Optical communications using multiplexed single sideband transmission and heterodyne detection

A method comprising:

• generating a first optical signal having a first carrier and a second optical signal having a second carrier;
• combining the first optical signal and the second optical signal into a first composite signal;
• filtering the first composite signal to produce a second composite signal, wherein said filtering includes selecting a lower optical sideband from the first optical signal and an upper optical sideband from the second optical signal;
• tapping the first composite signal;
• tapping the second composite signal;
• generating respective indications of the strengths of the first and second optical carriers in each of the first and second composite signals;
• and
• locking a frequency separation between the first and second optical carriers based on the respective indications of signal strength, wherein the frequency separation is substantially equal to the bandwidth of a filter that performs said filtering.

A communications system comprising:

• a transmission subsystem, wherein said transmission subsystem includes:
• first means for generating a first optical signal having a first optical carrier signal;
• second means for generating a second optical signal having a second optical carrier signal, wherein the first optical signal has a different polarization than the second optical signal;
• third means for combining the first and second optical signals into a first composite signal;
• fourth means for receiving the first composite optical signal and selecting a lower optical sideband from the first optical signal and an upper optical sideband from the second optical signal to produce a second composite signal;
• a first optical tap for tapping the first composite signal;
• a second optical tap for tapping the second composite signal;
• and
• fifth means for locking a frequency separation between the first optical signal and the second optical signal, said frequency separation being substantially equal to a filtering bandwidth of said fourth means, wherein said locking a frequency separation is performed based on respective indications of the signal strengths of the first and second optical carrier signals in the first composite signal and respective indications of signal strengths of the first and second optical carrier signals in the second composite signal.

A communications system comprising:

• a transmitter subsystem, wherein the transmitter subsystem includes:
• a first optical transmitter configured to generate a first optical signal that includes a first optical carrier signal;
• a second optical transmitter configured to generate a second optical signal that includes a second optical carrier signal;
• an optical combiner configured to optically combine the first and second optical signals into a first composite signal;
• an optical filter coupled to receive the first composite signal from the optical combiner and configured to produce a second composite signal by selecting a lower optical sideband from the first optical signal and an upper optical sideband from the second optical signal;
• a first optical tap configured to tap the first composite signal to produce a first sample;
• a second optical tap configured to tap the second composite signal to produce a second sample;
• and
• a wavelength-locking device coupled to the first optical tap and to the second optical tap and configured to generate indications of the strengths of the first and second optical carrier signals in the first and second samples and use the indications of the strengths of the first and second optical carrier signals to lock a frequency separation therebetween, wherein the frequency separation is substantially equal to the bandwidth of the optical filter.

A communications system comprising:

• a transmitter subsystem, wherein the transmitter subsystem includes:
• first and second optical transmitters configured to generate first and second optical signals, respectively, the first and second optical signals having first and second optical cater signals, respectively;
• first and second external optical modulators configured to receive and modulate the first and second optical signals, respectively, wherein each of the first and second optical modulators are Mach Zender Modulators (MZM's);
• an optical combiner coupled to receive the first and second optical signals from the first and second external optical modulators, respectively, and configured to optically combine the first and second optical signals into a first composite signal;
• an optical filter coupled to receive the first composite signal from the optical combiner and configured to produce a second composite signal by selecting a lower optical sideband from the first optical signal and an upper optical sideband from the second optical signal;
• a first optical tap configured to tap the first composite signal to produce a first sample;
• a second optical tap configured to tap the second composite signal to produce a second sample;
• and
• a wavelength-locking device coupled to the first optical tap and to the second optical tap and configured to lock a frequency separation between the first and second carrier signals, wherein the frequency separation is substantially equivalent to the bandwidth of the optical filter, wherein the wavelength locking device is configured to generate respective indications of the strength of each of the first and second optical carrier signals in each of the first and second samples, and configured to lock the frequency separation based on the respective indications of strength.

A communications system comprising:

• a transmitter subsystem comprising:
• a first optical transmitter configured to generate a first optical signal having a first optical carrier;
• a second optical transmitter configured to generate a second optical signal having a second optical carrier;
• a wavelength locking device, wherein the wavelength locking device is coupled to receive a first composite signal and a second composite signal, wherein the first composite signal includes the first optical signal and the second optical signal;
• an optical filter configured to produce the second composite signal by filtering the first composite signal such that a lower optical sideband of the first optical signal and a an upper optical sideband of the second optical signal are suppressed during said filtering;
• wherein the wavelength locking device is configured to generate respective indications of the strengths of the first and second optical carriers in each of the first and composite signals and lock a frequency separation therebetween based on the respective indications of strength, wherein the frequency separation is substantially equal to the bandwidth of the optical filter;
• and
• a receiver subsystem coupled to the transmitter subsystem by an optical fiber and configured to recover subbands of information from the second composite signal, wherein the receiver subsystem comprises:
• a polarizing splitter module configured to split the second composite optical signal according to polarization of the subbands;
• and
• a plurality of heterodyne receivers coupled to the polarizing splitter module and configured to recover the subbands.

A transmission subsystem for a communications system, the transmission subsystem comprising:

• a first optical transmitter configured to generate a first optical signal including a first optical carrier signal;
• a second optical transmitter configured to generate a second optical signal including a second optical carrier signal;
• and
• a wavelength locking device, wherein the wavelength locking device is coupled to a first optical tap in order to receive a first composite signal comprising the first optical signal and the second optical signal, and further coupled to a second optical tap in order to receive a second composite signal, wherein the wavelength locking device includes:
• a first synchronous detector coupled to receive a first sample taken from the first optical tap and configured to generate respective indications of the strengths of the first and second optical carriers in the first sample;
• a second synchronous detector coupled to receive a second sample taken from the second optical tap and configured to generate respective indications of the strengths of the first and second optical carriers in the second sample;
• and
• comparison circuitry coupled to the synchronous detectors configured to compare the indication of strength of the first optical carrier in the first sample to the indication of strength of the first optical carrier in the second sample, and further configured to compare the indication of strength of the second optical carrier in the first sample to the indication of strength of the second optical carrier in the second sample, and generate errors signals for the first and second optical transmitters based on the indications of the strengths of the first and second optical carrier signals in each of the first and second samples.