Abstract:
An optical subassembly includes two active optical elements on the same submount mounted at an angle to one another to provide optical coupling between them. The submount may include a portion bent upward in a principal plane thereof to provide the angle. The submount may serve as a heat sink. The active optical elements on the submount may be directly electrically connected to a circuit board on which the submount is provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/276,130 entitled “Optical Subassembly” filed Mar. 16, 2001, which is hereby incorporated by reference in its entirety for all purposes. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to an optical subassembly, and specifically to an optical subassembly, and method of manufacture, having an active transmitting optical device and an active detecting device collocated on a single submount.  
         BACKGROUND OF THE INVENTION  
         [0003]    The increasing demand for high-speed voice and data communications has led to an increased reliance on optical communications, especially optical fiber communications. The use of optical signals as a vehicle to carry channeled information at high-speeds is preferred in many instances to carrying channeled information at other electromagnetic wavelengths/frequencies in media such as microwave transmission lines, coaxial cable lines and twisted-pair transmission lines. Advantages of optical media include higher channeled capacities (bandwidth), a greater immunity to electromagnetic interference, and a lower propagation loss. In fact, it is common for high-speed optical signals to have signal rates in the range of approximately several megabites per second (Mbits/sec) to approximately several tens of gigabites per second (Gbits/sec), and greater. However, while particularly advantageous in effecting high signal rates, optical subassemblies have proven difficult to produce in mass production at acceptable yield levels.  
           [0004]    Such electro-optical systems have typically included more than one active element that are to be in optical communication with one another. The orientations required for effective optical coupling typically result in vertical stacking arrangements, individual submounts for each active element, and/or use of reflective surfaces. What is needed is a technique for fabricating optical subassemblies in an automated manner without sacrificing performance that result from inaccuracies of conventional automated processing techniques.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention relates to an optical subassembly and its method of manufacture which overcomes at least one of the above disadvantages.  
           [0006]    Advantageously, the present invention provides a reduction in cost due to the elimination of extra parts, the accurate location of parts on the assembly, and the heat dissipation of the emitter, which is required for suitable operation.  
           [0007]    According to an exemplary embodiment of the present invention, a handling vehicle becomes part of final product, and allows for the elimination of separate submounts for certain devices, such as an emitter and a detector. In particular, the devices are provided on the same submount at an angle to one another, allowing both integration and effective optical coupling.  
           [0008]    At least one of the above and other objects may be realized by providing an optical subassembly including a first active optical element having a first plane for optical coupling, a second active optical element having a second plane for optical coupling, wherein when the first active optical element and the second active optical element are oriented to optically couple, the first plane and the second plane are substantially not parallel, and a submount on which the first and second active optical elements are mounted, the submount having a principal surface, one of the first and second active elements being mounted on the principal surface of the submount and another of the first and second active optical elements being mounted on the submount at an angle from the principal surface, thereby allowing optical coupling between the first and second active optical elements.  
           [0009]    The optical subassembly may be part of an electro-optical package including a circuit board on which the submount is mounted and direct electrical connections between the first and second active optical elements and the circuit board.  
           [0010]    At least one of the above and other objects may be realized by providing a method of forming an optical subassembly including providing a first active optical element having a first plane for optical coupling on a submount, providing a second active optical element having a second plane for optical coupling on the submount, wherein when the first active optical element and the second active optical element are oriented for optical coupling, the first plane and the second plane are substantially not parallel, mounting one of the first and second active elements on a principal surface of the submount, and mounting another of the first and second optical elements on the submount at an angle from the principal surface, thereby allowing optical coupling between the first and second active elements.  
           [0011]    The submount may be part of an array of submounts and a first active element for at least two submounts in the array of submounts are simultaneously provided. The method may further include separating the array of submounts into individual submounts to thereby form the optical subassembly. The method may further include simultaneously testing at least two active elements in the array of submounts. The providing of the first and second active optical elements may include mounting both the first and second active optical elements on the principal surface of the submount and bending a portion of the submount containing one of the first and second active elements to allow optical coupling between the first and second active optical elements.  
           [0012]    These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.  
         [0014]    [0014]FIG. 1 is top-view of an optical subassembly on a circuit board according to an illustrative embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of an optical subassembly according to an illustrative embodiment of the present invention.  
         [0016]    [0016]FIG. 3 is a top-view of a lead frame according to an illustrative embodiment of the present invention.  
         [0017]    [0017]FIG. 3A is a top view of the detail A in FIG. 3.  
         [0018]    [0018]FIG. 4 is a top detailed view of another configuration of the lead frame. 
     
    
     DETAILED DESCRIPTION  
       [0019]    In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention.  
         [0020]    Turning to FIG. 1, a top-view of an optical subassembly  100  mounted on a circuit board  150  is shown. The optical subassembly  100  includes an emitter  101 , e.g., a laser or a light emitting diode (LED), in optical communication with a passive optical device  102 , illustratively a lens element. The optical subassembly  100  also includes a photodetector  103 . The photodetector  103  is illustratively a surface detector such as a PIN photodetector. Of course, this is illustrative, and other photodetectors may be used. Moreover, a thermistor  104  may be used to monitor the operating temperature of the laser  101 . As shown in FIG. 1, various wire or ribbon bonds  105  are used to make certain electrical connections from the optical subassembly  100  to the circuit board  150 , while metal traces  155  on the circuit board  150  are also used for this purpose.  
         [0021]    The emitter  101  shown in FIG. 1 emits radiation from two planes, a front facet and a rear facet. The front facet of the emitter  101  typically emits much more radiation than the rear facet and the radiation emitted from the front facet is output to a desired application. The radiation emitted from the rear facet may be output to the photodetector  103  to monitor the power of the radiation output by the emitter  101 . This information, in conjunction with information regarding temperature provided by the thermistor  104 , if present, may be used to control the operation of the emitter  101  in a known manner.  
         [0022]    Turning to FIG. 2, a cross-sectional view of the optical subassembly  100  according to an illustrative embodiment of the present invention is shown. Illustratively, a substrate  202  is an electrical lead frame assembly or any substrate material, such as Kovar, providing heat sinking for the active elements. A top layer  201  may be disposed over the substrate  202 , and is illustratively an oxide material. As shown in FIG. 2, the optical element  102  may be inserted into a v-groove or other feature in the substrate  202 . The optical element  102  is aligned to couple the light from the front facet of the emitter  101  to an end use. The optical element  102  could be a ball lens, a tube with a first optic and isolator or a beam splitter for front facet monitoring, or any number of drop-in optical elements. The photodetector  103  is mounted on an angled portion or tab  203  of the substrate  202 . The photodetector is aligned to receive light from a rear facet of the emitter  101  to monitor the power of the emitter  101 . This allows for the elimination of separate submounts for the emitter  101  and the photodetector  103 . A thermistor, which not seen in this view, may be included in the optical subassembly  100 .  
         [0023]    The assembly of the optical subassembly  100  may be done in planar form, i.e., the emitter  101  bonded to the lead frame  202 , the monitor  103  bonded to the lead frame, then the wire/ribbon bonding performed. After the planar structure is completed, the monitor  103  would be bent into position to receive light from the rear facet.  
         [0024]    According to one advantageous aspect of the present invention, as shown in FIG. 3, automated processing of a plurality of optical subassemblies  100  may be facilitated by simultaneously populating a mount  250 . As shown in FIG. 3, and as seen in more detail in FIG. 3A, the mount  250  includes a plurality of substrates  202  to be separated after population thereof to form the optical subassemblies  100  and alignment holes  252  to facilitate to automated processing of the mount  250 . As shown in FIG. 3, the mount  250  is a lead frame, containing a plurality of substrates  202 . Attachment of the substrate  202  to the mount  250  may be performed in a conventional manner, i.e., the substrate  202  may be thermo-compression bonded, soldered or epoxied to the lead frame  250 . Attachment points  254  may be designed to minimize heat conduction from the substrate  202  to the lead frame  250  itself. Another configuration of attachment points  254  is shown in FIG. 4. The attachment points  254  allow temperature testing or curing of epoxy on a single device site because it allows a single location to be cycled in temperature without affecting the other devices. These spaces between the optical substrates  202  on the mount  250  allow for thermal expansion and contraction without distorting the mount  250 . To this end, the mount  250  containing a plurality of substrates  202  may be used as a handling vehicle is part of the final optical subassembly  100  or the optical subassembly  100  may be completely removed from the mount  250 .  
         [0025]    The use of the mount  250  in this manner allows for various cost advantages in the elimination of extra parts, the accurate location of parts on substrate  202  and the provision of a heat sink necessary for proper operation of the active devices, particularly the emitter  101 . Also, direct testing of devices may be carried out on the mount  250  before the mount  250  is separated into respective optical subassemblies  100 .  
         [0026]    The mount  250  also enables the proper positioning of the photodetector  103  through relatively straightforward manipulation of the mount  250  during automated processing. When using lead frames as mounts, these lead frames have holes or slots along the side that allows the frame to be moved from position to position as it travels through a piece of equipment. After the planar configuration is completed, the photodetector  103  is bent into position as noted above. Rather than bending, the detector  103  could be mounted on a molded part  203 , although this adds parts to the assembly and makes the planer assembly more difficult. The coating  201  may be patterned during deposition so that when the tab containing the detector  103  is bent up to form  203 , there would be no material to crack. Since continuity of this coating  201  is not critical to the design, patterning a gap where the tab needs to bend is viable approach to solving this concern.  
         [0027]    The angled portion  203  may be adjusted to capture light from the rear facet of the emitter  101 . This could be an active process if the coupling to the back monitor needed to be precisely controlled. In most cases, an angle would be determined through experimentation or through computer modeling and the detector  103  would be mechanically bent to the desired angle after completing the assembly. This angle will depend on the far-field angle for the emitter  101  and could be anywhere from 0° to ˜ 140°. As such, increased monitor currents, depending on the asymmetry of the laser chip and the distances involved, may be achieved, allowing for more accurate monitoring of the device.  
         [0028]    There are many ways to use extra parts to get light coupled from the emitter  101  to the detector  103 . For example, reflective parts mounted above or directly behind the bent portion  203  to direct more light into the detector  103 , although this adds parts and thus increases cost and assembly requirements. Detectors  103  could be mounted to blocks at 90° to the emitter  101 , although again this would require more parts and handling plus it is often very difficult to provide electrical connection for such a configuration. Additionally, the photodetector  103  could be is directly behind the laser to provide planar coupling, although this provides limited coupling for low power lasers and can result in signal to noise issues with the very low powers detected.  
         [0029]    The subassembly  100 , according to the exemplary embodiments shown in FIGS. 1 and 2, have certain clear advantages over conventional optical subassemblies. For example, when active elements have been processed on lead frames by conventional methods, the lead frames are then used to provide electrical connection from the active devices to other devices. According to an illustrative embodiment of the present disclosure, the optical subassembly  100 ,  200  is separated from or sheared from the supporting frame, and thereafter mounted directly onto a circuit board or package body. To this end, the structure shown in FIG. 2 would be ready for direct mounting onto a circuit board or package body, as shown in FIG. 1. Further, both active elements are provided on the same submount. This enables subassembly test and characterization in mass or by individual site without need for costly packaging. It also allows for migration to plastic and over-molded packages as well.  
         [0030]    It will be obvious that the invention may be varied in a plurality of ways. For example, the optical subassemblies may include more than one detector-emitter pair. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.