Control apparatus and control method for an optical modulator

• US 20030184838A1
• Filed: 03/20/2003
• Published: 10/02/2003
• Est. Priority Date: 03/26/2002
• Status: Active Grant

• Current Assignee: Fujitsu Limited
• Original Assignee: Fujitsu Limited
• Inventors: Akiyama, Yuichi, Ishikawa, George
• Granted Patent: US 6,809,849 B2
• Time in Patent Office: Days
• US Class Current: 359/239
• CPC Class Codes: G02F 1/0121 Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
• CPC Class Codes: G02F 1/0322 Arrangements comprising two or more independently controlled crystals
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/5051 using a series, i.e. cascade, combination of modulators
• CPC Class Codes: H04B 10/5057 using a feedback signal generated by analysing the optical output
• CPC Class Codes: H04B 10/50577 to control the phase of the modulating signal
• CPC Class Codes: H04B 10/5162 Return-to-zero modulation schemes
• CPC Class Codes: H04B 10/5165 Carrier suppressed; Single sideband; Double sideband or vestigial

Abstract

The present invention aims at providing a control technique for an optical modulator, which can accurately detect a phase shift between signals of a drive system of an optical modulator, to feedback control. To this end, a control apparatus of the present invention comprises, for example, in an optical modulator which generates a signal light corresponding to the CS-RZ modulation method by two LN modulators connected in series: a monitor section that extracts a specific frequency component from a spectrum of the CS-RZ signal light output from a latter LN modulator to detect the optical intensity thereof, and a control CPU that determines a phase shift between first and second drive signals corresponding to a clock signal supplied to the latter LN modulator based on the optical intensity detected by the monitor section, and controls a phase difference between the drive signals so that the phase shift is minimized.

Claims

1. A control apparatus for an optical modulator which generates a signal light corresponding to the CS-RZ modulation method, comprising:

a monitor section that extracts a specific frequency component from a spectrum of the signal light output from said optical modulator to detect the optical intensity thereof; and

a control section that determines a phase shift between a plurality of drive signals supplied to said optical modulator based on the optical intensity detected by said monitor section, and controls a phase difference between said drive signals so that said phase shift is minimized.

2. A control apparatus for an optical modulator according to claim 1,

wherein said optical modulator includes a data side optical modulation section to which a drive signal corresponding to a data signal is supplied, and a clock side optical modulation section to which at least two drive signals corresponding to a clock signal with a frequency of ½

times the bit rate of said data signal are supplied, and

said control section determines, based on the optical intensity detected in said monitor section, at least one of a phase shift between the drive signals supplied to said clock side optical modulation section, and a phase shift between the data signal and the clock signal, to feedback control a phase difference between these signals so that said phase shift is minimized.

3. A control apparatus for an optical modulator according to claim 2,

wherein when the feedback control for the phase shift between the drive signals supplied to said clock side optical modulator is performed,

said monitor section includes an optical filter capable of extracting a frequency component apart by n×

B Hz from a central frequency of the spectrum of the signal light output from said optical modulator, where the bit rate of the data signal is Bb/s and n is a positive number of one or greater, and detects the optical intensity of the frequency component extracted from said signal light by said optical filter, and

said control section feedback controls the phase difference between the drive signals supplied to said clock side optical modulation section so that the optical intensity detected by said monitor section is minimized.

4. A control apparatus for an optical modulator according to claim 2,

wherein when the feedback control for the phase shift between the drive signals supplied to said clock side optical modulator is performed,

said monitor section includes a first optical filter capable of extracting a frequency component apart by −

(2n−

1)×

B/2 Hz from a central frequency of the spectrum of the signal light output from said optical modulator, and a second optical filter capable of extracting a frequency component apart by +(2n−

1)×

B/2 Hz from the central frequency of the spectrum of said signal light, where the bit rate of the data signal is Bb/s and n is a positive number of one or greater, and detects the optical intensities of the respective frequency components extracted from said signal light by said optical filters, and

said control section feedback controls the phase difference between the drive signals supplied to said clock side optical modulation section so that the respective optical intensities detected by said monitor section are approximately equal.

5. A control apparatus for an optical modulator according to claim 2,

wherein when the feedback control for the phase shift between the data signal and the clock signal is performed,

said monitor section includes an optical filter capable of extracting a frequency component coinciding with a central frequency of the spectrum of the signal light output from said optical modulator, and measures the optical intensity of the frequency component extracted from said signal light by said optical filter, and

said control section feedback controls the phase difference between the data signal and the clock signal so that the optical intensity measured by said monitor section is maximized.

6. A control apparatus for an optical modulator according to claim 1,

wherein said monitor section includes an optical power meter measuring the optical intensity of a specific frequency component extracted from the spectrum of the signal light output from said optical modulator.

7. A control apparatus for an optical modulator according to claim 1,

wherein said monitor section includes a light receiving element that receives a specific frequency component extracted from the spectrum of the signal light output from said optical modulator, to generate an electrical signal corresponding to the optical intensity, and an electric power sensor that measures the power of electrical signal generated by said light receiving element.

8. A control apparatus for an optical modulator comprising:

an optical modulator incorporating a section for branching an optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a section for combining these first and second branch optical waveguides, that controls the refractive indexes of said first and second branch optical waveguides using a first electrode and a second electrode which are provided in said first and second branch optical waveguides respectively, to obtain periodic optical intensity characteristics corresponding to a difference between the refractive indexes;

a drive circuit that applies a voltage to said first and second electrodes so that a modulation operation is performed based on one cycle of optical intensity characteristics of said optical modulator;

a phase controller that controls a phase of said drive circuit; and

a detector that detects a specific optical wavelength component in an output of said optical modulator,

wherein the phase of said drive circuit is controlled by said phase controller based on the detection result from said detector.

9. An optical transmitter for transmitting a signal light corresponding to the CS-RZ modulation method, comprising:

a light source that generates continuous light;

a modulator that modulates the continuous light from said light source, in accordance with the CS-RZ modulation method;

a monitor section that extracts a specific frequency component from a spectrum of the signal light output from said optical modulator to detect the optical intensity thereof; and

a control section that determines a phase shift between a plurality of drive signals supplied to said optical modulator based on the optical intensity detected by said monitor section, and controls a phase difference between said drive signals so that said phase shift is minimized.

10. A method of controlling an optical modulator which generates a signal light corresponding to the CS-RZ modulation method, comprising:

extracting a specific frequency component from a spectrum of the signal light output from said optical modulator to detect the optical intensity thereof;

determining a phase shift between a plurality of drive signals supplied to said optical modulator, based on said optical intensity; and

controlling a phase difference between said drive signals so that said phase shift is minimized.