Abstract:
A system and method for electronically controlling the temperature of a pump laser or laser diode controls temperature uses methods of sourcing drive current. Further, the system shuts off the laser diode and/or a thermoelectric cooler when the temperature of the pump laser exceeds a predetermined amount.

Description:
GOVERNMENT LICENSE RIGHTS  
       [0001] The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00014-00C-0117 awarded by the U.S. Navy. 
     
    
     
       RELATED APPLICATIONS  
         [0002]    The present application is related to and claims the benefit of U.S. Provisional Application No. 60/309,817 filed Aug. 3, 2001, in the names of John FINN and Chia-Chi TENG, titled METHOD OF ELECTRONIC CONTROL OF A LASER DIODE AND THERMO-ELECTRIC COOLER USING ULTRA-LOW POWER CONSUMPTION TECHNIQUES and U.S. Provisional Application No. 60/340,957 filed Dec. 19, 2001, in the names of John FINN, Renfeng GAO, and Chia-Chi TENG, titled SYSTEM AND METHOD OF ELECTRONIC CONTROL OF A LASER DIODE AND THERMO-ELECTRIC COOLER USING ULTRA-LOW POWER CONSUMPTION TECHNIQUES, the entire contents of which are relied on and fully incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0003]    The present invention relates generally to the control of active components, and more particularly to a method and system for controlling the temperature of lasers and laser diodes.  
         BACKGROUND OF THE INVENTION  
         [0004]    Lasers, and laser diodes in particular, contain three elements that define their properties. The first, the laser medium, is the material from which light is generated and can be composed of a gas, solid, or liquid. The conditions under which the laser will emit light and the properties of the beam of light depend on the particular laser medium. The second, the power supply, excites the laser medium sufficiently to emit light. The third, the resonator, concentrates the light to stimulate emission of laser radiation. Together, these elements generate an emission of light that is both monochromatic and coherent, distinguishing laser diodes from light emitting diodes (LEDs). LEDs emit light that is the result of spontaneously recombining photons, and as such, the spectrum of the emitted light is much broader than the spectrum of the light emitted by a laser.  
           [0005]    Presently, lasers diodes are used in a wide variety of applications, for example, laser pointers, bar code readers, and CD players. With respect to the power requirements for laser diodes, as more power is needed to excite the laser medium, the temperature at which the laser diode operates increases. Laser diodes are sensitive to power overshoots and fluctuations, so that laser diodes require stable power supplies. If the supplied power exceeds a threshold requirement of the laser diode, even for a small period of time, the laser diode will likely fail. Therefore, laser diodes require that the circuitry used to implement their power supplies be chosen with emphasis on protecting the laser diode from excessive current or temperature.  
           [0006]    Analog control loops can be used to control and monitor a laser diode&#39;s temperature. A general approach to designing analog control loops for active optical devices is to use power operational amplifiers and power transistors. However, using power operational amplifiers and transistors has several drawbacks. For example, this approach is costly. Second, power operational amplifiers and power transistors use a large amount of printed circuit board area. Therefore, it is inefficient to use these devices in applications where space is critical such as, for example, in telecommunications systems, pump controllers, continuous wave distributed feedback (CW DFB) laser controllers, Bragg gratings, temperature controllers, heater element controls, thermoelectric (TEC) controllers, L Band/C Band/S Band drivers, Raman amp controls, and semiconductor optical amplifier (SOA) driver controls. Third, using power operational amplifiers and power transistors gives rise to thermal inefficiencies, which may lead to the degradation of the laser diode. Since many applications using laser diodes require lower power consumption, as in telecommunication systems, the existing hardware is difficult to integrate into these applications.  
           [0007]    It is therefore desirable to provide a temperature control system for lasers and laser diodes that overcomes the above described problems and disadvantages of present systems.  
         SUMMARY OF THE INVENTION  
         [0008]    There is provided a system for electronically controlling a temperature of a pump laser that includes a laser diode, having an on-state and an off-state, a laser diode driver for providing a first input current to operate the laser diode, a cooler, and a cooler driver for providing a second input current to operate the cooler, the system comprising: a temperature detector to determine the pump laser temperature; and a temperature controller, coupled to receive the first input current and responsive to the pump laser temperature, for permitting the laser diode driver to supply the first input current if the pump laser temperature is within a first predetermined temperature range, and to sufficiently block the first input current to render the laser diode inoperable if the pump laser temperature is outside the first predetermined temperature range.  
           [0009]    There is also provided a system for electronically controlling a temperature of a laser diode that is operable in an on-state and inoperable in an off-state, comprising: means for determining the laser diode temperature; a cooler for providing heat transfer to lower the laser diode temperature; a laser diode driver for supplying a first input current to drive the laser diode; a temperature controller, coupled to receive the first input current and responsive to the laser diode temperature, for permitting the laser diode driver to supply the first input current if the laser diode temperature is within a first predetermined temperature range, and to reduce the first input current sufficiently to render the laser diode inoperable if the laser diode temperature is outside the first predetermined temperature range; and a cooler driver for supplying a second input current to drive the cooler.  
           [0010]    Additional features and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the claims. The features and advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings:  
         [0012]    [0012]FIG. 1 is a block diagram of a system for electronically controlling the temperature of a pump laser consistent with the invention.  
         [0013]    [0013]FIG. 2 is a circuit diagram further detailing a system for electronically controlling the temperature of a pump laser consistent with the invention.  
         [0014]    [0014]FIG. 3 is a flowchart of a method for electronically controlling the temperature of a laser diode. 
     
    
       [0015]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.  
       DETAILED DESCRIPTION  
       [0016]    Referring now to the drawings, in which the same reference numbers will be used throughout the drawings to refer to the same or like parts, FIG. 1 is a block diagram of a system  100  for electronically controlling the temperature of a pump laser. System  100  includes a pump laser  102 . Pump laser  102  may include a laser diode  104 , a thermistor  105 , which monitors and detects the temperature of pump laser  102 , and a cooler  106 , to provide heat transfer away from laser diode  104 . System  100  may also include a detector  108 , which detects and produces a signal in accordance with and as a function of the temperature of pump laser  102 . System  100  may further include a laser diode driver  110  to supply current to laser diode  104 , a cooler driver  112  to supply current to cooler  106 , and a temperature controller  114 , which allows drivers  110  and  112  to supply current or to block the supplied current. Cooler driver  112  is coupled to temperature controller  114 . Temperature controller  114  is also coupled to cooler  106  and laser diode driver  110 .  
         [0017]    As pump laser  102  operates, thermistor  105 , which is physically mounted near laser diode  104 , monitors the temperature of pump laser  102 . Thermistor  105  includes a resistance that varies as a function of temperature and that variation is used to vary a voltage that represents the temperature of pump laser  102 . This function of monitoring the temperature can also be performed and implemented by other devices such as a semiconductor-type sensor that varies a voltage as a function of temperature. Thermistor  105  is coupled to detector  108 , so that detector  108  can detect the temperature of pump laser  102  and produce a signal representative of the temperature of pump laser  102 . The signal output of detector  108  is sent to cooler driver  112 , which supplies current to cooler  106  for heat transfer away from laser diode  104 . Cooler  106 , for example, can be a thermoelectric cooler (TEC) with a maximum required current of 1.50 A. When system  100  is implemented with a TEC having the maximum required current of 1.50 A, cooler driver  112  and cooler  106  can be designed to maintain the pump laser temperature at a near constant temperature of +25° C.+/−1° C. Temperature controller  114  receives the signal, produced by detector  108 , representative of the temperature of pump laser  102  via cooler driver  112  and compares the temperature of pump laser  102  with a first predetermined temperature range. The first predetermined temperature range is selected to allow pump laser  102  to operate without burnout due to excessive temperature, so long as the temperature of pump laser  102  remains within the first temperature range. Temperature controller  114  sends a signal to laser diode driver  110  to allow or prevent current from being supplied to laser diode  104 , and may also allow or prevent current from being supplied to cooler  106  by cooler driver  112 .  
         [0018]    The selection of the first predetermined temperature range depends upon the particular operational characteristics and specifications of pump laser  102 , for example, a 980 nm pump laser. The first predetermined temperature range can be between 0° C. and 70° C., the operational temperature range for a  980  nm pump laser when laser diode driver  110  is operating in a constant current mode of 0.600 A. In addition, if operating in the constant current mode, backfacet photodiode current does not have to be monitored. A backfacet photodiode is a diode that can produce an electrical signal proportional to the light incident upon it from, for example, laser diode  104 . Since the backfacet photodiode current is not being monitored, it eliminates the need for further control circuitry, which decreases printed circuit board size and power consumption requirements. Also, because of the decrease in component count, circuit reliability is increased. If the temperature of pump laser  102  is outside the first predetermined temperature range, temperature controller  114  prevents the operation of pump laser  102 . Specifically, if the temperature of pump laser  102  is outside the first predetermined temperature range, temperature controller  114  sends a signal to laser diode driver  110  to sufficiently reduce or block the current supplied to laser diode  104  so that laser diode  104  is rendered inoperable. In addition, temperature controller  114  may also sufficiently reduce or block the drive current to cooler  106 , to render cooler  106  inoperable. Conversely, if the pump laser temperature is within the first predetermined temperature range, temperature controller  114  sends a signal to allow laser diode driver  110  to supply sufficient current for operation of laser diode  104  and allows cooler  106  to operate.  
         [0019]    The first predetermined temperature range can also be +15° C. to +30° C. and, as described in further detail with regard to FIG. 2, the pump laser temperature can be maintained at a near constant temperature of +25° C.+/−1° C.  
         [0020]    [0020]FIG. 2 is a circuit diagram illustrating the features of a system  200 , consistent with the present invention, for electronically controlling the temperature of a pump laser. In system  200 , thermistor  105  has an electrical resistance  202  that varies as a function of temperature in a predictable manner.  
         [0021]    Detector  108  may include a non-inverting amplifier  204 . Non-inverting amplifier  204  may include a fixed voltage reference input  206 , an amplifier input  208 , and an amplifier output  210 .  
         [0022]    Laser diode driver  110  may include a step down regulator  220  and an amplifier  236 . Step down regulator  220  may include a regulator output  224 , a driver input  226 , and an enable input  228 . Amplifier  236  may include a fixed voltage reference input  238 , a laser diode current setpoint input  240 , a laser diode driver output  242 , and a laser diode driver input  244 . Depending on the characteristics of laser diode  104 , a laser diode current setpoint applied to input  240  is determined by the user to drive laser diode  104  at a level acceptable for the operational characteristics of laser diode  104 . Laser diode driver output  242  is coupled to driver input  226 , regulator output  224  is coupled to laser diode  104 , and laser driver input  244  is also coupled to laser diode  104 , such that a current loop is formed between amplifier  236 , step down regulator  220 , and laser diode  104 .  
         [0023]    Cooler driver  112  may include a non-inverting amplifier  250 , a non-inverting amplifier  252 , and a pull down resistor  254 . The purpose of cooler driver  112  is to supply input current to cooler  106  (through temperature controller  114 ), if the pump laser temperature is above a second predetermined temperature. The second predetermined temperature can be selected so that pump laser  102  operates at a near constant temperature, for example 25° C.+/−1° C. Thereby, laser diode  104  is cooled if the pump laser temperature is above the second predetermined temperature. The effect of cooler  106  providing heat transfer away from laser diode  104  is to maintain the overall laser temperature at a near constant value (e.g., 25° C.+/−1° C.). Like laser diode  104 , cooler  106  can be rendered inoperable as well, by sufficiently reducing or blocking the current applied to cooler  106  by cooler driver  112 , if the temperature of pump laser  102  is outside the first predetermined temperature range as previously described.  
         [0024]    Non-inverting amplifier  250  may include an amplifier input  256 , a cooler temperature setpoint input  258 , and an amplifier output  260 . Amplifier output  210  is coupled to input  256  of non-inverting amplifier  250 . The signal on amplifier input  256  is compared against a signal applied to cooler temperature setpoint input  258 , which is the second predetermined temperature selected by the user according to the desired temperature at which pump laser  102  is to be maintained (e.g., 25° C.+/−1° C.). Cooler temperature setpoint input  258  can be generated from a voltage divider circuit which derives its source voltage from a fixed voltage reference.  
         [0025]    Non-inverting amplifier  252  may include a cooler input  262 , an amplifier input  264 , and an amplifier output  265 . Cooler input  262  is coupled to cooler  106  and pull down resistor  254 . Pull down resistor  254  facilitates stable operation of cooler input  262  by eliminating noise or spurious signals that may affect amplifier  252 . Amplifier input  264  is coupled to amplifier output  260 . The signal on amplifier output  260  that is representative of the temperature of pump laser  102  is amplified by non-inverting amplifier  252  to produce a signal on amplifier output  265 . For example, non-inverting amplifier  250  generates a difference voltage which is then used to drive non-inverting amplifier  252 . Using a difference voltage allows step down regulators  222  and  266  of temperature controller  114  to respond to differences in values of thermistor  105  and also for limits to be placed on non-inverting amplifier  252 , as previously described. Therefore, when cooler  106  is in operation, heat is transferred away from laser diode  104  and the temperature of pump laser  102  is lowered. Pull down resistor  254  also ensures that signal noise or spurious signals do not interfere with the operation of non-inverting amplifier  252 .  
         [0026]    Temperature controller  114  may include a comparator  212 , having a comparator input  214 , a fixed voltage reference input  216 , a comparator output  218 , an input resistor  246 , and a feedback resistor  248 . Comparator input  214  is coupled to input resistor  246 , and input resistor  246  is coupled to amplifier output  260  of non-inverting amplifier  250 . Comparator input  214  is also coupled to feedback resistor  248  and feedback resistor  248  is coupled to comparator output  218 , so that a feedback gain section is formed on comparator  212 . Temperature controller  114  may further include step down regulator  222  and step down regulator  266 . Step down regulator  222  includes a regulator output  230 , a driver input  232 , and an enable input  234 . Driver input  232  is coupled to receive the drive current from cooler driver  112 . Enable input  234  is coupled to receive a signal on comparator output  218 . In addition, comparator output  218  is also coupled to enable input  228  of step down regulator  220 . Step down regulator  266  includes a regulator output  268 , a driver input  270 , and enable input  272 . Driver input  270  is coupled to driver  232 , enable input  272  is coupled to enable input  234 , and regulator output  268  is coupled to regulator output  230 . Although FIG. 2 shows two shut down regulators for cooler  106  (shutdown regulators  222  and  266 ), more or fewer shutdown regulators could be used depending on the current requirements of the application. For example, in higher current applications, such as a 980 nm pump laser, two or more shutdown regulators are desirable. If the application only requires low current delivery, a single stepdown regulator is sufficient.  
         [0027]    As system  200  operates, pump laser  102  is in its “on” state, whereby laser diode  104  is being supplied current by laser diode driver  110 , cooler  106  is being supplied current by cooler driver  112 , and thermistor  105  is monitoring the temperature of pump laser  102 . Thermistor  105  is physically mounted near laser diode  104  to monitor the temperature of pump laser  102 . Depending on the monitored temperature of pump laser  102 , the value of variable resistance  202 , coupled between amplifier input  208  and amplifier output  210 , varies as a function of the temperature. Therefore, thermistor  105  forms a non-inverting feedback gain section for non-inverting amplifier  204 . As a result, a signal on amplifier output  210  is representative of the temperature of pump laser  102  and is coupled to cooler driver  112 .  
         [0028]    Cooler  106  is controlled by using non-inverting amplifier  250  as a difference amplifier. The signal on amplifier input  256  may have a scaled voltage based on variable resistance  202 , which is a signal representative of the temperature of pump laser  102 . Cooler temperature setpoint input  258  can be chosen by the user according to the desired application. As discussed earlier, cooler temperature setpoint input  258  can be generated from a voltage driver circuit which derives its source voltage from a fixed voltage reference. For ordinary applications, cooler temperature setpoint input  258  can be fixed. If the difference between amplifier input  256  and cooler temperature setpoint input  258  is large, then there will be a corresponding increase in the driving current supplied to cooler  106 . If there is a small variation in the difference voltage, then the corresponding value for the drive current supplied to cooler  106  will be small. The output of non-inverting amplifier  250  provided on amplifier output  260 , is a signal representative of the temperature of pump laser  102 , i.e., a signal value that varies as a function of the temperature of pump laser  102 . For example, if cooler  106  is trying to maintain the temperature of pump laser  102  at +25° C., then variable resistance  202  is 10.0 kΩ and a nominal value of a voltage sample for thermistor  105  is 3.58V+/−0.5 V. The value of the voltage sample output should be limited to a value that is near the midpoint of a corresponding sample amplifier&#39;s output range.  
         [0029]    Operating cooler driver  112  as a function of the difference between two voltages, e.g., the difference between the voltage representative of the temperature of pump laser  102  and voltage representative of cooler temperature setpoint input  258 , rather than a single control voltage, allows for better rejection of control voltages that are caused by random noise or elevated noise levels from extraneous offset values.  
         [0030]    Temperature controller  114  operates in the following manner. Comparator  212  may output one of two signals (e.g., a logic level low or logic level high) on comparator output  218 . This signal is provided to step down regulator  220  through enable input  228 , to step down regulator  222  through enable input  234 , and to step down regulator  266  through enable input  272 . Whether the signal on comparator output  218  is a logic level high or low depends on whether the temperature of pump laser  102  is within the first predetermined temperature range or outside the first predetermined temperature range.  
         [0031]    Generally, comparator output  218  may be a logic level low, if the pump laser temperature is outside the first predetermined temperature range. If comparator output  218  is a logic level low, enable input  228 , enable input  234 , and enable input  272  also receive a logic level low signal. In response, step down regulator  220  is disabled and this prevents amplifier  236  from supplying current sufficient to allow operation of laser diode  104 , thereby rendering laser diode  104  inoperable. Also, since enable input  234  and enable input  272  receive logic level low signals, step down regulators  222  and  266  are disabled, preventing cooler driver  112  from supplying current sufficient for operation of cooler  106  and rendering cooler  106  inoperable. Thermistor  105  continues to monitor the temperature of pump laser  102 , so that detector  108  produces a signal on amplifier output  210  that corresponds to the temperature of pump laser  102 .  
         [0032]    Comparator output  218  may be a logic level high if the pump laser temperature is within the first predetermined temperature range. If comparator output  218  is a logic level high, enable input  228 , enable input  234 , and enable input  272  also receive a logic level high signal. As a result, step down regulator  220  is enabled, allowing amplifier  236  to supply sufficient current for operation of laser diode  104 . Also, since enable input  228  and enable input  234  receive logic level high signals, step down regulators  222  and  266  are also enabled, allowing cooler driver  112  to supply sufficient current for operation of cooler  106 . Detector  108  continues to monitor the pump laser temperature, and so long as the pump laser temperature is within the first predetermined range, the signal on amplifier output  210  will cause comparator output  218  to be a logic level high.  
         [0033]    The operation of comparator  212  and the selection of the temperature levels corresponding to the first predetermined range are implemented by using the equation for an inverting zero crossing detector, set forth as the following equation (1):  
           V   zh =( R   i   *V   s )/( R   i   +R   f );  (1)  
         [0034]    where V zh  is a threshold voltage for comparator  212 ; R i  is the input resistance for comparator  212  (input resistor  246 ); R f  is the feedback resistance into comparator  212  (feedback resistor  248 ); and V s  is the maximum output voltage for comparator  212 . An input voltage to comparator  212 , e.g., the voltage on amplifier output  260  is compared against V zh . R i  and R f  are chosen by the user so that V zh  represents the voltage levels at which comparator output  218  will transition from a logic level high to logic level low or vice versa and depends upon the design selections and requirements of R i , R f , and V s . For example, in the case of the shutdown limits for pump laser  102  (e.g., the first predetermined temperature range between +15° C. and +30° C.), where fixed voltage reference  216  is 0.0 V, V zh  is +/−0.379 V. Therefore, when the voltage on amplifier output  260  is +/−0.379 V, comparator output  218  will transition from a logic level low to a logic level high. Further, in this example, at +15° C. or below, comparator  212  outputs a logic level low, corresponding to a −0.379 V or below on amplifier output  260 , thereby shutting down regulators  220 ,  222 , and  266 . At +30° C. or higher, comparator  212  outputs a logic level low, corresponding to a +0.379 V or higher on amplifier output  260 , thereby shutting down regulators  220 ,  222 , and  266 . At temperatures between +15° C. and +30° C., e.g., at +25° C., comparator  212  outputs a logic level high, corresponding to a voltage of 0.0 V on amplifier output  260 , allowing regulators  220 ,  222 , and  266  to operate. This, in turn, allows pump laser  102  to operate.  
         [0035]    System  200  can be operated using a power supply  274  that has an input  276  of 5V DC with a current of up to 1 A when system  200  operates at +70° C. Power supply  274  can also include a dual channel charge pump  278 , a low drop out regulator (LDO)  280 , an unfiltered output  282 , a filter  284 , and a filtered output voltage  286 . Power supply  274  can be filtered for low frequency noise using an LC filter at unfiltered output  282  of the LDO regulator  280 . Output  282  is filtered through filter  284 , and filtered output voltage  286  can be used for supply voltages of detector  108 , laser diode driver  110 , and comparator  212 . Filter  284  can be provided as an LC filter. Specifically, output voltage  286  can be used for fixed voltage reference input  206 , fixed voltage reference input  238 , cooler temperature setpoint input  258 , and fixed voltage reference input  216 . With this configuration and values, system  200  has a power consumption that is less that 1 W at 25° C. System  200  is also versatile in that the blocks shown as thermistor  105 , detector  108 , laser diode driver  110 , cooler driver  112  and temperature controller  114  can be substituted for other universal control blocks known in the art.  
         [0036]    [0036]FIG. 3 is a flowchart  300  of a method for electronically controlling the temperature of a pump laser. A method for electronically controlling the temperature of a pump laser begins at a stage  302 , where current is supplied to a laser diode and cooler, e.g., from a first input current coupled to the laser diode and a second input current coupled to the cooler. As current is supplied to operate the laser diode and cooler, the temperature of the pump laser is determined at a stage  304 . As the pump laser continues to operate and the pump laser temperature is continuously determined, the pump laser temperature is compared to a predetermined temperature range, e.g., a range selected by the user to prevent the burn out of the laser diode, as shown at a stage  306 . Two outcomes may result from stage  306 . First, if the pump laser temperature is within the predetermined temperature range, current continues to be supplied to the laser diode and cooler sufficient for their operation, as previously described at stage  302 . Second, if the pump laser temperature is not within the predetermined temperature range, the supplied current is blocked sufficiently so that the laser diode and/or cooler are inoperable, as shown at a stage  308 .  
         [0037]    Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the claims disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit being indicated by the following claims.