Abstract:
A method and apparatus are provided for providing an electro-optic interface for exchanging information signals. The method includes the steps of disposing an optical array adjacent a first side of an optically transparent substrate, such that a plurality of transmission paths of the optical array pass directly through the substrate, applying an optically transparent underfill between the substrate and adjacent optical array with the plurality of transmission paths of the optical array passing directly through the underfill and coupling a plurality of optical signals of the optical array through the optically transparent underfill and optically transparent substrate between the optical array and an optical connector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/282,561 filed Apr. 9, 2001, and U.S. Provisional Application No. 60/317,391 filed Sep. 5, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     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. Other mounting substrates could include metals such as invar or plastics housings such as LCP. As shown in the prior art of FIG. 1, a TO can assembly  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 tend to vary, variance exists in resistance, inductance, or capacitance of the lines. 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  22  through a lens  24 , and may be appropriately coupled into a waveguide (not shown). 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 adds 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 surface of light emission. As a result, the coupling efficiency between the active optical device and waveguide/optical fiber could increase. 
     As stated above, a VCSEL laser die often contains electrical contacts on the same surface of light emission. Flip chipping a laser die to a substrate can eliminate the need for complicated lensing devices necessary to capture enough light. Because flip chipping can eliminate use of wire bonds on an active optical surface, an optical fiber or waveguide can be closer to the optical port. If the distance from a coupling device to the optical port decreases, more divergent light can be collected before being obstructed by covering features or interfering with adjacent optical devices. This in turn may preserve signal integrity. 
     Flip chipping of IC&#39;s is widely understood. Yet, the flip chipping of VCSEL or photodiode dies is a newer practice with room for modifications and improvements. Typically, conventional stud bumps or solder bumps establish electrical connections between conductive traces and optical devices. A solder bump can structurally attach the optical device to a substrate or similar device, but a stud bump is typically used in conjunction with an adhesive (the structural member). Adhesive selection becomes important under large temperature variations. Given an assembly going through a tin-lead solder reflow oven, bonded surfaces could shift in relation to each other, and the adhesive, or solder bump, must hold the positions of the devices in relation to each other. 
     If an adhesive is placed on the sides of the die and not on the optical device&#39;s surface attached to the substrate, a number of problems could arise if an air-gap remains between the substrate and the optical device. Foreign materials could possibly find their way into the open space surrounded by the solder, contaminate the optical ports, and interfere with signal integrity. During aqueous washing of the assembly, unwanted chemicals may enter the region of the optical array, contaminating the optical array and depreciating signal integrity. For this reason, an adhesive is better suited between the two surfaces of contact. Yet, if attaching a VCSEL die to a substrate, where the optical emission surface of the optical array is attached to the substrate, the adhesive must allow optical signals to pass through. 
     As flip chipping an optical device to a substrate can enable closer proximity of a waveguide to the port, this can enable coupling of more divergent optical radiation, thus increasing the total amount of light gathered and eliminate the need for lens mechanisms. As it is desirable to uniformly collect light over the optical source&#39;s total angular emission field, 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. 
     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 beyond a certain range of light energy. Additionally, if the photodetector has not already saturated, a 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 control an optical signal. 
     In this invention we provide a novel way to couple light from an optical device, into a waveguide, and subsequently into an optical fiber. The invention may simultaneously function as a waveguide, a structural member, a protective means for optical ports, and an optical attenuator. It may allow coupling of divergent light while transmitting an appropriate amount of optical energy to a receiving device. In addition, the invention may promote eye safety while maintaining signal integrity. 
     SUMMARY OF THE INVENTION 
     A method and apparatus are provided for providing an electro-optic interface for exchanging information signals. The method includes the steps of disposing an optical array adjacent a first side of an optically transparent substrate, such that a plurality of transmission paths of the optical array pass directly through the substrate, applying an optically transparent underfill between the substrate and adjacent optical array with the plurality of transmission paths of the optical array passing directly through the underfill and coupling a plurality of optical signals of the optical array through the optically transparent underfill and optically transparent substrate between the optical array and an optical connector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of the prior art related to the invention; 
     FIG. 2 is a profile view of an electro-optic communications assembly under an illustrated embodiment of the invention; 
     FIG. 3 is a side view of an optical array of FIG. 2 attached to a optically transparent substrate; 
     FIG. 4 is a front view of the optical array of FIG. 2 attached to a substrate; and 
     FIG. 5 is a profile view of the substrate and a pattern recognition system used in aligning the assembly of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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  52  attached to the PCB  42 , an active optical array  44  attached to the substrate  52 , and an optical connector  54  for holding optical fibers  58  in alignment with the optical array  44 . Alignment apertures  74  may be disposed in the substrate  52  to allow guide pins  48 , inserted through the alignment apertures  74 , to align the optical fibers  58  of the optical connector  54  to the optical array  44 . 
     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). The optically transparent substrate  52 , having first and second sides  64  and  66  respectively (FIG.  3 ), may comprise an L-shaped glass or a glass-like structure having desirable optical and structural properties. The substrate  52  could be about 100 microns in thickness. The second side  66  of the substrate  52  may be attached to the PCB  42  by a conductive adhesive, solder/stud bumps, or a similar material. Attached to the first surface  64  of the substrate  52  may be the optical array  44 . In a preferred embodiment of the present invention, the substrate  52  may also contain a right angle bend  68  at an appropriate location to allow planarity of optical signals  46  of the optical array  44  with respect to the PCB  42 . 
     FIG. 3 illustrates a cut-away side view of a portion of the optical array  44  attached to the optically transparent substrate  52 . The optical array  44  may have disposed on a first surface  64  of the substrate  52 . Conventional electrical contacts  72  (i.e., solder or stud bumps), and electrical traces (not shown) may be used for electrically connecting the optical array  44  to a signal processing device (not shown). In a preferred embodiment of the invention, stud bumps electrically attach the optical array  44  to the substrate  52 . An optically transparent underfill  62  mechanically attaches the optical array  44  to the first surface of the substrate  64 . The details of the optically transparent underfill  62  will be described in further detail below. 
     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, P-I-N diode, PN diode, MSM 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  44  may have a number of optical ports  76 , and each optical port  76  may be a photonics transmitter, receiver, or a combination transmitter/receiver. FIG. 4 is a view of the active surface of the optical array  44 , viewed through the transparent substrate  52 . Also shown, through the substrate  52 , are optical ports  76 , the optically transparent underfill  62 , and electrical contacts  72 . (FIG. 4 shows 6 optical ports and 14 electrical contacts, yet the number of optical ports and electrical contacts used in the invention are not limited in any way). 
     Turning back to FIG. 3, the optically transparent underfill  62  may mechanically attach the optical array  44  to a first surface  64  of the substrate  52 . The thickness of the underfill  62  may be 50-60 microns, or about the thickness of conventional stud bumps  72 . In addition, the thickness of the underfill  62  may be changed by appropriately changing the height of the stud bumps  72 . The underfill  62  may be applied to the region between the first surface  64  of the substrate  52  and the active optical surface of the optical array. It could also be applied to the substrate  52  before the optical array  44  and the substrate  52  are connected, (i.e., the underfill  62  could be applied before or after the optical array  44  is attached to the substrate  52 ). The underfill  62  could be applied using a conventional syringe technique. The preferred method of application is a conventional pin transfer technique. Upon applying the underfill, the underfill cures for an appropriate amount of time. Use of an epoxy underfill for IC&#39;s is common, and details of composition, application, or curing will not be discussed in detail. 
     In a preferred embodiment of the present invention, the underfill  62  is an epoxy supplied by Epoxy Technology of Billerica, Mass. (commonly referred to as Epo-Tek). Two epoxies that have been used in the invention are Epo-Tek&#39;s U300 and OE121. Additional epoxies displaying adequate optical and thermal properties could be used for this application as well. The optical signal&#39;s transmission paths  46  originating from the optical ports  76  may sequentially pass directly through the underfill  62  and the optically transparent substrate  52 . The underfill  62  may also function as a hermetic encapsulant, thus protecting the optical ports  76  of the optical array  44  from unwanted harsh chemicals, debris, and the like. 
     The underfill  62  could also minimize light reflections between the optical array  44  and transparent substrate  52 . Reflected light coupled back into the optical ports  76  could reduce the performance of the optical array  144 , further increasing optical noise. By choosing an optical underfill with a refractive index reasonably close to that of the substrate&#39;s, this could reduce the effects of, if not preventing, a standing-wave cavity from forming between the substrate  52  and optical array  44 . This in turn could increase the optical signal integrity by minimizing reflections back into the optical ports  76  of the optical array  44 . 
     The underfill  62  shown in FIGS. 3 and 4 may include an additive dye  63  used to block a portion of the optical signal  46  transmitting through the underfill  62 . The dye  63  may attenuate an optical signal to any appropriate level. Details of the dye  63  will now be described in further detail. 
     The additive dye  63  could be a liquid or powder additive mixed with the epoxy adhesive  62 , by any conventional mixing techniques, before being applied to the region between the optical array  44  and the first surface  64  of the substrate  52 . In a preferred embodiment of the invention, the dye  63  is a convention infrared absorptive powder dye  63  supplied by American Dye Source, Inc. It blocks light of appropriate wavelengths while allowing other wavelengths to pass. For example, the preferred dye attenuates a portion of the 850 nanometer optical signal, while allowing light of other wavelengths to pass, including light in the visible spectrum (as will be described later, light in the visible spectrum is used for proper alignment/placement of the optical array). The amount of light the dye  63  blocks is directly proportional to the amount of dye  63  added to the underfill. That is, more dye  63  added to the underfill  62  could block a greater amount of light. In addition, a thicker amount of underfill  62  can block a greater amount of light. As previously stated, the thickness of the underfill  62  can be controlled by the thickness of the stud bumps  72 . 
     An underfill dye  63  will usually attenuate a range of wavelengths of light. For a given dye  63 , a “light frequency vs. amount of light blocked” plot could be modeled by a bell-shaped curve. A dye used in light attenuation applications is described/marketed as the absorptance of light over a wavelength range. As the wavelength of light passing through the dye/underfill deviates from the dye&#39;s nominal or rated absorption range, the amount of light blocked will decrease. By choosing a dye  63  with a rated wavelength and magnitude of light attenuation as close as possible to the wavelength of the optical signal, less dye  63  could be dissolved in the underfill to achieve a certain optical attenuation. That is, by matching the optical signal wavelength to the rated wavelength of a dye  63 , the amount of dye  63  used could be minimized. 
     In a preferred embodiment of the present invention, the additive dye  63  is used between the transparent substrate  52  and an optical transmitting port  76 . Thus, the additive dye  63  blocks optical signals on the transmitting end of the transmission path, and not on the receiving end of the signals. 
     As previously stated, the dye  63  could be mixed with the underfill  62  using a conventional procedure. Once thoroughly mixed, the underfill  62  could appropriately be applied in the gap between the optical array  44  and the optically transparent substrate  52 . 
     Turning again to FIG. 4, a set of alignment apertures  74  may be formed in the transparent substrate  52  for receiving the alignment guide pins  48  described earlier. The alignment apertures  74  may properly align the optical ports  76  of the optical array  44  to the optical fibers  58  of the fiber holding alignment mechanism  54 , as shown in FIG.  2 . 
     The alignment guide pins  48 , held in place by an alignment pin holder  50  shown in FIG. 2, could then be inserted concurrently through guide pin apertures  56  formed on a first surface  60  of the fiber holding alignment mechanism  54 . This could collinearly align optical ports  76  of the optical array  44 , to the respective optical fibers  58  of the fiber holding alignment mechanism  54 . (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). 
     To form the alignment apertures  74  in the substrate  52 , a boring fixture  82  may be used (FIG.  5 ). The boring fixture  82  may include a pattern recognition module  84  and lasers  86 ,  88 . The pattern recognition module  84  may include software adapted to recognize and position itself over a line of targets (not shown). The module may use a camera (not shown) to detect certain registration targets located on the optical array  44 . The camera may detect light in the visible spectrum, hence the use of an underfill dye  63  that allows the transmission of light in the visible spectrum. 
     Once recognition of targets has occurred, the pattern recognition module  84  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  84  then positions its own transverse line and center point with the identified transverse line and center point. The lasers  86 ,  88  may be precisely aligned along the transverse line of the pattern recognition module  84 . The lasers  86 ,  88  are also positioned a precise distance on either side of the center point of the pattern recognition module  84 . 
     The pattern recognition module  84  may be programmed to view the array  44  through the transparent substrate  52  and identify the set of alignment targets (e.g., the alignment targets on opposing ends of the array  44 ). Once the pattern recognition module  84  has aligned itself with the recognition targets (and also the lasers  86 ,  88  on either side of the targets), the boring fixture  82  activates the lasers  86 ,  88  to ablate the holes  74  in precise alignment with the ports  76 . 
     While a specific embodiment of the invention has been shown and described, 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. 
     
       
         
               
               
             
           
               
                   
               
               
                   
                 Optical Attenuating Underchip Encapsulant 
               
               
                 Numbering List 
                 83653 
               
               
                   
               
             
             
               
                 10 
                   
               
               
                 11 
               
               
                 12 
                 TO can 
               
               
                 13 
               
               
                 14 
                 optical device 
               
               
                 15 
               
               
                 16 
                 wire bonds 
               
               
                 17 
               
               
                 18 
                 spacer 
               
               
                 19 
               
               
                 20 
                 header 
               
               
                 21 
               
               
                 22 
                 can 
               
               
                 23 
               
               
                 24 
                 lense 
               
               
                 25 
               
               
                 26 
                 normal transmission axis 
               
               
                 27 
               
               
                 28 
               
               
                 29 
               
               
                 30 
               
               
                 31 
               
               
                 32 
               
               
                 33 
               
               
                 34 
               
               
                 35 
               
               
                 36 
               
               
                 37 
               
               
                 38 
               
               
                 39 
               
               
                 40 
                 optical converter assembly 
               
               
                 41 
               
               
                 42 
                 PCB 
               
               
                 43 
               
               
                 44 
                 optical array/devices 
               
               
                 45 
               
               
                 46 
                 transmission paths 
               
               
                 47 
               
               
                 48 
                 guide pins 
               
               
                 49 
               
               
                 50 
                 guide pin holder 
               
               
                 51 
               
               
                 52 
                 glass substrate 
               
               
                 53 
               
               
                 54 
                 MT ferrule/waveguide 
               
               
                 55 
               
               
                 56 
                 apertures in the ferrule 
               
               
                 57 
               
               
                 58 
                 ribbon fiber 
               
               
                 59 
               
               
                 60 
                 1st surface of the waveguide 
               
               
                 61 
               
               
                 62 
                 optically clear underfill 
               
               
                 63 
                 additive dye 
               
               
                 64 
                 1st surface of the substrate 
               
               
                 65 
               
               
                 66 
                 2nd surface of the substrate 
               
               
                 67 
               
               
                 68 
                 90 degree bend 
               
               
                 69 
               
               
                 70 
                 conductive traces 
               
               
                 71 
               
               
                 72 
                 conductive pads/bumps 
               
               
                 73 
               
               
                 74 
                 alignment apertures in the substrate 
               
               
                 75 
               
               
                 76 
                 optical ports 
               
               
                 77 
               
               
                 78 
               
               
                 79 
               
               
                 80 
                 darkened zones 
               
               
                 81 
               
               
                 82 
                 boring fixture 
               
               
                 83 
               
               
                 84 
                 recognition module 
               
               
                 85 
               
               
                 86 
                 laser 1 
               
               
                 87 
               
               
                 88 
                 laser2 
               
               
                 89 
               
               
                 90