Automatic control of laser diode current and optical power output
View Patent ↗An automatic power control system provides a control signal that regulates the output power of at least one laser diode. Coarse adjustment of the control signal is provided by a first means, preferably a digital variable resistor, while fine adjustment and compensation is provided by a second means, preferably by a digital-to-analog converter that receives an input signal proportional to a sensed control system parameter. The control system includes an operational amplifier having a first input coupled to sense output power, and a second input coupled to a DAC to provide finer resolution control. Memory can store system parameter or system parameter variations that can be coupled to the DAC and/or variable resistor to enhance system stability over ambient variations.
1. A method of compensating a system variable in response to a system parameter comprising:
providing a look-up table with a plurality of system parameter offset ranges wherein for each of said plurality of system parameter offset ranges said look-up table associates a separate compensation constant with each of said plurality of system parameter offset ranges;
sampling said system parameter;
calculating a first compensation point associated with said system parameter, wherein said calculating a first compensation point provides a first compensation point;
looking-up said compensation constant associated with said first compensation point; and
adjusting said system variable using said compensation constant.
2. A method according to claim 1 , wherein:
said calculating a first compensation point comprises averaging a plurality of system parameter samples wherein said averaging a plurality of system parameter samples provides an average of a plurality of system parameter samples; and
rounding said average of a plurality of system parameter samples to the nearest midpoint value of said ranges.
3. A method according to claim 2 , wherein:
said system variable comprises voltage; and
said system parameter comprises temperature.
4. The method according to claim 2 , wherein:
the system variable comprises voltage; and
the system parameter comprises resistance of a variable resistor.
5. A control system for a laser diode, comprising:
a memory storing a look-up table containing a plurality of system parameter offset ranges, and a plurality of compensation constants, each of said plurality of compensation constants associated with one of said plurality of system parameter offset ranges;
a sensor operative to sample the system parameter;
control logic operative to receive the system parameter from the sensor and further operative calculate to a first compensation point associated with said system parameter; wherein
the control logic is further operative to classify the compensation point as falling within a one offset range of the plurality of system parameter offset ranges in the look-up table;
the control logic is further operative to retrieve a particular compensation constant corresponding to the one offset range; and
at least one adjusting element operative to adjust a system variable using the compensation constant, the at least one adjusting element operationally connected to the control logic.
6. The control system of claim 5 , wherein the at least one adjusting element is a digital-to-analog converter.
7. The control system of claim 6 , further comprising a second adjusting element operative to further adjust the system variable and operationally connected to the control logic.
8. The control system of claim 7 , wherein the second adjusting element is a programmable resistor.
9. The control system of claim 7 , wherein the at least one adjusting element and the second adjusting element may operate simultaneously to adjust the system variable.
10. The control system of claim 6 , wherein the digital-to-analog converter may operate in a first configuration to adjust the system variable and a second configuration to adjust the system variable.
11. The control system of claim 10 , wherein:
the first configuration is an optical feedback configuration; and
the second configuration is a current feedback configuration.
12. The control system of claim 5 , wherein the system parameter is chosen from the group comprising: a laser diode current; a photodiode current; and optical power.