Optical communication using duobinary modulation

• US 20050286908A1
• Filed: 06/15/2005
• Published: 12/29/2005
• Est. Priority Date: 06/15/2004
• Status: Active Grant

• Current Assignee: Snell Holdings, LLC (Treq Labs, Inc.)
• Original Assignee: Vello Systems Incorporated
• Inventors: Way, Winston I.
• Granted Patent: US 7,577,367 B2
• Time in Patent Office: Days
• US Class Current: 398/186
• CPC Class Codes: H04B 10/2575 Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
• CPC Class Codes: H04B 10/50 Transmitters
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/506 Multiwavelength transmitters
• CPC Class Codes: H04B 10/5161 Combination of different modulation schemes
• CPC Class Codes: H04B 10/5165 Carrier suppressed; Single sideband; Double sideband or vestigial
• CPC Class Codes: H04B 10/5167 Duo-binary; Alternative mark inversion; Phase shaped binary transmission
• CPC Class Codes: H04B 10/61 Coherent receivers

Abstract

Optical techniques, devices and systems for combining duobinary modulation and optical subcarrier multiplexing in optical communication applications. An analog mixer is used to mix a duobinary signal for a data channel and a local oscillator signal to produce a modulation control signal for controlling the subsequent optical subcarrier multiplexing modulation. Various optical subcarrier multiplexing modulation techniques may be used including optical single sideband modulators and optical double sideband modulators.

Claims

1. A device, comprising:

a plurality of analog signal mixers to respectively produce a plurality of analog modulation control signals that respectively carry a plurality of data channels, each analog signal mixer configured to receive and mix a data channel encoded as a duobinary encoded signal and a local oscillator signal at a local oscillator frequency different from local oscillator frequencies received by other analog signal mixers to produce a corresponding analog modulation control signal; and

an optical modulator to receive an input CW laser beam at an optical carrier frequency and to modulate the input CW laser beam in response to the analog modulation control signals to produce an optical output beam which comprises a plurality of different optical subcarriers at optical subcarrier frequencies different from the optical carrier frequency and respectively related to the local oscillator frequencies of the local oscillator signals, wherein each optical subcarrier carries a baseband signal comprising information of a corresponding data channel of the data channels so that the different optical subcarriers carry baseband signals corresponding to the plurality of data channels, respectively.

2. The device as in claim 1, wherein the optical modulator is an optical amplitude modulator.

3. The device as in claim 1, wherein the optical modulator is an optical Mach-Zehnder modulator.

4. The device as in claim 3, wherein the Mach-Zehnder modulator is configured as an optical single sideband modulator.

5. The device as in claim 4, wherein the Mach-Zehnder modulator comprises:

an optical splitter which splits the input CW laser beam into a first optical carrier beam and a second optical carrier beam, both at the optical carrier frequency;

a first AC phase modulator to apply the analog modulation control signals with a 90-degree phase shift between two analog modulation control signals adjacent in frequency to modulate the first optical carrier beam to produce a first modulated optical signal;

a second AC phase modulator to apply the analog modulation control signals with a 90-degree phase shift between two analog modulation control signals adjacent in frequency to modulate the second optical carrier beam to produce a second modulated optical signal, each analog modulation control signal in the first AC phase modulator being phase shifted by 90 degrees relative to each corresponding analog modulation control signal in the second AC phase modulator;

first and second DC phase modulators to modulate the first and the second optical signals, respectively, and configured to modulate an optical carrier component at the optical carrier frequency of the first modulated optical signal to be phase shifted by 90 degrees relative to an optical carrier component at the optical carrier frequency of the second modulated optical signal; and

an optical combiner which combines the first and second modulated optical signals to form the output optical beam carrying the plurality of data channels, wherein each data channel is carried by only one of the different optical subcarriers and the optical subcarrier frequencies are different from the optical carrier frequency by amounts corresponding to the local oscillator frequencies of the local oscillator signals.

6. The device as in claim 1, wherein the optical modulator is an optical double sideband modulator.

7. The device as in claim 6, wherein the optical modulator comprises:

an optical carrier modulator to modulate the input CW laser beam to produce an optical beam having first and second carrier frequencies on two opposite sides of the optical carrier frequency;

an optical splitter which splits the optical beam into a first optical beam and a second optical beam;

a first optical filter to filter the first optical beam to transmit light at the first optical carrier frequency as a first optical carrier beam while rejecting light at the second carrier frequency and the optical carrier frequency;

a second optical filter to filter the second optical beam to transmit light at the second optical carrier frequency as a second optical carrier beam while rejecting light at the first carrier frequency and the optical carrier frequency;

a first optical Mach-Zehnder modulator to apply the analog modulation control signals and a DC bias to two optical modulation paths to cause a 180-degree phase shift between the optical modulation paths and to produce a first output optical signal which carries the plurality of different optical subcarriers at optical subcarrier frequencies at both sides o of the first optical carrier frequency and different from the first optical carrier frequency by amounts corresponding to the local oscillator frequencies of the local oscillator signals and in which light at the first optical carrier frequency is suppressed;

a second optical Mach-Zehnder modulator to apply the analog modulation control signals and a DC bias to two optical modulation paths to cause a 180-degree phase shift between the optical modulation paths and to produce a second output optical signal which carries the plurality of different optical subcarriers at optical subcarrier frequencies at both sides of the second optical carrier frequency and different from the second optical carrier frequency by amounts corresponding to the local oscillator frequencies of the local oscillator signals and in which light at the second optical carrier frequency is suppressed;

an optical combiner to combine the first and second output optical signals into a single combined beam; and

an optical filtering device to filter the single combined beam to transmit optical subcarriers at frequencies between the first optical subcarrier and the second optical carrier frequency while rejecting other optical subcarriers to produce the optical output beam.

8. The device as in claim 7, wherein the optical carrier modulator is an optical modulator which applies a microwave CW tone at a frequency of (½

)(c/λ

1−

c/λ

2) to modulate the input CW laser beam, wherein λ

1 and λ

2 are the first and second optical carrier frequencies in wavelength, respectively.

9. The device as in claim 8, wherein the optical modulator is an optical amplitude modulator.

10. The device as in claim 8, wherein the optical modulator is an optical phase modulator.

11. The device as in claim 1, further comprising:

a plurality of duobinary modulators respectively receive a plurality of binary data signals that respectively correspond to the data channels and to convert the binary data signals into the duobinary encoded signals sent to the analog signal mixers.

12. The device as in claim 11, wherein one of the duobinary modulators comprises a signal splitter to split a binary data signal into a first binary data signal and a second binary data signal, a first signal path to receive the first binary data signal, a second signal path comprising a bit delay device to receive the second binary data signal, and a signal combiner to combine the first and the second binary signals from the first and second signal paths into a corresponding duobinary encoded signal.

13. The device as in claim 11, wherein each binary signal is a NRZ binary signal and one of the duobinary modulators comprises a low-pass filter whose 3-dB bandwidth is 25% of a data rate of the NRZ binary signal.

14. A method, comprising:

using a first analog signal mixer to mix a first duobinary signal which represents a first data channel signal and a first local oscillator signal at a first local oscillator frequency to produce a first modulation control signal;

using a second analog signal mixer to mix a second duobinary signal which represents a second data channel signal and a second local oscillator signal at a second local oscillator frequency different from the first local oscillator frequency to produce a second modulation control signal; and

applying the first and second modulation control signals to modulate a CW laser beam at an optical carrier frequency to produce an optical output beam which comprises optical subcarriers at optical subcarrier frequencies different from the optical carrier frequency to carry to carry the first and the second data channels.

15. The method as in claim 14, further comprising

using at least a third analog signal mixer to mix a third duobinary signal which represents a third data channel signal and a third local oscillator signal at a third local oscillator frequency to produce a third modulation control signal;

and

modulating the a CW laser beam to carry at least the first, the second, and the third data channels in the optical output beam.

16. The method as in claim 14, further comprising:

transmitting the optical output beam through an optical medium to a destination; and

using an optical receiver at the destination to directly convert a signal carried by each optical subcarrier into an electrical signal that represents a corresponding data channel signal without a duobinary decoder.

17. A method, comprising:

modulating at least two binary electronic signals to produce duobinary encoded signals;

modulating a CW laser beam at an optical carrier frequency in response to the duobinary encoded signals to produce two optical single sideband subcarriers at optical frequencies different from the optical carrier frequency as an optical output; and

transmitting the optical output through an optical transmission link or network.

18. The method as in claim 17, further comprising:

filtering the optical output at a receiver to produce a filtered optical signal at one subcarrier; and

converting the filtered optical signal into an electronic signal for data detection or signal processing.

19. The method as in claim 18, further comprising selectively amplifying different frequency components in the electronic signal to reduces differences in amplitudes of frequency components.

20. A device, comprising:

a plurality of electronic duobinary signal modulators to respectively receive and modulate input binary signals and to output duobinary encoded signals;

a plurality of local oscillators to produce a plurality of local oscillator signals corresponding to the electronic duobinary signal modulators, respectively;

a plurality of electronic signal mixers, each coupled to mix a duobinary encoded signal with a local oscillator signal from a corresponding local oscillator to produce a modulation control signal; and

an optical single sideband modulator to receive an input CW beam at an optical carrier frequency and to modulate the beam in response to the modulation control signals from the electronic signal mixers to produce an optical output comprising the optical carrier, optical single sideband subcarriers at frequencies different from the optical carrier.