Feedback for electronic pre-distortion in an optical transmitter
View Patent ↗We disclose an optical transponder, in which one or more all-electronic feedback paths are used to obtain a relatively accurate estimate of the device-specific signal distortions in the transmitter portion thereof. The obtained estimate is used to enable the digital signal processor of the optical transponder to carry out electronic pre-distortion (EPD) that can significantly reduce or cancel these signal distortions without the use of detailed factory-calibration measurements or optics dedicated to feedback purposes. The use of all-electronic feedback paths may enable a beneficial reduction in the cost of the EPD functionality, e.g., by eliminating a significant extra cost associated with the implementation of optically generated feedback.
1. An apparatus comprising:
an optical modulator configured to generate a modulated optical signal based on an electrical digital signal, wherein the optical modulator comprises:
an optical waveguide;
a first electrode positioned along a length of the optical waveguide and having a first end and a second end, with the first end being electrically connected to receive an electrical drive signal generated using the electrical digital signal;
a digital signal processor configured to apply electronic pre-distortion to generate the electrical digital signal in a manner that reduces an amount of signal distortion in the modulated optical signal caused by hardware of the optical modulator; and
a first all-electronic feedback path between the optical modulator and the digital signal processor, wherein the first all-electronic feedback path comprises a transmission line electrically connected to the second end of the first electrode to transmit a first feedback signal to an analog-to-digital converter connected between the transmission line and the digital signal processor; and
wherein the digital signal processor is configured to apply the electronic pre-distortion based on a digital form of the first feedback signal generated by the analog-to-digital converter.
2. The apparatus of claim 1 , wherein the electronic pre-distortion causes an effective signal-transfer function exhibited by the optical modulator to differ from a signal-transfer function imposed by the hardware of the optical modulator.
3. The apparatus of claim 1 , further comprising a second all-electronic feedback path between the optical modulator and the digital signal processor that is electrically connected to transmit a second feedback signal to the digital signal processor, wherein the digital signal processor is further configured to apply the electronic pre-distortion based on a digital form of the second feedback signal.
4. The apparatus of claim 3 , further comprising a first partially optical feedback path between the optical modulator and the digital signal processor connected to an optical output of the optical modulator to transmit a third feedback signal to the digital signal processor, wherein the digital signal processor is further configured to apply the electronic pre-distortion based on a digital form of the third feedback signal.
5. The apparatus of claim 4 , further comprising a second partially optical feedback path between the optical modulator and the digital signal processor connected to an optical output of the optical modulator to transmit a fourth feedback signal to the digital signal processor, wherein the digital signal processor is further configured to apply the electronic pre-distortion based on a digital form of the fourth feedback signal.
6. The apparatus of claim 1 , further comprising a first partially optical feedback path between the optical modulator and the digital signal processor connected to an optical output of the optical modulator to transmit a second feedback signal to the digital signal processor, wherein the digital signal processor is further configured to apply the electronic pre-distortion based on a digital form of the second feedback signal.
7. The apparatus of claim 6 , wherein the first partially optical feedback path comprises:
an optical tap configured to tap out a portion of an optical power of the modulated optical signal;
an optical-to-electrical converter configured to convert the portion of the optical power of the modulated optical signal into a corresponding electrical signal; and
an electrical conductor configured to direct the corresponding electrical signal to a respective analog-to-digital converter connected between the electrical conductor and the digital signal processor.
8. The apparatus of claim 7 , wherein the optical-to-electrical converter comprises a photodiode.
9. The apparatus of claim 7 , wherein the optical-to-electrical converter comprises a coherent optical detector coupled to an optical local-oscillator source.
10. The apparatus of claim 1 , wherein the electrical drive signal causes the first electrode to operate as a traveling-wave electrode.
11. The apparatus of claim 1 ,
wherein the optical modulator further comprises a second electrode positioned along a length of the optical waveguide; and
wherein the transmission line is electrically connected to the second electrode.
12. The apparatus of claim 1 , wherein the first all-electronic feedback path comprises the analog-to-digital converter.
13. An electronic pre-distortion method comprising:
configuring an optical modulator to generate an optical output signal carrying a training data sequence, wherein the optical modulator comprises:
an optical waveguide;
a first electrode positioned along a length of the optical waveguide and having a first end and a second end, with the first end being electrically connected to receive an electrical drive signal generated using the training data sequence;
receiving, via a first all-electronic feedback path between the optical modulator and a digital signal processor, a first feedback signal, wherein the first all-electronic feedback path comprises a transmission line electrically connected to the second end of the first electrode and an analog-to-digital converter connected between the transmission line and the digital signal processor;
determining one or more parameter values for electronic pre-distortion based on the training data sequence and the first feedback signal; and
configuring the optical modulator to generate an optical signal carrying payload data by applying to the optical modulator a corresponding electrical drive signal generated using the electronic pre-distortion.
14. The method of claim 13 , further comprising receiving, via a second all-electronic feedback path, a second feedback signal, wherein the second all-electronic feedback path electrically connects the optical transmitter and the digital signal processor; and
wherein the step of determining comprises determining the one or more parameter values for the electronic pre-distortion based on the second feedback signal.
15. The method of claim 13 , further comprising receiving, via a first partially optical feedback path, a second feedback signal, wherein the first partially optical feedback path is connected between an optical output of the optical modulator and the digital signal processor; and
wherein the step of determining comprises determining the one or more parameter values for the electronic pre-distortion based on the second feedback signal.
16. The method of claim 13 , wherein the step of determining comprises:
estimating, based on the first feedback signal, a frequency response of the optical modulator at an optical output thereof; and
determining the one or more parameter values for the electronic pre-distortion based on the estimated frequency response.
17. The apparatus of claim 1 , wherein an impedance of the transmission line matches an impedance corresponding to the first electrode.