Patent Publication Number: US-6911599-B2

Title: Header assembly for optoelectronic devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/231,395, filed Aug. 29, 2002 now U.S. Pat. No. 6,703,561, entitled “Header Assembly Having Integrated Cooling Device” which is a continuation-in-part of the following applications: U.S. patent application Ser. No. 10/077,067, filed Feb. 14, 2002, entitled “Ceramic Header Assembly” (now U.S. Pat. No. 6,586,678); and U.S. patent application Ser. No. 10/101,260, filed Mar. 18, 2002, (claiming priority to U.S. Provisional Patent Application Ser. No. 60/317,835, filed Sep. 6, 2001), entitled “Compact Laser Package with Integrated Temperature Control.” All of the aforementioned patents and patent applications are incorporated herein in their respective entireties by this reference. 
    
    
     BACKGROUND 
     1. Technological Field 
     This invention is generally concerned with the field of opto-electronic systems and devices. More specifically, embodiments of the present invention relate to a header assembly for use in various optoelectronic devices. 
     2. Related Technology 
     Transistor headers, or transistor outlines (“TO”), are widely used in the field of opto-electronics, and may be employed in a variety of applications. As an example, transistor headers are sometimes used to protect sensitive electrical devices, and to electrically connect such devices to components such as printed circuit boards (“PCB”). 
     With respect to their construction, transistor headers often consist of a cylindrical metallic base with a number of conductive leads extending completely through, and generally perpendicular to, the base. A glass hermetic seal between the conductive leads and the base provides mechanical and environmental protection for the components contained in the TO package, and electrically isolates the conductive leads from the metallic material of the base. Typically, one of the conductive leads is a ground lead that may be electrically connected directly to the base. 
     Various types of devices are mounted on one side of the base of the header and connected to the leads. Generally, a cap is used to enclose the side of the base where such devices are mounted, so as to form a chamber that helps prevent contamination or damage to those device(s). The specific characteristics of the cap and header generally relate to the application and the particular device being mounted on the base of the header. By way of example, in applications where an optical device is required to be mounted on the header, the cap is at least partially transparent so to allow an optical signal generated by the optical device to be transmitted from the TO package. 
     Although transistor headers have proven useful, typical configurations nevertheless pose a variety of unresolved problems. Some of such problems relate specifically to the physical configuration and disposition of the conductive leads in the header base. As an example, various factors conspire to compromise the ability to precisely control the electrical impedance of the glass/metal feedthru, that is, the physical bond between the conductive lead and the header base material. One such factor is the fact that there is a relatively limited number of available choices with respect to the diameter of the conductive leads that are to be employed. Further, the range of dielectric values of the sealing glass typically employed in these configurations is relatively small. And, with respect to the disposition of the conductive leads, it has proven relatively difficult in some instances to control the position of the lead with respect to the through hole in the header base. 
     Yet other problems in the field concern those complex electrical and electronic devices that require many isolated electrical connections in order to function properly. Typically, attributes such as the size and shape of such devices and their subcomponents are sharply constrained by various form factors, other dimensional requirements, and space limitations within the device. Consistent with such form factors, dimensional requirements, and space limitations, the diameter of a typical header is relatively small and, correspondingly, the number of leads that can be disposed in the base of the header, sometimes referred to as the input/output (“I/O”) density, is relatively small as well. 
     Thus, while the diameter of the header base, and thus the I/O density, may be increased to the extent necessary to ensure conformance with the electrical connection requirements of the associated device, the increase in base diameter is sharply limited, if not foreclosed completely, by the form factors, dimensional requirements, and space limitations associated with the device wherein the transistor header is to be employed. 
     A related problem with many transistor headers concerns the implications that a relatively small number of conductive leads has with respect to the overall performance of the device wherein the transistor header is used. Specifically, devices such as semiconductor lasers operate more efficiently if their driving impedance is balanced with the impedance at the terminals. Impedance matching is often accomplished through the use of additional electrical components such as resistors, capacitors and transmission lines such as microstrips or striplines. However, such components cannot be employed unless a sufficient number of conductive leads are available in the transistor header. Thus, the limited number of conductive leads present in typical transistor headers has a direct negative effect on the performance of the semiconductor laser or other device. 
     In connection with the foregoing, another aspect of many transistor headers that forecloses the use of, for example, components required for impedance matching, is the relatively limited physical space available on standard headers. In particular, the relatively small amount of space on the base of the header imposes a practical limit on the number of components that may be mounted there. In order to overcome that limit, some or all of any additional components desired to be used must instead be mounted on the printed circuit board, some distance away from the laser or other device contained within the transistor header. Such arrangements are not without their shortcomings however, as the performance of active devices in the transistor header, such as lasers and integrated circuits, depends to some extent on the physical proximity of related electrical and electronic components. 
     The problems associated with various typical transistor headers are not, however, limited solely to geometric considerations and limitations. Yet other problems relate to the heat generated by components within, and external to, the transistor header. Specifically, transistor headers and their associated subcomponents may generate significant heat during operation. It is generally necessary to reliably and efficiently remove such heat in order to optimize performance and extend the useful life of the device. 
     However, transistor headers are often composed primarily of materials, Kovar® for example, that are not particularly good thermal conductors. Such poor thermal conductivity does little to alleviate heat buildup problems in the transistor header components and may, in fact, exacerbate such problems. Various cooling techniques and devices have been employed in an effort to address this problem, but with only limited success. 
     By way of example, solid state heat exchangers may be used to remove some heat from transistor header components. However, the effectiveness of such heat exchangers is typically compromised by the fact that, due to variables such as their configuration and/or physical location relative to the primary component(s) to be cooled, such heat exchangers frequently experience a passive heat load that is imposed by secondary components or transistor header structures not generally intended to be cooled by the heat exchanger. The imposition on the heat exchanger of such passive heat loads thus decreases the amount of heat the heat exchanger can effectively remove from the primary component that is desired to be cooled, thereby compromising the performance of the primary component. 
     As suggested above, the physical location of the heat exchanger or other cooling device has various implications with respect to the performance of the components employed present in the transistor header. On particular problem that arises in the context of thermoelectric cooler (“TEC”) type heat exchangers relates to the fact that TECs have hot and cold junctions. The cold junction, in particular, can cause condensation if the TEC is located in a sufficiently humid environment. Such condensation may materially impair the operation of components in the transistor header, and elsewhere. 
     Another concern with respect to heat exchangers is that the dimensions of typical transistor headers are, as noted earlier, constrained by various factors. Thus, while the passive heat load placed on a heat exchanger could be at least partly offset through the use of a relatively larger heat exchanger, the diametric and other constraints imposed on transistor headers by form factor requirements and other considerations place practical limits on the maximum size of the heat exchanger. 
     Finally, even if a relatively large heat exchanger could be employed in an attempt to offset the effects of passive heat loads, large heat exchangers present problems in cases where the heat exchanger, such as a TEC, is used to modify the performance of transistor header components such as lasers. For example, by virtue of their relatively large size, such heat exchangers are not well suited to implementing the rapid changes in laser performance that are required in many applications because such large heat exchangers heat up and cool down relatively slowly. Moreover, the performance of the laser or other component may be further compromised if the heat exchanger is located relatively far away from the laser because the rate at which heat can be transferred with respect to the laser or other component is at least partially a function of the distance between the component and the heat exchanger. 
     In view of the foregoing discussion, what is needed is a transistor header having features directed to addressing the foregoing exemplary concerns, as well as other concerns not specifically enumerated herein. An exemplary transistor header should implement a relatively high I/O density without increasing the relative diameter of the header. Moreover, the exemplary transistor header should be configured to precisely control the electrical impedance and permit location of various components in relatively close proximity to the active components, such as a laser, within the header without violating applicable form factors or other geometric and dimensional standards. Finally, the exemplary transistor header should include features directed to facilitating a relative improvement in heat management capability within the transistor header. 
     BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION 
     In general, embodiments of the invention are concerned with a transistor header including various features directed to enhancing the reliability and performance of various electronic devices, such as lasers, included in the transistor header. 
     In one exemplary embodiment, a transistor header is provided that includes a substantially cylindrical metallic base as well as a platform disposed in a substantially perpendicular orientation with respect to the base and extending through both sides of the base. The platform is constructed from an insulating material such as a ceramic. The platform is hermetically sealed to the base, and flat surfaces defined by the platform on either side of the base are configured to receive multiple electrical components. Moreover, the platform includes a plurality of conductive pathway(s) extending between the ends of the platform so that components on opposite sides of the base may be electrically connected with each other. On one end of the platform, a connector is provided that is in electrical communication with some or all of such conductive pathways. 
     In this exemplary embodiment, a laser is disposed on top of a TEC, which, in turn, is mounted to the platform. A cup having a transparent portion is situated on the base cooperates with the platform and the base to define a hermetic chamber enclosing the laser and the TEC. Power is supplied to the TEC by way of a laser control system that communicates both with a light intensity measuring device optically coupled to the laser and with a temperature sensing device thermally coupled to the laser. 
     In operation, power is supplied to the laser by way of the connector on the platform and the laser emits light through the transparent portion of the cup. The light intensity measuring device and the temperature sensing device provide data on the light intensity of the laser as a function of laser temperature and transmit the data to a control circuit which adjusts the power applied to the TEC, thereby raising or lowering the temperature of the laser as necessary to meet the laser performance requirements. 
     These and other, aspects of embodiments of the present invention will become more fully apparent from the following description and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1A  is a perspective view illustrating various aspects of the device side of an exemplary embodiment of a header assembly; 
         FIG. 1B  is a perspective view illustrating various aspects of the connector side of an exemplary embodiment of a header assembly; 
         FIG. 2A  is a perspective view illustrating various aspects of the device side of an alternative embodiment of a header assembly; 
         FIG. 2B  is a perspective view illustrating various aspects of the connector side of an alternative embodiment of a header assembly; 
         FIG. 3A  is a perspective view illustrating various aspects of the device side of another alternative embodiment of a header assembly; 
         FIG. 3B  is a perspective view illustrating various aspects of the connector side of another alternative embodiment of a header assembly; 
         FIG. 4A  is a top perspective view of an exemplary embodiment of a header including active devices mounted on a TEC disposed within a hermetic chamber; 
         FIG. 4B  is a bottom perspective view of the exemplary embodiment illustrated in  FIG. 4A ; 
         FIG. 4C  is a cross-section view illustrating various aspects of the exemplary embodiment presented in  FIGS. 4A and 4B ; 
         FIG. 4D  is a cross-section view taken along line  4 D— 4 D of FIG.  4 C and illustrates various aspects of an exemplary arrangement of a TEC in a header assembly; 
         FIG. 4E  is a side view illustrating aspects of an exemplary electrical connection scheme for the header assembly and a printed circuit board; 
         FIG. 4F  illustrates various aspects of an alternative platform/TEC configuration where the TEC is located outside the hermetic chamber; and 
         FIG. 5  is a schematic diagram illustrating various aspects of an exemplary embodiments of a laser control system. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Reference will now be made to figures wherein like structures will be provided with like reference designations. It is to be understood that the drawings are diagrammatic and schematic representations of various embodiments of the claimed invention, and are not to be construed as limiting the scope of the present invention in any way, nor are the drawings necessarily drawn to scale. 
     Reference is first made to  FIGS. 1A and 1B  together, which illustrate perspective views of one presently preferred embodiment of a header assembly, designated generally at  200 . In the illustrated example, the header assembly  200  includes a substantially cylindrical metallic base  10 . The base  10  includes two flanges  90  used to control angular or rotational alignment of the header  200  to a receptacle (not shown) on a higher level opto-mechanical assembly. The base can be formed of Alloy  42 , which is an iron nickel alloy, as well as cold-rolled steel, or Vacon VCF-25 Alloy. The base  10  also includes a ceramic platform  70  extending perpendicularly through the base as shown. The ceramic platform is hermetically sealed to the base to provide mechanical and environmental protection for the components contained in the TO package. 
     The hermetic seal between the base  10  and the platform  70  is created by electrically insulating glass-to-metal seals. Alternatively, the platform  70  may incorporate two additional ceramic outer layers to electrically isolate the outermost conductors. In this second case, a metal braze or solder can be used to hermetically seal the platform  70  to the metal base. This solution overcomes the principal shortcomings of glasses, namely their low strength, brittleness, and low thermal conductivity. 
     The platform  70  is structured to house multiple electrical components  50  and  100 , and active devices  60  on either side of the base. In the illustrated embodiment, the active device  60  comprises a semiconductor laser, and the components  50  and  100  are resistors, capacitors, and inductors that are used to balance the driving impedance of the laser with the component impedance. As it is important for a semiconductor laser to be precisely positioned perpendicularly to the base  10 , platform  70  is, therefore, precisely positioned perpendicularly with respect to the base  10 . 
     Where active device  60  comprises a semiconductor laser, a small deviation in the position of active device  60 , in relation to base  10  can cause a large deviation in the direction of the emitted laser beam. Accurate perpendicularity between the platform and the base can be achieved by incorporating a vertical pedestal feature in the base material, as shown on FIG.  1 A. The vertical pedestal houses the photodiode  30  in the embodiment shown in FIG.  1 A. Such feature can be machined, stamped, or metal injection molded directly with the base thus providing a stable and geometrically accurate surface for mating with the platform. 
     The platform  70  further includes multiple electrically isolated conductive pathways  110  extending throughout the platform  70  and consequently through the base  10 . The conductive pathways  110  provide the electrical connections necessary between electrical devices or components located throughout the platform  70 . The conductive pathways  110  form a connector on that side of the base that does not include the semiconductor laser  60 , also referred to herein as the “connector side” of the base. Note in connection with the foregoing that the side of the base where the active device  60  is located may in some instances be referred to herein as the “device side” of the base. 
     The connector formed by the conductive pathways  110  is used to electrically connect the header assembly  200  to a second electrical subassembly, such as a printed circuit board, either directly (for example, by solder connection) or indirectly by an intermediary device such as a flexible printed circuit. The semiconductor laser  60  is electrically connected to the electrical components  50  and  100  via the conductive pathways  10 . In one embodiment, the platform  70  is itself a printed circuit board having conductive pathways  10  formed therein. 
     The use of advanced ceramic materials, examples of which include aluminum nitride and beryllia, allows the header assembly  200  to achieve substantially lower thermal resistances between the devices inside the package and the outside world where heat is ultimately transferred. As discussed in further detail below in the context of an alternative embodiment of the invention, a cooling device, such as a thermoelectric cooler (“TEC”), a heat pipe or a metal heat spreader, can be mounted directly on the platform, thereby providing for a very short thermal path between the temperature sensitive devices on the platform and a heat sink located outside the header assembly. 
     As is further shown in  FIGS. 1A and 1B , the header assembly  200  can additionally include two conductive leads  40  extending through and out both sides of the base  10 . The conductive leads  40  are hermetically sealed to the base  10  to provide mechanical and environmental protection for the components contained in the TO package between the conductive leads  40  and the base  10 . The hermetic seal between the conductive leads  40  and the base  10  is created, for example, by glass or other comparable hermetic insulating materials that are known in the art. The conductive leads  40  can also be used to electrically connect devices and/or components located on opposite sides of the base. 
     In the illustrated embodiment at least, the conductive leads  40  extend out from the side of the base  10  that does not contain the semiconductor laser  60 , in a manner that allows for the electrical connection of the header assembly  200  with a specific header receptacle located on, for example, a printed circuit board. It is important to note that conductive pathways  110  and conductive leads  40  perform the same function and that the number of potential conductive pathways  110  is far greater than the potential number of conductive leads  40 . Therefore, alternative embodiments can incorporate even more conductive pathways  110  than shown in the illustrated embodiment. 
     The platform  70  further includes steps and recessed areas that permit mounting devices with various thicknesses flush with the metal pads on the ceramic. This allows the use of the shortest electrical interconnects, wire bonds for example, having improved electrical performance and characteristics. 
     The photodiode  30  is used to detect the signal strength of the semiconductor laser  60  and relay this information back to control circuitry (see  FIG. 5 ) of the semiconductor laser  60 . In the illustrated embodiment, the photodiode can be directly connected to the conductive leads  40 . Alternatively, the photodiode can be mounted directly onto the same platform as the laser, in a recessed position with respect to the light emitting area. This recessed position allows the photodiode to capture a fraction of the light emitted by the laser, thus allowing the photodiode to perform the same monitoring function. In yet another configuration, as shown in  FIG. 4C , a monitor photodiode  1004  with an angled facet can be mounted in a plane behind the laser diode. The angled facet deflects the light emitted from the back-facet of the laser upwards toward the sensitive area of the detector. 
     The configurations of the monitoring photodiode discussed in the previous paragraph allow for eliminating the need of conductive leads  40 , and lends themselves to simplified electrical connections, such as wire bonds, to the conductive pathways  110  of the platform  70 . In an alternative embodiment, the photodiode light gathering can be increased by positioning an optical element on the base for focusing or redirecting light, such as a mirror, or by directly shaping and/or coating the base metal to focus additional light onto the photodiode 
     As is further shown in  FIG. 1A , the base  10  includes a protruding portion  45  that is configured to releasably position or locate a cap (not shown) over one side of the base  10 . A cap can be placed over the side of the base  10  containing the semiconductor laser  60  for the purpose of protecting the semiconductor laser  60  from potentially destructive particles. A transparent cap is preferable for the illustrated embodiment so as to allow the laser light to escape the region between the cap and the base  10 . 
     Reference is next made to  FIGS. 2A and 2B , which illustrate perspective views of an alternative embodiment of a header assembly, designated generally at  300 . This alternative embodiment shows an optical receiver  360  mounted horizontally on the platform  370  perpendicularly bisecting the base  310  of the header assembly  300 . The optical receiver can be a photodetector or any other device capable of receiving optical signals. The optical receiver  360  is mounted flat on the platform  370  and detects light signals through the side facing away from the base  310 . This type of optical receiver is sometimes referred to as an “edge detecting” detector. The base  310  and platform  370  are described in more detail with reference to  FIGS. 1A and 1B . The platform  370  contains electrical components  350 ,  400  on either side of the base for operating the optical receiver  360 . The platform  370  also includes conductive pathways  410  for electrically connecting devices or components on either side of the base  310 . This embodiment of a header assembly does not contain conductive leads and therefore all electrical connections are made via the conductive pathways  410 . 
     Reference is next made to  FIGS. 3A and 3B , which illustrate perspective views of yet another alternative embodiment of a header assembly, designated generally at  500 . This alternative embodiment also shows an optical receiver  530  mounted vertically on the base  510 . The optical receiver can be a photodetector or any other device capable of receiving optical signals. This is an optical receiver  530  which detects light signals from the top of the device. The base  510  and platform  570  are described in more detail with reference to  FIGS. 1A and 1B . The platform  570  contains electrical components  550 ,  600  on either side of the base for operating the optical receiver  530 . The platform  570  also includes conductive pathways  510  for electrically connecting devices or components on either side of the base  510 . This embodiment of a header assembly does not contain conductive leads and therefore all electrical connections are made via the conductive pathways  410 . 
     Directing attention now to  FIGS. 4A through 4D , various aspects of an alternative embodiment of a header assembly, generally designated at  700 , are illustrated. The embodiment of the header assembly illustrated in  FIGS. 4A through 4D  is similar in many regards to one or more of the embodiments of the header assembly illustrated in  FIGS. 1A through 3B . Accordingly, the discussions of  FIGS. 4A through 4D  will focus primarily on certain selected aspects of the header assembly  700  illustrated there. Note that in one embodiment of the invention, header assembly  700  comprises a transistor header. However, header assembly  700  is not limited solely to that exemplary embodiment. 
     As indicated in  FIGS. 4A through 4D , header assembly  700  generally includes a base  702  through which a platform  800  passes. Platform  800  may comprise a printed circuit board or, as discussed herein, may comprise other materials and/or configurations as well. The platform  800  is configured to receive a cooling device  900  upon which various devices and circuitry are mounted. Note that while it may be referred to herein as a “cooling” device  900 , the cooling device  900  may, depending upon its type and the application where it is employed, serve both to heat and/or cool various components and devices. Finally, a cap  704  mounted to, and cooperating with, base  702 , serves to define a hermetic chamber  706  which encloses cooling device  900  and the mounted devices and circuitry. 
     As discussed in further detail below, a variety of means may be employed to perform the functions disclosed herein, of a cooling device. Thus, the embodiments of the cooling device disclosed and discussed herein are but exemplary structures that function as a means for transferring heat. Accordingly, it should be understood that such structural configurations are presented herein solely by way of example and should not be construed as limiting the scope of the present invention in any way. Rather, any other structure or combination of structures effective in implementing the functionality disclosed herein may likewise be employed. 
     With continuing attention to  FIGS. 4A and 4B , and directing attention also to  FIGS. 4C and 4D , further details are provided concerning various aspects of platform  800 . In the illustrated embodiment, platform  800  is disposed substantially perpendicularly with respect to base  702 . In particular, base  702  includes a device side  702 A and a connector side  702 B, and platform  800  passes completely through base  702 , so that an inside portion  801 A of platform  800  is disposed on device side  702 A of base  700  and outside portion  801 B of platform  800  is disposed on connector side  702 B of base  702 . However, this arrangement of platform  800  is exemplary only, and various other arrangements of platform  800  may alternatively be employed consistent with the requirements of a particular application. 
     In the illustrated embodiment, platform  800  includes a first feedthru  802  having a multi-layer construction that includes one or more layers  804  of conductive pathways  806  (see FIG.  4 A). In general, conductive pathways  806  permit electrical communication among the various components and devices (removed for clarity) disposed on platform  800 , while also permitting such components and devices to electrically communicate with other components and devices that are not a part of platform  800 . Moreover, conductive pathways  806  cooperate to form a connector  810  situated on the outside portion  801 B of platform  800 , on the connector side  702 B of base  700 . In general, connector  810  facilitates electrical communication between header assembly  700  and other components and devices such as, but not limited to, printed circuit boards (see FIG.  4 E). In one embodiment, connector  810  comprises an edge connector, but any other form of connector may alternatively be used, consistent with the requirements of a particular application. As discussed in further detail below, first feedthru  802  may include cutouts  811  or other geometric features which permit direct access to, and electrical connection with, one or more conductive pathways  806  disposed on an inner layer of first feedthru  802 . 
     In addition to the first feedthru  802 , platform  800  further includes a second feedthru  812  to which the first feedthru  802  is attached. Note that in the exemplary illustrated embodiment, first feedthru  810 , with the exception of conductive pathways  806 , may comprise a material that is generally resistant to heat conduction, such as a ceramic with low thermal conductivity, such as alumina for example. Low thermal conductivity ceramics may be more desirable in some instances than high thermal conductivity ceramics, such as aluminum nitrade or beryllia, due to the relatively lower cost of such low thermal conductivity ceramics, as well as the ease with which such low thermal conductivity ceramics can be brazed to various metals such as may be used in the construction of header assembly  700 . In contrast, second feedthru  812  in the illustrated embodiment comprises a material that is generally useful as a heat conductor, such as a metal. Various copper-tungsten alloys are examples of metals that are suitable in some applications. Thus, platform  800  is generally configured to combine heat conductive elements with non-heat conductive elements so as to produce a desired effect or result concerning the device wherein platform  800  is employed. 
     In connection with the foregoing, it should be noted further that ceramics and metals are exemplary materials only and any other material or combination thereof that will facilitate implementation of the functionality disclosed herein may alternatively be employed. Moreover, other embodiments of the invention may employ different arrangements and numbers of, for example, conductive and non-conductive feedthrus, or feedthrus having other desirable characteristics. Accordingly, the illustrated embodiments are exemplary only and should not be construed to limit the scope of the invention in any way. 
     With respect to their configurations, the geometry of both first feedthru  802  and second feedthru  812  may generally be configured as necessary to suit the requirements of a particular application or device. In the exemplary embodiment illustrated in  FIGS. 4A through 4D , second feedthru  812  incorporates a step  812 A feature which serves to, among other things, provide support for cooling device  900  and, as discussed in further detail below, to ensure that devices mounted to cooling device  900  are situated at a desirable location and orientation. As further indicated in  FIG. 4D , for example, second feedthru  812  defines a semi-cylindrical bottom that generally conforms to the shape of cap  704  and contributes to the stability of cooling device  900 , as well as providing a relatively large conductive mass that aids in heat conduction to and/or from, as applicable, cooling device  900  and other devices. 
     As suggested earlier, platform  800  also serves to provide support to cooling device  900 . Directing renewed attention now to  FIGS. 4A through 4D , details are provided concerning various aspects of cooling device  900 . In particular, a cooling device  900  is provided that is mounted directly to platform  800 . In an exemplary embodiment, cooling device  900  comprises a thermoelectric cooler (“TEC”) that relies for its operation and usefulness on the Peltier effect wherein electrical power supplied to the TEC may, according to the requirements of a particular application, cause selected portions of the TEC to generate heat and/or provide a cooling effect. Exemplary construction materials for the TEC may include, but are not limited to, bismuth-telluride combinations, or other materials with suitable thermoelectric properties. 
     Note that the TEC represents an exemplary configuration only, and various other types of cooling devices may alternatively be employed as required to suit the dictates of a particular application. By way of example, where active temperature control of one or more electronic devices  1000 , aspects of which are discussed in more detail below, is not required, the TEC may be replaced with a thermally conductive spacer or similar device. 
     In addition to providing heating and/or cooling functionality, cooling device  900  also includes a submount  902  that supports various electronic devices  1000  such as, but not limited to, resistors, capacitors, and inductors, as well as optical devices such as mirrors, lasers, and optical receivers. Thus, cooling device  900  is directly thermally coupled to electronic devices  1000 . 
     In one exemplary embodiment, the electronic devices  1000  include a laser  1002 , such as a semiconductor laser, or other optical signal source. With regard to devices such as laser  1002 , at least, cooling device  900  is positioned and configured to ensure that laser  1002  is maintained in a desired position and orientation. By way of example, in some embodiments of the invention, cooling device  900  is positioned so that an emitting surface of laser  102  is positioned at, and aligned with, a longitudinal axis A-A of header assembly  700  (FIG.  4 C). 
     Note that although reference is made herein to the use of a laser  1002  in conjunction with cooling device  900 , it should be understood that embodiments employing laser  1002  are exemplary only and that additional or alternative devices may likewise be employed. Accordingly, the scope of the invention should not be construed to be limited solely to lasers and laser applications. 
     In at least some of those embodiments where a laser  1002  is employed, a photodiode  1004  and thermistor  1006  (see  FIG. 4D ) are also mounted to, or proximate, submount  902  of cooling device  900 . In general, photodiode  1004  is optically coupled with laser  1002  such that photodiode  1004  receives at least a portion of the light emitted by laser  1002 , and thereby aids in gathering light intensity data concerning laser  1002  emissions. Further, thermistor  1006  is thermally coupled with laser  1002 , thus permitting the gathering of data concerning the temperature of laser  1002 . 
     In some embodiments, photodiode  1004  comprises a  45  degree monitor photodiode. The use of this type of diode permits the related components, such as laser  1002  and thermistor  1006  for example, to be mounted and wirebonded on the same surface. Typically, the  45  degree monitor diode is arranged so that light emitted from the back of laser  1002  is refracted on an inclined surface of the monitor diode and captured on a top sensitive surface of the monitor diode. In this way, the monitor diode is able to sense the intensity of the optical signal emitted by the laser. 
     Note that in those embodiments where a laser  1002  is employed, cap  704  includes an optically transparent portion, or window,  704 A through which light signals generated by the laser  1002  are emitted. Similarly, in the event electronic device  1000  comprises other optical devices, such as an optical receiver, cap  704  would likewise include a window  704 A so as to permit reception, by the optical receiver, of light signals. As suggested by the foregoing, the construction and configuration of cap  704  may generally be selected as required to suit the parameters of a particular application. 
     In view of the foregoing general discussion concerning various electronic devices  1000  that may be employed in conjunction with cooling device  900 , further attention is directed now to certain aspects of the relation between such electronic devices  1000  and cooling device  900 . In general, cooling device  900  may be employed to remove heat from, or add heat to, one or more of the electronic devices  1000 , such as laser  1002 , in order to achieve a desired effect. As discussed in further detail herein, the capability to add and remove heat, as necessary, from a device such as laser  1002 , may be employed to control the performance of laser  1002 . 
     In an exemplary embodiment, the heating and cooling, as applicable, of electronic devices  1000  is achieved with a cooling device  900  that comprises a TEC. Various aspects of the arrangement and disposition of electronic devices  1000 , as well as cooling device  900 , serve to enhance these ends. By way of example, the fact that electronic devices  1000  are mounted directly to cooling device  900  results in a relatively short thermal path between electronic devices  1000  and cooling device  900 . Generally, such a relatively shorter thermal path between components translates to a corresponding increase in the efficiency with which heat may be transferred between those components. Such a result is particularly useful where devices whose operation and performance is highly sensitive to heat and temperature changes, such as lasers, are concerned. Moreover, a relatively short thermal path also permits the transfer of heat to be implemented relatively more quickly than would otherwise be the case. Because heat transfer is implemented relatively quickly, this exemplary arrangement can be used to effectively and reliably maintain the temperature of laser  1002  or other devices. 
     Another aspect of at least some embodiments relates to the location of cooling device  900  relative, not just to electronic devices  1000 , but to other components, devices, and structures of header assembly  700 . In particular, because cooling device  900  is located so that the potential for heat transmission, whether radiative, conductive, or convective, from other components, devices, and structures of header assembly  700  to cooling device  900  is relatively limited, the passive heat load imposed on cooling device  900  by such other components and structures is relatively small. Note that, as contemplated herein, the “passive” heat load generally refers to heat transferred to cooling device  900  by structures and devices other than those upon which cooling device  900  is primarily intended to exert a heating and/or cooling effect. Thus, in this exemplary embodiment, “passive” heat loads refers to all heat loads imposed on cooling device  900  except for those heat loads imposed by electronic devices  1000 . 
     The relative reduction in heat load experienced by cooling device  900  as a consequence of its location has a variety of implications. For example, the reduced heat load means that a relatively smaller cooling device  900  may be employed than would otherwise be the case. This is a desirable result, particularly in applications such as header assemblies where space may be limited. As another example, a relatively smaller cooling device  900 , at least where cooling device  900  comprises a TEC, translates to a relative decrease in the amount of electrical power required to operate cooling device  900 . 
     Another consideration relating to the location of cooling device  900  concerns the performance of laser  1002  and the other electronic components  1000  disposed in hermetic chamber  706 . In particular, the placement of cooling devices  900 , such as TECs that include a “cold” connection, in hermetic chamber  706  substantially forecloses the occurrence of condensation, and the resulting damage to other components and devices of header assembly  700 , caused by the cold connection, that might otherwise result if cooling device  900  were located outside hermetic chamber  706 . 
     In addition to the heat transfer effects that may be achieved by way of the location of cooling device  900 , and the relatively short thermal path that is defined between cooling device  900  and the electronic devices  1000  mounted to submount  902  of cooling device  900 , yet other heat transfer effects may be realized by way of various modifications to the geometry of cooling device  900 . In connection with the foregoing, it is generally the case that by increasing the size of cooling device  900 , a relative increase in the capacity of cooling device  900  to process heat will be realized. 
     In this regard, it should be noted that it is the case in many applications that the diameter of base  702  is often constrained to fit within certain predetermined form factors or dimensional requirements and that such form factors and dimensional requirements, accordingly, have certain implications with respect to the geometric and dimensional configuration of cooling device  900 . 
     By way of example, the diametric requirements placed on base  702  may serve to limit the overall height and width of cooling device  900  (see, e.g., FIG.  4 D). In contrast however, the overall length of header assembly  700  is generally not so rigidly constrained. Accordingly, certain aspects of cooling device  900 , such as its length for example, may desirably be adjusted to suit the requirements of a particular application. In the case of a TEC, for example, such a dimensional increase translates into a relative increase in the amount of heat that cooling device  900  can process. As noted earlier, such heat processing may include transmitting heat to, and/or removing heat from, one or more of the electronic components  1000 , such as laser  1002 . 
     Moreover, various dimensions and geometric aspects of cooling device  900  may be varied to achieve other thermal effects as well. By way of example, in the event cooling device  900  comprises a TEC, a relatively smaller cooling device  900  will permit relatively quicker changes in the temperature of electronic devices  1000  mounted thereto. In the case where electronic device  1000  comprises a laser, this capability is particularly desirable as it lends itself to control of laser performance through the vehicle of temperature adjustments. 
     Turning now to consideration of the power requirements for cooling device  900 , at least where it comprises a TEC, and electronic devices  1000 , it was suggested earlier herein that those devices typically rely for their operation on a supply of electrical power. Generally, the TEC must be electrically connected with platform  800  so that power for the operation of the TEC, transmitted from a power source (not shown) to platform  800 , can be directed to the TEC. Additionally, power is supplied to electronic devices  1000  by way of platform  800 , and electronic devices  1000  must, accordingly, be connected with one or more of the conductive pathways  806  of platform  800 . 
     The foregoing electrical connections and configurations may be implemented in a variety of ways. Various aspects of exemplary connection schemes are illustrated in  FIGS. 4A ,  4 B and  4 E. With reference first to  FIG. 4B , the underside of submount  902  of cooling device  900  is connected with conductive elements  814  disposed on the underside of first feedthru  802 , by way of connectors  816  such as, but not limited to, wire bonds. Such conductive elements  814  may be electrically connected with selected conductive pathways  806  (see  FIG. 4A ) and/or connector  810 , that are ultimately connected with an electrical power source (not shown). 
     Directing attention next to  FIG. 4A , details are provided concerning various aspects of the electrical connection of electronic devices  1000  disposed on submount  902 . As noted earlier, and illustrated in  FIG. 4A , some embodiments of platform  800  include one or more cutouts  811 , or other geometric feature, that permits direct connection of electronic devices  1000 , such as laser  1002  to one or more conductive pathways  806  disposed within first feedthru  802  of platform  800 . This connection may be implemented by way of connectors  818  such as bond wires, or other appropriate structures or devices. In addition to the aforementioned connection, and as illustrated in  FIG. 4E , at least some embodiments of the invention further include a flex circuit  820 , or similar device, which serves to electrically interconnect platform  800  of header assembly  700  with another device, such as a printed circuit board. 
     With attention now to  FIGS. 4A through 4D , details are provided concerning various operational aspects of header assembly  700 . In general, power is provided to laser  1002  and/or other electrical components  1000  by way of connector  810 , conductive pathways  806 , and connectors  818 . In response, laser  1002  emits an optical signal. Heat generated as a result of the operation of laser  1002 , and/or other electronic components  1000 , is continuously removed by cooling device  900 , which comprises a TEC in at least those cases where a laser  1002  is employed in header assembly  700 , and transferred to second feedthru  812  upon which cooling device  900  is mounted. Ultimately, second feedthru  812  transfers heat received from cooling device  900  out of header assembly  700 . 
     Because cooling device  900  is disposed within hermetic chamber  706 , the cold junction on cooling device  900 , where it comprises a TEC, does not produce any undesirable condensation that could harm other components or devices of header assembly  700 . Moreover, the substantial elimination of passive heat loads on cooling device  900 , coupled with the definition of a relatively short thermal path between electronic components  1000 , such as laser  1002 , and cooling device  900 , further enhances the efficiency with which heat can be removed from such electronic components and, accordingly, permits the use of relatively smaller cooling devices  900 . And, as discussed earlier, the relatively small size of cooling device  900  translates to a relative decrease in the power required to operate cooling device  900 . Yet other operational aspects of embodiments of the invention are considered in further detail below in the context of the discussion of a laser control system. 
     While, as noted earlier in connection with the discussion of  FIGS. 4A through 4D , certain effects may be achieved by locating cooling device  900  within hermetic chamber  706 , it is nevertheless desirable in some cases to locate the cooling device outside of the hermetic chamber. Aspects of an exemplary embodiment of such a configuration are illustrated in  FIG. 4F , where an alternative embodiment of a header assembly is indicated generally at  1100 . As the embodiment of the header assembly illustrated in  FIG. 4F  is similar in many regards to one or more of the embodiments of the header assembly discussed elsewhere herein, the discussion of  FIG. 4F  will focus primarily on certain selected aspects of the header assembly  1100  illustrated there. 
     Similar to other embodiments, header assembly  1100  includes a base  1102  having a device side  1102 A and a connector side  1102 B, through which a platform  1200  passes in a substantially perpendicular orientation. The platform  1200  includes an inside portion  1202 A and an outside portion  1202 B. One or more electronic devices  1300  are attached to inside portion  1202 A of platform  1200  so as to be substantially enclosed within a hermetic chamber  1104  defined by a cap  1106  and base  1102 . In the event that electronic device  1300  comprises an optical device, such as a laser, cap  1106  may further comprise an optically transparent portion, or window,  1106 A to permit optical signals to be transmitted from and/or received by one or more electronic devices  1300  disposed within hermetic chamber  1104 . 
     With continuing reference to  FIG. 4F , platform  1200  further comprises a first feedthru  1204 , upon which electronic devices  1300  are mounted, joined to a second feedthru  1206  that includes an inside portion  1206 A and an outside portion  1206 B. The outside portion  1206 B of second feedthru  1206  is, in turn, thermally coupled with a cooling device  1400 . In the illustrated embodiment, cooling device  1400  comprises a TEC. However, other types of cooling devices may alternatively be employed. 
     In operation, heat generated by electronic devices  1300  is transferred, generally by conduction, to second feedthru  1206 . The heat is then removed from feedthru  1206  by way of cooling device  1400  which, in some embodiments, comprises a TEC. As in the case of other embodiments, a TEC may also be employed, if desired, to add heat to electronic devices  1300 . 
     Thus positioned and arranged, cooling device  1400  is able not only to implement various thermal effects, such as heat removal or heat addition, with respect to electronic devices  1300  located inside or outside hermetic chamber  1104 , but also operates to process passive heat loads, which may be conductive, convective and/or radiative in nature, imposed by various components such as the structural elements of header assembly  1500 . As noted herein in the context of the discussion of various other embodiments, variables such as, but not limited to, the geometry, placement, and construction materials of platform  1200  and cooling device  1400  may be adjusted as necessary to suit the requirements of a particular application. 
     As suggested earlier, at least some embodiments of the cooling device may be usefully employed in the context of a laser control system. Directing attention now to  FIG. 5 , various aspects of an exemplary embodiment of a laser control system, indicated generally at  2000 , are illustrated. 
     As indicated in  FIG. 5 , laser control system  2000  includes a temperature sensing device  2002 , such as a thermistor, which is thermally coupled with a laser  2004 , such as a semiconductor laser. Laser control system  2000  further includes a light intensity sensing device  2006 , such as a photodiode, that is optically coupled with laser  2004 . Further, a TEC  2008  is thermally coupled with laser  2004 . In at least one embodiment, such thermal coupling is achieved by mounting laser  2004  directly to a submount of TEC  2008 . Laser control system  2000  further includes a control circuit  2010  configured to receive inputs from temperature sensing device  2002  and light intensity sensing device  2006 , and to send corresponding control signals to a power source  2012  in communication with TEC  2008 . 
     In general, operation of laser control system  2000  proceeds as hereafter described. In particular, the intensity of the optical signal emitted by laser  2004  is sensed, either directly or indirectly, by light intensity sensing device  2006 . Light intensity sensing device  2006  then transmits, from time to time, a corresponding signal to control circuit  2010 . In at least some embodiments, the temperature of laser  2004  may be regulated by TEC  2008  so as to achieve wavelength stabilization. This can be achieved by way of control circuit  2010  and power source  2012 . 
     Additionally, temperature sensing device  2002  is positioned and configured to measure the temperature of laser  2004  and transmit, from time to time, a corresponding signal to control circuit  2010 . Based upon inputs received from temperature sensing device  2002  and light intensity sensing device  2006 , control circuit  2010  is able to implement changes to the temperature of laser  2004  by way of power source  2012  and TEC  2008 . 
     In particular, because TEC  2008  may be configured to add and/or remove heat from laser  2004 , laser control system  2000  thus affords the ability to, among other things, change and/or maintain the temperature of laser  2004  as desired or required by a particular application. Thus control circuit  2010  cooperates with TEC  2008  to control both the direction and amount of heat flow with respect to laser  2004 . In this way, various operational parameters of the signal emitted by laser  2004  may desirably be adjusted. 
     That is, embodiments of laser control system  2000  are capable of not only maintaining the temperature of active devices such as laser  2004  below a critical value at which laser  2004  performance begin to degrade and reliability becomes an issue, but embodiments of laser control system  2000  also enable control of the temperature of active devices such as laser  2004  at a given value independent of ambient temperature conditions, so as to achieve certain ends such as, in the case of laser  2004  operation for example, wavelength stabilization. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.