Abstract:
The invention relates to an optical sub-assembly package for use in receiver optical sub-assemblies or transmitter optical sub-assemblies in which the electrical connections between the transducer chip, e.g. photo-detector or light source, and the device printed circuit board is made by a single flexible circuit conductor extending through the wall of the package. The package is comprised of a housing and a stiffening plate, which encloses and end of the housing and forms a mechanical support for an end of the flexible circuit conductor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority from U.S. Patent Application No. 60/539,219 filed Jan. 26, 2004, which is incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates to a compact optical sub-assembly (OSA), and in particular to an OSA including an integrated flexible circuit connector extending from an opto-electronic transducer chip within the OSA housing to printed circuit board (PCB) electronics of a transceiver. 
   BACKGROUND OF THE INVENTION 
   The driving forces in the transceiver manufacturing industry are reducing the form factor sizes, increasing the data transfer rates, and decreasing the costs. To achieve all of these goals, the conventional transistor outline (TO) can design approach must be replaced with a more exotic component packaging approach. However, to provide an OSA that can be used over a wide range of data transfer rates and products, the OSA must use controlled impedance connections for the high speed RF electrical signal path between the OSA chip and the transceiver electronics. Moreover, the total number of component parts must be reduced, and manufacturable from readily available materials. The assembly processes, including optical alignment, must be simplified and/or automated to reduce labor costs and increase production rates, and the fiber receptacle components should support a variety of wavelengths. 
   Conventional OSA designs, such as the one disclosed in U.S. Pat. No. 5,537,504, issued Jul. 16, 1996 to Cina et al and assigned to the present Applicant, include a opto-electronic (O/E) transducer  4  mounted in a container  25 , which is sealed by a window  26 . Solid metallic leads  23  and  24  extend through the rear of the container  25  for soldering to other electrical leads or directly to a transceiver PCB. The window  26  limits the relative positioning of the fiber, the lens and the O/E transducer, and the leads  23  and  24  limit the quality of the transmission and the positioning of the transceiver PCB. The use of flexible-tape conductive wiring has been disclosed in U.S. Pat. No. 5,005,939 issued Apr. 9, 1991 to Arvanitakis et al and assigned to the present Applicant, but only for connecting the existing leads of an OSA to the transceiver PCB. Moreover, the Arvanitakis et al device does not disclose the use of multi-layer micro strip transmission lines required for high-quality high-data rate signals. 
   An object of the present invention is to overcome the shortcomings of the prior art by providing an optical sub-assembly with an integrated flexible circuit conductor for reducing the number and length of electrical interfaces between the OSA chip and the transceiver electronics to reduce the strength of electrical signal reflections. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention relates to an optical sub-assembly comprising: 
   an optical connector for receiving an end of an optical fiber, which transmits a beam of light including an optical signal; 
   a housing coupled to the optical connector; 
   a stiffening plate mounted on an end of the housing forming an enclosure therewith; 
   a transducer mounted on the stiffening plate for converting the optical signal into an electrical signal or for converting an electrical signal into the optical signal; 
   a lens mounted in the housing for relaying the beam of light between the optical fiber and the transducer along an optical axis; and 
   a flexible circuit conductor for transmitting the electrical signal to or from the transducer, one end of which is supported by the stiffening plate and electrically connected to the transducer within the enclosure, the other end of which extends outside of the housing for electrical connection with control electronics of a host device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
       FIG. 1  is an isometric view of an optical sub-assembly according to the present invention; 
       FIG. 2  is a partially sectioned isometric view of the optical sub-assembly of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the optical sub-assembly of  FIGS. 1 and 2 ; 
       FIG. 4  is an alternative embodiment of an optical sub-assembly according to the present invention; 
       FIG. 5   a  to  5   d  are isometric views of a flip-chip bonded photo-detector for the optical sub-assembly of  FIGS. 1 to 4 ; 
       FIG. 6  is an isometric view of the back end of the optical sub-assembly of  FIGS. 1 to 3 ; 
       FIG. 7  is an isometric view of the back end of the optical sub-assembly of  FIGS. 1 to 3  with an alternative photo-detector; 
       FIG. 8  is a top view of the flexible circuit conductor of the optical sub-assembly of  FIGS. 1 to 4 ; 
       FIG. 9  is a bottom view of the flexible circuit conductor of the optical sub-assembly of  FIGS. 1 to 4 ; 
       FIG. 10  is an alternative embodiment of the optical sub-assembly according to the present invention; 
       FIG. 11  is an alternative embodiment of the optical sub-assembly according to the present invention; and 
       FIG. 12  is an optical transceiver device with the optical sub-assembly of  FIGS. 1 to 3  mounted therein. 
   

   DETAILED DESCRIPTION 
   With reference to  FIGS. 1 to 4 , an OSA, generally indicated at  1 , according to the present invention includes an optical connector  2 , at the front end thereof, a housing  3 , and a stiffening plate ring  4 , at the rear end thereof with a flexible circuit  5  extending outwardly therefrom. The optical connector  2  includes a bore  6  for receiving a ferrule on an end of an optical fiber, as is well known in the art, which transmits an beam of light  10  containing an optical signal to or from the optical sub-assembly  1 . The housing  3  includes a mounting flange  9 , and a lens  7 , which relays the beam of light  10  between the optical fiber an opto-electronic (O/E) transducer, generally indicated at  8 . The OSA  1  could be either a receiver optical sub-assembly (ROSA) or a transmitter optical sub-assembly (TOSA). For a ROSA, the O/E transducer  8  includes a photo-detector  11  with a trans-impedance amplifier (TIA)  12  connected thereto. For a TOSA, the O/E transducer  8  includes a light source, e.g. a vertical cavity surface emitting laser (VCSEL), with a laser driver connected thereto. Preferably the optical connector  2 , the housing  3 , the lens  7  and the mounting flange  9  are all integrally formed from an optical grade plastic, e.g. ULTEM1010. 
   The stiffening plate ring  4  includes a base  13 , on which the O/E transducer  8  is mounted, an annular flange  14 , and a slot  16 . The annular flange  14  mates with or surrounds the mounting flange  9 , which are secured together using an adhesive  17  or other suitable means.  FIG. 5  illustrates an embodiment of an OSA  1 ′ according to the present invention, in which the annular flange  14 ′ includes teeth  18  for mating with teeth  19  extending from the mounting flange  9 ′. The interlocking teeth  18  and  19  provide a much more robust housing structure. 
   The stiffening plate ring  4 , which provides a solid support for an end of the flexible circuit  5 , can be constructed out of a material, e.g. zinc, aluminum, with high thermal conductivity (TC), i.e. between 100 and 500 W/m°K or several times more than that of the conventional TO can housing, which enables the OSA  1  to run at higher operating temperatures before thermally induced noise becomes a factor. For example: TC A1 =237 W/m°K, TC Zn =116 W/m°K, TC Cu =410 W/m°K To reduce back reflections in a ROSA, the O/E transducer  8  is mounted at a non-normal angle to the incoming optical beam of light  5 , so that any reflected light will not be reflected directly back through the lens  7  into the optical fiber. The base  13  is at a nominal angle of between −4° and −10°, preferably −7°, from a plane normal to the incoming optical beam  10 , i.e. the inner surface of the base  13  is at an acute angle of between 80° and 86° from a central optical axis of the beam of light  10 . 
   To further limit back reflections as the beam of light  10  exits the optical fiber, an index-matching optical insert  21  is mounted on a front surface of the lens  7 . The optical insert  21  has an index of refraction closely matching that of the optical fiber. Preferably, the optical insert  21  is a rectangular or cylindrical block of silica, BK7, or Borosilicate float glass. Ideally the optical insert  21  is fixed to the front surface of the lens  7  using an index-matching adhesive, preferably having an index of refraction midway between the index of refraction of the optical insert  21  and the index of refraction of the lens  7 . Alternatively, the optical insert  21  can be mounted to the front surface of the lens  7  by some other means, such as press fitting. 
   Ideally the optical insert  21  projects outwardly into the bore  6  of the optical connector  2  forming a trough  22  therearound. The trough  22  will provide an area for collecting any dust or foreign particles entering the bore  6  to prevent this material from being embedded into the optical insert  21 . 
   Since the optical fiber is silica based, the reflection at the optical fiber/optical insert  21  interface is negligible. The difference in refractive index at the optical insert  21 /plastic lens  7  interface does result in a small amount of back reflection. However, as is illustrated in  FIG. 4 , the beam of light  10  expands prior to hitting the front surface of the lens  7 , and continues to expand as it is reflected back to the optical fiber. Accordingly, the overlap between the back reflected light and the optical fiber mode is relatively small, i.e. only a small fraction of the beam of light  10  is reflected back to the optical fiber. To reduce the back reflection even further, the size of the optical insert  21  can be increased beyond the usual 0.8 mm length. 
   With reference to  FIGS. 5   a  to  5   d , the photo-detector  11  is preferably a rear-illuminated reverse-biased photodiode, which responds to an incident optical signal by generating a current with both an AC and a DC component. Electrical contacts  28  on a mounting surface of the TIA  12  are connected to corresponding electrodes  29  on the photo-detector  11  using any one of many known methods, such as the use of solder bumps in a flip chip bonding process. Preferably a redistribution layer  27 , with the pre-amplifier contacts  28 , is preferably added to the TIA  12  after initial processing to match the electrical contacts  29  on the photo-detector  11 . The flip chip bonding process provides very low package parasitics, while enabling the photo-detector  11  to be aligned with high precision. Alternatively, a wiring layout with contacts  28  can be added to the metallurgy of the TIA  12  during initial processing; however, this method precludes the TIA  12  from being used with standard wire bonds, as well. 
   With reference to  FIG. 6 , outer contacts  25  on the TIA  12  are electrically connected to corresponding contacts  31  on a rounded end  32  of the flexible circuit conductor  5  using short leads  33 . A portion of the end  32  is cut away leaving an opening  34 , through which the TIA  12  and the photo-detector  11  can extend, as the end  32  is fixed to the base  13  of the stiffening plate ring  4 . Other electrical components  36  can be positioned on the end  32  proximate the transducer  8 , e.g. capacitors used in low pass filters for the TIA  12  or inductive choke components, which enable DC current to be fed to a laser without a reduction in the AC RF signal. 
   A front-illuminated photo-detector  41  ( FIG. 7 ) could also be used, in which top-side contact pads  42  on the photodiode substrate connect to pads  43  on the TIA  44 . Newly developed front-illuminated photodiodes bring the substrate contact to the top surface of the photodiode, so that both contacts (Anode and Cathode) can be made with wire bonds. The TIA  44  and the flexible circuit conductor  5  each include six corresponding electrical trace leads, two for power transmission (+V, Gnd), two for differential data transmission (RF Out), and two for optical power monitoring. Alternatively, the photo-detector  11  and the TIA  12  could be attached beside each other on the stiffener plate  4 . 
   Since the OSA housing  3 /stiffening plate  4  is not hermetically sealed, the transducer  8  must be coated in order to survive under environmental stress conditions. Special chip level coatings, e.g. SiO 2 , can be applied during the fabrication of the transducer  8  or the transducer  8  can be coated or encapsulated during the assembly of the OSA  1 . With reference to the aforementioned flip chip assembly process, an encapsulation is used by under filling the cavity between the active surface of the photo-detector  11  and the top surface of the transimpedance amplifier  12 . If the active side is up, as in the front illuminated photo-detector  41  illustrated in  FIG. 7 , an encapsulation is applied over the active photo-detector chip  41 . 
   Since the transducer  8  is not hermetically sealed in its own container, the hermetic window found in the prior art devices is unnecessary. Accordingly, lens  7  can be positioned relatively close to the transducer  8 , which enables a small spot to be created on the photo-detector  11 , while maintaining a low numerical aperture. At higher data rates it is important to be able to provide a small spot, since the active region of the photo-detector is reduced to lower the capacitance-increasing bandwidth. In lower data rate photo-detectors, e.g. 2.5 Gb/s, the active regions have a diameter of 70 to 100 um, whereas in higher data rate photo-detectors, e.g. 10 Gb/s, 20 to 40 um diameter active regions are used. Since the diameter of multi-mode fiber is between 50 and 62.5 um, it is highly advantageous to be able to position the lens  7  proximate the photo-detector  11  in order to produce magnification less than unity, while still providing a relatively low numerical aperture. 
   With reference to  FIGS. 8 and 9 , the flexible circuit conductor  5  is a multi-layer micro-strip transmission line including a first conductive layer  51  ( FIG. 8 ) for the information signals, and a second conductive ground layer  52  ( FIG. 9 ), which enables the layout of controlled impedance transmission lines required for high-quality high data rate signals. 
   Another embodiment of the present invention in the form of OSA  61  is illustrated in  FIG. 10 , and includes an optical coupler  62 , a housing  63 , a stiffening plate  64 , and a flexible circuit conductor  65  extending out from the side thereof. As above, the optical coupler  62  and the housing  63  are integrally formed defining a bore  66  for an optical ferrule (not shown), and a lens  67 . The flexible circuit conductor  65  is electrically connected to a transducer  68 , e.g. photo-detector or VCSEL, and sandwiched between a mounting flange  69  of the housing  62  and the stiffening plate  64 . The flexible circuit  65  can include connecting portions on opposite surfaces thereof to facilitate attachment of the mounting flange  69  and the stiffening plate  64 . The stiffening plate  64 , like the stiffening plate ring  4 , is preferably cylindrical, although any other suitable shape is possible. As above, in the case of a ROSA, the lens  67  focuses a beam of light  70  onto a photo-detector  71  to convert the optical signal therein into an electrical signal, which is transmitted through TIA  72  to the transceiver PCB via flexible circuit conductor  65 . The end of the flexible circuit conductor  65  is supported by the stiffening plate  64  and surrounds the transducer  68  to provide easy access to a variety of contacts thereon. 
   The stiffening plate  64  can be constructed out of a material with high thermal conductivity, i.e. &gt;100 W/m°K or more than ten times that of the conventional TO can housing, e.g. zinc, aluminum, which enables the OSA  61  to run at higher operating temperatures before thermally induced noise becomes a factor. 
   Another embodiment of the present invention is illustrated in  FIG. 11 , in which an OSA  75  includes the a similar integrated optical coupler  62 /housing  63 , as above, but with the stiffening plate  64  replaced by a square stiffening plate  74  fabricated from a printed circuit board material, e.g. FR4. The stiffening plate  74  is attached to the mounting flange  69  with an adhesive  76  for enclosing the housing  63 . Accordingly, the stiffening plate  74  provides support for the end of the flexible circuit conductor  65 , while providing electrical communication between the transducer  68  and the flexible circuit conductor  65 . 
   An optical transceiver device  81 , illustrated in  FIG. 12 , includes a module casing  82  supporting a ROSA  83  (similar to OSA  1 ), a TOSA  84  and a PCB  85 . An OSA saddle adapter  86  provides extra support for the OSA&#39;s  83  and  84  in the casing  82 . Front end  87  of the casing  82  includes an optical connector adapted to receive a duplex optical connector, e.g. LC or SC, mounted on the end of a pair of optical fibers for transmitting optical signals to and from the OSAs  83  and  84 . An electrical connector, e.g. pins or card edge connector, (not shown) extends from the PCB  85  at the back end  88  of the casing  82  for connecting the transceiver to a PCB in a host device.