Abstract:
A method for controlling a laser includes determining an average component of a laser drive current and adjusting a modulation component of the drive current based on the average component and a threshold current signal at the present temperature. Determining the average component includes adjusting the drive current until an output power of the laser is approximately equal to a reference signal that varies according to temperature. The method further includes calibrating the threshold current signal, which includes adjusting the drive current until the laser output power is approximately equal to a second reference signal, and storing the drive current signal as the threshold current signal at the present temperature. The method further includes recalibrating the threshold current signal, which includes storing a difference between a new value and a stored value of the threshold current signal as an offset for other stored values of the threshold current signal.

Description:
FIELD OF INVENTION 
     This invention relates to control and calibration of lasers. 
     DESCRIPTION OF RELATED ART 
     Laser diodes are a common light source for optical transmitters. As the transmission rate of optical transmitter increases, the precision of the control of the laser diodes must also increase. The classic problems that plague laser diodes include (1) the variability of slope efficiency over temperature, between parts, and over time, (2) the variability of the threshold current over temperature, between parts, and over time, and (3) the balance between the need for speed and reliability for the amount of drive current. Thus, what are needed are a method and an apparatus that optimize the control of the laser diodes despite these difficulties. 
     SUMMARY 
     In accordance with one aspect of the invention, a method for controlling a laser includes determining an average component of a laser drive current and adjusting a modulation component of the drive current based on the average component and a threshold current signal at the present temperature. 
     In one embodiment of the invention, determining the average component includes adjusting the drive current until an output power of the laser is approximately equal to a reference signal that varies according to the temperature. 
     In one embodiment of the invention, the method further includes calibrating the threshold current signal, which includes adjusting the drive current until the laser output power is approximately equal to a second reference signal, and storing the drive current signal as the threshold current signal at the present temperature. 
     In one embodiment of the invention, the method further includes recalibrating the threshold current signal, which includes storing a difference between a new value and a stored value of the threshold current signal as an offset for other stored values of the threshold current signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic of a laser system in accordance with one embodiment of the invention. 
         FIG. 1B  is a schematic of a laser system in accordance with one embodiment of the invention. 
         FIG. 1C  is a cross-section of a laser subassembly for a laser system in one embodiment of the invention. 
         FIG. 1D  illustrates a reflected-to-transmitted power ratio vs. temperature graph of a laser system in one embodiment of the invention. 
         FIG. 2  illustrates a method for determining an average component of a current to the laser for generating a desired average power in one embodiment of the invention. 
         FIG. 2A  is an LOP (luminous output power) vs. current graph of a laser diode in one embodiment of the invention. 
         FIG. 3  illustrates a method for determining a modulation component of a current to the laser for generating desired modulated powers in one embodiment of the invention. 
         FIG. 4  illustrates a method for automatically calibrating the values of a threshold current for a range of temperatures in one embodiment of the invention. 
         FIG. 4A  is a threshold current vs. temperature graph of a laser diode in one embodiment of the invention. 
         FIG. 5  illustrates a method for recalibrating the values of a threshold current for a range of temperatures in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  illustrates a laser system  100 A in one embodiment of the invention. The anode of a laser  10  (e.g., a laser diode) is connected at a node  40  to a first terminal of an inductor  44 . The second terminal of inductor  44  is connected to a supply rail that provides supply voltage Vcc to laser diode  10 . The cathode of laser diode  10  is connected at a node  42  to a first terminal of an inductor  50 . The second terminal of inductor  50  is connected to a current source  20  that sinks a current Iavg from laser diode  10 . In operation, inductors  44  and  50  block alternating currents, and current source  20  provides current Iavg to laser diode  10 . Current Iavg forms an average component of a drive current Idrive for laser diode  10 . An Iavg control circuit  62  sets the value of current Iavg by outputting a gain signal to current source  20 . 
     A differential amplifier  60  receives data signal Data and its complement Data_b. In response, amplifier  60  outputs a control signal to control terminals of switches  61  and  63 . Switch  61  has a first terminal coupled to a first terminal of current source  22 , a second terminal coupled to a first terminal of a capacitor  54 , and a third terminal coupled to a first terminal of a capacitor  46 . Switch  63  has a first terminal coupled to a second terminal of current source  22 , a second terminal coupled to the first terminal of capacitor  46 , and a third terminal coupled to the first terminal of capacitor  54 . Depending on the control signal, switches  61  and  63  are configured so current source  22  either sources current Imod to capacitor  46  and sinks current Imod from capacitor  54 , or sinks current Imod from capacitor  46  and sources current Imod to capacitor  54 . 
     The second terminal of capacitor  46  is coupled to node  40  and the second terminal of capacitor  54  is coupled to node  42 . Thus, current Imod is either added to node  40  and subtracted from node  42 , or subtracted from node  40  and added to node  42 . In operation, capacitors  46  and  54  block direct currents to laser diode  10 , and current source  22  provides current Imod to laser diode  10 . Current Imod forms a modulation component of drive current Idrive. Accordingly, drive current Idrive for laser diode  10  is either (Iavg+Imod) or (Iavg−Imod). An Imod control circuit  64  sets the value of current Imod by outputting a gain signal to current source  22 . 
     A mirror  11  reflects a part of the light signal from laser diode  10  to a light detector  12  (e.g., a photodiode) and transmits a part of the light signal to a fiber  13  that carries the light signal to another component. Photodiode  12  is connected between supply rail Vcc and an Imon ADC (analog-to-digital converter)  18 . Photodiode  12  outputs an analog signal Imon to Imon ADC  18 . Analog signal Imon is proportional to the reflected power received by photodiode  12 . The reflected power is proportional to the transmitted power received by fiber  13  and the total output power of laser diode  10 . Imon ADC  18  outputs a digital signal IMON to control circuit  62 . 
     A temperature sensor  16  outputs a signal TEMP to control circuits  62  and  64 . Signal TEMP can be either digital or analog. Signal TEMP is proportional to the temperature of laser diode  10 . 
     An Iref source  19  outputs a reference signal IREF to control circuit  62 . Reference signal IREF can be either digital or analog. The values of reference signal IREF and signal IMON are typically compared in a feedback loop to control laser diode  10 . Control circuit  62  can use signals IMON, TEMP, and IREF to determine the proper value for current Iavg in a method  200  ( FIG. 2 ) described later. 
     A Kmod source  34  outputs a linearity coefficient signal Kmod to control circuit  64 . Signal Kmod can be either digital or analog. An Ith source  36  outputs a threshold current signal Ith to control circuit  64 . Signal Ith can be either digital or analog. Control circuit  64  can use signals Kmod and Ith to determine the proper value for current Imod in a method  300  ( FIG. 3 ) described later. 
     A memory  3  outputs parameters for operating laser system  100 A to control circuits  62  and  64 . These parameters may include maximum and minimum values for current Iavg. Memory  3  may be a programmable nonvolatile memory such as an EEPROM. 
       FIG. 1B  illustrates a laser system  100 B in one embodiment of the invention. Same or similar elements in  FIGS. 1A and 1B  use the same reference numerals. 
     In  FIG. 1B , the second terminal of inductor  50  is connected to an Iavg DAC (digital-to-analog converter)  120  in a driver circuit  104 . Iavg DAC  120  sinks current Iavg from laser diode  10 . 
     A differential amplifier  160  receives data signal Data and its complement Data_b. In response, amplifier  160  generates a signal Out to the first terminal of capacitor  46  and a complement Out_b to the first terminal of capacitor  54 . As signals Out and Out_b are alternatively switched between high and low states by data signals Data and Data_b, capacitors  46  and  54  add or subtract current Imod to or from drive current Idrive. An Imod DAC  122  in driver circuit  104  controls the gain of differential amplifier  160  to set the value of current Imod. 
     In the present embodiment, a controller  102  implements control circuits  62  and  64  ( FIG. 1A ) to adjust the values of currents Iavg and Imod. A memory  103  implements IREF source  19 , Kmod source  34 , Ith source  36 , and memory  3  (FIG.  1 A). An interface block  114  in driver circuit  104  provides the interface between controller  102  and temperature sensor  16 , Imon ADC  18 , Iavg DAC  120 , and Imod DAC  122 . Controller  102  communicates with interface block  114  through a bus  108 . 
     Iavg DAC  120  receives a control signal Iavg from controller  102  through interface block  114 . Control signal Iavg sets the value of current Iavg. 
     Imod DAC  122  receives a control signal Imod from controller  102 . Control signal Imod sets the gain for differential amplifier  160  that generates current Imod. 
       FIG. 1C  illustrates a configuration of a laser subassembly  178 . Laser diode  10  and photodiode  12  are typically located inside a laser header  180  having an angled window mirror  11 . Light emitted from laser diode  10  is partially transmitted through angled window mirror  11  to fiber  13  and partially reflected to photodiode  12 . From experimental results, it is known that the ratio of reflected power detected by photodiode  12  to the transmitted power received by fiber  13  in laser system  100  varies over temperature.  FIG. 1D  illustrates an exemplary plot  902  of the ratio of reflected power to transmitted power over temperature for laser system  100 . Thus, the variation of the reflected-to-transmitted power ratio should be compensated over temperature so the transmitted power received by fiber  13  remains constant over temperature. 
     In accordance with one aspect of the invention, controller  102  uses a reference signal IREF that varies over temperature to adjust current Iavg in a closed feedback loop. The variation of reference signal IREF over temperature offsets the variation of reflected power over temperature in order to keep the transmitted power received by fiber  13  constant. 
       FIG. 2  is a flowchart of a method  200  implemented by controller  102  ( FIG. 1B ) to adjust the value of current Iavg in one embodiment of the invention. Alternatively, method  200  can also be implemented using control circuit  62  (FIG.  1 A). 
     In step  202 , controller  102  reads the temperature of laser diode  10  by requesting interface block  114  for the value of signal Temp from temperature sensor  16 . 
     In step  204 , controller  102  determines the value of reference signal IREF at the present temperature. Controller  102  may look up the value of reference signal IREF at the present temperature stored in a table  132  ( FIG. 1B ) in memory  103 . The values of reference signal IREF can be determined from experimental data in which reference signal IREF is varied and the transmitted power received by fiber  13  is measured. Alternatively, controller  102  can calculate the value of reference signal IREF at the present temperature by using a function extrapolated from experimental data. 
     In step  206 , controller  102  reads the reflected average power of laser diode  10  by requesting from interface block  114  the value of signal IMON from Imon ADC  18 . 
     In step  207 , controller  102  determines if signal IMON is greater than a safety warning. This ensures that laser diode  10  does not generate a level of light that is harmful to human operators. If signal IMON is greater than the safety warning, step  207  is followed by step  208 . Otherwise step  207  is followed by step  209 . 
     In step  208 , controller  102  decrements current Iavg. Step  208  is followed by step  202  and method  200  is repeated. 
     In steps  209  to  218 , controller  102  adjusts current Iavg until the value of signal IMON is approximately equal to the value of reference signal IREF. When this occurs, the reflected power of laser diode  10  is approximately equal to a desired value that indicates laser diode is generating a consistent transmitted average power to fiber  13 . 
     In step  209 , controller  102  determines if the value of reference signal IREF is greater than the value of signal IMON by a predetermined tolerance. If so, the reflected power of laser diode  10  is unacceptably smaller than its desired value, and step  209  is followed by step  210 . If the value of reference signal IREF is not greater than the value of signal IMON by a predetermined tolerance, step  209  is followed by step  214 . 
     In step  210 , controller  102  determines if the value of current Iavg is greater than a maximum value. The maximum value is a function of the temperature of laser diode  10  and can be stored in memory  3 . The maximum value ensures that drive current Idrive is not too high as to make laser diode  10  unreliable. Controller  102  can read the value of current Iavg from memory  103 . Alternatively, controller  102  can read the value of current Iavg by requesting from interface block  114  the value of control signal Iavg from Iavg DAC  120 . If the value of current Iavg is greater than a maximum value, step  210  is followed by step  202  so that current Iavg is not incremented, and method  200  is repeated in a feedback loop. Otherwise step  210  is followed by step  212 . 
     In step  212 , controller  102  increments current Iavg by instructing interface block  114  to increment the value of control signal Iavg to Iavg DAC  120 . Iavg DAC  120  then increments current Iavg. Step  212  is followed by step  202  and method  200  is repeated in a feedback loop. 
     In step  214 , controller  102  determines if the value of signal IMON is greater than the value of reference signal IREF by the predetermined tolerance. If so, the reflected power of laser diode  10  is unacceptably greater than the desired value, and step  214  is followed by step  216 . If the value of signal IMON is not greater than the value of reference signal IREF by the predetermined tolerance, then step  214  is followed by step  202  and method  200  is repeated in a feedback loop. 
     In step  216 , controller  102  determines if the value of current Iavg is less than a minimum value. The minimum value is a function of the temperature of laser diode  10  and can be stored in memory  3 . The minimum value ensures that the laser output maintains a minimum edge speed. If the value of current Iavg is less than a predetermined minimum, step  216  is followed by step  202  so that current Iavg is not decremented, and method  200  is repeated in a feedback loop. Otherwise step  216  is followed by step  218 . 
     In step  218 , controller  102  decrements the value of current Iavg by instructing interface block  114  to decrement the value of control signal Iavg to Iavg DAC  120 . Iavg DAC  120  then decrements the value of current Iavg. Step  218  is followed by step  202  and method  200  is repeated in a feedback loop. 
     In accordance with another aspect of the invention, controller  102  uses the values of the temperature of laser diode  10  and current Iavg to adjust the value of current Imod in an open-loop scheme based on the following formula:
 
 Imod=Kmod *( Iavg−Ith )  (1)
 
Kmod is a linearity coefficient for laser diode  10  that depends on the temperature, and Ith is the threshold current of laser diode  10  that also depends on the temperature. Formula 1 is based on the relationship between the power, the drive current, and the temperature of laser diode  10  as shown in an exemplary graph  600  in FIG.  2 A.
 
     Graph  600  shows a plot  602  of LOP vs. drive current of laser diode  10  at a temperature Temp 1 . The vertical axis of graph  600  identifies a power Pth that is a predetermined reflected power representing a minimum threshold, a power P 0  that is a predetermined reflected power representing a first logical state (e.g., a logical “0”), a power P 1  that is a predetermined reflected power representing a second logical state (e.g., a logical “1”), and a power Pavg that is the average of reflected powers P 0  and P 1 . 
     Power Pth is selected at a point where the reflected power is stable and linear. Power P 0  is typically selected to be approximately 10% greater than power Pth. Power P 1  is typically selected to produce an extinction ratio of P 1 /P 0  of approximately 7 to 13. Such an extinction ratio provides the proper edge speed and reliable data recovery. 
     For plot  602 , the horizontal axis of graph  600  identifies a current Ith (e.g., 2 milliamps) that produces power Pth, a current I 0  (e.g., 2.3 milliamps) that produces power P 0 , a current I 1  (e.g., 7.7 milliamps) that produces power P 1 , and current Iavg (e.g., 5 milliamps) that produces power Pavg. The difference between I 1  and I 0  (e.g., 5.4 milliamps) is twice the current Imod (e.g., 2.7 milliamps). 
     As plot  602  is linear between currents Ith and I 1 , current Imod can be made a fraction of the difference between currents Iavg and Ith where that fraction depends on the slope of plot  602 . That fraction is represented by linearity coefficient Kmod in formula 1 for temperature Temp 1 . Coefficient Kmod can be determined for a range of temperatures by plotting LOP vs. drive current of laser diode  10  at these temperatures. For each plot, coefficient Kmod at the temperature of the plot is determined by using formula 1 from the values of currents Iavg, Ith, and Imod. With our exemplary values described above, Kmod for temperature Temp 1  is calculated as follows:
 
 Kmod=Imod /( Iavg−Ith )=2.7/(5−2)=0.9  (2)
 
     Graph  600  also shows a plot  604  of LOP vs. current of laser diode  10  at temperature Temp 2 . The horizontal axis of graph  600  identifies a current Ith′ that produces power Pth, a current I 0 ″ that produces power P 0 , a current I 1 ′ that produces power P1, and a current Iavg′ that produces power Pavg. The difference between currents I 1 ′ and I 0 ′ is twice the current Imod′. Coefficient Kmod at temperature Temp 2  can be determined by using formula 2 from the values of currents Iavg′, Ith′, and Imod′. 
       FIG. 3  is a flowchart of a method  300  for controller  102  ( FIG. 1B ) to adjust the value of current Imod using formula 1 in one embodiment of the invention. Alternatively, method  300  can also be implemented using control circuit  64  (FIG.  1 A). 
     In step  302 , controller  102  reads the temperature of laser diode  10 . 
     In step  304 , controller  102  determines the value of current Iavg. In the present embodiment, controller  102  uses method  200  described above to determine the value of current Iavg. 
     In step  306 , controller  102  determines the value of current Imod from the values of currents Iavg and Ith using the relationship described by formula 1. Current Ith is a function of the temperature of laser diode  10 . 
     In one embodiment of step  306 , controller  102  determines the values of coefficient Kmod and threshold current Ith at the current temperature by looking up their values in respective tables  134  and  136  in memory  103 . Controller  102  then calculates the value of current Imod from the values of current Iavg, coefficient Kmod, and threshold current Ith using formula 1. 
     In another embodiment of step  306 , the values of threshold current Ith and current Imod are stored in respective tables  136  and  138  in memory  103  (FIG.  1 B). Controller  102  first looks up threshold current Ith at the current temperature in table  136 . Controller  102  then calculates the difference between the values of current Iavg and threshold current Ith at the current temperature. Controller  102  finally uses the difference to look up the value of current Imod in table  138 . 
     In step  308 , controller  102  adjusts current Imod by instructing interface block  114  to set the value of signal Imod to Imod DAC  122 . Imod DAC  122  then adjusts the gain of amplifier  160  to increment or decrement current Imod to its desired value. Step  308  is followed by step  302  and method  300  repeats in an open loop. 
     In method  300 , threshold current Ith plays an important role. Thus, the calibration of this parameter over a range of temperatures should be carefully accomplished.  FIG. 4  is a flowchart of a method  400  for controller  102  to calibrate threshold current Ith over a range of temperatures without human intervention in one embodiment of the invention. Alternatively, method  400  can also be implemented using control circuit  62 . 
     In step  402 , laser system  100  is put into an auto-calibration mode. In the auto-calibration mode, controller  102  turns off current Imod. Laser system  100  is placed into an oven. The oven is then slowly heated and cooled over the range of operating temperatures of laser system  100 . 
     In step  404 , controller  102  determines the value of a reference signal IREF-TH for a threshold reflected power Pth of laser diode  10 . Controller  102  may look up the value of reference signal IREF-TH in memory  103 . 
     In step  406 , controller  102  reads the temperature of laser diode  10 . 
     In steps  408  to  416 , controller  102  adjusts the value of current Iavg until the value of signal IMON is approximately equal to the value of reference signal IREF-TH. When this occurs, the reflected power of laser diode  10  is approximately equal to a desired value that indicates laser diode  10  is generating a desired power Pth. 
     In step  408 , controller  102  reads the reflected power of laser diode  10 . 
     In step  410 , controller  102  determines if the value of reference signal IREF-TH is greater than the value of signal IMON by a predetermined tolerance. If so, the reflected power of laser diode  10  is unacceptably smaller than the desired power Pth of laser diode  10 , and step  410  is followed by step  412 . If the value of reference signal IREF-TH is not greater than the value of signal IMON by the predetermined tolerance, then step  410  is followed by step  414 . 
     In step  412 , controller  102  increments current Iavg. Step  412  is followed by step  408 . 
     In step  414 , controller  102  determines if the value of signal IMON is greater than the value of reference signal IREF−TH by the predetermined tolerance. If so, the reflected power of laser diode  10  is unacceptably greater than the desired power Pth of laser diode  10 , and step  414  is followed by step  416 . If the value of signal IMON is not greater than the value of reference signal IREF-TH by the predetermined tolerance, then step  414  is followed by step  418 . 
     In step  416 , controller  102  decrements the value of current Iavg. Step  416  is followed by step  408 . 
     In step  418 , the reflected power of laser diode  10  is approximately equal to the desired power Pth. Thus, controller  102  stores the value of current Iavg as the value of threshold current Ith at the present temperature in table  136 . Step  418  is followed by step  420 . 
     In step  420 , controller  102  determines if the end of auto-calibration mode has been reached. The end of the auto-calibration mode has been reached when a specific operating temperature of laser system  100  has been reached or instructed by an external command signal. If so, step  420  is followed by the end of method  400  in step  422 . If the end of the auto-calibration mode has not been reached, then step  420  is followed by step  406  and method  400  is repeated to determine the value of threshold current Ith at another temperature. 
       FIG. 4A  shows that over time, a plot  702  of threshold current Ith vs. temperature for laser diode  10  can shift to a plot  704 . The shift can be generally described by an offset Ith-offset. Thus, the threshold current Ith should be recalibrated over time as laser system  100  ages to account for the shift of threshold Ith vs. temperature. 
       FIG. 5  is a flowchart of a method  500  for controller  102  ( FIG. 1B ) to recalibrate threshold current Ith each time laser system  100  is turned on or instructed by an external command signal in one embodiment of the invention. Alternatively, method  500  can also be implemented using control circuit  62  (FIG.  1 A). 
     In step  502 , controller  102  puts laser system  100  into a recalibration mode where current Imod is turned off. Controller  102  does so when laser system  100  is turned on or when instructed by an external command signal. 
     In step  504 , controller  102  determines a threshold current Ith−new at the current temperature. In the present embodiment, controller  102  uses method  400  described above to determine threshold current Ith-new for the current temperature. 
     In step  506 , controller  102  calculates a difference between the values of threshold current Ith−new and the threshold current Ith stored in table  136  for the present temperature. Controller  102  stores difference Ith-offset in memory  103  as an offset for all the values of threshold current Ith stored in table  136 . 
     In step  508 , controller  102  puts laser transmitter  100  into a normal operation mode. During normal operation mode, controller  102  can adjust current Iavg and current Imod as described in methods  200  and  300  above with the exception that difference Ith−offset is added to all the values of Ith that are looked up from table  136 . 
     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.