additional steps to manufacture a fiber optic sub-assembly. One such method involves providing a printed circuit board having first and second surfaces and separately providing a TIR sub-assembly consistent with the embodiment mentioned above. The TIR sub-assembly is then supported on the printed circuit board, which is shaped to receive the carrier of the TIR sub-assembly proximate the second surface so that at least a portion of the optical path between the active device and lens of the TIR sub-assembly is located between the first and second surfaces of the printed circuit board. Testing may be performed on the TIR sub-assembly prior to this step to verify performance of the active device and optical path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the objects, advantages, and principles of the invention. In the drawings: 
         FIG. 1  is a perspective view, partially cut-away, of a connector for an active optical cable, wherein the connector includes a fiber optic sub-assembly having a known configuration; 
         FIG. 1A  is an enlarged view of the area circled in  FIG. 1 ; 
         FIG. 2  is a perspective view, partially cut-away, of a connector including a fiber optic sub-assembly according to one embodiment; 
         FIG. 3  is a perspective view, partially cut-away and similar to  FIG. 2 , but showing an opposite side of the connector and fiber optic sub-assembly; 
         FIG. 4  is a perspective view showing the fiber optic sub-assembly of  FIGS. 2 and 3  in isolation; 
         FIG. 5  is a perspective view of a portion of the fiber optic sub-assembly of  FIGS. 2 and 3 ; 
         FIG. 6  is an exploded perspective view of a total internal reflection sub-assembly included in the fiber optic sub-assembly shown in  FIG. 5 ; 
         FIG. 7  is a perspective view of the fiber optic sub-assembly of  FIGS. 2 and 3 , wherein the fiber optic sub-assembly is illustrated in isolation and from a different angle; 
         FIG. 8  is an exploded perspective view of the fiber optic sub-assembly shown in  FIG. 7 ; 
         FIG. 9  is a cross-sectional perspective view taken along line  9 - 9  in  FIG. 7 ; 
         FIG. 10  is a cross-sectional elevation view taken along line  10 - 10  in  FIG. 7 ; 
         FIG. 11  is a side elevation view of a fiber optic sub-assembly having a known configuration; 
         FIG. 12  is a perspective view of a total internal reflection sub-assembly according to another embodiment; 
         FIG. 13  is a perspective view of an embodiment of a fiber optic sub-assembly including the total internal reflection assembly of  FIG. 12 ; and 
         FIG. 14  is a cross-sectional side view of a portion of the fiber optic sub-assembly shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to fiber optic sub-assemblies for active optical cable assemblies, with examples of the latter being illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and description to refer to the same or like parts. 
     Some of the drawings show the fiber optic sub-assemblies within a connector of an active optical cable assembly. The active optical cable assemblies may be used in the consumer electronics field. For example, the connectors may be USB, Thunderbolt, HDMI, or PCI Express connectors. However, the disclosure is not limited to such connectors or consumer electronics applications. Other optical cable assemblies and applications are possible for the fiber optic sub-assemblies described herein. 
     With this mind,  FIGS. 1 and 1A  illustrate a known arrangement for a fiber optic sub-assembly  20  within a connector  22  of an active optical cable assembly  10 . The fiber optic sub-assembly  20  includes a printed circuit board  24  and total internal reflection (TIR) sub-assembly  26  supported on the printed circuit board  24 . The TIR sub-assembly  26  includes one or more active devices  28  (four are shown in the illustrated embodiment) electrically coupled to the printed circuit board  24  and optically coupled to a respective optical fiber  30 . To this end, the TIR sub-assembly  20  defines optical paths between the active devices  28  and optical fibers  30 , which extend into the connector  20  from a cable  12  that bundles and protects the optical fibers  30 . 
     The active devices  28  may be light sources, such as vertical-cavity surface-emitting lasers (VCELs), or light detectors, such as photodiodes. Light traveling from the active devices  28  to the optical fibers  30  is collected by lenses (not shown in  FIGS. 1 and 1A ) in each of the optical paths and directed to an optical turning member  32  (typically an angled mirror), which then reflects the light approximately 90 degrees toward ends of the optical fibers  30 . Conversely, light traveling from the optical fibers  30  to the active devices  28  is reflected by the optical turning member  32  towards the lenses, which concentrate and direct the light at the active devices  28 . Thus, there is a change in direction in the optical paths between the optical fibers  30  and active devices  28 . The distance of the optical fibers  30  from the optical turning member  32 , which may either be a common or respective optical turning member  32 , depends on the design of the optical system (e.g., the type of active device  28 , size of lens in the optical path, etc.). 
     The lenses in a TIR sub-assembly like the one shown in  FIG. 1  are spaced a specific distance from the active devices  28 , with an air gap maintained therebetween, based on the properties of the active devices  28  (e.g., the optical power) and other considerations familiar to persons skilled in the design of optical systems. The need for this spacing and the presence of the optical turning member  32  (e.g., one or more angled mirrors) for changing the direction of the optical paths influences the overall height of the TIR sub-assembly  26 . This height may even be greater than other components on the printed circuit board  24  and thereby influence the overall profile, or “stack height,” of the fiber optic sub-assembly  20 . Furthermore, the need to support the lenses over the active devices  28  may restrict or limit the placement of passive devices  34  close to the active devices  28 . Such passive devices  34  may include transimpedance amplifiers, resistors, capacitors, inductors, and other circuit components electrically coupled to one or more chips  36  on the printed circuit board  42 . Increasing the distance between the active devices  28  and passive devices  34  may increase losses beyond acceptable limits at high transmission rates. 
     For example, passive components such as a capacitor are sometimes needed to reduce noise in the signal from an active component like a photodiode. The capacitor provides a low impedance at high frequencies so that power supply noise does not couple through the photodiode&#39;s internal capacitance and reach other passive components connected to the photodiode, such as a transimpedance amplifier. But the wires or traces that connect the capacitor to the photodiode tend to negate the low impedance by acting as a series inductor, which has an impedance that rises linearly with frequency. The longer the wires or traces, the more they negate the desired low impendance (and resulting short circuit) provided by the capacitor at high frequencies. Additionally, sufficient inductance may introduce an unwanted resonant frequency in the circuit. 
       FIGS. 2 and 3  illustrate an exemplary embodiment of new arrangement for a fiber optic sub-assembly  40  intended to address some of the above-mentioned challenges. Like  FIG. 1 ,  FIGS. 2 and 3  illustrate the fiber optic sub-assembly  40  within the connector  22  of an active optical cable assembly  10 . Different views are provided to show opposite sides of a printed circuit board  42  in the fiber optic sub-assembly  40 .  FIG. 4  illustrates the fiber optic sub-assembly  40  in isolation for both views. In general, the printed circuit board  42  has opposed first and second surfaces  44 ,  46 . A total internal reflection (TIR) sub-assembly  48  is at least partially integrated into the printed circuit board  42  and supported thereby. The configuration is such that the overall stack height (i.e., profile) defined by the printed circuit board  42  and integrated TIR sub-assembly  48  is less than a non-integrated configuration involving the same components. In other words, the distance between the first and second surfaces  44 ,  46  defines a printed circuit board height. The distance between lowermost and uppermost portions of the TIR sub-assembly  48  defines a nominal height of the TIR sub-assembly  48 . The overall stack height referred to above is less than the sum of the printed circuit board height the nominal height of the TIR sub-assembly. 
     Reference will now be made to  FIGS. 5-10  to describe the fiber optic sub-assembly  40  in further detail. As mentioned above, the embodiment is merely an example; different embodiments of the new arrangement mentioned above will be appreciated by persons skilled in the art. As shown in  FIGS. 5 and 6 , the TIR sub-assembly  48  in this embodiment includes four active devices  28  and four lenses  52 . Each active device  28  and lens  52  is associated with a corresponding optical path defined by the TIR sub-assembly  48 . Four active devices  28  and four lenses  52  are provided in the embodiment shown (e.g., for four different optical paths), although more or fewer may be provided in alternative embodiments. 
     At least a portion of the optical path between each active device  28  and lens  52  is located between the first and second surfaces  44 ,  46  of the printed circuit board  42 . For example, the printed circuit board  42  may include an opening or hole  54  to allow components of the TIR sub-assembly  48  to be supported on opposite sides of the printed circuit board  42 . In the embodiment shown, the TIR sub-assembly  48  includes a body  56  coupled to the side of the printed circuit board  42  that includes the first surface  44 . The body  56  supports the optical fibers  30 , optical turning member  32 , and lenses  52 . The optical fibers  30  may be supported in V-grooves (not numbered in  FIGS. 5-10 ) on an upper surface of the body  56 , for example, and may extend in a plane substantially parallel to the first surface  44  of the printed circuit board  42 . The lenses  52  are supported on an underside surface  58  ( FIG. 10 ) that faces the first surface  44  of the printed circuit board  42 . The body  56  is positioned on the printed circuit board  42  so that lenses  52  are aligned with the opening  54  in the printed circuit board  42 .  FIGS. 7-10  illustrate these aspects in further detail. 
     As shown in  FIGS. 7-10 , the body  56  may be designed to rest on the first surface  44  of the printed circuit board  42 . The body  56  may also be shaped so that the underside surface  58  with the lenses  52  at least partially covers or overhangs the opening or hole  54 . Fiducial features  60  (e.g., holes or other reference structures) are provided on the printed circuit board  42  to properly align and position the body  56  (and lenses  52  supported thereby) relative to the printed circuit board  42 . As will be described in greater detail below, the body  56  may include alignment features  64  (e.g., fiducial holes, projections, or other structures) configured to cooperate with the fiducial features  60  for this purpose. 
       FIG. 10  illustrates how at least a portion of each lens  52  extends into the opening  54  in the printed circuit board  42  so as to be offset from the first surface  44  in a direction towards the second surface  46 . This is due to the body  56  resting on the first surface  44 . In alternative embodiments, the printed circuit board  42  may include a recess or slot that receives the body  56  so that a lower surface of the body  56  is also offset from the first surface  44  of the printed circuit board  42  in a direction towards the second surface  46 . The lenses  54  in such embodiments may therefore be positioned closer to the active devices  28 , if necessary based on the design of the optical system. Arrangements will also be appreciated where the body  56  is shaped to support the lenses  54  above the first surface  44 . 
     Referring back to  FIGS. 5-10  in general, the TIR sub-assembly  48  in the embodiment shown further includes a carrier  66  coupled to the second surface  46  of the printed circuit board  42 . The carrier  66  at least partially covers the opening  54  in the printed circuit board  42  and supports the active devices  28  on an upper surface  68  ( FIGS. 9 and 10 ) that faces the lenses  52 . The active devices  28  may be, for example, wire-bonded to pads on the carrier  66 . Conductors or other electrical links or traces (not shown) on the carrier  66  and printed circuit board  42  may be used to electrically couple the active devices  28  to one or more passive devices  34 . The active devices  28  may alternatively or additionally be bonded to the carrier  66  using conductive epoxy. 
     Alignment of the active devices  28  and lenses  52  may be achieved by using the contours of the carrier  66  as a reference for position registration of the active devices  28 . The carrier  66  may then be coupled to the printed circuit board  42  with a vision system that uses the fiducial features  60  as a reference. Because the alignment features  64  on the body  56  cooperate with the fiducial features  60  to position the body  56  on the printed circuit, the lenses  52  on the body  56  are, in effect, located using the fiducial features  60  as references as well. Other alignment schemes are possible, however, including those using a “look up/look down” optical alignment system. 
     For example, the carrier  66  with the active devices  28  may first be coupled to the printed circuit board  42 . A beam splitter (not shown) may then be positioned somewhere between the active devices  28  and the body  56 , with the latter being moved in a horizontal plane (i.e., X and Y-directions in a reference coordinate system) until the active devices  28  are aligned with the lenses  52 . At this point the beam splitter may be removed and the body  56  and/or printed circuit board  42  may be moved vertically (i.e., in a Z-direction) until the two contact each other. The geometries are such that upon contact, the proper distance is present between the active devices  28  and lenses  52  for the particular optical system design. The body  56  may be bonded in place to the printed circuit board  42  after this positioning using a quick-curing adhesive, such as a UV-curing adhesive, or fixed in position using other known techniques. 
     In alternative embodiments not shown herein, the opening  54  in the printed circuit board  42  may be a recess or well with a bottom surface. The active devices  28  may be positioned on the bottom surface of the recess or well such that a carrier is not needed. Persons skilled in the art will appreciate other variations of the types of arrangements described above, where the active devices  28  are offset from the first surface  44  of the printed circuit board  42  in a direction towards the second surface  46  and the printed circuit board  42  is shaped so that a space is maintained between the active devices  28  and lenses  52 . 
     As can be appreciated, the TIR sub-assembly  48  makes use of space in the printed circuit board  42  to provide the fiber optic sub-assembly  40  with a lower profile than known arrangements. This can best be appreciated by comparing  FIG. 10 , which illustrates a portion of the fiber optic sub-assembly  40 , to  FIG. 11 , which illustrates a portion of the fiber optic sub-assembly  20 . Positioning the active devices  28  proximate or otherwise closer to the second surface  46  of the printed circuit board  42  enables the lenses  54  to be supported proximate or below the first surface  44  (like in  FIG. 10 ) rather than above the first surface  46  (like in  FIG. 11 ). The overall stack height is reduced considerably while maintaining the required space between the active devices  28  and lenses  52 , thereby helping the fiber optic sub-assembly  40  meet the difficult space requirements for standard connector packages in the consumer electronics field or the like. 
     Moreover, positioning the component of the TIR sub-assembly  48  that supports the optical fibers  30  and lenses  52  (i.e, the body  56 ) on an opposite side of the printed circuit board  42  than the active devices  28  allows the passive devices  34  ( FIGS. 7 and 8 ) to be positioned much closer to the active devices  28 . Short electrical path lengths are possible, which results in improved signal to noise performance. The increased freedom to position the passive devices  34  is particularly advantageous for TIR sub-assemblies including more two or more optical fibers (and, therefore, two or more optical paths/channels) because the number of passive devices required increases with the number of optical fibers. Positioning the increased number of passives devices close enough to the active devices so that signal to noise losses remain within acceptable levels can be a challenge, especially when there are four or more optical paths/channels. By not having the body  56  limit the positioning of the passive devices  34  and by providing a low profile/stack height, this challenge can be met in a manner so that a large number of optical paths/channels (e.g., four or more) may be provided with the fiber optic sub-assembly still fitting within standard connector packages. 
       FIG. 12  illustrates a TIR sub-assembly  80  according to an alternative embodiment, and  FIGS. 13 and 14  illustrate the TIR sub-assembly  80  as part of a fiber optic sub-assembly  82 . The TIR sub-assembly  80  may be at least partially integrated with a printed circuit board  42  of the fiber optic sub-assembly  82  such that the advantages mentioned above apply equally to this embodiment. The manner in which the integration is achieved is different, however, due to the TIR sub-assembly  80  having a different configuration. 
     In particular, the carrier  66  supporting the active devices  28  is coupled to the body  56  of the TIR sub-assembly  80  rather than to the printed circuit board  42 . The body  56  is shaped to support the carrier  66  on opposite sides of the active devices  28  and to maintain a space between the active devices  28  and lenses  52 . In the embodiment shown, the body  56  includes first and second portions  84 ,  86  extending from the underside surface  58  on opposite sides of the lenses  52 . The first portion  84  is prismatic or block-like and provides support for the portion of the body  56  on which the optical fibers  30  are disposed. The second portion  86  is also prismatic or block-like, but has a smaller width or thickness than the first portion  84 . To this end, the first portion  84  may be considered a main support for the body  56  while the second portion  86  may be considered a support rim that is spaced from the main support. The space maintained between the upper surface  68  of the carrier  66  on which the active devices  28  are disposed and the underside surface  58  of the body  56  on which the lenses  52  are disposed forms a passage between the first and second portions  84 ,  86 . In alternative embodiments, the first and second portions  84 ,  86  may be joined so that the space takes the form of hole or well in the body  56  (e.g., with the underside surface  58  being a bottom surface of the hole or well). Other shapes and configurations of the body  56  that allow the upper surface  68  of the carrier  66  to be coupled to the body  56  on opposite sides of the active devices  28  will be appreciated by persons skilled in the art. 
     The printed circuit board  42  is shaped to receive the TIR sub-assembly  80 , as shown in  FIGS. 13 and 14 . To this end, a slot or recess (not numbered) may be formed in the first surface  44  of the printed circuit board  42  to accommodate the first and second portions  84 ,  86  of the body  56 . Additionally, an opening  90  extends through the printed circuit board  42  to the second surface  46  thereof to accommodate the carrier  66 . 
     The alignment features  64  on the body  56  cooperate with corresponding fiducial features  60  on the printed circuit board  42  to help enable proper positioning of the TIR sub-assembly  80  (and particularly the active devices  28 ) relative to the printed circuit board  42 . The alignment features  64  may be in the form of fiducial holes or alignment pins, for example. Persons skilled in the art of electric-optical systems will appreciate more detailed aspects of positioning processes that use such alignment features and fiducial features. In terms of the TIR sub-assembly  80 , however, note that the active devices  28  may be positioned relative to the lenses  52  prior to supporting the TIR sub-assembly  80  on the printed circuit board  42 . This pre-alignment may be achieved, for example, by using the alignment features  64  (e.g., fiducial holes) as a positional reference when coupling the carrier  66  to the body  56 . The alignment features  64  have an accurate location with respect to lenses  52  and are used to position the body  56  relative to the printed circuit board  42  (as discussed above). Positioning the active devices  28  relative to the lenses  52  in this manner may result in a smaller tolerance stack-up and thereby provide better alignment between the active devices  28  and lenses  52 . Moreover, aligning the active devices  28  and lenses  52  only with reference to the TIR sub-assembly  80  directly may reduce or relax the accuracy required for positioning the body  56  relative to the printed circuit board  42 . 
     As can be appreciated from  FIG. 13 , the fiber optic sub-assembly  82  still allows passive devices  34  to be positioned in close proximity to the active devices  28 . Conductors  92  may be electrically coupled to the active devices  28  and extend through the carrier  66  to lower surface  94  of the carrier  66 . Additional conductors or electrical leads/traces (not shown) then electrically couple the conductors to the passive devices  34  on the printed circuit board  42 . If desired, some passive devices  34  may even be supported on a lower surface  94  of the carrier  66 . 
     One of the benefits of the fiber optic sub-assembly  82  is that it is a self-contained sub-assembly including the active devices  28  and lenses  52 . This not only allows the active devices  28  and lenses  52  to be pre-aligned (i.e., aligned before supporting the TIR sub-assembly  80  on the printed circuit board  42 ) as discussed above, but also allows the optical system to be tested independently of the printed circuit board  42 . If for some reason the optical system does not function properly, only the TIR sub-assembly  80  is lost. The printed circuit board  42  and its electronic components are not affected by the failure/loss because they were never connected the TIR sub-assembly  80 . In other words, losses the entire fiber optic sub-assembly  82  need not be replaced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention. For example, the TIR sub-assembly  80  is illustrated with the carrier  66  having a smaller footprint area than the body  56 . No portion of the carrier  66  extends transversely beyond the body  56 . In alternative embodiments, portions of the carrier  66  may extend in this manner such that the portions do not face/confront the body  56 . 
     Since these and other modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents.