Abstract:
An optoelectronic device includes a header having a plurality of pins extending therethrough, a thermo-electric cooling device mounted adjacent to a side of the header and an optoelectronic assembly mounted on the thermo-electric cooling device. The optoelectronic assembly includes a light emitting device operable to emit an optical signal in response to an electric signal received by at least one of the plurality of pins, and a lens assembly operable to receive at least some of the light emitted by the light emitting device, the lens assembly having a lens. A cap substantially encloses the thermo-electric cooling device and the optoelectronic assembly. The cap has a window operable to transmit light emitted by the optoelectronic assembly. The lens is the only optical component in the lens assembly.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to optoelectronic converters, and more particularly to integrated laser assemblies or modules that provide a communications interface between a computer or communications unit having an electrical connector or interface and an optical fiber, such as used in fiber optic communications links. 
       BACKGROUND OF THE INVENTION 
       [0002]    A variety of optoelectronic transceivers are known in the art. Such devices typically include an optical transmitter portion (that converts an electrical signal into a modulated light beam that is coupled to an optical fiber), and a receiver portion (that receives an optical signal from an optical fiber and converts it into an electrical signal). Traditionally, optical receiver sections include an optical assembly to focus or direct the light from the optical fiber onto a photodetector, which in turn, is connected to an amplifier/limiter circuit on a circuit board. The photodetector or photodiode is typically packaged in a hermetically sealed package in order to protect it from harsh environmental conditions. 
         [0003]    Coaxial laser modules have seen some use in fiber optic telecommunication and CATV applications. Such modules typically use transistor outline (TO) packages and provide a relatively low cost solution in some markets. However, for applications where the laser consumes a relatively large amount of power or the laser is operated over a wide range of ambient temperatures, the laser diode (LD) and other optical components must be cooled in order to meet the requirements of an extremely narrow frequency spectrum and stable LD performance. External, forced air cooling has been the method of choice. 
         [0004]    The use of internal cooling with TO packages has proven difficult due to the limited space within the TO header and the size of the active and passive components found therein. One previous effort in this regard involved the cooling of a converter module using very small custom thermo-electric coolers (TECs) and was limited to the cooling of only the active components (i.e., the LD) and not the passive components (i.e., lens and isolator). The cooling of only the active components has been found to result in unstable optical performance where a wide range of operating temperatures is involved. 
         [0005]    U.S. Pat. No. 7,118,292 discusses a TO package housing a laser diode, a monitor photodiode (MPD) and a lens-isolator combo which are all mounted in thermal contact with a thermo-electric cooler. The use of a lens-isolator combo raises the thermal load, and also the cost due to the requirement of a relatively large clear aperture. In the TO package discussed in U.S. Pat. No. 7,118,292, light is directed from the rear facet of the LD to the MPD via a mirror mounted on a wedge. This has the advantage of lowering the profile of the components mounted on the cold plate, improving mechanical and thermal stability. But the addition of the mirror and the wedge also increases cost. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide an improved optical transmitter using an integrated thermal electric cooler and other optical subassemblies. 
         [0007]    This and other objects are provided by an optoelectronic device comprising a header having a plurality of pins extending therethrough, a thermo-electric cooler mounted adjacent to the second side of the header, an optoelectronic assembly mounted on the thermo-electric cooler, and a cap substantially enclosing the thermo-electric cooler and the optoelectronic assembly, the cap having a window operable to transmit light emitted by the optoelectronic assembly. The optoelectronic assembly includes a light emitting device operable to emit an optical signal in response to an electric signal received by at least one of the plurality of pins, and a lens assembly operable to receive at least some of the light emitted by the light emitting device. A lens is the only optical component in the lens ring assembly, so that the thermal load on the thermo-electric cooler is relatively small and the cost of the optoelectronic device is reduced. 
         [0008]    This and other objects are also provided by an optoelectronic device comprising a header having a plurality of pins extending therethrough, a light emitting device operable to emit an optical signal in response to an electric signal received by at least one of the plurality of pins, said light emitting device being operable to emit light from two opposing sides, a light detector, and a cap substantially enclosing the light emitting device and the light detector, the cap having a window operable to transmit light emitted from one of said two opposing sides of the light emitting device. Light emitted from the other of said two opposing sides of the light emitting device is emitted along a light path directly to the light detector, and the light detector is positioned perpendicular to a central axis of the light path for light emitted from said other of said two opposing sides away from said central axis of the light path. Alternatively, the light detector may be positioned oblique to the central axis of the light path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of the proposed coaxial cooled laser module. 
           [0010]      FIG. 2  (a) and (b) are typical views of the TO-can package and the cap of  FIG. 1  with a flat window; 
           [0011]      FIG. 3  shows the TO header of  FIG. 1  and its pin orientation; 
           [0012]      FIG. 4  shows the configuration of a laser diode and a monitor photodiode forming part of the module of  FIG. 1 ; 
           [0013]      FIG. 5  shows a side view of the laser diode and monitor photodiode configuration illustrated in  FIG. 4 ; 
           [0014]      FIG. 6  shows a side view of an alternative laser diode and monitor photodiode configuration; 
           [0015]      FIG. 7  shows an optical lens assembly forming part of the module of  FIG. 1 ; 
           [0016]      FIG. 8  shows an optoelectronic assembly including the laser diode and monitor photodiode configuration of  FIGS. 4 and 5  and the lens assembly of  FIG. 7 ; 
           [0017]      FIG. 9  shows an alternative optical lens assembly; and 
           [0018]      FIG. 10  shows an alternative optoelectronic assembly including the laser diode and monitor photodiode configuration of  FIG. 6  and the lens assembly of  FIG. 8 . 
       
    
    
       [0019]    It should be noted that the dimensions and scales shown in above figures are not accurate and are for illustration and explanation only. Similarly, the components shown in the figures also are for illustration and explanation purpose. Actual components may vary. For simplicity, the wirebonds between the components are omitted herein. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Details of present invention will now be described, including exemplary aspects and embodiments thereof. 
         [0021]    With this in mind,  FIG. 1  shows a coaxially cooled laser module having a TO-can  10  and a fiber pigtail module  12 , having a proximal end and a distal end, which functions to align an axis of light transmission between the TO-can  10  and an optical fiber. 
         [0022]    The TO-can  10  consists of a TO header subassembly  14 , as shown in  FIG. 2(   a ), and a sealing cap  16  with a flat window  18 , as shown in  FIG. 2(   b ). In this embodiment, the flat window  18  has a few degrees of tilt from the axis of the laser beam to reduce back reflection, although this is not essential. 
         [0023]    As shown in  FIG. 2(   a ), the TO header subassembly  14  includes a TO header  20  with a number of electrical conductor pins  22  extending through the TO header  20 . The TO header subassembly  14  also includes a thermo-electric cooler (TEC)  24  and an optoelectronic assembly  26  that supports active and passive optical components including a laser diode (LD), a monitor photodiode (MPD), and an optical lens assembly. The TO header  20  may be made of a number of different materials, like cold roll steel, Kovar or other alloys. The diameter of the header should be large enough to accommodate a selected TEC  24 . In this embodiment, the TO header  24  is 9 mm in diameter. 
         [0024]    In this embodiment, the header pins  22  are in an inline layout as shown in  FIG. 3 . Alternatively, the header pins  22  may be in a circular arrangement. In order to get better RF performance at high frequency ranges, an RF pin  300  is designed to have good impedance match. In this embodiment, the RF pin  300  is a coaxial pin consists of metal tubing and a glass filler. The diameters of the tubing and pin are determined by the matching impedance and dielectric constant of the glass filler. The metal tubing is brazed on the header. 
         [0025]    The TEC  24  may be any commercial available miniature cooler that consists of a hot plate, a cold plate and thermal electric semiconductor elements. The thermal electric semiconductor elements are placed in couples. The thermal capacity of the TEC  24  should be chosen so that it has a sufficient number of thermal couples to dissipate both the active heat load generated by LD and the passive heat load leaked into the package from the surrounding environment, while still keeping the cost low. In this embodiment, the TEC  24  has a footprint of approximately 5×4 mm. 
         [0026]    As discussed in U.S. Pat. No. 7,118,292, the whole contents of which are hereby incorporated herein by reference, two wirebond pads are located in opposite ends of the TEC  24 . Other configurations of the wirebond pads are possible. In this embodiment, the TEC  24  is soldered to the center of the TO header  20  with the hot plate adjacent to the TO Header  20 , and the wirebond pads are connected to pins  22  via a pair of wirebonds. 
         [0027]    The optoelectronic assembly  26  has a carrier  400  which is mounted onto the cold plate of the TEC  24 . In this embodiment, the carrier  400  is made of Kovar, but alternatively it could be made of stainless steel or any other suitable material with good thermal conductivity. As shown in  FIG. 4 , in this embodiment the LD  402  is mounted on a LD submount  404 , which is in turn mounted on the carrier  400 . The LD submount  404  is made of aluminum nitride (AIN), although a different material with good thermal conductivity could alternatively be used. In this embodiment, the LD  402  is an edge emitting laser and is soldered vertically on the submount. The edge emitting LD  402  emits laser light in two directions, one is in a forward direction from a front facet and the other is backward from a rear facet. 
         [0028]    Alternatively, a surface emitting laser is also suitable for this application with slight modification of the LD submount  404 . 
         [0029]    In a traditional uncooled TO laser package where the edge-emitting laser is used, an MPD is mounted directly beneath the LD to catch the laser light from the rear facet of the LD for purposes of monitoring laser performance. This configuration has a drawback in that it results in back reflection into the laser diode. As shown in  FIG. 5 , in this embodiment the MPD  406  is mounted on a substrate ( 408 ) which is not directly beneath the LD  402 , so that the MPD  406  is offset from being directly beneath the LD  402  in order to reduce back reflection into the LD  402 . In particular, the LD  402  emits laser light along a light path having a central axis. The MPD  406  is positioned perpendicular to the central axis of the light path at a position away from the central axis so that light travelling along the central axis does not impinge on the MPD  406 , but rather a portion of the light away from the central axis is incident on the MPD  406 . 
         [0030]      FIG. 6  shows an alternative configuration in which a MPD  606  is positioned directly beneath the rear facet of an LD  602 , but is mounted on a wedge  610  to reduce back reflection into the LD  602 . 
         [0031]      FIG. 7  shows a lens assembly  700  which is also mounted on the carrier  400 , as shown in  FIG. 8 . The lens assembly  700  consists of an optical lens  702  which is bonded into a metal housing  704 . The optical lens  702  is the only optical component in the lens assembly  700 . In this embodiment, the optical lens  702  is an aspheric lens having a numerical aperture (NA) of 0.4. Other values of NA could be used, and a ball lens could alternatively be used. The surface of the lens may have an anti-reflective (AR) coating. The metal housing  704  includes at one longitudinal end a metal ring portion  706  having notches. As shown in  FIG. 8 , the lens assembly  700  is mounted to the carrier  400  with the notched ring portion  706  adjacent the carrier  400 . 
         [0032]    The optical lens  702  may be pre-fixed to the metal housing  704  prior to assembly. Alternatively, the optical lens  702  may be slip-fit within the metal housing  704  to allow the position of the optical lens  702  to be adjustable during an alignment process. The optical lens  702  can be bonded to the metal housing  704  in various ways, for example using either epoxy or laser welding. Similarly, the lens assembly can be bonded to the carrier  400  in various ways, for example using either epoxy or laser welding. 
         [0033]    The placement of the lens assembly  700  onto the carrier  400  can be done using either active or passive alignment. In particular, by using an optical lens  702  with a relatively low NA, there is less sensitivity to lens placement allowing passive alignment to be used, and in addition the working distance from the laser to the lens is longer, allowing more room for component placement. 
         [0034]    In an alternative embodiment, as shown in  FIG. 9  the lens assembly  900  consists of an optical lens  902  which is bonded into a metal housing  904  having a metal ring portion  906  without notches at one longitudinal end. In this alternative embodiment, as shown in  FIG. 10 , the carrier  1000  has extruded portions and the lens assembly  900  is mounted on the extruded portions. 
         [0035]    By mounting the lens assembly on the carrier, this coaxial package provides cooling for both active components (i.e. the LD and the MPD) and passive components (i.e. the optical lens) on the cooled platform, ensuring stable performance over a wide range of operation temperature. 
         [0036]    As discussed above, the only optical component in the lens assembly is the optical lens, so that the optical lens is the only optical component in the light path between the LD and the window of the cap. The lens assembly does not include an optical isolator. In an embodiment, an optical isolator is mounted at the proximal end of the fiber pigtail module  12 . Alternatively, an inline optical isolator may be used. 
         [0037]    Compared with a traditional butterfly package, the coaxial package described herein consumes much less DC power than the butterfly package, for substantially the same laser output. Typically only half of the DC consumed by the butterfly package is needed by the module described above. Therefore, the package reliability is increased and thermal efficiency is also increased. 
         [0038]    The coaxial package simplifies the manufacturing processes and significantly reduces component and labor cost compared with the coaxial package discussed in U.S. Pat. No. 7,118,292. 
         [0039]    It will be understood that elements described above, or two or more together, may be replaced by functionally equivalent elements which satisfy the design requirements. For example, the photodiode could be replaced by an alternative photodetector.