Patent Publication Number: US-7914211-B2

Title: Optical assemblies

Description:
This application is a continuation of U.S. patent application Ser. No. 11/445,043, filed Jun. 1, 2006, now U.S. Pat. No. 7,597,488 which claims the benefit of priority to U.S. Provisional Patent Application No. 60/686,543, filed Jun. 1, 2005, the entire contents of both of which are incorporated herein by reference. 
    
    
     The present invention relates generally to the field of optoelectronics. In particular, the present invention relates to optical assemblies which allow for optical and mechanical connection between an optical bench and optical fiber connector. The invention finds particular applicability to the manufacture of a micro-optical component assembly which includes a housing for an optoelctronic subassembly such as a transmitter, receiver or transceiver optical subassembly, and which can be connected with an optical fiber connector. 
     Optoelectronic devices such as lasers and photodetectors are used for a variety of applications, for example, in the telecommunications industry. Existing commercial transmitter optical subassemblies (TOSAs) are manufactured using a process based on a TO-can header assembly containing a laser diode and monitor diode. A typical TO-can type package is described, for example, in U.S. Patent Application Publication No. US 2004/0240497 A1. The header assembly also has an optically transparent window or lens, allowing an optical signal to pass to or from the optoelectronic device. In order to focus the laser light into an optical fiber ferrule, conventional TO-can TOSA packages incorporate a variety of cylindrical machined subcomponents which must be aligned with each other, and welded or bonded in place. At least two separate alignment and welding or bonding steps are required for such a package. In devices requiring an optical isolator, further alignment and welding or bonding is required. In fabricating a conventional TO-can TOSA package, costly tooling in the form of laser welding or bonding systems is required. In addition, TO-can packages are generally not well suited for packaging of planar-type device structures such as silicon optical bench. 
     It would therefore be beneficial to provide an optical assembly in which the number of components, alignment steps and welding or bonding steps is reduced. For example, it would be desirable to reduce the number of alignment and welding or bonding steps to a single alignment and welding or bonding step. As a result, significant cost savings in materials, labor and tooling over conventional optical assemblies could be attained. It would also be desirable to provide an optical assembly which would be suitable for use with silicon optical bench structures. 
     In accordance with a first aspect of the invention, an optical assembly is provided. The optical assembly includes: an optical bench comprising an optoelectronic component affixed to a substrate; a housing in which the optical bench is disposed, the housing comprising a first mating region; an optical ferrule receptacle for optically coupling the optical bench with an optical fiber, the optical ferrule receptacle comprising a second mating region, wherein the housing and the optical ferrule receptacle are mated with each other at the first and second mating regions; and a flex circuit attached to the optical bench wherein the flex circuit is disposed in contact with an interior surface of the housing for relieving strain in the flex circuit. 
     As used herein, the terms “a” and “an” are inclusive of “one or more”. The term “on” and “over” are used interchangeably in defining spatial relationships, and encompass the presence or absence of intervening layers or structures. Also as used herein, the term “optical assembly” encompasses structures having optical and/or optoelectronic functionality. 
    
    
     
       The present invention will be discussed with reference to the following drawings, in which like reference numerals denote like features, and in which: 
         FIG. 1  illustrates an exemplary optical assembly in accordance with the invention; 
         FIG. 2  illustrates a first exploded view of the optical assembly shown in  FIG. 1 ; 
         FIG. 3  illustrates a second exploded view of the optical assembly shown in  FIG. 1  from a direction opposite that shown in  FIG. 2 ; and 
         FIG. 4  illustrates an upper housing portion of an optical assembly in accordance with a further aspect of the invention. 
     
    
    
     The present invention will now be described with reference to  FIGS. 1-3 , depicting an exemplary optical assembly  1  in accordance with the present invention. The optical assembly includes an optical bench  3 . The optical bench includes a substrate  5  having an upper surface  7  in/on which various surface features are formed. The substrate  5  is typically formed from a semiconductor material which may be in wafer or chip form, such as silicon, for example, &lt;100&gt; single-crystal-silicon, gallium arsenide, indium phosphide, or lithium niobate, or from a ceramic, polymer, or metal. One or more optical and/or optoelectronic components are bonded to the substrate upper surface  7 . Typical optical components include, for example, optical fibers, lenses such as ball lenses, filters, and optical isolators. The exemplified optical isolator includes permanent magnets  8   a  and garnet crystal  8   b . Typical optoelectronic devices include, for example, laser die and photodetectors. The optical and optoelectronic components of the optical bench are shown generally as a dashed line in the illustrations. 
     The substrate upper surface  7  includes one or more surface features formed therein or on for holding the various components. The surface features may include, for example, grooves such as V- or U-groove for holding an optical fiber, pits for holding lenses such as a ball lens, and metal features for electrical connection of the optoelectronic device. Typical metal features include, for example, contact pads to which the optoelectronic device is soldered, metal lines and bonding pads for connection to a power supply. Techniques for forming surface features are known to those skilled in the art. For example, grooves and pits may be formed using masking and wet and/or dry etching techniques, while metallization structures may be formed by sputtering, evaporation or plating techniques. These techniques may optionally be used to form a substrate master from which substrates may be formed in a molding process. A lid  9  may further be provided for the optical bench for forming a hermetically enclosed volume for the components. 
     Electrical connection to the optical bench  3  may be accomplished by any known means. In the exemplified embodiment, a flex circuit  11  is provided for this purpose. Bonding pads (not shown) of the flex circuit  11  are soldered to corresponding bonding pads on the underside of the optical bench  3 , which in turn are electrically connected to the optoelectronic device of the optical bench. 
     A housing  13  for the optical bench provides one or more functions including, for example, alignment when assembling the optical assembly, heat sinking to remove unwanted heat generated by the optoelectronic device, strain relief to relieve stress from the flex circuit, mechanical protection for the optical bench, mounting capability to a printed circuit board or other electronic substrate, and thermal isolation to prevent solder reflow in earlier-formed solder joints during later soldering processes. The optical bench housing  13  may be constructed, for example, of machined, molded or stamped metal or plastic, including thermally conductive plastic. The optical bench housing may be formed of a plurality of pieces as in the embodiment shown in  FIGS. 1-3 . In the exemplified embodiment, the housing includes an upper portion  15  and lower portion  17 . The housing includes an alignment hole  19  for receiving an optical ferrule receptacle  21 . In accordance with one embodiment of the invention, when connected together, a symmetrical relationship between the housing and optical ferrule receptacle mating portions is created. The housing parts may include surface features which allow for passive alignment between the housing parts and between the housing parts and other components. For example, alignment pins  23  may be provided for passively locating flex circuit  11 . A trench  25  and elevated regions  26  in the lower housing portion  13  for passively locating the optical bench  3  may be provided. 
     The flex circuit  11  is in contact with the lower housing portion  17 . A relief trench  28  may be provided in this section for providing an air gap between the flex circuit and lower housing portion. This air gap acts as a thermal stop to protect solder joints formed between the flex circuit and optical bench  3  from reflowing when subsequently soldering the optical bench to the housing. The lower and/or upper housing portions  17 ,  15  may further include a strain relief land surface  29  for relieving strain in the flex circuit  11 . As the housing upper and lower portions  15 ,  17  are assembled, surfaces  29  contact the flex circuit, providing strain relief for the flex circuit. Pins  31  shown protruding from the land surface  29  may be provided for mating with holes  32  in the flex circuit to provide an optional additional registration. Compliant materials such as foams or adhesives may also be added on relief land surface  29  and/or on the flex circuit  11  to sandwich and support the flex circuit as it exits the housing  13 . 
     The housing  13  may optionally include provision for holding one or more components, such as one or more of the above-described optical components (e.g., optical fibers, lenses such as ball lenses, filters, and optical isolators). Such a structure may allow for a package having a smaller footprint and/or provide improved manufacturability.  FIG. 4 , for example, illustrates an upper housing portion  15  in which a permanent magnet  8   a , which forms a portion of an optical isolator, has been incorporated. One may additionally or optionally include the garnet crystal  8   b  (see  FIGS. 2 and 3 ) of the optical isolator and/or multiple isolator magnets in the housing. 
     The optical ferrule receptacle  21  is provided for optically and mechanically mating to an optical fiber connector (not shown) containing an optical fiber or fiber stub. The optical fiber connector is attached to an optical fiber pigtail. Optical ferrule receptacle  21  is typically made of a metal or plastic on the outside, and typically contains a split ceramic sleeve to align with the connector. The optical receptacle may contain a double sided polished ferrule stub with a length of optical fiber in it, such as is commercially available, for example, from Kyocera Corporation. The optical receptacle may include a wick stop  27  for preventing excess bonding agent from entering the optical path. As illustrated, the wick stop may take the form of a closed groove in the face of the optical receptacle between the mating region and the optical path. While the illustrated wick stop  27  is circular, other geometries may be employed. 
     Connection of the housing  15  to the optical ferrule receptacle  21  is facilitated with a bonding agent. Suitable bonding agents include, for example, epoxies such as UV curing, thermal curing, and dual cure epoxies, and solders. Beneficially, the bonding agent may be kept out of the optical path of the assembly. In this way, bonding agents that would otherwise be unusable due, for example, to optical loss issues, may be employed. Application of the bonding agent may be controlled such that it may be symmetrically dispensed between the mating region  19  of the housing  15  and the mating region  30  of the optical receptacle assembly  21 . The symmetrical relationship between the housing and optical receptacle mating regions minimizes variations in shrink and swelling of the bonding agent during cure and due to environmental exposure, resulting in minimal impact on the alignment to the optical bench  3 .