# US 10,122,507 B2 · Optical transmitter and method of controlling optical modulator

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- Number: US 10,122,507 B2
- Document ID: US10122507B2
- Status: Granted
- Kind: B2
- Application number: 15/172,289
- Filed: 2016-06-03
- Published: 2017-02-02
- Granted: 2018-11-06
- Earliest claimed: 2015-07-28
- Assignees: Fujitsu Optical Components Limited
- Applicants: Fujitsu Optical Components Limited
- Inventors: Hiroshi Nishimoto
- Web page: https://patentgrep.com/patent/US10122507B2

## CPC

- H04L 5/0048: {Allocation of pilot signals, i.e. of signals known to the receiver (allocation of control signalling H04L5/0053; use of control signalling H04L5/0091)}
- H04B 10/5053: {using a parallel, i.e. shunt, combination of modulators}
- H04B 10/50572: {to control the modulating signal amplitude including amplitude distortion}
- H04B 10/50575: {to control the modulator DC bias}
- H04L 5/0008: {Wavelet-division}
- H04L 27/0008: {arrangements for allowing a transmitter or receiver to use more than one type of modulation (negotiating modulation type for two-way transmission paths H04L5/1453)}
- H04L 27/2096: {Arrangements for directly or externally modulating an optical carrier (optical modulation H04B10/503)}
- H04L 27/36: Modulator circuits; Transmitter circuits
- H04B 2210/075: using a pilot tone
- H04L 27/18: Phase-modulated carrier systems, i.e. using phase-shift keying (H04L27/32 takes precedence)
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)
- H04B: TRANSMISSION

## Abstract

An optical transmitter has an optical modulator with a Mach-Zehnder interferometer, a pilot signal generator configured to generate a pilot signal to be superimposed on a drive signal for driving the optical modulator or on a substrate bias voltage applied to the optical modulator, and a controller configured to detect a ratio between a pilot component and a direct current component contained in a light output from the optical modulator and control at least one of an amplitude of the drive signal and a level of the substrate bias voltage such that the ratio becomes a constant value.

## Claims

### Claim 1

An optical transmitter comprising:
an optical modulator with a Mach-Zehnder interferometer,
a pilot signal generator configured to generate a pilot signal to be superimposed on a drive signal for driving the optical modulator or on a substrate bias voltage applied to the optical modulator,
a monitor circuit that detects a ratio between a pilot component and a direct current component contained in a light output from the optical modulator; and
a processor that controls at least one of an amplitude of the drive signal for driving the optical modulator and a level of the substrate bias voltage applied to the optical modulator such that the ratio becomes a constant value,
wherein the ratio between the pilot component and the direct current component is maintained constant so as to achieve a predetermined optical phase modulation depth of a main signal output from the optical modulator, the ratio being determined according to a modulation scheme of the optical transmitter.

### Claim 2 (depends on 1)

The optical transmitter as claimed in claim 1,
wherein the monitor circuit has a photo detector configured to convert the light output from the optical modulator into an electrical signal, a first monitor configured to detect the direct current component from the electrical signal, and a second monitor configured to detect the pilot component from the electrical signal, and
the processor maintains the ratio between the pilot component and the direct current component at the constant value.

### Claim 3 (depends on 1)

The optical transmitter as claimed in claim 1, wherein
the processor maintains a power level of the direct current component at a constant level and controls a power level of the pilot component so as to be a predetermined level.

### Claim 4 (depends on 3)

The optical transmitter as claimed in claim 3, further comprising:
a light level adjuster configured to adjust an output level of the optical modulator;
wherein the processor adjusts a gain or an attenuation of the light level adjuster such that the power level of the direct current component becomes the constant level.

### Claim 5 (depends on 1)

The optical transmitter as claimed in claim 1,
wherein the optical modulator is a semiconductor optical modulator.

### Claim 6

A method of controlling an optical modulator, comprising:
superimposing a pilot signal onto a drive signal for driving an optical modulator with a Mach-Zehnder interferometer or onto a substrate bias voltage applied to the optical modulator;
detecting a ratio between a pilot component and a direct current component contained in a light output from the optical modulator; and
controlling an amplitude of the drive signal or a level of the substrate bias voltage such that the ratio becomes a constant value,
wherein the ratio between the pilot component and the direct current component is maintained constant so as to achieve a predetermined optical phase modulation depth of a main signal output from the optical modulator, the ratio being determined according to a modulation scheme of the optical transmitter.

### Claim 7 (depends on 6)

The method as claimed in claim 6, wherein
a power of the direct current component is controlled to a constant level and a power level of the pilot component is controlled so as to be a predetermined level.