Bias control of optical modulators

Patent metadata

Field	Value
Publication number	US-11086187-B2
Application number	US-201916683451-A
Country	US
Kind code	B2
Filing date	Nov 14, 2019
Priority date	Mar 15, 2017
Publication date	Aug 10, 2021
Grant date	Aug 10, 2021

Abstract

An optical waveguide modulator with automatic bias control is disclosed. A dither signal is applied to the modulator bias and its signature detected in light tapped from an output of the modulator using a phase sensitive dither detector such as a lock-in amplifier. The detected signal is processed using pre-recorded information defining the direction of the detected signal change relative to a change in the modulator bias, and the bias is adjusted in the direction determined using the information. An IQ bias of a quadrature modulator is controlled by dithering bias settings of two inner modulators at different dither frequencies, and detecting an oscillation at a sum frequency.

First claim

We claim:

1. An optical modulator apparatus comprising:
a substrate;
an optical modulator circuit formed in or upon the substrate and comprising:
an input optical port for receiving input light;
an output optical port configured to provide output modulator light and tapped light; and
one or more bias electrodes configured to control a modulator set point; and,

a bias control circuit electrically coupled to the one or more bias electrodes and comprising:
a controller configured to provide one or more bias control signals to the one or more bias electrodes, the one or more bias control signals comprising a first dither signal at a first dither frequency f1 and a first DC bias signal; and,
a phase-sensitive dither detection circuit comprising a photodetector (PD) disposed to receive the tapped light, the phase-sensitive dither detection circuit configured to detect in the tapped light a first oscillation at the first dither frequency f1, and to communicate to the controller two first feedback signals that represent two quadrature components of the first oscillation;
wherein the controller is configured to adjust the first DC bias signal based on the two first feedback signals.

2. The optical modulator apparatus of claim 1 wherein the phase-sensitive dither detection circuit comprises a lock-in detector electrically connected to the PD.

3. The optical modulator apparatus of claim 1 comprising memory configured to store first slope information that is indicative of a direction of change of at least one of the two first feedback signals relative to a change in the first DC bias signal, wherein the controller is configured to read the first slope information from the memory and to adjust the first DC bias signal in a direction determined using the first slope information.

4. The optical modulator apparatus of claim 3 , wherein the controller is further configured to:
collect first bias scan data by varying the first DC bias signal in a vicinity of a target set-point thereof;
record the two first feedback signals for two or more values of the first DC bias signal; and,
process the first bias scan data for determining the first slope information.

5. The optical modulator apparatus of claim 1 wherein:
the one or more bias electrodes comprises a first bias electrode, a second bias electrode, and a third bias electrode;
the optical modulating circuit comprises a first optical modulator including the first bias electrode and a second optical modulator including the second bias electrode; and,
the first and second optical modulators are connected in parallel forming an outer Mach-Zehnder interferometer (MZI) structure comprising the third bias electrode;
wherein the one or more bias control signals further comprise a second DC bias signal, a second dither signal at a second dither frequency f2, and a third DC bias signal; and
wherein the controller is configured to provide the first, second, and third DC bias signals to the first, second, and third bias electrodes.

6. The optical modulator apparatus of claim 5 wherein the controller is configured to:
provide the first dither signal to the first bias electrode and the second dither signal to the second bias electrode;
detect, from an output of the phase-sensitive dither detection circuit, a second oscillation at the second dither frequency; and
output two second feedback signals that represent two quadrature components of the second oscillation.

7. The optical modulator apparatus of claim 6 comprising memory configured to store first slope information that is indicative of a direction of change of at least one of the two first feedback signals relative to a change in the first DC bias signal, and to store second slope information that is indicative of a direction of change of at least one of the two second feedback signals relative to a change in the second DC bias signal; and,
wherein the controller is configured to read at least one of the first slope information or the second slope information from the memory, and to adjust at least one of the first DC bias signal or the second DC bias signal in a direction determined using at least one of the first slope information or the second slope information.

8. The optical modulator apparatus of claim 7 wherein the controller is configured to:
vary the first DC bias signal in a vicinity of a target set point thereof while keeping the second DC bias signal constant, and record the two first feedback signals for two or more values of the first DC bias signal;
vary the second DC bias signal in a vicinity of a target set point thereof while keeping the first DC bias signal constant, and record the two second feedback signals for two or more values of the second DC bias signal;
record first slope information indicating a direction of change of the two first feedback signals relative to a change in the first DC bias signal; and,
record second slope information indicating a direction of change of the two second feedback signals relative to a change in the second DC bias signal.

9. The optical modulator apparatus of claim 7 wherein:
the second dither frequency f2 is greater than the first dither frequency f1 and differs from any harmonic thereof;
the phase-sensitive dither detection circuit comprises a lock-in detector connected to the PD and configured to detect, in an electrical PD signal received from the PD, a dither product signal at a sum frequency fsum=(f1+f2) and to obtain at least one third feedback signal that is responsive to a phase of said dither product signal;
the memory is configured to store third slope information that is indicative of a direction of change of the at least one third feedback signal relative to a change in the third DC bias signal, and
wherein the controller is further configured to adjust the third DC bias signal in a direction determined using the third slope information and the third feedback signal.

10. The optical modulator apparatus of claim 9 wherein the controller is configured to
collect third bias scan data by varying the third DC bias signal in a vicinity of a target set-point thereof, and record the at least one third feedback signal for two or more values of the third DC bias signal; and,
process the third bias scan data to determine the third slope information.

11. The optical modulator apparatus of claim 2 wherein the controller is configured to receive, from the lock-in detector, the two first feedback signals comprising separate cosine and a sine components of the first oscillation.

12. The optical modulator apparatus of claim 9 wherein the controller is configured to receive, from the lock-in detector, two third dither signals comprising separate cosine and a sine components of the dither product signal.

13. The optical modulator apparatus of claim 6 wherein the phase-sensitive dither detection circuit comprises a lock-in detector electrically connected to the PD, and wherein the controller is configured to receive, from the lock-in detector, the two first feedback signals comprising separate cosine and sine components of the first oscillation, and the two second feedback signals comprising separate cosine and sine components of the second oscillation.

Assignees

• Elenion Tech Llc
• Nokia Solutions & Networks Oy

Inventors

• Padmaraju Kishore
• Baehr-Jones Thomas Wetteland
• Rohde Bernd-Harald Horst Jurgen
• Palmer Robert
• Streshinsky Matthew Akio
• Bohn Marc
• Wuth Torsten

Classifications

• G02F1/212

  Mach-Zehnder type · CPC title

• G02F1/225 Primary

  in an optical waveguide structure · CPC title

• G02F1/0123

  Circuits for the control or stabilisation of the bias voltage, e.g. automatic bias control [ABC] feedback loops · CPC title

• H04B10/516

  Details of coding or modulation · CPC title