Abstract:
A semiconductor laser diode for transmitting optical signals in a telecommunications network is housed within a module supported adjacent to a thermoelectric cooling element that is mounted in thermally conductive contact with a heat sink. The cooling element is positioned externally on the surface of the module. The adjacent surface areas of the module and the cooling element are brought into thermal conductivity by a thermally conductive filler occupying the space between the module and the cooling element. The filler conforms to the configuration of the adjacent surfaces of the module and the cooling element to increase the thermal conductivity therebetween for maximum efficiency in the transfer of heat from the laser diode through the module to the cooling element and the heat sink. In response to a change in the temperature of the laser diode, the current applied to the cooling element is adjusted to increase or decrease the current and accordingly sink heat from the laser diode or supply heat to the laser diode. By removing the cooling element from internally within the laser module, the manufacturing cost of the laser module is substantially reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to method and apparatus for assembling a laser module in a telecommunications network and, more particularly, to a thermoelectric cooling element positioned externally of the laser module for maintaining the operating temperature of the laser module. 
     2. Description of the Prior Art 
     In a conventional cable television (CATV) system optical transmitters are used to convert radio frequency (RF) signals to optical signals. The conversion is provided by a laser diode which transmits AM modulated or digitally modulated optical signals for communication over a fiber optic CATV distribution network. The laser diode is coupled to a fiber optic network suitable for use in optical transmissions. 
     Laser diode devices are sensitive to operating conditions such as temperature, modulating signal level, and the loss or improper application of current which biases the laser diode to its stimulated or laser emission state. As the ambient temperature of the optical transmitter increases or decreases, unequal thermal expansion of the components creates stresses on the components which can alter their optical characteristics. For example, the optical beam emitted from the laser diode is focused to a modulator. Thermal stresses applied to the laser diode misalign the optical beam, resulting in a reduction in the output of the optical transmitter. Therefore, the laser diode must be maintained at a stable operating temperature, i.e. within a range of plus or minus two degrees. 
     In a conventional optical communications system, the laser diode and the other optical components in the system are buried in underground conduits and the like, exposing them to extreme environmental conditions. The optical elements may be exposed to ambient temperatures ranging from −30° C. to about 50° C. Because the oscillation characteristics of a semiconductor laser diode have a large temperature dependency, if the module is not maintained at a constant temperature. shifts in the threshold current density or oscillation wavelength may take place depending on temperature variation. 
     In order to accommodate the temperature variations to which the laser diode is exposed and to avoid wavelength shifts, it is the conventional practice to hold the operating temperature of the laser diode by a Peltier-element electronic cooling device. By keeping the operating temperature of the laser diode at a constant temperature, the operating bias and, therefore, the total optical transmission of the optical signal is maintained at a constant level under extreme environmental conditions and in response to electrical disturbances. 
     An example of the prior art semiconductor laser module using an electronic cooling device in an optical communication system is disclosed in U.S. Pat. No. 6,181,718 in what is commonly referred to as a butterfly-type module package. A semiconductor laser diode is mounted on a carrier which is coupled internally to an electronic cooling device. The cooling device includes a Peltier-element having a pin junction sandwiched between a first dielectric plate substrate and a second dielectric plate substrate. To avoid a temperature increase in the diode, a temperature detector, such as a thermistor-resistor, is installed around the diode. With this arrangement, the temperature around the diode is maintained by the supply current to the Peltier-element. A change in the temperature is detected by the thermistor and in response actuates an increase or decrease of the electric current to the Peltier-element. The current is increased to increase the rate of heat flow from the diode and thereby cool the diode back to the operating temperature. When the temperature measured by the thermistor is lower than the operating temperature, the electric current to the Peltier-element is decreased to decrease the rate of heat flow from the laser diode. The heat generated in the diode raises the temperature thereof back to the operating temperature. 
     With the above-described butterfly-type module construction, the thermoelectric cooling element is mounted internally within the package in contact with the laser diode. The package is formed by a hermetically sealed metal-ceramic or metal-glass rectangular package with multiple leads protruding from opposite sides of the package. External electronic circuitry is used to control the operation of the internal cooling element. However, this configuration adds significantly to the cost of manufacture of the laser diode module. 
     Another example of a laser diode module having an internally packaged thermoelectric cooler (TEC) is disclosed in U.S. Pat. No. 5,181,214. All the elements of the laser diode module are mounted to a common, temperature stabilized base plate. The base plate is fabricated of a low thermal expansion material, such as copper-tungsten alloy. The base plate is mounted on a thermoelectric cooler, which in turn is mounted on a heat sink. The thermoelectric cooler controls the rate of heat flow between the base plate and the heat sink in order to maintain the temperature of the laser diode at a predetermined operating temperature. 
     In U.S. Pat. No. 5,379,145, an optical transmitter for light wave communications utilizes a thermistor thermally coupled with a laser diode in a module. Also, thermally coupled to the laser diode is a thermoelectric cooler. The TEC is connected to a controller responsive to voltage developed across the thermistor to turn current to the TEC on and off to cool the laser diode when its temperature exceeds a certain temperature. The other components are within the package. 
     In the multichannel analog optical fiber communication system disclosed in U.S. Pat. No. 5,034,334, the laser diode chip is mounted on a metallized carrier. The carrier is in turn attached to a copper stud cooled by a conventional thermoelectric cooling element to maintain the laser diode at about 20° C. 
     Conventional integrated optical transmitter in a CATV system includes an optical head assembly generating a formed optical beam and an optical modulator which receives the formed optical beam for modulation. An optical head assembly is maintained in a fixed relationship by an epoxy bonding to the modulator. The optical head includes a laser diode that is coupled to the modulator for transmitting an optical beam to the modulator. A thermal transfer plug couples a rear portion of the optical head assembly to a TEC to transfer heat therebetween. A second TEC is coupled by adhesive directly to the optical head. TECs are conventionally operable to remove or add heat from the modulator and optical head assembly to maintain optimum operating temperature. Further, it is disclosed that a thermistor is mounted in the transfer plug to monitor the temperature of the optical head assembly. All these components are contained in an integrated package. 
     Another example of a butterfly type module package for a semiconductor laser diode in an optical fiber telecommunications system is disclosed in U.S. Pat. No. 6,219,364. A laser diode chip and a thermistor are mounted via a heat sink on a submount. The submount is in turn mounted on a metal substrate. The metal substrate is bonded by a hard metal solder to the top of a Peltier-element. The Peltier-element is in turn sandwiched by ceramic panels so that the cooler element is internally mounted within the module beneath the laser diode. 
     Another example of a thermoelectric cooling element mounted internally within the laser diode package is disclosed in U.S. Pat. No. 6,018,536. An integrated laser package includes a gain element supported on a high thermal conductive submount in alignment with a fiber of an optical coupling means which is also supported on the submount. The submount is in turn supported on a TEC cooler. Thus, all the elements are heat sunk to the same support in an integrated package and maintained at the same temperature. 
     One disadvantage of an integrated laser module where the thermoelectric cooling element is contained within the module is the high manufacturing cost of the integrated design. Internally mounting the TEC element requires a support structure for positioning the laser diode on top of the TEC element. This requires hermetically sealing the TEC element in a metal-ceramic or metal-glass rectangular package and soldering the package to a substrate for supporting the laser diode above the TEC element. The manufacturing cost of a laser diode module would be substantially reduced by fabricating the module with the TEC element positioned externally of the module. This would eliminate many of the components required to support the TEC element internally in heat transfer relation with the laser diode. Therefore, there is need in a laser diode module for mounting the thermoelectric cooling element externally of the module to reduce the manufacturing cost of the module. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a laser diode assembly in a telecommunications network that includes a module of a preselected configuration housing the laser diode. A thermoelectric cooling element has opposing surfaces. A first of the opposing surfaces is a controlled surface maintained at a selected operating temperature. A second of the opposing surfaces is an uncontrolled surface for transfer of heat to and from the cooling element. A heat sink is positioned in thermal contact with the cooling element second surface. The module has a heat transfer surface positioned in spaced relation oppositely of the cooling element first surface. A thermally conductive material fills the space between the module heat transfer surface and the cooling element first surface. The thermally conductive material conforms to the surface configurations of the module and the cooling element to place the module in thermal contact with the cooling element for transfer of heat therebetween. A measuring device is supported by the conductive material for detecting a change in the temperature of the cooling element first surface from the operating temperature. The temperature applied to the cooling element is adjusted to transfer heat between the module, the cooling element, and the heat sink to maintain the cooling element at the operating temperature. 
     Further in accordance with the present invention there is provided a method for assembling a laser diode in a telecommunications network that includes the steps of housing a laser diode within a laser module having an outer surface fabricated of a thermally conductive material. The laser module is connected to a telecommunications network for the transmission of optical signals thereto. A thermoelectric cooling element is positioned oppositely of the laser module to transfer heat therebetween. The cooling element is positioned in thermal contact with a heat sink for transfer of heat between the cooling element and the heat sink. A thermally conductive material is inserted between and in contact with opposing surfaces of the laser module and the cooling element to place the laser module and the cooling element in thermal contact for the transfer of heat therebetween. The temperature of the cooling element is measured to detect a change in the temperature of the laser module from a preselected temperature in response to heat transferred from the laser module to the cooling element. The electric current supplied to the cooling element is adjusted to transfer heat between the laser module and the heat sink through the cooling element to maintain a preselected temperature of the laser module. 
     In addition the present invention is directed to apparatus for controlling the temperature of a semiconductor laser diode that includes a module housing the laser diode in thermal contact therewith. The module has a thermally conductive surface of a preselected configuration. A thermoelectric cooling element is positioned in heat transfer relation with the module. The cooling element has a thermally conductive surface of a preselected configuration. A heat sink is positioned in thermal contact with the thermoelectric cooling element. A thermally conductive filler material is positioned between the module and the cooling element in conformity with the configurations of the thermally conductive surfaces of the module and the cooling element to place the module and the cooling element in thermal contact for the transfer of heat between the laser diode and the heat sink. 
     Accordingly, a principal object of the present invention is to provide method and apparatus for maintaining the operating temperature of a semiconductor laser diode in a module or package by a cooling element independently controlled and positioned externally of the module and maintained in thermal conductivity with the module for a transfer of heat therebetween. 
     Another object of the present invention is to provide apparatus for housing a semiconductor laser diode in a package free of the electrical components associated with a temperature controller housed externally of the package. 
     Another object of the present invention is to provide method and apparatus for positioning a thermoelectric cooling element externally of a housing for a laser module where the adjacent surfaces of the cooling element and laser module are placed in thermal conductivity by a thermally conductive filler material conforming to the adjacent surfaces. 
     Another object of the present invention is to provide a package for housing a semiconductor laser diode used in a telecommunications network where the manufacturing cost of the package is reduced by using a TEC element externally of the package. 
     A further object of the present invention, is to provide a package for a semiconductor laser module in a telecommunications network having a temperature control device positioned externally in thermal contact with the module. 
     These and other objects of the present invention will be more completely disclosed and described in the following specification, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic of an optical communications system, illustrating a semiconductor laser diode for converting RF electrical signals to optical signals for transmission over a CATV distribution network. 
     FIG. 2 is a view in side elevation of a laser module thermally connected externally to a thermoelectric cooling element. 
     FIG. 3 is a top plan view of the laser diode shown in FIG. 1, illustrating the external positioning of the thermoelectric cooling element. 
     FIG. 4 is an electrical schematic of a control circuit for maintaining the temperature of the laser diode at a constant temperature over a wide ambient temperature range. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and particularly to FIG. 1, there is illustrated an optical communications system generally designated by the numeral  10  for conversion of an RF input signal to an optical output signal. The RF signal is converted by a laser module  12  that includes a semiconductor laser diode  14  maintained at a constant temperature by a thermoelectric cooling element  16  (shown in FIGS. 2 and 3) positioned externally of the laser module  12  and controlled by a control circuit  17 . The converted RF signal is transmitted from the laser module  12  through the output glass fiber  18  in a communications system, such as a CATV system. 
     The output optical signal has a magnitude which is proportional to the operating bias current of the laser diode  14 . Modulation of the bias current by an external signal varies the power of the optical output signal in proportion to the applied signal. With this arrangement, all of the signal information and the applied signal is transferred to the optical signal by intensity modulation. In accordance with the present invention, the RF signal converted by the laser diode  14  must be of low distortion and high quality, requiring that the laser diode  14  be held at a constant operating temperature and at a fixed operating bias current as the surrounding ambient temperature varies. 
     A RF modulating signal, such as a broadband CATV signal including a number of television channels, is applied to an input terminal  18  of an input attenuator  20 . The RF signal from the attenuator  20  is transmitted to an input coupler  22  which splits the signal for transmission in one direction for feedback through a monitor attenuator  24  to an input monitor  26 . The other path of the RF signal from the coupler  22  is directed to preamplifier  28  where the RF signal is amplified and transmitted to a variable attenuator  30  operated by a controller  32 . The attenuator  30  receives the amplified signal from the amplifier  28  and transmits an attenuated signal to an equalizer  34 , which equalizes the signal. The equalizer  34  then feeds the equalized signal to a laser driver-amplifier  36 . 
     The RF amplified signal from amplifier  36  is transmitted through a transformer  38  to the laser module  12  containing the laser diode  14  and an output monitor diode  40 . The laser module  12  also includes a laser bias electrical device  42 . The thermoelectric cooling element  16  is positioned externally in thermal contact with the laser module  12 , as shown in FIGS. 2 and 3. 
     The bias control device  42  controls the operating bias of the RF signal to ensure that the power of the optical output signal transmitted from the laser module  12  is maintained at a constant level. The thermoelectric cooling element  16  assures that the laser module and subsequently the laser diode  14  are maintained at a constant operating temperature. The RF signal is converted by the laser diode to the desired optical signal transmitted to the output fiber  18 . 
     Now referring to FIGS. 2 and 3, there is illustrated the laser module  12  positioned externally in thermal contact with the thermoelectric cooling element  16 . The cooling device  16  is positioned in thermal contact with a heat sink  44 . Therefore, the opposing surfaces of the cooling element  16  are positioned in heat transfer relation with the laser module  12  and the heat sink  44 . 
     In accordance with the present invention, the laser module  12  is manufactured without the provision for internal temperature control, i.e. without a TEC element housed within the module  12 . Consequently, without the addition of the cooling element  16  within the laser module  12 , there is no capability to control the temperature of the laser diode  14  housed within the module  12 . In this respect, without the provision of the cooling element  16 , the laser module  12  is subject to the deleterious effects of variations in ambient temperature. The exclusion of a TEC element  16  from internally within the laser module  12  eliminates the need to fabricate the module  12  to include electrical leads for connecting the cooling element to the control electronics for the element. 
     As illustrated in FIGS. 2 and 3, the only wiring required for the module  12  is for the laser bias and RF signal input leads  46  and  48  connected to input end  50  of module  12 . The optical signal generated by the laser diode  14  is transmitted to the output fiber  18  connected to an output end  52  of module  12 . No other leads are required to be connected externally to the module  12 . With the cooling element  16  positioned externally on the laser module  12 , electrical leads  54  and  56  connect the cooling element to the control electronics for controlling the temperature of the cooling element  16 . 
     The thermoelectric cooling element  16  has opposing surfaces  58  and  60  that are used to control and maintain a constant operating temperature of the laser diode  14  within module  12 . The surface  58  is in contact with the body of laser module  12 , and surface  60  is in contact with the heat sink  44 . The surface  58  in contact with the laser module  12  is designated the controlled side of element  16 . The surface  60  in contact with the heat sink  44  is designated the uncontrolled or environmental side of the cooling element  16 . 
     In operation, the thermoelectric cooling element  16  responds to temperature changes of the laser module  12 . In the event the temperature of the laser module  12  increases above a preset level, the cooling element  16 , being in heat transfer relation with module  12 , transfers or pumps heat from the module  12  to the heat sink  44 . The heat sink  44 , being in heat transfer relation with the cooling element  16 , absorbs the thermal energy transferred from the module  12  to the cooling element  16 . Thus, the heat sink  44  acts as a heat transfer medium and removes heat absorbed by the laser module  12  due to variations in ambient temperature and transfers the heat from the cooling element  16  to the ambient air surrounding the heat sink  44 . 
     The laser module  12  is in thermal contact with the temperature controlled surface  58  of the thermoelectric cooling element  16 . The efficiency of the cooling element  16  to transfer heat from the laser module  12  to the heat sink  44  is determined by the surface area of the laser module  12  in thermal contact with the temperature controlled surface  54  of the cooling element  16 . The greater the area of thermal contact of laser module  12  with the cooling element  16 , the greater the thermal conductivity between the respective surfaces and the greater the transfer of heat from the laser module  12 . Accordingly, by increasing the area of thermal contact between the laser module  12  and the cooling element  16 , the thermal conductivity between the laser module  12  and the cooling element  16  is increased. 
     The thermal conductivity between the laser module  12  and the cooling element  16  is maximized when the surface of the laser module  12  conforms to the flat surface  58  of the cooling element  16 . If the opposing surfaces are not in conformity, i.e. the thermal area of contact between the laser module  12  and the cooling element  16  is minimal, then the heat transferred between the respective surfaces will be minimal resulting in low thermal efficiency of the cooling element  16  to transfer heat from the laser module  12  to the heat sink  44 . In operation where the thermal efficiency of the cooling element is low, the electric current to the cooling element  16  must be increased to increase the rate of heat transfer from the laser module  12  through the cooling element  16  to the heat sink  44  to maintain the desired operating temperature of the laser module  12 . 
     The optimum or desired thermal conductivity between the laser diode  12  and the cooling element  16  is achieved when the opposing heat transfer surfaces are in conformity, i.e. maximum thermal conductivity between opposing surfaces. Most preferably, maximum thermal conductivity between the laser module  12  and the cooling element  16  is achieved when the opposing surfaces are flat. When a substantial portion of the surface of the laser module  12  is flat and conforms with the opposite flat surface  58  of the cooling element  16 , maximum heat transfer occurs. 
     If a substantial portion of the surface of the laser module  12  does not conform to the surface configuration of the cooling element  16 , then the thermal conductivity and heat transfer efficiency is substantially minimized. In one example, this occurs when the surface of laser module  12  is round or arcuate in configuration and the surface  58  of the cooling element  16  is flat, as shown in FIGS. 2 and 3. In this embodiment the thermal contact area between the opposing surfaces is minimal, resulting in low thermal conductivity. 
     Conventionally, the body of the laser module  12  is fabricated to include a combination of planar and radial surfaces, as shown in FIGS. 2 and 3. For example, a center body portion  62  of module  12  extending from end portion  50  has a radial configuration of a constant diameter. A conical section  64  extends from the body portion  62  to the output end  52  from which the laser fiber  18  extends. 
     The radial body portion  62  of the module  12  includes a mounting flange  66  for supporting the module  12  on the flat surface  58  of cooling element  16  (FIG.  2 ). The mounting flange  66  includes a rectangular base plate  68  having a planar surface abutting the planar surface  58  of cooling element  16 . Only the base plate  68  of the laser module  12  is in thermal contact with the cooling element  16 . Consequently, the thermal conductivity between the laser module  12  and the cooling element  16  is low. This results in low thermal efficiency of the cooling element  16  to withdraw and transfer heat from the laser module  12 . In accordance with the present invention the thermal efficiency is increased by increasing the area of thermal contact between the laser module  12  and the cooling element  16 . 
     To increase the thermal conductivity between the substantially radial body of the laser module  12  and the flat surfaces of the cooling element  16 , a filler  70  of thermally conductive material is positioned between the laser module  12  and the cooling element  16  in conformity with the opposing surfaces of module  12  and element  16 . The filler  70  is molded or machined, depending upon the material from which the filler is fabricated, into conformity with the opposing surfaces which contact the filler  70 . 
     Preferably, the filler  70  is fabricated from a thermally conductive material. Metal is one class of thermally conductive material used for the filler  70 . In one embodiment, aluminum is machined or molded to conform to the configuration of the surfaces of the module  12  and element  16  as the thermally conductive material therebetween. 
     In another embodiment, the thermally conductive filler  70 , such as a thermally conductive epoxy, is injected into the void between module  12  and element  16  and fills the void by conforming to the surfaces of the radial portion  62  of module  12 , the rectangular configuration of the module mounting flange  66  and base plate  68 , and the flat or planar surface  58  of the cooling element  16 . The thermally conductive filler  70  conforms substantially to all the surrounding thermal contact surfaces of the laser module  12  and the cooling element  16 . In this manner maximum thermal conductivity and consequently thermal efficiency is achieved to maintain the laser diode  14  at an operating temperature. 
     In accordance with the present invention, regardless the external configuration of the laser module  12  and the cooling element  16 , the opposing surfaces thereof are brought into maximum thermal contact with one another by filling the voids or spaces between the non-conforming surfaces with the thermally conductive material  70 . The filler material  70  is molded in place or shaped by machining to place the heat transfer surfaces of module  12  and element  16  in thermal contact. Maximizing the thermal efficiency of heat transfer from module  12  through element  16  to heat sink  44  minimizes the current that must be supplied to the cooling element  16  to maintain the laser module  12  at the desired operating temperature. A further function of the filler material  70  is to operate as a saddle to support the laser module  12  in thermal contact with the cooling element  16 . 
     The filler material  70  between the laser module  12  and the cooling element  16  is a thermally conductive material. Consequently, when the temperature of the laser module  12  changes, the temperature of the cooling element  16  changes. A temperature-sensing element  72 , such as a thermistor, is used to monitor the temperature of the controlled surface  58  of the cooling element  16 . The thermistor  72  is suitably secured to the surface of the thermally conductive filler  70 . For example as shown in FIGS. 2 and 3, the thermistor  72  is positioned in a hole formed on the surface of the filler  70 . A thermally conductive epoxy is inserted in the hole and in surrounding relation with the thermistor  72  to securely bond the thermistor  72  to the filler  70 . A pair of electrical leads  74  and  76  extend from the thermistor  72  to a 
     In operation, any change in the ambient temperature surrounding the laser module  12  leads to a change in the temperature of the conductive filler  70  and the temperature of surface  58  of the cooling element  16 . The temperature change is sensed by the thermistor  72 , which continually measures the cooling element surface  58 . The thermistor  72  is responsive to the temperature change and adjusts the electric current supplied to the cooling element  16 , accordingly. 
     When the temperature measured by the thermistor  72  exceeds the initially specified operating temperature of the laser module  12 , the electric current to the thermoelectric cooling element  16  is increased. This increases the rate of heat flow from the laser module  12  through the cooling element  16  to the heat sink  44 . In this manner, the laser module  12  is cooled to the specified operating temperature. 
     When the temperature measured by the thermistor  72  is lower than the operating temperature, the electric current to the thermoelectric cooling element  16  is decreased. This decreases the rate of heat flow from the laser module  12  to the heat sink  44 . In this manner, the heat generated in the laser module  12  raises the temperature of the module  12  back to its operating temperature. 
     With the above-described arrangement, the laser module  12  is maintained as a constant operating temperature without the provision of integrating the thermoelectric cooling element  16  within the laser module. The cooling element  16  is positioned externally of the laser module  12  and maintained in high thermal contact therewith. The desired thermal conductivity between the externally mounted cooling element  16  and the laser module  12  is enhanced by the provision of the thermally conductive filler material  70  to bring the nonconforming, displaced surfaces of the laser module  12  and cooling element  16  into thermal contact. The conductive filler material  70  bonds the opposing surfaces together irrespective of the surface contour of the opposing surfaces. Thus, the surfaces normally not in thermal contact are brought into thermal contact. 
     A reduced cost in the manufacture of the laser module  12  is realized by eliminating the expense of integrating a cooling element within the laser module. Specifically, the electrical leads  54  and  56  for the cooling element  16  are removed from internal connection to the laser module. Also, the temperature sensing element  72  is eliminated from the laser module  12  and incorporated within the filler material  70  in thermal conductivity with the surface of the cooling element  16 . 
     The cooling element  16  being in high thermal conductivity with the laser module  12  responds to changes in the temperature of the laser module. A change in the temperature of the temperature controlled surface  58  of the cooling element  16  is sensed by the thermistor  72  in response to a temperature change in the laser module  12 . This arrangement is effective to maintain the laser module  12  at a constant operating temperature over a wide range of ambient temperature changes. Thus, the laser module  12  is maintained at an operating temperature by temperature control devices positioned externally of the laser module  12 . 
     Now referring to FIG. 4, there is illustrated the circuit  17  for controlling the temperature of the thermoelectric cooling unit  16  to maintain the thermistor  72  and consequently the operating temperature of the laser diode  14  within the module  12  at a selective operating temperature. With the circuit  17 , the laser diode  14  is held substantially at a constant temperature over a wide ambient temperature range. The temperature is maintained at a +/− 2 degree window as the ambient temperature varies, for example, over a 65 degree or more range. The temperature is controlled by the circuit  17 , which is also maintained externally of the laser module  12 . The circuit  17  is operable as a temperature sensing and control feedback monitor circuit. 
     As above discussed, the Peltier cooling element  16  is externally mounted in thermal contact with the laser module  12 . The cooling element  16  is connected at its negative terminal by conductor  82  to output terminal  84  of one half of the H-bridge circuit  78  that is connected to an output terminal  88  of an operational amplifier  86 . The second half of the H-bridge circuit  78  is connected to the positive terminal of the cooling element  16  by conductor  98  which is connected to terminal  92  of operational amplifier  80 . Amplifier  80  is actuated by the thermistor  72  responding to a change in the temperature of the cooling element  16  in contact with the laser module  12 . 
     The operational amplifier  80  is connected to terminal  94  of the H-bridge circuit  78  and a temperature bridge circuit generally designated by the numeral  93 . The current supplied to the positive terminal of the cooling element  16  is either increased to increase the rate of heat flow from the laser module  12  (sourced) or decreased to decrease the rate of heat flow from the laser module  12  (sinked). The current to the cooling element  16  is either increased or decreased (sourced or sinked) depending upon whether the potential of the H-bridge circuit  78  at the terminal  88  is higher or lower than the potential of the H-bridge circuit  78  at terminal  94 . 
     Supplying current to the positive terminal of the cooling element  16  by operation of the H-bridge circuit  78  increases the heat transferred from the laser module  12  to cool the laser module. Decreasing the flow of current to the positive terminal of the cooling element  16  decreases the rate of heat flow from the laser module  12  to increase the temperature thereof. In this manner, heat is pumped in either direction between the cooling element  16  and the heat sink  44 . 
     As disclosed in FIGS. 2 and 3, the laser module  12  and the thermistor  72  are positioned in thermal contact through the conductive filler  70  to the temperature controlled surface  58  of the thermoelectric cooling element  16 . The uncontrolled side  60  of the cooling element  16  is mounted on the heat sink  44 . The temperature sensing thermistor  72  is connected through the operational amplifier  80  to the H-bridge circuit terminal  94 . The voltage at the terminal  94  is proportional to the resistance of thermistor  72 , which is a function of the absolute temperature of the laser module  12 . The opposite terminal  88  of the H-bridge circuit  78  is set at a voltage which is equal and opposite to the voltage at terminal  94 , which corresponds to the resistance of the thermistor  72 . 
     The laser module  12  is maintained at the desired operating temperature, for example 23° C. Any variation in the temperature of the laser module  12  unbalances the temperature bridge circuit  93 , producing an error voltage at driver input terminal  100 . The error voltage at the terminal  100  is applied by the operational amplifier  80  to terminal  94  of the H-bridge circuit  78 . The applied voltage results in a drive current through conductor  90  into and out of the positive terminal of the cooling element  16 . The drive voltage is inverted by the operational amplifier  86  at the terminal side  88  of H-bridge circuit  78 . This produces an equal but opposite current at the terminal side  88  of circuit  78 . 
     The outputs at each side of the H-bridge circuit  78  in the case of an increase in the rate of heat transfer from the laser module  12  increases the current to the cooling element  16 . In the case when the temperature measured by the thermistor  72  is lower than the laser diode operating temperature, the electric current to the cooling element  16  is decreased. This decreases the rate of heat flow from the cooling element  16  to the heat sink  44  and allows the temperature of the laser module  12  to rise back to the operating temperature. The amount and direction of the current is based upon the need to maintain the thermistor  72  and consequently the operating temperature of the laser diode  12  at an initial operating temperature. 
     According to the provisions of the patent statutes, we have explained the principle, preferred construction, and mode of operation of our invention and have illustrated and described what we now consider to represent its best embodiments. However, it should be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.