Control and calibration of laser systems

This invention relates to control and calibration of lasers.

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.

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.

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.

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.