Modulator for optical transmitter

Patent metadata

Field	Value
Publication number	US-9485028-B2
Application number	US-201114119744-A
Country	US
Kind code	B2
Filing date	Jun 1, 2011
Priority date	Jun 1, 2011
Publication date	Nov 1, 2016
Grant date	Nov 1, 2016

Abstract

An optical transmitter has a modulator for modulating data onto an optical signal for transmission to the receiver, the modulated signal having components at one or more constellations of points of different amplitudes and phases. The modulator is tunable such that distortions of the points of the one or more constellation can be tuned, and a tuning controller is provided for receiving a feedback signal from the receiver indicating a distortion measured at the receiver, and for tuning automatically the modulator to adjust the modulation based on the received feedback signal to pre-compensate for the measured distortion. Such pre-compensation can reduce the amount of distortion in the transmission system and thus enable more transmission capacity, without the need for a complex transmitter.

First claim

The invention claimed is:

1. An optical transmitter for an optical communications system having a remote optical coherent receiver, the optical transmitter comprising:
a modulator, configured to modulate data onto an optical signal for transmission to the coherent receiver, the modulated signal having components at one or more constellations of points of different amplitudes and phases, the modulator being tunable such that distortions of the one or more constellations can be tuned; and
a tuning controller configured to:
receive a feedback signal from the coherent receiver indicating a distortion measured at the coherent receiver, and
automatically tune the modulator to adjust a driving voltage of the modulator based on the received feedback signal to pre-compensate for a measured distortion, wherein the modulator uses a modulation scheme having a same number of constellation points and same geometric pattern of constellation points before and after the automatic tuning, where the automatic tuning tunes the modulator to apply pre-compensation to one or more of the constellation points.

2. The optical transmitter of claim 1 , wherein the modulator is arranged to output a single carrier modulated signal, and wherein the modulator is tunable to adjust positions of at least some of the points in the constellation for that carrier.

3. The optical transmitter of claim 2 , wherein the measured distortion comprising distortion in the positions of at least some of the points of the constellation.

4. The optical transmitter of claim 3 , wherein the modulator is arranged to modulate the data at multiple amplitude levels.

5. The optical transmitter of claim 4 , having a feedback path comprising a receiver for receiving an incoming optical signal, and a frame decoder for detecting a frame in the incoming optical signal and detecting a feedback in the detected frame, and passing the feedback to the tuning controller.

6. The optical transmitter of claim 5 , wherein the distortion comprising an amplitude non linearity, and wherein the tuning controller is arranged to tune the modulator to alter the amplitudes to pre-compensate for the measured amplitude non linearity.

7. The optical transmitter of claim 2 , wherein the modulator comprising a Mach-Zehnder modulator having a multilevel input signal, and circuitry for generating the multilevel input signal from the data, and wherein the modulator is tunable such that output levels of the modulated signal can be tuned by the tuning controller.

8. The optical transmitter of claim 7 , wherein the circuitry comprising a digital gain control circuit for producing a multibit digital representation of each of the levels, tunable according to the tuning controller, and a digital to analog converter for converting the multibit digital representation into an analog multilevel signal for driving the Mach-Zehnder modulator.

9. The optical transmitter of claim 8 , wherein the circuitry further comprising a tunable digital to analog converter, tunable by the tuning controller.

10. The optical transmitter of claim 8 , wherein the circuitry further comprising a digital to analog converter and an analog gain controller tunable by the tuning controller.

11. The optical transmitter of claim 4 , wherein the modulator having two or more modulator elements, each for generating a different one of the amplitudes, an optical part for combining optical outputs of the two or more modulator elements, and a tunable optical level controller for controlling relative levels of the optical outputs, under the control of the tuning controller.

12. The optical transmitter of claim 11 , wherein the tuning controller is arranged to determine a new tuning setting for the modulator based on the feedback, and on characteristics of the modulator.

13. The optical transmitter of claim 12 , wherein the modulator comprising a tunable phase rotator, and wherein the tuning controller is coupled to control a tuning of the phase rotator if the feedback indicates a phase distortion.

14. The optical transmitter of claim 1 , wherein the modulator is arranged to output a multicarrier modulated signal.

15. The optical transmitter of claim 14 , wherein the tuning controller is arranged to tune the modulator according to a part of the measured distortion which correlates with nonlinearities in the modulator to pre-compensate for the nonlinearities.

16. A method for tuning a modulator of an optical transmitter for an optical communications system having a remote optical coherent receiver, the method comprising operations of:
modulating data onto an optical signal for transmission to the coherent receiver, the modulated signal having components at one or more constellations of points of different optical amplitudes and phases;
receiving feedback from the coherent receiver indicating distortion measured at the coherent receiver; and
tuning the modulating operation by adjusting a driving voltage used for modulation based on the received feedback to pre-compensate for the measured distortion, wherein the modulating operation uses a modulation scheme having a same number of constellation points and same geometric pattern of constellation points before and after the tuning, where the tuning tunes the modulation operation to apply pre-compensation to one or more of the constellation points.

17. The method of claim 16 , wherein modulating operation further comprising outputting a single carrier modulated signal, and wherein tuning operation further comprising adjusting positions of at least some of the points in the constellation for that carrier.

18. The method of claim 17 , wherein the measured distortion in the received feedback comprising distortion in the positions of at least some of the points of the constellation.

19. The method of claim 18 , wherein the modulating operation further comprising modulating the data at multiple amplitude levels.

20. The method of claim 18 , wherein the feedback is incorporated in an incoming optical signal, and wherein the method further comprising an operation of detecting a frame in the incoming optical signal and detecting a feedback in the frame.

21. The method of claim 19 , wherein the distortion comprising an amplitude nonlinearity, and wherein the method further comprising an operation of tuning the modulating operation to alter the amplitudes to pre-compensate for the measured amplitude nonlinearity.

22. The method of claim 20 , wherein the modulating operation further comprising using a Mach-Zehnder modulator having a multilevel input signal, and generating the multilevel input signal from the data, and wherein the tuning operation further comprising tuning of output levels of the modulated signal.

Assignees

• Magri Roberto
• Deho Alberto
• Ericsson Telefon Ab L M

Inventors

• Magri Roberto
• Deho Alberto

Classifications

• H04B10/58

  Compensation for non-linear transmitter output · CPC title

• H04B10/541 Primary

  Digital intensity or amplitude modulation · CPC title

• H04B10/50572

  to control the modulating signal amplitude including amplitude distortion · CPC title

• H04B10/516 Primary

  Details of coding or modulation · CPC title