Optical Transmission Device

• US 20080075470A1
• Filed: 12/26/2005
• Published: 03/27/2008
• Est. Priority Date: 01/25/2005
• Status: Active Grant

• Current Assignee: Panasonic Corporation (Panasonic Holdings Corporation)
• Original Assignee: Panasonic Corporation (Panasonic Holdings Corporation)
• Inventors: Ohira, Tomoaki, Masuda, Kouichi
• Granted Patent: US 8,000,612 B2
• Time in Patent Office: Days
• US Class Current: 398/183
• CPC Class Codes: G02F 1/0123 Circuits for the control or stabilisation of the bias voltage, e.g. automatic bias control [ABC] feedback loops
• CPC Class Codes: G02F 2203/21 Thermal instability, i.e. DC drift, of an optical modulator; Arrangements or methods for the reduction thereof
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/5053 using a parallel, i.e. shunt, combination of modulators
• CPC Class Codes: H04B 10/50575 to control the modulator DC bias
• CPC Class Codes: H04B 10/5165 Carrier suppressed; Single sideband; Double sideband or vestigial

Abstract

An optical transmission device is provided with an external optical modulator including three or more Mach-Zehnder interferometers and outputs a single-sideband optical intensity-modulated signal with suppressed optical carrier component, wherein an optical carrier component and an unnecessary one sideband component contained in the outputted optical signal are suppressed. A light branching section (51) branches an optical signal outputted by an external optical modulator (4) into two signals. A photodetector section (52) converts one of the optical signals to an electric signal. An optical carrier component extracting section (54) extracts an optical carrier component signal that is in the vicinity of a frequency f₁ from the electric signal. An optical carrier component suppressing section (56) applies a bias voltage for suppressing the optical carrier component to the external optical modulator (4) based on the signal level of the optical carrier component. A vestigial sideband component extracting section (57) extracts a vestigial sideband component signal in the vicinity of a frequency 2×f₁ from the electric signal. A vestigial sideband component suppressing section (59) applies a bias voltage for suppressing an unnecessary one sideband component to the external optical modulator (4) based on the signal level of the vestigial sideband component.

Claims

1. An optical transmission device for outputting a single-sideband optical intensity-modulated signal with suppressed optical carrier component, the optical transmission device comprising a first, second and third Mach-Zehnder interferometer, each of the first and second Mach-Zehnder interferometers receiving one of two optical signals obtained by branching an optical carrier having a frequency f₀ outputted from a light source and outputting a phase-modulated signal obtained by modulating a phase of the input optical signal with an inputted electric signal having a frequency f₁, and the third Mach-Zehnder interferometer for further phase-modulating and combining together the two phase-modulated signals outputted from the first and second Mach-Zehnder interferometers, the optical transmission device comprising:

a light branching section for branching the optical intensity-modulated signal into an optical signal transmitted along an optical transmission path and a monitoring optical signal;

a photodetector section for converting the monitoring optical signal to an electric signal and outputting the electric signal as a monitoring signal;

a branching section for branching the monitoring signal into two signals;

an optical carrier component extracting section for passing therethrough only a signal component of one of the branched monitoring signals that is in a vicinity of the frequency f₁;

an optical carrier component level detecting section for detecting a level of the signal outputted by the optical carrier component level extracting section to output an optical carrier component monitoring signal according to the detected level;

a vestigial sideband component extracting section for passing therethrough only a signal component of the other one of the branched monitoring signals that is in a vicinity of a frequency 2×

f₁;

a vestigial sideband component level detecting section for detecting a level of the signal outputted by the vestigial sideband component level extracting section to output a vestigial sideband component monitoring signal according to the detected level; and

a suppressing section for controlling bias voltages to be applied to the first and second Mach-Zehnder interferometers based on the optical carrier component monitoring signal so as to suppress an optical carrier component of the optical intensity-modulated signal, and for controlling a bias voltage to be applied to the third Mach-Zehnder interferometer based on the vestigial sideband component monitoring signal so as to suppress an unnecessary one sideband component of the optical intensity-modulated signal.

2. The optical transmission device according to claim 1, wherein:

the suppressing section comprises;

an optical carrier component suppressing section for controlling bias voltages to be applied to the first and second Mach-Zehnder interferometers based on the inputted optical carrier component monitoring signal; and

a vestigial sideband component suppressing section for controlling a bias voltage to be applied to the third Mach-Zehnder interferometer based on the inputted vestigial sideband component monitoring signal, wherein;

the optical carrier component suppressing section comprises;

a first bias voltage supply section for supplying a bias voltage to be applied to the first Mach-Zehnder interferometer;

a second bias voltage supply section for supplying a bias voltage to be applied to the second Mach-Zehnder interferometer;

an optical carrier component reference value storing section for storing a reference value for a level of the optical carrier component monitoring signal;

an optical carrier component level storing section for storing a level of the inputted optical carrier component monitoring signal;

a first bias voltage storing section for storing the bias voltages supplied by the first and second voltage supply sections;

an optical carrier component level comparing section for comparing a level of the newly-inputted optical carrier component monitoring signal with the reference value stored in the optical carrier component reference value storing section, and for comparing the level of the newly-inputted optical carrier component monitoring signal with the level of the optical carrier component monitoring signal stored in the optical carrier component level storing section; and

a first voltage control section for controlling bias voltages to be applied to the first and second Mach-Zehnder interferometers based on a comparison result from the optical carrier component level comparing section; and

the vestigial sideband component detecting section comprises;

a third bias voltage supply section for supplying a bias voltage to be applied to the third Mach-Zehnder interferometer;

a vestigial sideband component reference value storage section for storing a reference value for a level of the vestigial sideband component monitoring signal;

a vestigial sideband component level storing section for storing a level of the inputted vestigial sideband component monitoring signal;

a third bias voltage storing section for storing the bias voltage supplied by the third voltage supply section;

a vestigial sideband component level comparing section for comparing a level of the newly-inputted vestigial sideband component monitoring signal with the reference value stored in the vestigial sideband component reference value storage section, and for comparing the level of the newly-inputted vestigial sideband component monitoring signal with the level of the vestigial sideband component monitoring signal stored in the vestigial sideband component level storing section; and

a second voltage control section for controlling a bias voltage to be applied to the third Mach-Zehnder interferometer based on a comparison result from the vestigial sideband component level comparing section.

3. The optical transmission device according to claim 1, wherein the suppressing section comprises:

a first bias voltage supply section for supplying a bias voltage to be applied to the first Mach-Zehnder interferometer;

a second bias voltage supply section for supplying a bias voltage to be applied to the second Mach-Zehnder interferometer;

a third bias voltage supply section for supplying a bias voltage to be applied to the third Mach-Zehnder interferometer;

an optical carrier component reference value storing section for storing a reference value for a level of the inputted optical carrier component monitoring signal;

a vestigial sideband component reference value storage section for storing a reference value for a level of the inputted vestigial sideband component monitoring signal;

a monitoring signal level storing section for storing the level of the optical carrier component monitoring signal and that of the vestigial sideband component monitoring signal;

a bias voltage storing section for storing the bias voltages supplied by the first and second voltage supply sections;

a monitoring signal level comparing section for comparing level of the newly-inputted optical carrier component monitoring signal with the reference value stored in the optical carrier component reference value storing section and the level of the optical carrier component stored in the optical carrier component level storing section, and for storing the level of the newly-inputted vestigial sideband component monitoring signal with the reference value stored in the vestigial sideband component reference value storage section and the level of the optical carrier component monitoring signal stored in the vestigial sideband component level storing section; and

a voltage control section for controlling bias voltages to be applied to the first, second and third Mach-Zehnder interferometers based on a result from the monitoring signal level comparing section.

4. The optical transmission device according to claim 1, wherein the bias voltage control section controls bias voltages to be applied to the first and second Mach-Zehnder interferometers so that the level of the optical carrier component monitoring signal is less than or equal to the reference value, and then controls a bias voltage to be applied to the third Mach-Zehnder interferometer so that the level of the vestigial sideband component monitoring signal is less than or equal to the reference value.

5. The optical transmission device according to claim 1, wherein the bias voltage control section controls a bias voltage to be applied to the third Mach-Zehnder interferometer so that the level of the vestigial sideband component monitoring signal is less than or equal to the reference value, and then controls bias voltages to be applied to the first and second Mach-Zehnder interferometers so that the level of the optical carrier component monitoring signal is less than or equal to the reference value.

6. The optical transmission device according to claim 1, wherein:

the light branching section is a directional coupler provided in the third Mach-Zehnder interferometer for making two of the phase-modulated signals, which have been further phase-modulated, interfere with each other and combining the phase-modulated signals together, so as to output two of the optical intensity-modulated signals; and

the optical transmission device further comprises a first port for outputting one of the optical intensity-modulated signals outputted by the directional coupler to an optical transmission path, and a second port for outputting the other one of the optical intensity-modulated signals outputted by the directional coupler to the photodetector section.

7. The optical transmission device according to claim 1, wherein the light branching section branches the optical intensity-modulated signal so that an intensity of the optical signal transmitted along an optical transmission path is greater than that of the monitoring optical signal.

9. The optical transmission device according to claim 3, wherein the bias voltage control section controls bias voltages to be applied to the first and second Mach-Zehnder interferometers so that the level of the optical carrier component monitoring signal is less than or equal to the reference value, and then controls a bias voltage to be applied to the third Mach-Zehnder interferometer so that the level of the vestigial sideband component monitoring signal is less than or equal to the reference value.

10. The optical transmission device according to claim 3, wherein the bias voltage control section controls a bias voltage to be applied to the third Mach-Zehnder interferometer so that the level of the vestigial sideband component monitoring signal is less than or equal to the reference value, and then controls bias voltages to be applied to the first and second Mach-Zehnder interferometers so that the level of the optical carrier component monitoring signal is less than or equal to the reference value.

11. The optical transmission device according to claim 2, wherein the light branching section branches the optical intensity-modulated signal so that an intensity of the optical signal transmitted along an optical transmission path is greater than that of the monitoring optical signal.

12. The optical transmission device according to claim 3, wherein the light branching section branches the optical intensity-modulated signal so that an intensity of the optical signal transmitted along an optical transmission path is greater than that of the monitoring optical signal.

8. A method for suppressing an optical carrier component and an unnecessary one sideband component of a single-sideband optical intensity-modulated signal with suppressed optical carrier component, which is outputted from an optical transmission device, the optical transmission device comprising a first, second and third Mach-Zehnder interferometer, each of the first and second Mach-Zehnder interferometers receiving one of two optical signals obtained by branching an optical carrier having a frequency f₀ outputted from a light source and outputting a phase-modulated signal obtained by modulating a phase of the input optical signal with an inputted electric signal having a frequency f₁, and the third Mach-Zehnder interferometer for further phase-modulating and combining together the two phase-modulated signals outputted from the first and second Mach-Zehnder interferometers, the method comprising the steps of:

branching the optical intensity-modulated signal into an optical signal transmitted along an optical transmission path and a monitoring optical signal;

converting the monitoring optical signal to an electric signal and outputting the electric signal as a monitoring signal;

branching the monitoring signal into two signals;

passing only a signal component of one of the branched monitoring signals that is in a vicinity of the frequency f₁;

detecting a level of the signal component in a vicinity of the frequency f₁ and outputting an optical carrier component monitoring signal according to the detected level;

passing only a signal component of the other one of the branched monitoring signals that is in a vicinity of a frequency 2×

f₁;

detecting a level of the signal component in a vicinity of the frequency 2×

f₁ and outputting a vestigial sideband component monitoring signal according to the detected level; and

controlling bias voltages to be applied to the first and second Mach-Zehnder interferometers based on the optical carrier component monitoring signal so as to suppress the optical carrier component, and controlling a bias voltage to be applied to the third Mach-Zehnder interferometer based on the vestigial sideband component monitoring signal so as to suppress the unnecessary one sideband component.