Abstract:
A method and apparatus are provided for transmitting an optical communications signal. The method includes the steps of securing an optical device onto an optically transparent substrate with an optically transparent adhesive such that an axis of transmission of the optical device passes directly through the optically transparent adhesive and a portion of the body of the optically transparent substrate, darkening a portion of the optically transparent adhesive with a laser; and transmitting light from the optical device through the darkened portion of the optically transparent adhesive such that at least some of the light from the optical device is absorbed by the darkened portion.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to optical communication devices, and more specifically to the structure of opto-electronic couplers.  
         BACKGROUND OF THE INVENTION  
         [0002]    This application claims the benefit of U.S. Provisional Patent Application No. 60/317,138, filed on Sep. 6, 2001.  
           [0003]    Traditionally, VCSEL dies are vertically mounted to a printed circuit board, or PCB, with light emitting from the same surface as the electrical contacts. The PCB is usually made of FR4 or ceramic. As shown in the prior art of FIG. 1, a TO can  12  has wire bonds  16  used in electrically connecting the VCSEL die  14 . Wire bonds  16  are more susceptible to damage than solder bumps, and are generally avoided if possible. In addition, wire bonding is inconsistent in terms of variance in electrical properties. As the wire lengths vary, the resistance, inductance and capacitance also varies.  
           [0004]    As shown in FIG. 1, the TO can&#39;s base comprises a header  20  and a conductive spacer  18 . A metallic structure  22 , referred to as a can, provides a hermetic seal for a VCSEL laser array  14 . Optical signals  26  exit the TO can  10  through a lens  24 , and may be appropriately coupled into a waveguide (not shown). Lensing mechanisms are often needed to couple light as desired into a waveguide or optical fiber. For example, a VCSEL laser die contains electrical contacts on the same surface of light emission, and wire bonding to that surface will increase the minimum distance from the active surface of the laser to the optical fiber. As a result, the signal may require lensing to gather diverging light.  
           [0005]    A method of attaching the VCSEL die using metal to metal contacts on the pads such as solder bumps or stud bumps can make closer connections that are more consistent in electrical variance and offer greater structural stability than wire bonds. This method of attaching is commonly referred to as flip chipping. Wire bonding will add to the overall height in the package more so than flip chipping, as shown in FIG. 1. In addition, flip chipping allows for a waveguide and/or lens structure to be placed closer to the surface of light emission. Thus, more light could be gathered from an optical port into a waveguide or optical fiber. This in turn may preserve signal integrity. In this invention is provided a novel way to couple light from an optical device, into a waveguide, and subsequently into an optical fiber. The invention may allow coupling of a more uniform optical beam profile while transmitting an appropriate optical energy amount to a receiving device. In addition, the invention may promote signal integrity.  
         SUMMARY OF THE INVENTION  
         [0006]    A method and apparatus are provided for transmitting an optical communications signal. The method includes the steps of securing an optical device onto an optically transparent substrate with an optically transparent adhesive such that an axis of transmission of the optical device passes directly through the optically transparent adhesive and a portion of the body of the optically transparent substrate, darkening a portion of the optically transparent adhesive with a laser; and transmitting light from the optical device through the darkened portion of the optically transparent adhesive such that at least some of the light from the optical device is absorbed by the darkened portion.  
           [0007]    When moving a waveguide or optical fiber closer to an optical port of a VCSEL, a more uniform LASER beam profile can be coupled, and the total amount of light gathered will increase. As it is desirable to couple a uniform optical profile, it is not necessarily advantageous to gather as much light as possible. Capturing too much light through an optical fiber or waveguide could cause a few problems, one of which is eye safety. As a laser can cause permanent damage to the human eye, it is imperative to ensure that a laser&#39;s output does not come in contact with a human eye in a hazardous manner. To do this, the net amount of light output could be decreased or absorbed in order to promote an eye safe device. That is, enough light output could be decreased or absorbed to maintain signal integrity but promote eye safety.  
           [0008]    Another possible consequence in gathering too much light involves the inability of a receiving optical device to process the light energy. A photodetector may provide an electrical output that is proportional to the amount of light energy from a transmitting device. If the input signal to a photodetector contains too much light energy, the photodetector could become saturated. That is, the linear proportionality between the incoming light energy and the outgoing electrical signal could diminish, and the photodetector may not respond accordingly to a certain range of light energy. Additionally, if the photodetector has not already saturated, the signal processor receiving the electrical signal from the photodetector could become saturated. That is, the signal processor&#39;s limits will have been reached because the value of the input electrical signal could be too high. Because of these two consequences in gathering too much light energy, it is necessary to appropriately attenuate an optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a front view of a prior art optical device;  
         [0010]    [0010]FIG. 2 is a profile view of an electro-optic communications assembly under an illustrated embodiment of the invention;  
         [0011]    [0011]FIG. 3 is a side view of an optical array attached to the optically transparent substrate of FIG. 2;  
         [0012]    [0012]FIG. 4 is a top view of an optical array of attached to the substrate of FIG. 2; and  
         [0013]    [0013]FIG. 5 is a profile view of the substrate of FIG. 2 and a pattern recognition system. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 2 shows an electro-optic communications assembly  40  in accordance with the invention and in a context of use. The assembly  40  may include a common substrate  42 , or printed circuit board (PCB), an optically transparent substrate assembly  52  attached to the PCB  42 , and an optical connector assembly  54  generally holding a plurality of optical fibers  72 .  
         [0015]    The PCB  42 , may be any suitable material such as FR4, ceramic interconnect, or the like. The PCB  42  may have a plurality of electrical and optical devices for signal processing, as well as electrical traces and electrical pads (not shown in the figure).  
         [0016]    The optically transparent substrate assembly  52  may comprise a glass or a glass-like structure having desirable optical and structural properties, and could be about 110 microns in thickness. The substrate assembly  52  generally includes first and second optically transparent substrates  58 ,  60  joined along a common edge and an optical array  44 . The optical array  44  may be mechanically and electrically attached to the first substrate  58 . The second substrate  60  may be attached to the PCB  42  by a conductive adhesive, solder/stud bumps, or a similar material. As shown in FIG. 2, the planar elements  58 ,  60  of the substrate assembly  52  may be separated by a right angle bend at an appropriate location to allow planarity of optical signals  46  of the optical array  44  with respect to the PCB  42 . Further details of the substrates  58 ,  60  will be discussed below.  
         [0017]    The optical connector assembly  54  may generally comprise a plug  62  holding a plurality of optical fiber ends  72  in alignment with a set of apertures  70  as shown in the figure. In a preferred embodiment of the invention, the fiber holding alignment mechanism  50  could be a standard MT connector, or ferrule, manufactured by US Conec or Nippon Telephone &amp; Telegraph; US Conec Part number MTF-12MM7.  
         [0018]    [0018]FIG. 3 illustrates a side view of a portion of the first substrate  58 . The substrate  58  may have disposed on a first surface  78  conventional electrical bumps (or stud bumps)  86  and electrical traces (not shown) for electrically connecting the optical array  44  to a signal processing device (not shown). The electrical attachment method is not limited to stud bumps but may include solder bumps or a similar method. In a preferred embodiment of the invention, stud bumps electrically attached the optical array  44  is to the substrate  58 , and an optically transparent adhesive  76 , or underfill, mechanically attaches the optical array  44  to the first surface  78  of the substrate  58 . The details of the optically transparent underfill  76  will be described in further detail below.  
         [0019]    It will be understood that the active optical array  44  can be any suitable photonic device or array of photonic devices including photo-transmitters, photo-receivers, or a combination thereof. A photo-transmitter can be any suitable device such as a vertical cavity surface emitting laser (VCSEL), light emitting diode (LED), or the like. Furthermore, any suitable photo-receiving device can be used, such as a photodiode, i.e., P-I-N diode, PN diode, or the like. Thus, the active optical array  44  can be a broad range of photoactive devices with transmitting and receiving capabilities. The optical array may have a number of optical ports  90 , and each optical port  90  may be a photonics transmitter, receiver, or a combination transmitter/receiver, (FIG. 4 shows eight optical ports  90 , yet the number of ports  90  is not limited to a specific value).  
         [0020]    As shown in FIG. 3, the optically clear underfill may mechanically attach the optical array  44  to the substrate  58 . The thickness of the underfill  76  may be 50-60 microns, or about the thickness of the stud bumps  86 . In a preferred embodiment of the present invention, the underfill  76  is a conventional epoxy supplied by Epoxy Technology (commonly referred to as Epo-Tek). Yet, an addition epoxy displaying adequate optical and thermal properties could be used in the application. The optical signal&#39;s transmission paths  46  originating from the optical ports  90  may sequentially pass directly through the underfill  76  and the optically transparent substrate  58 .  
         [0021]    Also illustrated in FIG. 3 is a region of the optically clear underfill  76  that has been darkened,  94 . As light emits from the optical port  90  in direction  46  shown, the darkened region  94  may absorb a portion of light. The darkened region  94  may then attenuate an optical signal as desired.  
         [0022]    Details of the darkened region  94  will now be described in further detail. In a preferred embodiment of the invention, the darkened region  94  may be formed in the underfill  76  after the underfill  76  has cured. Thus, relative movement between the optical array  44  and the substrate  58  may be minimized when the darkened region  94  is being formed. This way, the darkened region  94  is formed at a precise location with respect to the transmission paths  46  of the optical array  44 .  
         [0023]    The darkened region  94  of the underfill  76  may be formed by using a conventional recognition module  110  comprising a photodetector  112  and a laser  114 . The recognition module  110  may include software adapted to recognize and position itself in the line of the transmission path  46  of the optical signal from an optical port  90 . Once recognition of the optical signal has occurred, the photodetector  112  may measure the light output from the optical port  90  and compare it with a nominal output programmed into a computing system. Upon reading the optical power output, the optical signal from the optical port  90  is disabled, and the laser  114  may be positioned in the line of the transmission path  46  of the optical signal.  
         [0024]    The laser  114  is activated, and its output energy may be concentrated on a precise area of the optically clear underfill  76 . When exposed to a specific amount of energy for a precise amount of time, the underfill  76  may darken accordingly. Essentially, the underfill  76  may over-cure and darken when exposed to energy from the laser  114 . The amount of darkening desired coincides with the deviation of optical power output of the port  90  from the nominal output, (i.e., an optical output that is substantially higher than the nominal output may receive greater underfill darkening  94  along its transmission path  46 ).  
         [0025]    The laser  114  used to darken the underfill  76  may be an excimer laser capable of emitting a controlled energy density. Similar lasers could be used to treat the underfill as appropriate.  
         [0026]    Shown in FIG. 4 is a front view of a portion of the second surface  80  of the optically transparent substrate  58 . Shown through the substrate are the plurality of darkened regions  94  as described above. In a preferred embodiment of the invention, one darkened region  94  exists for each optical port  90 . Accordingly, each darkened region  94  could be fine tuned according to the output of each optical port  90  such that a desired optical energy is transmitted into an optical fiber  72 . That is, for each optical port  90 , the darkened region may be darkened to a specific amount as desired to reduce output energy to an appropriate level. Thus, the optical energy entering each optical fiber  72  could substantially be the same based on the level of darkening in each region.  
         [0027]    In an alternate embodiment of the invention, a larger portion of the optically transparent underfill  76  could be darkened, such that all of the optical signals  46  of the optical array  44  pass substantially through the one darkened region. Using one darkening region has the advantage of less manufacturing time, but it does not allow a unique attenuation for each optical signal from each optical port.  
         [0028]    Also shown in FIG. 4 is a set of apertures  88  formed in the transparent substrate  58  for receiving a set of alignment guide pins  64 . The apertures  88  may properly align the optical port  90  of the optical array  44  to the optical fibers  72  of the optical connector assembly  54 , as shown in FIG. 2.  
         [0029]    The alignment pins  64 , held in place by a pin holder assembly  50  shown in FIG. 2, are concurrently inserted through the apertures  88  in the substrate  58 , and then through a corresponding set of apertures  70  formed on a first surface  74  of the plug  62 . This is turn collinearly aligns the optical port  90  of the optical array  44  to the respective optical fibers  72  of the optical connector assembly  54 .  
         [0030]    The pin holder assembly  50  may generally comprise a pin holder block  56  and a plurality of guide pins  64 . The pin holder block may be fabricated by a conventional machining or casting process, out of a suitable material having desirable thermal conductivity and thermal expansion properties. Materials found to exhibit such characteristics include but are not limited to stainless steel and molybdenum. The guide pins may in turn be glued or press fit into the pin holder block  56 .  
         [0031]    To form the alignment apertures  88  in the substrate  58  for receiving the alignment guide pins  64 , a boring fixture  96  may be used (FIG. 5). The boring fixture  96  may include a pattern recognition module  98  and lasers  100 ,  102 . The pattern recognition module  98  may include software adapted to recognize and position itself over a line of targets (not shown).  
         [0032]    Once recognition of targets has occurred, the pattern recognition module  98  functions to identify a transverse line passing through the line of targets as well as a center point of the line of targets. The pattern recognition module  98  then positions its own transverse line and center point with the identified transverse line and center point. The lasers  100 ,  102  may be precisely aligned along the transverse line of the pattern recognition module  98 . The lasers  100 ,  102  are also positioned a precise distance on either side of the center point of the pattern recognition module  98 .  
         [0033]    The pattern recognition module  98  may be programmed to view the array  44  through the transparent substrate  58  and identify the set of alignment targets (e.g., the alignment targets on opposing ends of the array  44 ). Once the pattern recognition module  98  has aligned itself with the recognition targets (and also the lasers  100 ,  102  on either side of the targets), the boring fixture  96  activates the lasers  100 ,  102  to ablate the holes  88  in precise alignment with the port  90 .  
         [0034]    A specific embodiment of a method and apparatus for attenuating an optical signal used in communications devices has been described for the purpose of illustrating the manner in which the invention is made and used. It should be understood that the implementation of other variations and modifications of the invention and its various aspects will be apparent to one skilled in the art, and that the invention is not limited by the specific embodiments described. Therefore, it is contemplated to cover the present invention and any and all modifications, variations, or equivalents that fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.