Abstract:
VCSEL array configurations for use with parallel WDM transmitters are disclosed. Transmitters that use several wavelengths of VCSELs are built up out of multiple die to avoid the difficulty of manufacturing monolithic arrays of VCSELs with different optical wavelengths. VCSEL configurations are laid out to insure that VCSELs of different wavelengths that destined for the same waveguide are close together.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is related to the patent application entitled “APPARATUS AND METHOD FOR A FILTERLESS PARALLEL WDM MULTIPLEXER”, Attorney Docket Number 10020888 filed on the same day and assigned to the same assignee.  
         BACKGROUND OF INVENTION  
         [0002]    Two optical communications techniques that enable increased bandwidth density in communications systems are parallel optics and wavelength division multiplexing (WDM). In parallel optics, multiple optical data signals are typically transmitted along a multi-fiber ribbon, with a single optical signal being transmitted on each fiber. In WDM, multiple optical data signals are combined and transmitted along a single optical fiber, with each optical signal being carried on a different wavelength.  
           [0003]    Parallel WDM combines the two communications techniques by transmitting multiple optical wavelengths through each fiber of the multi-fiber ribbon. Parallel optical transmitters are typically constructed from monolithic arrays of vertical cavity surface emitting lasers (VCSELs) operating at single wavelength. Because it is typically difficult to manufacture monolithic arrays of VCSELs operating at different wavelengths, a parallel WDM transmitter that operates VCSELs at several wavelengths is typically built out of multiple dies. It is typically advantageous for VCSELs of different wavelengths to be close together because the light from several different wavelength VCSELs typically must be combined into a single optical fiber using an optical multiplexer.  
         SUMMARY OF INVENTION  
         [0004]    In accordance with the invention, specific VCSEL array configurations of VCSEL die are described that are consistent with compact filterless optical multiplexers and low-cost manufacturing for realizing parallel WDM transmitters. VCSELs of different wavelengths whose light is destined for the same waveguide are configured to be close together. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 shows a simplified conceptual view of an element of a parallel WDM transmitter in accordance with the invention.  
         [0006]    [0006]FIG. 2 a  shows an embodiment in accordance with the invention.  
         [0007]    [0007]FIG. 2 b  shows a simplified view of solder bumps on two embodiments in accordance with the invention.  
         [0008]    [0008]FIG. 3 a  shows a group of VCSEL die in accordance with the invention.  
         [0009]    [0009]FIG. 3 b  shows a first plane of lenses forming part of an optical multiplexer in accordance with an embodiment of the invention.  
         [0010]    [0010]FIG. 3 c  shows a second plane of lenses forming part of an optical multiplexer in accordance with an embodiment of the invention.  
         [0011]    [0011]FIG. 4 shows a simplified view of a parallel WDM transmitter in accordance with an embodiment of the invention.  
         [0012]    [0012]FIG. 5 shows an embodiment in accordance with the invention.  
         [0013]    [0013]FIG. 6 shows an embodiment in accordance with the invention.  
     
    
     DETAILED DESCRIPTION  
       [0014]    In accordance with the invention, a configuration of VCSELs may be arranged as linear arrays, n by n arrays or other geometries in accordance with the invention. FIG. 1 shows a conceptual view of an embodiment in accordance with the invention. Filterless parallel WDM multiplexer element  75  is used to direct light from VCSEL configuration  70 , comprising VCSELs  10 ,  20 ,  30 ,  40  to optical fiber  50 . The number of VCSELs and lenses may be increased along with the number of optical fibers. Using two or more filterless parallel WDM multiplexer elements  75  with two or more optical fibers, respectively, results in a filterless parallel WDM multiplexer as shown by, for example, filterless parallel WDM multiplexer  201  in FIG. 4.  
         [0015]    Lenses in plane  1  such as lenses  11 ,  21 ,  31 ,  41  are typically made just large enough to collect most of the light emitted by VCSELs  10 ,  20 ,  30 ,  40  such as beams  60 ,  61 ,  62 ,  63 , respectively. The general design considerations are as follows. Because a VCSEL typically emits a vertical cone of light, the center of the lens aperture in plane  1  should be aligned with the VCSEL aperture to capture the VCSEL light. In order to direct light from a first lens in a first plane to the appropriate lens in the a second plane, the vertex of the first lens must lie on the line connecting the VCSEL aperture to the center of the appropriate lens aperture in the second plane. This results in an offset between the center of the first lens aperture and the vertex of the first lens. Therefore, the first lens is an off-axis section of a lens. The appropriate lens in the second plane needs to be large enough to capture most of the light incident on it and focus this light into the optical fiber. The lens in the second plane focuses the incident light into the optical fiber which is positioned to minimize the overall range of angles of the incident light going into the optical fiber. Because the lens in the second plane needs to focus the incident into the optical fiber, the line connecting the optical fiber center with the lens vertex needs to be parallel to the incident light which by design is parallel to the line connecting the VCSEL aperture to the center of the lens in second plane. This requires that there be an offset between the center of the lens aperture in the second plane and the lens vertex. Hence, the lens in second plane is also off-axis. The other lenses of the multiplexer and any additional optical fibers are similarly positioned.  
         [0016]    The general design considerations discussed above assume that the VCSEL is a point source which is an approximation. Additional assumptions have neglected diffraction and lens aberrations. The design implementation of WDM multiplexer corrects for these factors and the implementation typically will differ from the above description that, however, results in a baseline design that is qualitatively similar to the actual implementation. In practice, the qualitative description provides a starting configuration that may be iteratively modified using ray tracing software packages such as ZEMAX® or CODE V® until the amount of VCSEL light reaching the optical fiber has been optimized.  
         [0017]    With respect to FIG. 1, for example, VCSEL  10  is lined up with the center of lens  11  and lens  11  needs to be large enough to capture most of the light from VCSEL  10 . The vertex of lens  11  lies in plane  1  on the line defined by VCSEL  10  and the center of lens  12 . Hence, the vertex of lens  11  and the center of the aperture of lens  10  are offset from each other and lens  11  is an off-axis lens. Lens  12  in plane  2  needs to be sufficiently large to collect most of the light incident on it and focus that light into optical fiber  50 . Lens  12  focuses most of the incident light into optical fiber  50  which is positioned to minimize the overall range of angles of the incident light that is entering optical fiber  50 . Because lens  12  focuses the light into optical fiber  50 , the line connecting the center of optical fiber  50  needs to be parallel to the incident light. By design, the incident light is parallel to the line connecting the aperture of VCSEL  10  to the center of lens  12  in plane  2 . This requires that the vertex of lens  12  and the center of the aperture of lens  12  are offset from each other. Hence, lens  12  is also an off-axis lens. Similar considerations apply for lenses  21 ,  31 ,  41  in plane  1  and lenses  22 ,  32 ,  42  in plane  2 .  
         [0018]    In FIG. 1, each of VCSELs  10 ,  20 ,  30 ,  40  operates at a separate wavelength to generate light beams  60 ,  61 ,  62 ,  63 , each light beam being at a particular wavelength. VCSELs  10 ,  20 ,  30 ,  40  typically reside on separate die. Light beams  60 ,  61 ,  62 ,  63  enter filterless parallel WDM multiplexer  75  having two planes of lenses. In FIG. 1, VCSELs  10 ,  20 ,  30 ,  40  transmit light beams  60 ,  61 ,  62 ,  63  to lenses  11 ,  21 ,  31 ,  41  residing in first lens plane  1 . Lenses  11 ,  21 ,  31 ,  41  function to redirect beams  60 ,  61 ,  62 ,  63  into lenses  12 ,  22 ,  32 ,  42 , respectively. Lenses  12 ,  22 ,  32 ,  42  residing in second lens plane  2  function to direct light beams  60 ,  61 ,  62 ,  63 , respectively, into optical fiber  50 . Hence, light of four different wavelengths is multiplexed into optical fiber  50 .  
         [0019]    [0019]FIG. 2 a  shows an embodiment of a VCSEL array configuration for a parallel WDM transmitter in accordance with the invention. Configuration  100  shown in FIG. 2 a  is a four wavelength, twelve optical fiber or waveguide configuration constructed from two-dimensional single wavelength monolithic VCSEL arrays. In this embodiment, there are three groups  101 ,  102 ,  103  of four square dies  121 ,  122 ,  123 ,  124  corresponding to two by two VCSEL arrays  150 ,  160 ,  170 ,  180 , respectively. The number of die and groups in the configuration may be increased in accordance with the invention to allow for both more wavelengths and optical fibers or waveguides. VCSEL arrays  150 ,  160 ,  170 ,  180  each operate at a different wavelength. Dies  121 ,  122 ,  123 ,  124  are arranged such that each group of square die  101 ,  102 ,  103  contain VCSEL arrays for each of the four wavelengths. This arrangement ensures that devices of different wavelengths are sufficiently close together to avoid the need for large angle deflections within multiplexer element  75  (i.e. between planes  1  and  2 ) to direct the light beams into optical fiber  50 . The need for large angle deflections using refractive lenses presents a cost issue and using diffractive lenses results in higher light losses.  
         [0020]    The substantially square aspect ratio of dies  121 ,  122 ,  123 ,  124  improves handleability in the manufacturing environment and reduces handling breakage. VCSEL material is typically brittle and VCSEL structures with a high aspect ratio are inherently more susceptible to damage than VCSEL structures with a low aspect ratio. Long VCSEL arrays (high aspect ratio) have proportionally more surface area than square VCSEL arrays (low aspect ratio). For example, a three by three VCSEL array on a 250 μm pitch has nine devices with a perimeter of 3000 μm whereas a one by nine VCSEL array also has nine devices but for the same pitch has a 5000 μm perimeter. Because cracks usually start on the die perimeter, reducing the die perimeter typically increases the VCSEL array yield. Additionally, long VCSEL arrays are typically subject to more stress due to thermally induced stresses resulting from attachment to the substrate material.  
         [0021]    Conventional production tooling is typically designed to handle parts that have a low aspect ratio. The majority of semiconductor devices have a relatively low aspect ratio (typically an approximately square shape when viewed from the top or bottom) and as a result the conventional production tooling is typically designed to accommodate such low aspect ratio shapes.  
         [0022]    Using two by two VCSEL arrays  150 ,  160 ,  170 ,  180  located on die  121 ,  122 ,  123 ,  124 , respectively, the arrangement of the bond-pads (not shown) on each die  121 ,  122 ,  123 ,  124  allows the use of solder reflow self-alignment during alignment and attachment decreasing assembly costs. Typically, solder reflow self alignment is more effective for two by two arrays such as VCSEL arrays  150 ,  160 ,  170 ,  180 . FIG. 2 b  shows a simplified view of solder bumps  199  and  599  on the bottom of die  121  and die  515 , respectively (see FIG. 5). Solder bumps  199  and  599  act to self align die  121  and die  515 , respectively, during reflow.  
         [0023]    The self-alignment mechanism is due to minimization of the surface tension at each of the individual solder attachment sites so that at each solder attachment site the surface tension is minimized. Each solder bump has a somewhat different volume and wets the bonding pads somewhat differently. The differences are relatively small but cause each solder bump to pull dies  121  and  515  in a different direction. A vector summing of the various forces occurs resulting in the final positioning of die  121  and  515 . Because a two by two VCSEL array has a higher degree of symmetry than a one by twelve VCSEL array, better alignment typically results for a two by two VCSEL array or other VCSEL arrays having a higher degree of symmetry than a one by twelve array VCSEL array.  
         [0024]    The size of two by two VCSEL arrays  150 ,  160 ,  170   180  can be reduced in size to the minimum size needed for solder bumps to attach VCSEL arrays  150 ,  160 ,  170 ,  190  to the substrate. For example, if sufficiently small solder bumps are used to attach two by two VCSEL arrays  150 ,  160 ,  170 ,  180  that are 150 μm on a side, the VCSEL array size will work with filterless parallel WDM multiplexer  201  (see FIG. 4) even if the pitch of the optical fiber array is 250 μm. In contrast, for one by twelve VCSEL arrays  511 ,  521 ,  531 ,  541  (see FIG. 5), the pitch of the VCSEL array is constrained by and must match the pitch of the optical fiber array. Because the cost of VCSEL die is proportional to their area cost may be reduced by reducing area. In addition, having a relatively small number of devices per die increases the yield per die. For example, if 5% of the VCSELs in a one by twelve VCSEL arrays  511 ,  521 ,  531 ,  541  are defective, the array yield will be about 54% if the defects are random. For two by two VCSEL arrays  150 ,  160 ,  170 ,  180  with the same defect rate of 5%, the array yield will be 81%. Because yield per die is proportional to cost, smaller arrays are much cheaper.  
         [0025]    [0025]FIG. 3 a  shows group  101  of FIG. 2 with VCSEL apertures  150   a - 150   d ,  160   a - 160   d ,  170   a - 170   d ,  180   a - 180   d  of VCSEL arrays  150 ,  160 ,  170 ,  180 , respectively, labeled to illustrate how light is optically directed from the individual VCSELs into the optical fibers. FIG. 3 b  shows the portion of first lens plane  210 , corresponding to group  101 , used to multiplex the light from VCSEL arrays  150 ,  160 ,  170 ,  180  into optical fibers for the embodiment in FIG. 1. Each of lenses  151   a - d ,  161   a - d ,  171   a - d ,  181   a - d  in first lens plane  210  is offset in the horizontal plane with respect to VCSEL apertures  150   a - d ,  160   a - d ,  170   a - d ,  180   a - d , respectively. This allows light coming from the VCSEL apertures  150   a - d ,  160   a - d ,  170   a - d ,  180   a - d  through lenses  151   a - d ,  161   a - d ,  171   a - d ,  181   a - d  to be directed at an angle to intersect corresponding lenses  152   a - d ,  162   a - d ,  172   a - d ,  182   a - d  in second lens plane  220  (see FIG. 3 c ).  
         [0026]    [0026]FIG. 3 c  shows how light from first lens plane  210  is mapped into lenses  152   a - d ,  162   a - d ,  172   a - d ,  182   a - d  in second lens plane  220  as viewed from the optical fiber side. Lenses  152   a - d ,  162   a - d ,  172   a - d ,  182   a - d  in second lens plane  220  are positioned so that light from lens groups  301 ,  302 ,  303 ,  304  is focused into optical fibers  352 ,  362 ,  372 ,  382 , respectively. Starting from the nine o&#39;clock position in each group and going clockwise, lens group  301  has lenses  152   a ,  162   c ,  182   a ,  172   c ; lens group  302  has lenses  152   c ,  162   a ,  182   c ,  172   c , lens group  303  has lenses  152   b ,  162   d ,  182   b ,  172   d ; lens group  304  has lenses  152   d ,  162   b ,  182   d ,  172   b . The axis of each optical fiber  352 ,  362 ,  372 ,  382  is aligned with the center of lens groups  304 ,  303 ,  302 ,  301 , respectively. Lenses in each lens group  304 ,  303 ,  302 ,  301  are positioned such that the four lenses in each group focus the light into optical fibers  352 ,  362 ,  372 ,  382 , respectively. Light from lens  151   a  is directed to lens  152   a ; light from lens  151   b  is directed to lens  152   b ; light from lens  151   c  is directed to lens  152   c ; light from lens  151   d  is directed to  152   d ; light from lens  161   a  is directed to lens  162   a ; light from lens  161   b  is directed to lens  162   b ; light from lens  161   c  is directed to lens  162   c ; light from lens  161   d  is directed to lens  162   d ; light from lens  171   a  is directed to lens  172   a ; light from lens  171   b  is directed to lens  172   b ; light from lens  171   c  is directed to  172   c ; light from lens  171   d  is directed to  172   d ; light from lens  181   a  is directed to lens  182   a ; light from lens  181   b  is directed to lens  182   b ; light from lens  181   c  is directed to lens  182   c ; light from lens  181   d  is directed to lens  182   d.    
         [0027]    The mapping of the light beams between first lens plane  210  and second lens plane  220  is designed to minimize the largest required angular bending of the light within the configuration constraints. FIG. 4 shows a side view of the configuration shown in top view in FIGS. 3 a - 3   c  showing VCSEL arrays  150  and  160  and the position of optical fibers  352 ,  362 ,  372 ,  382 . VCSEL arrays  170  and  180  are not shown to aid clarity. Dashed lines in FIG. 4 relate to the hidden VCSEL apertures  160   d ,  160   c ,  150   d ,  150   c  and corresponding hidden lenses  161   d ,  161   c ,  151   d ,  151   c.    
         [0028]    [0028]FIG. 4 shows the mapping by optical multiplexer  201  of light beams  410 ,  411 ,  412 ,  413 ,  414 ,  415 ,  416 ,  417  from lens plane  210  to lens plane  220  and into optical fibers  352 ,  362 ,  372 ,  382 . Light beam  410  originates from VCSEL aperture  160   b  passing through lens  161   b  to lens  162   b  and into optical fiber  352 . Light beam  411  originates from VCSEL aperture  160   d  passing through lens  161   d  to lens  162   d  and into optical fiber  362 . Light beam  414  originates from VCSEL aperture  160   a  passing through lens  161   a  to lens  162   a  and into optical fiber  372 . Light beam  416  originates from VCSEL aperture  160   c  passing through lens  161   c  to lens  162   c  and into optical fiber  382 . Light beam  412  originates from VCSEL aperture  150   d  passing through lens  151   d  to lens  152   d  and into optical fiber  352 . Light beam  413  originates from VCSEL aperture  150   b  passing through lens  151   b  to lens  152   b  and into optical fiber  362 . Light beam  415  originates from VCSEL aperture  150   c  passing through lens  151   c  to lens  152   c  and into optical fiber  372 . Light beam  417  originates from VCSEL aperture  150   a  passing through lens  151   a  to lens  152   a  and into optical fiber  382 .  
         [0029]    [0029]FIG. 5 shows an embodiment of VCSEL array configuration  501  in accordance with the invention, each VCSEL array being a one by twelve array. Each of die  510 ,  520 ,  530 ,  540  contains the same number of VCSELs  515 ,  525 ,  535 ,  545 , respectively, as the number of optical fibers (not shown). The embodiment shown is for use with 12 optical fibers and four wavelengths. Hence, there are four monolithic  12  element VCSEL arrays  511 ,  521 ,  531 ,  541 , with each VCSEL array operating at a different wavelength. To have VCSELs of different wavelengths closely spaced, the optimal configuration for the four die approach is for each die  510 ,  520 ,  530 ,  540  to be a one by twelve linear array where the width is minimized. Typically, VCSEL arrays  511 ,  521 ,  531 ,  541  might be about 3000 μm long and only about 200 μm wide. This embodiment minimizes the number of single-wavelength die that need to be used and is amenable to use with a number of multiplexing schemes such as the interference-filter-based zigzag geometry described in U.S. Pat. No. 6,198,864 used in the reverse direction as a multiplexer. The high aspect ratio of die  510 ,  520 ,  530 ,  540  has the disadvantage that the die are difficult to manipulate, susceptible to breakage and the relatively large number of VCSELs per die reduces the overall yield per die.  
         [0030]    [0030]FIG. 6 shows an embodiment of VCSEL configuration  601  in accordance with the invention. The embodiment shown is for use with 12 optical fibers (not shown) and four wavelengths and uses one die for each VCSEL resulting in 48 die. Configuration  601  provides the tightest possible spacing of VCSELs of different wavelengths subject to the constraint of one VCSEL per die. Device yield is maximized because there is only one device per die. However, due to the maximization of the number of die, time and cost for assembly are increased.  
         [0031]    VCSEL apertures  615 ,  625 ,  635 ,  645 , one aperture for each VCSEL, reside on die  620 ,  630 ,  640 ,  650 , respectively. The group of four VCSEL apertures  615 ,  625 ,  635 ,  645 , each typically emitting at a different wavelength, is repeated 12 times to provide a total of 48 VCSEL apertures. Typically, in order to keep different wavelength emitting VCSEL apertures  615 ,  625 ,  635 ,  645  as close together as possible, die  620 ,  630 ,  640 ,  645  are arranged into closely spaced two by two groups  661 ,  662 ,  663 ,  664 ,  665 ,  666 ,  667 ,  668 ,  669 ,  670 ,  671 ,  672 . Note that VCSEL apertures  615 ,  625 ,  635 ,  645  are positioned so that they are at the inner corners of die  620 ,  630 ,  640 ,  650 . However, configuration  601  is merely exemplary and may be modified to accommodate a different number of optical fibers and wavelengths. For example, if more than four wavelengths are to be used, non-rectangular die such as those described in U.S. patent application Ser. No. 10/370,853 filed Feb. 21, 2003 (attorney docket number 10011197-1) and incorporated by reference may be used in accordance with the invention.  
         [0032]    While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.