Systems and methods for reducing clipping in multichannel modulated optical systems
View Patent ↗A system for reducing clipping may be used between a multichannel RF source and a laser to reduce or correct clipping that might occur in the laser as a result of negative spikes or peaks in a multichannel RF signal. The system generally includes a clipping correction circuit that receives the multichannel RF signal and responsive to the RF signal, prevents one or more of the negative peaks in the RF signal from causing clipping. The clipping correction circuit may either detect an envelope of the RF signal and/or may detect one or more peaks in the RF signal. One or more negative peaks may be prevented from causing clipping by adjusting a bias current provided by a bias control circuit and/or by modifying the RF signal with one or more clipping correction pulses coinciding with one or more negative peaks.
1. A system for reducing clipping in a modulated optical system, the system comprising:
a laser including an RF input configured to receive a multichannel RF signal and an optical output to provide a modulated optical signal in response to the multichannel RF signal, the multichannel RF signal including a superposition of multiple modulated carriers and occupying a bandwidth across a range of frequencies of the multiple modulated carriers;
a peak detector configured to receive the multichannel RF signal and to detect at least one peak in the multichannel RF signal; and
a pulse generator coupled to the peak detector, the pulse generator being configured to generate a series of clipping correction pulses in response to at least one detected peak and to combine the clipping correction pulses with the multichannel RF signal, wherein consecutive clipping correction pulses in the series of clipping correction pulses are spaced at an interval corresponding to an interval between consecutive negative peaks in the multichannel RF signal such that the clipping correction pulses coincide with a corresponding series of subsequent negative peaks in the multichannel RF signal and increase an amplitude of the RF signal at the time of respective ones of the corresponding series of subsequent negative peaks to prevent the subsequent negative peaks in the multichannel RF signal from causing clipping when the laser receives the multichannel RF signal combined with the clipping correction pulses.
2. The system of claim 1 further comprising a delay element configured to provide a delay before triggering the pulse generator, wherein the delay is based on an interval between peaks in the multichannel RF signal that cause clipping.
3. The system of claim 1 wherein the pulse generator includes a digital timer configured to provide a delay before generating at least one of the clipping correction pulses, wherein the delay is based on an interval between peaks in the multichannel RF signal that cause clipping.
4. The system of claim 1 wherein the multichannel RF input signal occupies a bandwidth over a frequency range of about 50 MHz to 1000 MHz.
5. The system of claim 1 wherein the pulse generator is configured to generate the series of clipping correction pulses following an initial detected peak.
6. The system of claim 1 wherein the peak detector is configured to detect a plurality of peaks in the multichannel RF signal and wherein the pulse generator is configured to generate each clipping correction pulse in the series of clipping correction pulses in response to and following a respective detected peak of the plurality of detected peaks.
7. The system of claim 1 wherein the peak detector is configured to detect a negative peak that falls below a peak detection threshold, and wherein the pulse generator is configured to generate the series of clipping correction pulses following a detected negative peak that falls below the threshold.
8. The system of claim 7 wherein the peak detection threshold corresponds to a threshold current that causes clipping.
9. The system of claim 7 wherein the peak detection threshold corresponds to a percentage of a threshold current that causes clipping.
10. The system of claim 1 wherein the peak detector is configured to detect at least one positive peak, and wherein the pulse generator is configured to generate the series of clipping correction pulses following at least one detected positive peak.
11. The system of claim 1 wherein each of at least some of the multiple modulated carriers have an optical modulation index (OMI) of at least about 4%.
12. A method of reducing clipping in a modulated optical system, the method comprising:
receiving a multichannel RF signal, the multichannel RF signal including a superposition of multiple modulated carriers and occupying a bandwidth across a range of frequencies of the multiple modulated carriers;
detecting at least one peak in the multichannel RF signal;
generating a series of clipping correction pulses in response to detecting the at least one peak, wherein consecutive clipping correction pulses in the series of clipping correction pulses are spaced at an interval corresponding to an interval between consecutive negative peaks in the multichannel RF signal;
combining the series of clipping correction pulses with the multichannel RF signal such that the series of clipping correction pulses coincide with a corresponding series of subsequent negative peaks, and wherein the clipping correction pulses increase an amplitude of the RF signal at the time of respective ones of the corresponding series of subsequent negative peaks;
providing the multichannel RF signal combined with the series of clipping correction pulses; and
providing a modulated optical signal output from the laser.
13. The method of claim 12 further comprising providing a delay after detecting the at least one peak before generating at least one of the clipping correction pulses, wherein the delay is based on an interval between peaks in the multichannel RF signal that cause clipping.
14. The method of claim 12 wherein detecting the at least one peak includes detecting at least one negative peak that falls below a peak detection threshold.
15. The optical system of claim 14 wherein the peak detection threshold corresponds to a threshold current of the laser.
16. The method of claim 14 wherein the peak detection threshold corresponds to a percentage of a threshold current of the laser.
17. The method of claim 12 wherein detecting the at least one peak includes detecting at least one positive peak that exceeds a peak detection threshold.
18. The method of claim 12 wherein detecting the at least one peak includes detecting an initial detected peak and wherein the series of clipping correction pulses is generated following the initial detected peak.
19. The method of claim 18 wherein the amplitude of the series of clipping correction pulses is fixed such that the clipping correction pulses are capable of preventing the series of subsequent negative peaks from causing clipping.
20. The method of claim 12 wherein generating the series of clipping correction pulses includes generating at least one clipping correction pulse having an amplitude proportional to an amplitude of the detected peak.
21. The method of claim 12 wherein detecting the at least one peak includes detecting a plurality of peaks, and wherein generating the series of clipping correction pulses in response includes generating each clipping correction pulse in the series of clipping correction pulses in response to and following a respected detected peak of the plurality of detected peaks.
22. The method of claim 12 wherein the series of clipping correction pulses includes pulses with a pulse width of about 0.1 μs.
23. The system of claim 1 wherein the pulse generator is configured to generate the series of clipping correction pulses with a pulse width of about 0.1 μs.