Digitally integrated self-trained pre-distortion curve finder for linearly frequency swept semiconductor laser systems
View Patent ↗The present disclosure provides a unique digitally integrated, self-trained pre-distortion curve generation method and apparatus for semiconductor lasers (SCLs) to generate linearly swept optical signals that are applicable to a wide range of sweep velocities and semiconductor laser types. The method requires no prior knowledge of the frequency response of the laser and is highly accurate.
1. A pre-distortion curve finder for a frequency-swept semi-conductor laser comprising:
a baseline pre-distortion curve stored in a memory;
an optical frequency discriminator (OFD) receiving an optical signal from said semiconductor laser and outputting an analog RF signal linearly proportional to the sweep velocity;
a digitizer receiving output from said OFD and converting said output into digital signals;
a digital phase frequency detector (PFD) receiving said digital signals from said digitizer, and generating an expected digital signal phase and a digital phase error signal;
a central loop controller receiving said expected digital phase signal and said digital phase error signal and generating a compensation signal to modify said baseline pre-distortion curve through a plurality of iterations, said loop controller counting iterations of digitized signals received;
a digital adder receiving said compensation signal;
a delay compensation loop providing a delayed version of said pre-distortion curve from a previous compensation iteration,
wherein said delayed version of said previous predistortion curve is summed with said current compensation signal to generate a newly modified pre-distortion curve which is written to said memory, and
wherein the laser diode is driven based on the newly modified iterative version of said pre-distortion curve stored in memory.
2. The pre-distortion curve finder of claim 1 , wherein said OFD includes a photodetector.
3. The pre-distortion curve finder of claim 1 , wherein said digitizer comprises an automatic grain control amplifier (AGC) and a comparator.
4. The pre-distortion curve finder of claim 1 wherein the loop controller is configured and arranged to cycle through at least 300 iterations.
5. The predistortion curve finder of claim 1 wherein the loop controller is configured and arranged to generate a new compensation signal every 1 ms.
6. The predistortion curve finder of claim 5 wherein the loop controller is configured and arranged to generate a compensation signal every 1 ms.
7. A linearly frequency swept semiconductor laser, comprising:
a laser diode;
a laser driver circuit operating the laser diode;
a baseline pre-distortion curve stored in a memory;
an optical frequency discriminator (OFD) receiving an optical signal from said semiconductor laser and outputting an analog RF signal linearly proportional to the sweep velocity;
a digitizer receiving output from said OFD and converting said output into digital signals;
a digital phase frequency detector (PFD) receiving said digital signals from said digitizer, and generating an expected digital signal phase and a digital phase error signal;
a central loop controller receiving said expected digital phase signal and said digital phase error signal and generating a compensation signal to modify said baseline pre-distortion curve through iterations, said loop controller further counting iterations of digitized signals received;
a digital adder receiving said compensation signal;
a delay compensation loop proving a delayed version of said pre-distortion curve from a previous compensation iteration,
wherein said delayed version of said previous predistortion curve is summed with said current compensation signal to generate a newly modified pre-distortion curve which is written to said memory, and
wherein the laser driver circuit uses the newly modified iterative version of said pre-distortion curve stored in memory to drive said laser diode.
8. The semiconductor laser of claim 7 , wherein said OFD includes a photodetector.
9. The semiconductor laser of claim 7 , wherein said digitizer comprises an automatic grain control amplifier (AGC) and a comparator.
10. The semiconductor laser of claim 7 wherein the loop controller is configured and arranged to cycle through at least 300 iterations.
11. The semiconductor laser of claim 7 wherein the loop controller is configured and arranged to generate a new compensation signal every 1 ms.
12. The semiconductor laser of claim 11 wherein the loop controller is configured and arranged to generate a compensation signal every 1 ms.
13. A method of finding the pre-distortion curve of a semiconductor laser comprising the steps of:
driving a semiconductor laser diode to generate an optical output;
storing a baseline pre-distortion curve store in a memory;
receiving an optical signal from said semiconductor laser and outputting an analog RF signal linearly proportional to a sweep velocity;
receiving output from said OFD and converting said output into digital signals;
receiving said digital signals and generating an expected digital signal phase and a digital phase error signal;
receiving said expected digital phase signal and said digital phase error signal and generating a compensation signal to modify said baseline pre-distortion curve;
providing a delayed version of said pre-distortion curve from a previous compensation iteration,
summing said delayed version of said previous predistortion curve with said current compensation signal to generate a newly modified pre-distortion curve;
writing said newly modified pre-distortion curve to said memory; and
driving said semiconductor laser diode to generate an optical output using the newly modified iterative version of said pre-distortion curve stored in memory.
14. The method of claim 13 , wherein said steps of generating a compensation signal, providing said delayed version of said pre-distortion curve from the previous iteration, summing the delayed version with the compensation signal and generating a newly modified pre-distortion curve is repeated every 1 ms.
15. The method of claim 13 , wherein the steps of generating a compensation signal, providing said delayed version of said pre-distortion curve from the previous iteration, summing the delayed version with the compensation signal and generating a newly modified pre-distortion curve is repeated every 1 ms for at least 300 iterations.
16. The method of claim 13 , wherein the steps of generating a compensation signal, providing said delayed version of said pre-distortion curve from the previous iteration, summing the delayed version with the compensation signal and generating a newly modified pre-distortion curve is repeated every 1 ms for at least 500 iterations.
17. The method of claim 13 , wherein the steps of generating a compensation signal, providing said delayed version of said pre-distortion curve from the previous iteration, summing the delayed version with the compensation signal and generating a newly modified pre-distortion curve is repeated every 1 ms for at least 1000 iterations.