Abstract:
A system and method for maintaining a set point temperature of a laser. The required operating temperature range of laser may be reduced by determining whether to increase or decrease the amount of heating to the laser. A controller compares the desired set point temperature with a measured temperature which is provided as feedback to produce an output voltage. A heater transistor is driven by the output voltage of the controller which is proportional to the amount of heat to be generated by the transistor in order to maintain the laser at the desired set point temperature.

Description:
RELATED APPLICATION  
       [0001]     The present U.S. application is related to U.S. application entitled “LASER HEATER ASSEMBLY”, (Attorney Docket No. 10227), which is incorporated herein by reference, and having been filed concurrently with the present application. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to lasers and, more particularly, relates to controlling the temperature of a laser exposed to an ambient environment by determining when to heat the laser.  
       BACKGROUND OF THE INVENTION  
       [0003]     Due to cost, many lasers do not have built-in heaters or coolers. Unless an internal thermoelectric heater/cooler is included, the case temperature of an operating laser is substantially equivalent to ambient temperature. In some cases, for example outdoor HFC applications such as CATV nodes, the case temperature can be quite extreme. Outside temperatures may reach as cold as −40 degrees Celsius.  
         [0004]     Moreover, laser performance characteristics are very dependent upon operating case temperature. Not only do parameters such as slope efficiency and output power vary with operating temperature, but output wavelength varies as well. This can cause issues if the laser output wavelength drifts outside of the bandwidth of the combining and splitting optical passives in the network. Therefore, it is desirable to maintain the laser at as constant a temperature as possible or at least within a range that the device is designed to operate within. What is needed is an economical means to minimize performance degradation due to cold temperatures and to extend the lowest temperature a laser can operate at. 
     
    
     BRIEF DISCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  illustrates a perspective view of one embodiment of a thermostatically controlled heater assembly of the present invention.  
         [0006]      FIG. 2  illustrates a perspective view of the heater assembly of  FIG. 1  utilized in an optical transmitter of a CATV node.  
         [0007]      FIG. 3  illustrates an exploded view of the heater assembly of  FIG. 1 .  
         [0008]      FIG. 4  illustrates a generalized block diagram of one embodiment of the present invention.  
         [0009]      FIG. 5  illustrates one embodiment of a controller circuit for controlling the temperature of a laser according to the present invention.  
         [0010]      FIG. 6  illustrates one embodiment of a temperature dependent laser enable circuit according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0011]     The present invention will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which an exemplary embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The present invention is described more fully hereinbelow.  
         [0012]      FIG. 1  illustrates one embodiment of a thermoelectric heater assembly  10  of the present invention. The heater assembly  10  of  FIG. 1  is utilized, for example, in a CATV node as a coarse wave division multiplexing (CWDM) reverse transmitter  12  as shown in  FIG. 2 . The transmitter  12  includes a top cover  14  and a bottom cover  16  for housing a transmitter printed circuit board  18  with fiber optic cable having a fiber connector  20 . However, the heater assembly  10  may be implemented as part of other laser designs where it is desirable to reduce the overall temperature range that a laser must efficiently operate at and where an economical solution is desired.  
         [0013]      FIG. 3  illustrates an exploded view of the heater assembly  10  of  FIG. 1 . One embodiment of the present invention allows the heater assembly  10  to be a plug-in type modular assembly which can be used with many different frequency lasers for different applications. The heater assembly  10  includes a laser  22  adapted to be electrically and mechanically coupled to a laser printed circuit board  24  as shown in  FIG. 1 . The heater assembly  10  also includes a heating element or heater such as heater transistor  30  which is also electrically and mechanically coupled to the circuit board  24 . A thermal transfer member or heat sink, such as a metal plate or block  40 , is positioned in between the mounted laser  22  and heater transistor  30  as best shown in  FIG. 1 . Preferably, the heater transistor  30  and the laser  22  directly abut opposite sides of the block  40  for the best thermal connection. Heat generated from the transistor  30  is directly absorbed by the block  40  and then transferred to the laser  22 . Therefore, the laser  22  is indirectly heated by the transistor  30  external to the laser  22 .  
         [0014]     The block  40  is preferably thin aluminum to facilitate transferring heat from the heater transistor  30  to the laser  22  through low thermal impedance and to minimize heating delay through thermal mass. The block  40  may be mechanically coupled to the circuit board  24  with a fastener such as screw  42  which is received in opening  44  defined between protruding portion  46  and protruding portion  48  of the block  40 . However, other means for mechanically securing the block  40  to the circuit board  24  exist depending on the type and configuration of the thermal transfer plate or heat sink used. The block  40  may also be mechanically secured to a printed circuit board  18  of the transmitter  12 . The circuit board  24  and the block  40  are preferably mounted in substantially a vertical manner on the circuit board  18  of the transmitter  12  in order to lift the laser  22  and the transistor  30  away from the circuit board  18  to economize the space on the circuit board  18 .  
         [0015]     Preferably, the laser  22  is directly mounted to the block  40  with mechanical fasteners such as screws  52  through laser flange members  54  as best shown in  FIG. 3 . Also, the heater transistor  30  is directly mounted to the block  40  with a mechanical fastener such as a screw  58 . However, other means of securing the laser  22  and the heater transistor  30  to the block  40  may be used.  
         [0016]     As best shown in  FIG. 3 , the laser assembly  10  may include a right angle header  62  to electrically couple the circuit board  24  to the circuit board  18  of the transmitter  12 . The laser assembly may also include a temperature sensor  66  positioned underneath the block  40  and electrically coupled to the circuit board  18 . The temperature sensor  66  measures the temperature of the block  40 , and thus the temperature of the laser  22 , and feeds this information to the heater controller describer below. Preferably, a thermally conductive material such as grease or a thermal pad  70  is used between the block  40  and the sensor  66  to improve thermal conductivity as well as absorb any dimensional tolerances between the block  40  and the temperature sensor  66 .  
         [0017]      FIG. 4  illustrates in a generalized manner the laser  22  with thermal connections to the heater transistor  30  and the temperature sensor  66 . The temperature sensor measures the temperature of the laser  22  and feeds this information as an input to a heater control circuit  76 . The heater control circuit  76  determines whether to increase or decrease the amount of heating. The output of the heater control circuit  76  is fed to the heater transistor  30  which produces heat in proportion to the control signal from the heater control circuit  76 .  
         [0018]      FIG. 5  illustrates one embodiment of the heater control circuit  76  having controller circuitry  78  and voltage to current converter circuitry  80 . The controller circuitry  78  of the heater control circuit  76  includes a controller  82 . The controller  82  compares the temperature measured by the temperature sensor  66  to a desired set point temperature and produces an output voltage that produces more or less heat in an effort to make the temperature of the block  40  equal to the set point temperature. When ambient temperatures rise above the set point temperature, the heater transistor  30  will turn off and becomes passive until the ambient temperature drops back below the set point temperature.  
         [0019]     Depending on the requirements of the circuit, the controller  82  may be an integrating or proportional controller, or both. If an integrating controller is selected, the time constant of the integrator must be set long enough to compensate for thermal lag due to heating the block  40  and the laser  22 . Proportional controllers have the advantage of less settling time, but may have a static error between laser temperature and set point temperature the magnitude of which is dependent on loop gain.  
         [0020]     Still referring to  FIG. 5 , the controller  82  has two inputs, a reference voltage (REF) produced by a resistive divider on the positive input, and the output of the temperature sensor  66  on the negative input. The temperature sensor  66  is input to the controller  82  through an input resistor  68  with feedback provided from the output to the negative input through impedance  84 . The output voltage of the controller  82  is used to drive the heater transistor  30  and is proportional to the amount of heat generated in the heater transistor  30 . As shown in  FIG. 5 , the controller  82  output voltage is scaled through a simple voltage divider to ground provided by resistors  86  and  88 . The output of the voltage divider is connected to the noninverting input of op amp  90 . The output of op amp  90  is connected to the base of transistor  92  through resistor  94 .  
         [0021]     However, transistor  92  is not required but can be used if needed to drive the heater transistor  30 . If the heater transistor  30  is a BJT transistor and if either op amp  90  has a low drive current or the gain, β, of the heater transistor  30  is low, transistor  92  may be used for additional current gain. This is accomplished by connecting transistor  92  and transistor  30  in the Darlington configuration in which the emitter of transistter  92  connects to the base of the heater transistor  30  and the collectors of both transistors  30 ,  92  are connected to the voltage supply Vcc. Current passing through heater transistor  30  produces the desired heat and is measured by the voltage created by passing the current of the heater transistor  30  through a current sampling resister  96 . Negative feedback from resistor  98  is provided to op amp  90  through resistor  98 . Heat produced by the heater transistor  30  is conducted to the laser  22  through the thermal connection provided by the block  30  which conducts heat to the temperature sensor  66  that feeds a signal back to the controller  82  as described above.  
         [0022]     In cases where the laser  22  should not be allowed to operate below a certain temperature, a temperature based enable circuit should be employed.  FIG. 6  illustrates one embodiment of a temperature based enable circuit  110 , commonly referred to as a comparator circuit, for use with the laser assembly  10  and the control circuit  76 . The circuit  110  precludes the laser  22  from turning on until the temperature of the laser  22  is above the set point temperature. A simple comparator circuit can be used to pull the laser power control loop reference to a voltage which will force laser output power to zero.  
         [0023]     Still referring to  FIG. 6 , the temperature based enable circuit  110  includes a comparator  120  which monitors and compares input voltages from a reference trip point and the output of the temperature sensor  66 . The reference trip point voltage is set by the resistive divider of resistor  122  and resistor  124 . The output of comparator  120  is indicative of whether the temperature of laser  22  is above or below the reference trip point. The gate of a transistor  130  is driven by the output of comparator  120  through resistor  132  allowing transistor  130  to function as a switch. The switch function of transistor  130  acts upon the laser power control loop reference, Vref, of the integrator circuit  140 . When the transistor  130  is “ON”, or saturated, the laser power control loop reference voltage, Vref, is brought to zero volts or slightly negative which forces the laser power to zero. The Vref voltage when transistor  130  is “ON” is shifted by the value of Vee and the resistor  134 . The Vref voltage when the transistor  130  is “OFF” is set by the voltage divider of resistors  136  and  138  from Vcc. Turning transistor  130  on and off effectively changes the reference voltage of the circuit that controls laser output power.  
         [0024]     The foregoing has broadly outlined some of the more pertinent aspects and features of the present invention. These should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Other beneficial results can be obtained by applying the disclosed information in a different manner or by modifying the disclosed embodiments. Accordingly, other aspects and a more comprehensive understanding of the invention may be obtained by referring to the detailed description of the exemplary embodiments taken in conjunction with the accompanying drawings, in addition to the scope of the invention defined by the claims.