Frequency domain multiplex optical transmission

1. A method of transmitting a data signal in an optical communications system, the method comprising:

processing the data signal to generate In-Phase and Quadrature analog drive signals, the processing including mixing In-Phase and Quadrature components of the data signal with a mixing signal having a first predetermined frequency to generate corresponding In-Phase and Quadrature composite signals, and bandpass filtering the In-Phase and Quadrature composite signals using a filter characteristic having a cut-off frequency equal to the first predetermined frequency to generate the In-Phase and Quadrature analog drive signals such that frequency components of the data signal lay in at least two separated spectral bands, each spectral band having a respective bandwidth that is less than a bandwidth of the data signal; and

modulating an optical carrier light using the In-Phase and Quadrature analog drive signals.

2. The method of claim 1 , further comprising:

detecting In-Phase and Quadrature components of the modulated optical carrier light, and generating corresponding In-Phase and Quadrature detector signals; and

processing the In-Phase and Quadrature detector signals to recover the analog sub-band signals; wherein the processing comprises:

supplying the In-Phase and Quadrature detector signals to at least two paths, each path corresponding to a respective one of the data signals; and

for at least one of the data signals:

band-pass filtering each of the In-Phase and Quadrature detector signals using a filter characteristic having a second predetermined pass-band to generate corresponding filtered detector signals;

mixing each of the filtered detector signals with an Rx mixing signal having a second predetermined frequency to generate corresponding composite signals; and

low-pass filtering the composite signals using a filter characteristic having a cut-off frequency equal to a third predetermined frequency to generate corresponding In-Phase and Quadrature filtered sub-band signals.

3. The method of claim 2 , wherein the first and second predetermined frequencies are equal.

4. The method of claim 2 , wherein the second and third predetermined frequencies are equal.

5. The method of claim 2 , wherein the first predetermined frequency is greater than the second predetermined frequency.

6. The method of claim 2 , wherein the first and second predetermined pass-bands are equal.

7. A transmitter configured to transmit a data signal in an optical communications system, the transmitter comprising:

a processor configured to process the data signal to generate In-Phase and Quadrature analog drive signals, including respective mixers configured to mix In-Phase and Quadrature components of the data signal with respective Tx mixing signals having a first predetermined frequency to generate corresponding In-Phase and Quadrature composite signals, and respective band-pass filters configured to filter the In-Phase and Quadrature composite signals to generate the In-Phase and Quadrature analog drive signals such that frequency components of the data signal lay in at least two separated spectral bands, each spectral band having a respective bandwidth that is less than a bandwidth of the data signal; and

a modulator configured to modulate an optical carrier light using the In-Phase and Quadrature analog drive signals,

wherein the respective band-pass filters have a first predetermined pass-band having a cut-off frequency corresponding to the first predetermined frequency.

8. The transmitter of claim 7 , wherein the processor comprises:

a digital signal processor configured to digitally processing each one of the set of the two or more data signals to generate respective In-Phase and Quadrature sample streams for each digital data signal; and

a respective digital-to-analog converter configured to process each sample stream to generate corresponding In-Phase and Quadrature analog sub-band signals.

9. A receiver configured to receive an optical signal received through an optical communications system from a transmitter, the receiver comprising:

a photodetector block configured to detect In-Phase and Quadrature components of the optical signal, and to generate corresponding In-Phase and Quadrature detector signals, the optical signal comprising two or more frequency multiplexed sub-band signals, at least one sub-band signal having frequency components lying in at least two separated spectral bands of the optical signal, each of the at least two separated spectral bands having a respective bandwidth that is less than a bandwidth of the sub-band signal; and

a complex demixer configured to process the In-Phase and Quadrature detector signals to recover the sub-band signals, wherein the complex demixer comprises:

at least two paths, each path corresponding to a respective one of the sub-band signals and receiving the In-Phase and Quadrature detector signals; and

in each path corresponding to a sub-band signal having frequency components lying in at least two separated spectral bands of the optical signal:

a respective band-pass filter configured to filter each of the In-Phase and Quadrature detector signals, each band-pass filter having a filter characteristic including a predetermined pass-band to generate corresponding filtered detector signals;

a respective mixer configured to mix each of the filtered detector signals with an Rx mixing signal having a first predetermined frequency to generate corresponding composite signals; and

a respective low-pass filter configured to filter each of the composite signals, each low-pass filter having a filter characteristic including a cut-off frequency equal to a second predetermined frequency to generate corresponding In-Phase and Quadrature filtered sub-band signals.

10. A method of frequency demultiplexing two or more sub-band signals modulated on an optical signal received through an optical communications system from a transmitter, the method comprising:

a photodetector block detecting In-Phase and Quadrature components of the optical signal, and generating corresponding In-Phase and Quadrature detector signals, the optical signal comprising two or more frequency multiplexed sub-band signals, at least one sub-band signal having frequency components lying in at least two separated spectral bands of the optical signal, each of the at least two separated spectral bands having a respective bandwidth that is less than a bandwidth of the sub-band signal; and

a complex demixer processing the In-Phase and Quadrature detector signals to recover the sub-band signals, wherein the processing comprises:

supplying the In-Phase and Quadrature detector signals to at least two paths, each path corresponding to a respective one of the sub-band signals; and

for a sub-band signal having frequency components lying in at least two separated spectral bands of the optical signal:

band-pass filtering each of the In-Phase and Quadrature detector signals using a filter characteristic having a predetermined pass-band to generate corresponding filtered detector signals;

mixing each of the filtered detector signals with an Rx mixing signal having a first predetermined frequency to generate corresponding composite signals; and

low-pass filtering the composite signals using a filter characteristic having a cut-off frequency equal to a second predetermined frequency to generate corresponding In-Phase and Quadrature filtered sub-band signals.

11. The receiver as claimed in claim 9 , wherein the complex demixer further comprises a second path corresponding to a sub-band signal having frequency components lying in a single spectral band of the optical signal, the second path having a respective low-pass filter configured to filter each of the In-Phase and Quadrature detector signals, each low-pass filter having a filter characteristic including a cut-off frequency equal to the second predetermined frequency to generate corresponding In-Phase and Quadrature filtered sub-band signals.

12. The receiver as claimed in claim 11 , further comprising a digital signal processor configured to process the In-Phase and Quadrature filtered sub-band signals from each path of the complex demixer to recover a data signal encoded in the sub-band signals modulated on the modulated optical carrier light.

13. The method as claimed in claim 10 , further comprising, for sub-band signals having frequency components lying in a single spectral band of the optical signal, low-pass filtering each of the In-Phase and Quadrature detector signals using a filter characteristic having a cut-off frequency equal to the second predetermined frequency to generate corresponding In-Phase and Quadrature filtered sub-band signals.

14. The method as claimed in claim 13 , further comprising a digital signal processor processing the In-Phase and Quadrature filtered sub-band signals corresponding to each sub-band signal to recover a data signal encoded in the sub-band signals modulated on the modulated optical carrier light.