Abstract:
An electro-optical subassembly generally includes a base supporting at least one lead and a lens unit having a lens. The lead supports an electro-optical component. The lens unit is through transmission laser welded to the base such that the lens is aligned with the electro-optical component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of non-provisional application Ser. No. 10/904,223 filed Oct. 29, 2004 now U.S. Pat. No. 7,149,405, which is incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   Electro-Optical (EO) components, like single mode transmitter/receiver optical sub-assemblies used in transmitters and receivers in fiber communication, are usually packaged utilizing transistor outline construction (sometimes referred to as a “TO can”). The EO components inside a TO can are wire-bonded to a number of leads that protrude through the package and allow signals to be routed to the EO components. These leads are bent and soldered onto a PCB board that contains the electronic components and circuitry to drive the EO components. 
   A TO can has several disadvantages. The packaging process requires significant human labor with multiple alignment processes. Current methods are very much cottage industry like—often described a being akin to building a ship in a bottle. A large fraction of the cost of single mode transmitter/receiver optical subassemblies is associated with the packaging process. 
   The leads on known TO can structures are typically a few millimeters in length and can cause a degradation of the frequency response of the subassembly. The leads also have to be bent and soldered onto the PCB board. This process is difficult to automate and is typically performed by hand. Yet another disadvantage is the mechanical tolerances stack up, e.g. the tolerance for the lens placement is affected by die placement. This requires that each component be positioned using a dedicated three—alignment system: one for die placement; one for lens placement; and one for the receptacle. 
   The present inventors have recognized a need for an electro-optical subassembly wherein the packaging process can be automated while avoiding at least some of the disadvantages of known TO cans. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a lead frame in accordance with an embodiment of the present invention. 
       FIG. 2  is an isometric view of a pre-molded lead frame in accordance with an embodiment of the present invention. 
       FIG. 3  is an isometric view of a pre-molded lead frame in accordance with an embodiment of the present invention. 
       FIG. 4  is an isometric view of a partial optical sub-assembly in accordance with an embodiment of the present invention. 
       FIG. 5  is an isometric view of an optical sub-assembly in accordance with an embodiment of the present invention. 
       FIG. 6  is an isometric view of an electro-optical subassembly in accordance with an embodiment of the present invention. 
       FIG. 7  is an isometric view of an optical unit in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 1  is an isometric view of a lead frame in accordance with an embodiment of the present invention. Fabrication of an electro-optical subassembly  100  (see  FIG. 5 ) starts with the stamping of a set of leads from, for example, copper or tungsten copper. The central lead  10  supports one or more optical electrical components, such as edge emitting lasers, PIN detectors, Fabry Perot cavity lasers and VCSELs. The two flanking leads  12  and  14  may supply power or, depending on the desired configuration, act as ground. 
     FIG. 2  is an isometric view of a pre-molded lead frame in accordance with an embodiment of the present invention. A body  20  is molded over the leads  10 - 14 . The body  20  is generally circular and encompasses the central portion of the leads  10 ,  12 , and  14 . The body  20  may be provided several features to assist with alignment. For example a cavity  22  and projections  24  and  26  may be used to assist with alignment of a laser with a lens unit. 
   The body  20  may be molded from a variety of materials including plastics. However, it may prove advantageous to use liquid crystal polymer (“LCP”) for stability over a broad range of temperatures. LCP is a thermoplastic fiber with exceptional strength and rigidity (five times that of steel), and about 15 times the fatigue resistance of aramid. Very good impact resistance. LCP doesn&#39;t absorb moisture, has very low stretch, doesn&#39;t creep like UHMW-PE fibre, and has excellent abrasion, wear, and chemical resistance. LCP&#39;s high melting-point (320 C.) allows the retention of these properties over broad ranges of temperatures. 
   LCP has an unusual property of anisotropic coefficient of thermal expansion due to a molecular structure comprising highly ordered linear chains which are melt oriented. The polymer chain undertake a regular orderly crystal like orientation during solidification in a mold, By gating at the rear of the body  20 , the flow direction can be aligned in the Z-axis (parallel to the longitudinal axis of the leads  12  and  14 ) of the subassembly reducing expected drifts to an acceptable level based on the low CTE of LCP (&lt;5 ppm). The expansion in the X-Y directions should closely match the expected expansion of a polymer lens (˜50 ppm)—effectively minimizing movement due to temperature shifts about the optical axis of the lens. 
     FIG. 3  is an isometric view of a pre-molded lead frame in accordance with an embodiment of the present invention.  FIG. 3  illustrates the placement of an edge-emitting laser  30  and a PIN monitor  32  on the central leads  10 . The laser  30  and monitor  32  may be electrically connected to the leads  12  and  14  with wire bonding. 
     FIG. 4  is an isometric view of a partial optical sub-assembly in accordance with an embodiment of the present invention. A spacer in the form of a molded hollow cylinder  40  is formed and inserted into the groove  22  of the body  20 . A cavity  42  of the cylinder  40  surrounds the portion of the leads  10 ,  12 , and  14  extending from the face of the body  20  along the Z-axis thereby protecting the laser  30  and monitor  32 . The length of the cylinder  40  is determined by the optical properties of the laser  30 , monitor  32  and the lens used in the electro-optical subassembly  100  (see element  52  in  FIG. 5 ). 
   As with the body  20 , the cylinder  40  may be molded from a variety of materials including plastics. However, it may prove advantageous to use LCP. Further, it may prove advantageous to mold the spacer/cylinder  40  as part of the body  20  or the lens unit  50  (see  FIG. 5 ). 
     FIG. 5  is an isometric view of an optical sub-assembly in accordance with an embodiment of the present invention. To complete the electro-optical subassembly  100 , a lens unit  50  is attached to the cylinder  40 . The lens unit is provided with a lens  52 , such as an aspherical lens. The lens unit  52  is generally shaped like a cap and slides over the cylinder  40 . By closely controlling the surfaces of the body  20 , the cylinder  40  and the lens unit  50 , the laser  30  and the lens  52  can be precisely aligned within acceptable tolerances (in the sub-micron range). 
   It may prove beneficial to mold the lens unit  50  using a polymer. The use of polymer, as opposed to traditional glass, permits the formation of intricate 3-D geometry for registration purposes as well as the ability to couple more light. Further, polymer lens fend themselves to mass production—reducing the overall cost of the electro-optical subassembly  100 . 
   The cylinder  40  may be affixed to the base  20  and the lens unit  50  using epoxy. However, it should be noted that the use of epoxy may increase the possibility of drift (especially during cure) and may prove difficult to apply to the small parts comprising the electro-optical subassembly  100 . For example, the overall length of the electro-optical subassembly  100  may be smaller than 6 mm while the diameter of the cylinder  40  may be smaller than 5 mm. 
   A more suitable joining technique is through transmission laser welding (TTLW). TTLW, well known method for joining two thermoplastics part, is undergoing a renaissance with the introduction of IR absorbing dyes allowing clear-to-clear polymer transmission welding. Older TTLW techniques required that the first part to be joined had to be optically transparent and the second part had to absorb the laser energy. With the new IR absorbing dyes, the second part can also be optically transparent. 
   In TTLW, a laser passes though the optically clear part impinging on the second part with the IR absorbing dye. The second part absorbs the laser creating heat that in turn leads to plastification. The resultant local increase in volume of the second (absorbent) part causes a surface contact with the first part (translucent) that causes plastification of the second part creating the weld. With the use of appropriate jigs, movement between the two parts during the welding process may be minimized. Further, as the heat is localized to the joint, the parts experience little or no heat based distortion. The strength of the joint is quite high and may exceed that of the individual parts. 
   In the present invention, the second part may comprise the cylinder  40  which can be doped with IR absorbing dye. The first part could be one or both of the body  20  and the lens unit  50 . TTLW techniques and apparatus are well suited for the cylindrical shape of the joints between the base  20 , the cylinder  40  and the lens unit  50  such that it is possible to create a hermitically sealed electro-optical subassembly  100 . Further, as TTLW operations are suitable for large batch operations, the manufacturing of the electra-optical subassembly  100  can be automated to a level similar to that found in the microelectronic industry. 
   It will be appreciated by those skilled in the art that changes may be made in the described embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents. For example, while the base  20 , cylinder  40  and lens unit  50  have been described as being cylindrical, different shapes and configurations may prove beneficial. 
     FIG. 6  is an isometric view of an alternative configuration which may prove beneficial. The electro-optical assemble  600  generally comprises a base  610 , an optical unit  620 , The base  610  generally comprises a collection of leads  612  partially encased by a wedge shaped molded body  614 . Electro-optical components, such as a laser  616  and a PIN detector  618 , are fixed to one or more leads  612   n . The optical unit  620  has a wedge shaped cavity  622  that accepts the base  610  and facilitates alignment of the electro-optical components (such as  616  and  618 ) with an optical lens (not visible) formed as part of the optical unit  620 . The electro-optical subassembly  610  mates with a port  630  that facilitates alignment of the optical unit  620  with an optical cable (not shown). The optical unit  620  and the base  610  are joined using TTLW. 
   The electro-optical assembly  600  provides many advantageous. The body  614  has two wedge shaped arms defining a central opening for holding the optical components. The leads  612   n  can be formed using standard technologies and, if desired, can be configured to facilitate surface mounting the electro-optical subassembly  600  onto a PCB board (not shown). The design of the base  61   0  allows the overall size of the electro-optical subassembly  600  to be reduced as compared to a TO-can. This size reduction minimizes disruptive thermal expansions and reduces the distance between the electro-optical components and the optical lens. Further, as the leads  612   n  are anchored into the modeled body  614 , overall rigidity is increased. The emitting surface of the laser  616  can be accurately positioned relative to the optical lens making Z-alignment of the port  630  redundant. Since the optical lens and the laser  616  are referenced against the same base, XY-alignment of the lens may be redundant, reducing the typical three-alignment process to a two-alignment process. 
     FIG. 7  is an isometric view of an optical unit  620  in accordance with an embodiment of the present invention. The optical unit  620  generally comprises a body portion  702  and a lens  704 . The body portion  702  generally comprises a frustum having two opposing flat surfaces  708   a  and  708   b . The opposing flat surfaces  708   a  and  708   b  may be molded or ground into the body and serve as alignment features. The lens  704  may comprise an aspherical lens. The exact configuration of the lens  704  will be determined by the required function, for example coupling the light from a laser with an optical fiber and/or coupling the light from the optical fiber to the PIN detector. The lens  704  may be molded with the body  702  and then provided with a clear optical surface. Alternatively, the lens  704  may be turned after the body  702  has been molded.