# US 10,120,210 B2 · Feed-forward optical equalization using an electro-optic modulator with a multi-segment electrode and distributed drivers

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- Number: US 10,120,210 B2
- Document ID: US10120210B2
- Status: Granted
- Kind: B2
- Application number: 15/385,409
- Filed: 2016-12-20
- Published: 2017-12-07
- Granted: 2018-11-06
- Earliest claimed: 2016-06-03
- Assignees: International Business Machines Corporation
- Applicants: International Business Machines Corporation
- Inventors: Nicolas Dupuis; Tam N. Huynh; Benjamin G. Lee; Jonathan E. Proesel; Renato Rimolo-Donadio; Alexander V. Rylyakov; Clint L. Schow
- Web page: https://patentgrep.com/patent/US10120210B2

## CPC

- G02F 1/0121: {Operation of devices; Circuit arrangements, not otherwise provided for in this subclass}
- G02F 1/2257: {the optical waveguides being made of semiconducting material}
- H04B 10/5059: {using a feed-forward signal generated by analysing the optical or electrical input}
- H04B 10/516: Details of coding or modulation
- H04B 10/541: {Digital intensity or amplitude modulation}
- G02F 1/212: {Mach-Zehnder type}
- G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- H04B: TRANSMISSION

## Abstract

A device and method of optical equalization using an optical modulator is provided. An electrical modulation signal is split into a first modulation signal and a second modulation signal. The second modulation signal is delayed relative to the first modulation signal. An amplitude of the second modulation signal is attenuated relative to the first modulation signal. The first modulation signal is applied to a first waveguide segment of the optical modulator. The second modulation signal that is delayed and attenuated relative to the first modulation signal is applied to a second waveguide segment of the optical modulator. Both the applied first and second modulation signals generate a feed-forward equalized optical signal that is recombined in the optical domain.

## Claims

### Claim 1

An optical communication system, comprising:
an optical modulator device including a first and a second waveguide segment, which is configured to modulate an incident optical signal; and
a feed-forward equalization circuit including a first tap and a second tap, wherein:
the first tap is coupled to the first segment of the optical modulator,
the second tap is coupled to the second segment of the optical modulator,
the first tap is configured to generate a first modulation signal,
the second tap is configured to generate a second modulation signal,
the first modulation signal is attenuated relative to the second modulation signal,
the first and the second modulation signals are time-delayed with respect to each other,
the second tap comprises a plurality of paths, each path driving a corresponding segment of the second segment,
a recombination of the first tap and the second tap is in the optical domain,
the feed-forward equalization circuit comprises a pre-amplifier that is shared between the first tap and the second tap; and
the first tap of the feed-forward equalization circuit comprises:
a tunable delay buffer device coupled to an output of the shared pre-amplifier;
a tunable tap weight buffer device coupled to an output of the tunable delay buffer device; and
a first output driver having an input coupled to the output of the tunable delay buffer device and an output coupled to the first segment of the optical modulator.

### Claim 2 (depends on 1)

The optical communication system of claim 1, wherein the first and second modulation signals are inverted with respect to each other.

### Claim 3 (depends on 1)

The optical communication system of claim 1, wherein the optical modulator comprises a Mach Zehnder Modulator (MZM) having a first arm and a second arm, the first arm including the first and the second segment.

### Claim 4 (depends on 1)

The optical communication system of claim 1, wherein the optical modulator comprises an Electro-Absorption Modulator (EAM) having an electro-absorption region including the first and the second segment.

### Claim 5 (depends on 1)

The optical communication system of claim 1, wherein each path generates a change in a refractive index in its corresponding segment that it is driving.

### Claim 6 (depends on 1)

The optical communication system of claim 1, wherein a delay and amplitude relations between the first tap and the second tap generate an equalization signal, which is encoded onto a phase or amplitude of the incident optical signal passing through the optical modulator.

### Claim 7 (depends on 1)

The optical communication system of claim 1, wherein:
the first modulation signal is complementary,
the second modulation signal is complementary, and
each of the first and second modulation signals is operative to control the optical modulator device in a push-pull configuration.

### Claim 8 (depends on 1)

The optical communication system of claim 1, wherein the first and second modulation signals are time-delayed with respect to each other to compensate for an optical delay between the first and second waveguide segments.

### Claim 9 (depends on 1)

The optical communication system of claim 1, wherein the second tap of the feed-forward equalization circuit comprises:
a second tunable delay buffer device coupled to an output of the shared pre-amplifier; and
a second output driver having an input coupled to the output of the second tunable delay buffer device and an output coupled to the second segment of the optical modulator.

### Claim 10 (depends on 9)

The optical communication system of claim 9, wherein the second tap of the feed-forward equalization circuit further comprises an inverter coupled between the second tunable delay buffer and the second driver.

### Claim 11 (depends on 9)

The optical communication system of claim 9, wherein the first and second output drivers are in BiCMOS technology.

### Claim 12

A method of optical equalization using an optical modulator, comprising:
splitting an electrical modulation signal into a first modulation signal and a second modulation signal;
delaying the second modulation signal relative to the first modulation signal;
attenuating an amplitude of the second modulation signal relative to the first modulation signal;
applying the first modulation signal to a first waveguide segment of the optical modulator for providing optical modulation;
applying the second modulation signal that is delayed and attenuated relative to the first modulation signal, to a second waveguide segment of the optical modulator;
splitting the second modulation signal into a plurality of additional modulation signals, each additional modulation signal driving a corresponding segment of the second segment; and
sharing a pre-amplifier between a first tap and a second tap of the electrical modulation signal;
coupling a tunable delay buffer device to an output of the pre-amplifier;
coupling a tap weight buffer device to an output of the tunable delay buffer; and
coupling an input of a first output driver to the output of the tunable delay buffer device and an output to the first waveguide segment of the optical modulator, wherein:
both the applied first and second modulation signals generate a feed-forward equalized optical signal that is recombined in the optical domain,
the first modulation signal is driven by a first tap of the optical modulator, and
the second and the additional modulation signals are driven by a second tap of the optical modulator.

### Claim 13 (depends on 12)

The method of claim 12, further comprising inverting the second modulation signal upon delaying the second modulation signal relative to the first modulation signal.

### Claim 14 (depends on 12)

The method of claim 12, wherein the optical modulator comprises a Mach Zehnder Modulator (MZM) having a first arm and a second arm, the first arm including the first and the second waveguide segments.

### Claim 15 (depends on 12)

The method of claim 12, wherein the optical modulator comprises an Electro-Absorption Modulator (EAM) having an electro-absorption region including the first and the second waveguide segments.

### Claim 16 (depends on 12)

The method of claim 12, wherein each modulation signal generates a change in a refractive index in its corresponding waveguide segment.

### Claim 17 (depends on 12)

The method of claim 12, wherein the feed-forward equalized optical signal equalization signal, is encoded onto a phase or amplitude of an incident optical signal passing through the optical modulator based on a delay and amplitude relations between the first and second modulation signals.