Abstract:
An optical coupling system includes a unitary separation-setting member for establishing precise spatial relationships among a microlens array, an array of light sources, and an array of optical fibers. The separation-setting member includes an interior region with a shoulder against which the lens array is positioned. The shoulder is at a precisely controlled distance from a target plane along which the ends of the optical fibers are aligned. The target plane is defined by the front surface of the separation-setting member. Thus, the dimensions of the separation-setting member control the distance of the fiber ends from the microlenses. Moreover, a back surface of the separation-setting member is at a precisely controlled distance from the shoulder, so that when the back surface is rested against a substrate on which the light sources are mounted, the light sources are precisely positioned relative to the microlenses.

Description:
TECHNICAL FIELD 
     The invention relates generally to optoelectronic devices and more particularly to maintaining source-to-lens alignment along three perpendicular axes. 
     BACKGROUND ART 
     Transmitting data using optical signals is increasingly taking the place of the traditional approach of exchanging data via electrical signals. An optoelectronic module provides the interface between an optical transfer medium and an electrical medium. For example, the optical transfer medium may be a fiber cable that terminates with a connector that exposes ends of an array of optical fibers. Laser diodes, such as Fabry-Perot lasers or Vertical Cavity Surface Emitting Lasers (VCSELs), are commonly used to generate optical signals in response to electrical excitation signals. Laser diodes are preferred in many applications, since they provide high performance signaling in a miniaturized environment. 
     FIG. 1 illustrates key components of an optoelectronic system. In the illustrated embodiment, the system is a twelve-channel parallel fiber arrangement. Light sources  10 , such as VCSELs, are fabricated on a substrate  12 . The substrate may be a semiconductor die, such as a gallium arsenide chip. A lens array  14  resides between the light sources and an array of parallel optical fibers  16 . The lens array is shown as including a number of optical elements  18 , which are used to manipulate light rays passing from the sources  10  to the fibers  16 . For example, the optical elements may be diffractive elements. 
     While not shown in FIG. 1, an optoelectronic module includes hardware components that secure the light sources  10 , the lens array  14 , and the optical fibers  16 . As is well known in the art, the optical components should be aligned along x and y axes to ensure integrity of signal exchanges. Often, guide pins are used to provide the alignment. For example, guide pins extending along the z axis may have central regions that pass through the lens array  14 , so that end portions can extend into both the substrate  12  and the removable connector that supports the optical fibers  16 . U.S. Pat. No. 5,917,976 to Yamaguchi describes an optical transmission path coupling apparatus that includes guide pins and guide pin holes to provide alignment of fibers to microlenses and light receivers/emitters, with the alignment being along the x and y axes. U.S. Pat. No. 5,867,621 to Luther et al. describes the use of guide pins to properly position two optical fiber connectors, so that the fibers of the connectors are aligned along the x and y axes. 
     Alignment along the z axis is also important to achieving desired performance in a high speed application. In one example, the desired distance between the light sources  10  and the optical elements  18  may be 2.0 millimeters, with a tolerance of ±35 microns in order to pass a sufficient percentage of emitted light to maintain performance. Z-axis alignment is set in some products by lowering the lens array  14  over the array of light sources  10  while sensing the light that is transmitted through the optical elements  18 . The lens array is fixed in position relative to the light sources when maximum light is transmitted through the optical elements. 
     Another z-axis alignment that is critical to optimal performance is the alignment of the ends of the fibers  16  from the optical elements  18 . As one example, the target distance may be 0.475 millimeters, with a tolerance of ±25 microns. This may be achieved by using an alignment tool to join a connector receptacle to another component of the optoelectronic module to which the lens array  14  is attached. 
     While the use of known alignment tools and procedures may provide the target results, the process is often time consuming, so that production throughput is lowered. What is needed is a system and method that provide repeatable precision alignments for an optoelectronic module, with alignments along three axes being achieved without the need of alignment fixtures. 
     SUMMARY OF THE INVENTION 
     An optical coupling system utilizes a one-piece, separation-setting member for defining precise spatial relationships from a lens array to both an array of light sources and an array of optical fibers. The optical fibers are arranged along an end face of a fiber connector that abuts an exterior connector-contacting surface of the separation-setting member. The connector-contacting surface is configured to locate and align the optical fibers along a “target” plane. 
     The separation-setting member includes an interior region in which the lens array resides. The lens array abuts a shoulder having a precisely controlled distance from the target plane having the ends of the optical fibers when the fiber connector is seated against the connector-contacting surface. This precisely controlled distance is based upon maximizing the light transfer through the optical lenses of the lens array to the fibers. The lens array is seated in a manner in which it is parallel to the target plane and is exposed to the target plane through an opening within the separation-setting member. 
     The separation-setting member also includes a back surface that has a precisely controlled distance from the shoulder against which the lens array is seated. During assembly, the back surface is positioned against a substrate that supports the array of light sources. For example, the substrate may be a flex circuit having conductive traces to a semiconductor chip on which light sources, such as VCSELs, are integrated. Because the back surface is at the precisely controlled distance from the shoulder and because the back surface is parallel to the shoulder and the target plane, the light sources will have a desired orientation and distance relative to the lens array. In the preferred embodiment, the portion of the back surface that abuts the light source-supporting substrate is comprised of a number of feet that are strategically positioned to ensure that the parallelism is maintained while providing some access to the interior for a bonding step. 
     An advantage of the invention is that fabricating the separation-setting member to tight tolerances enables the spatial relationships to be achieved without the use of special z-axis alignment steps or tools. Axial alignments along x and y axes are achieved using conventional techniques. For example, the lens array is precisely positioned along the shoulder using a visual alignment system prior to gluing the lens array to the shoulder. Subsequently, x-direction alignment and y-direction alignment between the lens array and the array of light sources may be achieved using active alignment in which power through the lenses from the light sources is monitored while the relative positioning of the two arrays is stepped in increments of one micron. The separation-setting member is fixed in the position at which power is at a maximum. Guide pins are used to provide x and y axes alignment of the connector. It should be noted that the use of guide pins requires exacting positional tolerances of the guide pin holes. The guide pins should extend through the separation-setting member into holes of both the fiber connector and the light source-supporting substrate. 
     The preferred embodiment of the optical coupling system includes a connector receptacle that releasably attaches the fiber connector such that the fiber ends are aligned along the target plane. That is, the receptacle should position the connector to abut the separation-setting member. In this preferred embodiment, the receptacle has both a locked position and a release position relative to the separation-setting member. In the locked position, the receptacle physically engages the separation-setting member, so that the one-piece components are in a desired orientation. However, by rotating the receptacle, the receptacle is moved to its release position in which it can be removed from the separation-setting member. 
     In accordance with the method, the lens array is seated within the interior region of the separation-setting member, so as to contact the shoulder. The back surface of the separation-setting member is then placed in contact with the substrate on which the light sources reside. In one example of the method, the lens array is 2.0 millimeters from VCSELs fixed to a flexible circuit. The distance from the VCSELs to the array is maintained within a tolerance of 35 microns by forming the separation-setting member with a shoulder-to-back surface tolerance of ±10 microns. Moreover, the parallelism of the shoulder and the rear surface is maintained by the fabrication processing during the formation of the separation-setting member. 
     The connector receptacle is rotated into its locking position onto the front surface of the separation-setting member. In this position, insertion of a fiber connector into the receptacle places the connector end in abutment with the connector-contacting surface of the separation-setting member. Thus, the fiber ends are aligned along the target plane that is at a controlled distance from the lens array. In one embodiment, the connector is a Mechanical Transfer Plug (MTP) connector that is spring biased into contact with the separation-setting member. The use of MTP connectors and other spring-biased connectors is well known in the art. The distance between the fiber ends and the lens array may be held to 0.475 millimeters, ±25 microns, using the invention. 
     In one embodiment, the optical coupling system also includes a housing having a first portion adapted to receive the substrate after it has been connected to a heatsink or other component. A second portion of the housing is adapted to securely hold the connector receptacle. For example, the connector receptacle may be simultaneously locked to the separation-setting member and the housing by rotating the receptacle from its release position to its locking position. In a final step, the assembled system is coated with an adhesive to provide extra strength. An advantage of the housing is that it eliminates the need for fixturing to hold the parts together while they are being glued and cured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic top view of a lens array between an array of light sources and an array of optical fibers. 
     FIG. 2 is an exploded view of an optical coupling system in accordance with the invention. 
     FIG. 3 is a perspective view of a one-piece, separation-setting member that is used in the system of FIG.  2 . 
     FIG. 4 is rear perspective view of the separation-setting member of FIG.  3 . 
     FIG. 5 is a front view of the separation-setting member of FIG.  4 . 
     FIG. 6 is a side sectional view of the separation-setting member of FIG.  5 . 
     FIG. 7 is a perspective view of a flex circuit assembly of FIG.  2 . 
     FIG. 8 is a perspective view of a connector receptacle of FIG.  2 . 
     FIG. 9 is a rear view of the connector receptacle of FIG.  8 . 
     FIG. 10 is a perspective view of a housing for securing the optical coupling system of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 2, an optical coupling system  20  is shown in an exploded view. The system includes a receptacle  22  that releasably locks to both a connector  24  and a separation-setting member  26 . The configuration of the receptacle  22  depends upon the type of connector. In one embodiment, the connector is a Mechanical Transfer Plug (MTP) connector, which is also known as a Mechanically Transferrable Plug. Such a connector includes a mating end  28 , a cable entry end  30  and a spring-biased body  32 . A fiber cable  34  enters the connector  24  and the ends of the individual fibers are exposed at a ferrule  36 . 
     When the spring-biased body  32  of the connector  24  is moved rearwardly relative to the mating end  28 , indentations are exposed which mate with latches  38  of the receptacle  22 . Thus, the connector is locked in position within the receptacle by operation of the latches. As a result, the position of the ferrule  36  is such that there is contact with the front surface  40  of the separation-setting member  26 . 
     As will be explained more fully below, a lens array  42  resides within the interior region of the separation-setting member  26 . The lens array includes a number of optical elements that corresponds to the number of fibers exposed at the ferrule  36  of the connector  24 . When the coupling system  20  is fully assembled, the fibers are optically aligned with the lenses of the lens array  42 . By fabricating the separation-setting member  26  using exacting tolerances, the components are precisely aligned with regard to positioning along the z axis. 
     In a conventional manner, guide pins  44  pass through the separation-setting member  26  and into the mating end  28  of the connector  24  to ensure positioning of the connector along the x and y axes. While only one guide pin  44  is shown in FIG. 2, there are typically at least two such pins. 
     The guide pins  44  also extend at least partially into a substrate assembly  46  that includes a flex circuit  48  and a pair of metallic members  50  and  52 . The flex circuit is formed of a flexible material, such as polyimide, having an array of bond pads on the portion of the flex circuit that is attached to the horizontal metallic member  52 . Thus, the bond pads are used for connection to external circuitry which provides drive signals and power for operating light sources on a semiconductor chip  54  that is connected to the portion of the flex circuit on the vertical metallic member  50 . Conductive traces from the bond pads to the semiconductor chip  54  are used to conduct the drive signals and to provide the necessary power to the light sources. Typically, the flex circuit  48  also includes processing circuitry, such as power amplifiers, but this is not critical. 
     A heatsink  56  is thermally coupled to the vertical and horizontal metallic members  50  and  52 . The function of the heatsink is to maintain the circuitry along the flex circuit  48  at a desirable operating temperature. The use of the heatsink is not critical to the invention. 
     A perspective view of the separation-setting member  26  is shown in FIG.  3 . The dimensions of the member  26  are not critical. In one application, the vertical height, as viewed in FIG. 3, is 8.325 mm and the width is 7.3 mm. The member may be formed of a molded plastic material that may be shaped to provide features which satisfy exacting tolerances. 
     The separation-setting member  26  is also shown in FIGS. 4-6, with FIG. 4 being a rear view, FIG. 5 being a front view, and FIG. 6 being a side sectional view. The main function of the member  26  is to secure the lens array  42  of FIG. 2 in a precise location. The location of the lens array is critical, since it must receive the light from the light sources on the semiconductor chip  54  and must focus the light on the fibers that are exposed at the end of the connector  24 . In another embodiment, the fibers release light that is focused upon photodetectors on the semiconductor chip  54 . 
     The member  26  includes an interior region that receives the lens array, so that the lens array is placed flat against a rectangular shoulder  58 , as best seen in FIGS. 4 and 6. The rectangular shoulder includes a central opening  60  for the passage of light from the lens array to the optical fibers, or from the optical fibers to the lens array. Other features  62  within the interior region are provided to aid in applying epoxy to fix the lens array against the shoulder  58 . 
     As noted with regard to FIG. 2, the ferrule  36  at the end of the fiber connector  24  is held against the front surface  40  of the separation-setting member  26 . The distance between the front surface  40  and the shoulder  58  is precisely controlled to define the z-axis alignment of the lens array and fiber array. The distance between the two surfaces is controlled to ±20 microns. Moreover, the parallelism of the shoulder  58  to the front surface  40  is tightly controlled. Alignments in the x direction and the y direction are provided using conventional techniques, such as active alignment. A visual alignment system monitors power that passes through the lenses as the relative positioning of the lens array is moved incrementally. The lens array is glued to the shoulder  58  when the position of maximum power is detected. This procedure is also followed to achieve x axis alignment and y axis alignment of the separation-setting member to the array of light sources. The guide pins  44  of FIG. 2 are used to provide repeatable x axis and y axis alignment of the connector to the lens array and light array. The guide pins pass through cylindrical openings  64  that are only slightly larger in diameter than the guide pins. To ensure that the connector is firmly and repeatedly positioned such that the fiber ends are aligned with the lenses, the guide holes  64  must be precisely located and must be perpendicular to the shoulder  58  and the front surface  40 . 
     As described with reference to FIG. 2, the light sources are integrated into a semiconductor chip  54  on a flex circuit  48 . The flex circuit is joined to two metallic members  50  and  52 . This assembly  46  is shown in greater detail in FIG. 7, with some features being deleted for the purpose of providing clarity. The flexible substrate  48  includes a number of bond pads  65 . Drive circuitry may be formed on a separate circuit board or semiconductor chip that resides within a recess  68 . By locating the circuitry substrate within the recess  68 , the substrate is in better thermal engagement with the metallic member  50 , as compared to mounting the circuitry substrate on the surface of the insulative flex circuit. The traces that extend from the bond pads  65  to the circuitry or the light sources are not shown. 
     An opening  70  through the flex circuit  48  and the metallic member  52  is used to seat a memory chip to the flex circuit. Bonding material may be formed on the memory chip and the surface of the flex circuit  48  to securely hold the chip in position. 
     After the lens array  42  of FIG. 2 has been precisely located within the separation-setting member  26 , the member  26  is placed into contact with the surface of the flex circuit  48 . As best seen in FIG. 3, the rear surface of the separation-setting member  26  includes three feet  72 ,  74  and  76  that extend outwardly and that will contact the surface of the flex circuit  48 . A fixturing tool is not necessary for z axis alignment. Rather, the member  26  may be turned upside down, so that the feet rest flat against the surface of the flex circuit  48 . The feet provide a planar rear surface that is a precisely controlled distance from the shoulder  58  against which the lens array rests. As with the spatial and orientational relationship of the shoulder  58  to the front surface  40 , the parallelism and distance between the lens array and the light sources are ensured by the precise fabrication of the separation-setting member  26 . With regard to alignment along the x and y axes, the active alignment techniques are employed. Features  80  on the flex circuit  48  may be used for proper positioning and for fixing the flexible substrate to the separation-setting member. For example, an adhesive or other bonding material may be used. 
     Referring again to FIG. 2, the connector receptacle  22  must be properly fit to the separation-setting member  26  to ensure that the fibers in the connector  24  are aligned with the microlenses of the lens array  42 . The main function of the receptacle  22  is to guide the connector  24  into the precise location necessary to ensure proper optical communication. Referring now to FIGS. 2,  8  and  9 , the receptacle is dimensioned to receive the connector such that the guide pins  44  extend into guide pin holes within the connector. The spring-biased body  32  of the connector is pressed rearwardly to expose the indentations in which the latches  38  extend to lock the connector into contact with the receptacle and with the separation-setting member  26 . The receptacle is formed of a material which allows the latches to move short distances toward and away from each other without undue material fatigue. 
     Referring now to FIGS. 3,  8  and  9 , the end of the receptacle opposite to the latches  38  extends around the outwardly projecting front portion  82  of the separation-setting member  26 . That is, the outwardly projecting portion  82  is dimensioned to enter the interior of the receptacle  22 . As represented by the arrows  84  in FIG. 9, the receptacle rotates relative to the separation-setting member. The receptacle is shown in its locking position in FIG.  9 . During assembly, the receptacle is rotated to a position in which an ear  86  on the separation-setting member (FIG. 3) is able to freely enter the receptacle. As can be seen in FIG. 5, there is an ear in the upper left hand corner of the outwardly projecting portion  82 , but there is no similar ear in the upper right hand corner. A second ear  88  is located in the corner of the projecting portion  82  diagonal from the first ear  86 . While not apparent in FIG. 5, the second ear  88  is slightly spaced away from the non-projecting lower portion of the member  26 . In the rotated release position of the receptacle of FIG. 8, the ears  86  and  88  enter into the interior of the receptacle. However, when the receptacle is rotated to its locking position, the ears attach the member  26  to the receptacle  22  by rotating behind the regions  90  and  92  that have been blackened in FIG. 9 to more clearly identify the locking arrangement. With the ears  86  and  88  of FIG. 5 residing behind the locking areas  90  and  92  of the receptacle  22 , the receptacle and separation-setting member are properly aligned. In this alignment, the top surface of the receptacle is parallel to the centerline that connects the two guide pins  44 . Thus, the connector is precisely positioned when the connector is inserted into the receptacle. 
     Referring now to FIGS. 2 and 10, the optical coupling system  20  is preferably maintained within a housing  94  having bottom projections  96  that can extend into a printed circuit board or the like to stabilize the system. A rearward portion  98  of the housing  94  is open and is dimensioned to receive the heatsink  56  after it has been adhered to the two metallic members  50  and  52  and after the separation-setting member  26  with the lens array  42  has been seated against the surface of the flex circuit  48 . While not shown in FIG. 2, the heatsink  56  includes an open region that allows a rearward projection  100  of the housing  94  to enter the heatsink. Thus, the projection  100  prevents the heatsink from inadvertently being moved rearwardly. 
     With the heatsink  56 , flex circuit  48 , lens array  42  and separation-setting member  26  assembled onto the housing  94 , the connector receptacle  22  is rotated and locked onto the separation-setting member  26 . As previously noted, the rotation of the receptacle relative to the ears  86  and  88  of FIG. 5 causes the ears and the major portion of the member  26  to be on opposite sides of the locking areas  90  and  92  of FIG. 9, thereby locking the member  26  to the receptacle  22 . Simultaneously, the rotation of the receptacle  22  locks the receptacle into a front portion  102  of the housing  94 . Referring to FIGS. 8 and 10, diagonally opposite ears  104  and  106  on the receptacle  22  are used to lock the receptacle to the front portion  102  of the housing. The front portion  102  has a key  108  on only one side of the top surface. The ear  106  of the receptacle (FIG. 8) is able to rotate into the key  108  of the housing. A similar key (not shown) resides on the lower surface of the front portion  102  of the housing  94 . This second key is positioned to receive the ear  104  (FIG. 8) that is diagonally opposite to the ear  106 . Therefore, the receptacle  22  brings all of the parts together and holds them in place while a gluing step is practiced to provide extra strength. Thus, the use of the receptacle  22  eliminates the need of fixturing to hold the parts together while they are being glued and cured.