Method and apparatus for stabilization of optical transmitter

Patent metadata

Field	Value
Publication number	US-9692518-B2
Application number	US-201314415602-A
Country	US
Kind code	B2
Filing date	Jul 17, 2013
Priority date	Jul 20, 2012
Publication date	Jun 27, 2017
Grant date	Jun 27, 2017

Abstract

A DP-QPSK optical transmitter includes an outer MZM comprising a first parent MZM comprising a first child MZM and a second child MZM that modulates a QPSK signal with a first polarization. A second parent MZM includes a first child MZM and a second child MZM that modulates a QPSK signal with a second polarization. The outer Mach-Zehnder modulator multiplexes the first and second polarization embedded into a dual-polarization QPSK signal generation. A first optical detector detects the QPSK signal generated by the first parent MZM with the first polarization. A second optical detector optical detects the QPSK signal generated by the second parent Mach-Zehnder modulator with the second polarization. A bias control circuit generates bias signals on at least one output that stabilize the DP-QPSK signal in response to signals generated by the first and second optical detector using electrical time division multiplexing.

First claim

We claim:

1. A dual polarization optical modulator comprising:
a) a first Mach-Zehnder modulator superstructure comprising two parallel child Mach-Zehnder optical modulators, the first Mach-Zehnder modulator superstructure modulating a first optical signal with a first polarization;
b) a second Mach-Zehnder modulator superstructure comprising two parallel child Mach-Zehnder optical modulators, the second Mach-Zehnder modulator superstructure being optically coupled in parallel with the first Mach-Zehnder modulator superstructure to form a dual polarization optical modulator having an optical input that receives an optical beam from a laser source, the second Mach-Zehnder modulator superstructure modulating a second optical signal with a second polarization and the dual polarization optical modulator multiplexing the first optical signal with the first polarization and the second optical signal with the second polarization to form a dual polarization optical signal;
c) a first optical detector coupled to an output of the first Mach-Zehnder modulator superstructure;
d) a second optical detector coupled to an output of the second Mach-Zehnder modulator superstructure;
e) a third optical detector connected to an output of the dual polarization optical modulator; and
f) a bias control circuit having a first, second, and third input electrically connected to an output of a respective one of the first, second, and third optical detectors and a first output electrically connected to a bias input of the first Mach-Zehnder modulator superstructure and a second output electrically connected to a bias input of the second Mach-Zehnder modulator superstructure, the bias control circuit generating bias signals on at least one of the first and second outputs in response to both DC and AC signals detected by at least one of the first, second, and third optical detectors that stabilizes the dual polarization signal, wherein the bias control circuit generates an electrical signal on at least one of the first and second outputs that maximizes average DC photocurrent generated by at least one of the first, second, and third optical detectors.

2. The dual polarization optical modulator of claim 1 wherein the laser source comprises a tunable laser source.

3. The dual polarization optical modulator of claim 1 wherein the first and second Mach-Zehnder modulator superstructures comprise Mach-Zehnder modulators formed of lithium niobate.

4. The dual polarization optical modulator of claim 1 wherein the first and second Mach-Zehnder modulator superstructures comprise Mach-Zehnder modulators formed of a III-V semiconductor.

5. The dual polarization optical modulator of claim 1 wherein the first and second Mach-Zehnder modulator superstructures comprise Mach-Zehnder modulators formed of a II-VI semiconductor.

6. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises a proportional integral controller.

7. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises a pure integral controller.

8. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises an analog processor that generates a voltage on at least one of the first and second outputs that corresponds to a DC photocurrent generated by at least one of the first, second, and third optical detectors.

9. The dual polarization optical modulator of claim 1 wherein the bias control circuit generates electrical signals on at least one of the first and second outputs that stabilize the first and second Mach-Zehnder modulator superstructure to a quadrature point.

10. The dual polarization optical modulator of claim 1 wherein at least one of the first, second, and third optical detectors comprises a PIN photodetector with an optical-electrical bandwidth of a least 1 GHz.

11. The dual polarization optical modulator of claim 1 wherein at least one of the first, second, and third optical detectors comprises a PIN photodetector with an optical-electrical coupling efficiency of at least 0.05%.

12. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises a digital processor and an analog-to-digital converter, the analog-to-digital converter converting AC electrical signals generated by at least one of the first, second, and third optical detector in response to modulated digital data, the digital processor processing the AC electrical signals to determine bias signals that are generated by at least one of the first and the second outputs that stabilize the dual polarization modulator.

13. The dual polarization optical modulator of claim 1 wherein the bias control circuit monitors a pilot tone received by at least one of the first, second, and third optical detectors and generates a bias control signal on at least one of the first and second outputs at a time that stabilize the dual polarization optical modulator in response to a monitored pilot tone.

14. The dual polarization optical modulator of claim 1 wherein the bias control circuit generates a bias control signal on at least one of the first and second outputs that either maximizes or minimizes DC and minimizes AC signals detected by the first, second, and third optical detectors.

15. The dual polarization optical modulator of claim 1 wherein the bias control circuit generates a bias control signal on at least one of the first and second outputs that minimize a fundamental harmonic of an AC pilot tone applied to a bias signal and that is detected by at least one of the first, second, and third optical detectors.

16. The dual polarization optical modulator of claim 1 wherein the bias control circuit generates an electrical signal on at least one of the first and second outputs that suppresses a fundamental harmonic of a pilot tone.

17. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises a first analog processor connected to the first optical detector, a second analog processor connected to the second optical detector, and a third analog processor connected to the output of the third optical detector.

18. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises a first differential amplifier with a non-inverting output and an inverse output electrically connected to the bias input of the first Mach-Zehnder modulator superstructure and a second differential amplifier with a non-inverting output and an inverse output electrically connected to the bias input of the second Mach-Zehnder modulator superstructure.

19. The dual polarization optical modulator of claim 1 wherein the bias control circuit comprises at least one analog processor or at least one digital processor that calculates an error value of a difference between a measured value at an input and a desired setpoint and corrects the bias signals generated in response to the errors.

Assignees

• Finisar Corp

Inventors

• Bhandare Suhas P
• Ereifej Heider N
• Kim Kyu
• Colyar Mark

Classifications

• H04B10/5561

  Digital phase modulation · CPC title

• H04J14/06

  Polarisation multiplex systems · CPC title

• H04B10/50575

  to control the modulator DC bias · CPC title

• H04B10/532

  Polarisation modulation · CPC title

• H04B10/5053 Primary

  using a parallel, i.e. shunt, combination of modulators · CPC title