Apparatus and method for monitoring signal quality of a modulated optical signal

Patent metadata

Field	Value
Publication number	US-9729244-B2
Application number	US-201414518262-A
Country	US
Kind code	B2
Filing date	Oct 20, 2014
Priority date	Oct 20, 2014
Publication date	Aug 8, 2017
Grant date	Aug 8, 2017

Abstract

A transmitter with at least one optical modulator adapted to modulate the optical signal output by a laser source to generate a modulated optical signal, wherein the optical signal output by the laser source is tapped and supplied to a monitoring circuit comprising an optical front end configured to select signal components of the tapped modulated optical signal and to convert the selected signal components of the tapped modulated optical signal into analog signals, and comprising at least one analog-to-digital converter, ADC, adapted to perform equivalent-time sampling of the analog signals to provide digital signals processed by a processing unit to monitor signal quality of the modulated optical signal.

First claim

The invention claimed is:

1. A transmitter with at least one optical modulator adapted to modulate the optical signal output by a laser source to generate a modulated optical signal, wherein the optical signal output by the laser source is tapped and supplied to a monitoring circuit, comprising:
an optical front end configured to select signal components of a tapped modulated optical signal, via optical coherent detection, and to convert the selected signal components of the tapped modulated optical signal into analog signals; and
at least one analog-to-digital converter (ADC) adapted to perform equivalent-time sampling of the analog signals to provide digital signals to be processed by a processing unit to monitor signal quality of the modulated optical signal,
wherein the unmodulated optical signal output by the laser source is tapped by a first polarization maintaining coupler and supplied as a reference signal to the optical front end of the monitoring circuit,
wherein the modulated optical signal output by the optical modulator is tapped by a second polarization maintaining coupler and supplied to the optical front end of the monitoring circuit, and
wherein the unmodulated optical signal tapped by the first polarization maintaining coupler is supplied as a reference signal to a first polarization beam splitter of the optical front end, and the modulated optical signal tapped by the second polarization maintaining coupler is supplied to a second polarization beam splitter of the optical front end.

2. The transmitter according to claim 1 , wherein the optical modulator is configured to modulate an amplitude and/or a phase and/or a polarization of the optical signal output by said laser source to generate the modulated optical signal output by said transmitter.

3. The transmitter according to claim 1 , wherein the processing unit of said monitoring circuit comprises a digital signal processor, DSP, adapted to calculate a pre-distortion function and/or correction parameters to be used by the transmitter to compensate for nonlinear transfer functions and/or bandwidth limitations of transmitter components.

4. The transmitter according to claim 3 , wherein the monitoring circuit supplies a feedback control signal to adjust driving signals applied to the optical modulator according to the calculated pre-distortion function and/or correction parameters.

5. The transmitter according to claim 3 , wherein the calculated optimal pre-distortion function and/or correction parameters are continuously adjusted by the monitoring circuit to compensate changes of characteristics of transmitter components due to time-dependent variations of operation parameters and/or aging of transmitter components.

6. The transmitter according to claim 1 , wherein: a first split signal output by the first polarization beam splitter is supplied to a first input of a first 90-degree hybrid coupler of the optical front end and a second split signal output by the first polarization beam splitter is supplied to a second input of a second 90-degree hybrid coupler of the optical front end, and a first split signal output by the second polarization beam splitter is supplied to a second input of the first 90-degree hybrid coupler of the optical front end and the second split signal output by the second polarization beam splitter is supplied to a first input of the second 90-degree hybrid coupler of the optical front end.

7. The transmitter according to claim 6 , wherein: the first 90-degree hybrid coupler of the optical front end outputs an in-phase signal component and a quadrature signal component in a first polarization state to two photo detectors converting both optical signal components into corresponding analog signals, and the second 90-degree hybrid coupler of the optical front end outputs an in-phase signal component and a quadrature signal component in a second polarization state to two photo detectors converting both optical signal components into corresponding analog signals.

8. The transmitter according to claim 7 , wherein the analog signals provided by the four photo detectors of the optical front end are applied to four analog-to-digital converters adapted to perform equivalent time sampling of the received analog signals to provide digital signals processed by the processing unit of the monitoring circuit.

9. The transmitter according to claim 1 , wherein the unmodulated optical signal tapped by the first polarization maintaining coupler is supplied as a reference signal to a polarization rotator of the optical front end configured to either not rotate the polarization axis of the tapped unmodulated optical signal or to rotate said axis by 90 degrees.

10. The transmitter according to claim 9 , wherein:
the at least one analog-to-digital converter includes two analog-to-digital converters; and
the rotated or un-rotated tapped unmodulated optical signal output by the polarization rotator is supplied to a first input of a 90-degree hybrid coupler of the optical front end and the tapped modulated optical signal output by a second polarization maintaining coupler is supplied to a second input of said 90-degree hybrid coupler which outputs an in-phase signal component and a quadrature signal component in a selected polarization state to two photo detectors converting the optical signals to analog signals applied to the two analog-to-digital converters, said two analog-to-digital converters adapted to perform equivalent-time sampling of the received analog signals to provide digital signals processed by the processing unit of the monitoring circuit.

11. The transmitter according to claim 1 , wherein the unmodulated optical signal tapped by the first polarization maintaining coupler is supplied as a reference signal to a polarization rotator of the optical front end configured to either not rotate the polarization axis of the tapped unmodulated optical signal or to rotate said axis by 90 degrees and to shift simultaneously the optical phase of the tapped unmodulated optical signal by either 0 degrees or 90 degrees.

12. The transmitter according to claim 11 , wherein the rotated or un-rotated unmodulated optical signal output by the polarization rotator is mixed by means of an optical coupler with the modulated optical signal tapped by a second polarization maintaining coupler and supplied to a single photo detector of the optical front end converting the mixed optical signal to an analog signal applied to a single analog-to-digital converter adapted to perform equivalent-time sampling of the received analog signal to provide a corresponding digital signal processed by the processing unit of the monitoring circuit.

13. The transmitter according to claim 1 , wherein the processing unit is configured to construct a transfer function of the transmitter used to calculate a pre-distortion function to be used by the transmitter.

14. The transmitter according to claim 1 , wherein the monitoring circuit comprises an interface to report parameters related to the monitored signal quality of the modulated optical signal output by the transmitter to a network operator.

15. The transmitter according to claim 1 , wherein the optical modulator comprises at least one Mach-Zehnder modulator MZM.

16. The transmitter according to claim 1 , wherein the modulated optical signal output by the transmitter is an optical polarization division multiplexed, PDM, quadrature amplitude modulation, QAM, signal.

17. The transmitter according to claim 1 , wherein the modulated optical signal output by the transmitter comprises a data rate of more than

Assignees

• Adva Optical Networking Se

Inventors

• Alfiad Mohammad
• Tibuleac Sorin

Classifications

• H04B10/50575

  to control the modulator DC bias · CPC title

• H04B10/588

  in external modulation systems · CPC title

• H04B10/50593

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

• H04B10/615

  Arrangements affecting the optical part of the receiver · CPC title

• H04B10/50595

  to control the modulator DC bias · CPC title