Methods of bi-directional optical communication and optical transceiver

1. A method of bi-directional optical communication, the method comprising:

generating a first optical communication signal for transmission in one direction through an optical fiber, wherein generating the first optical communication signal comprises:

receiving information for transmission and generating a baseband signal comprising a representation of the information;

performing digital upconversion of the baseband signal to form an upconverted baseband signal;

performing optical modulation of an optical carrier signal with the upconverted baseband signal; and

restricting an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot by performing one of digital filtering in addition to digital upconversion and optical filtering after optical modulation;

and receiving a second optical communication signal transmitted in an opposite direction through the optical fiber, the second optical communication signal having an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

2. The method of claim 1 , wherein the baseband signal is a real baseband signal and digital filtering is performed in addition to digital upconversion to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

3. The method of claim 2 , wherein the digital filtering comprises upper-single sideband modulation.

4. The method of claim 1 , wherein the baseband signal is a complex baseband signal and optical filtering is performed after the optical modulation to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

5. The method of claim 4 , wherein the second optical communication signal is generated using the same method as for generating the first optical communication signal and wherein one of optical high-pass filtering and optical low-pass filtering is performed after the optical modulation for generating the first optical communication signal and the other of optical high-pass filtering and optical low-pass filtering is performed after the optical modulation for generating the second optical communication signal.

6. The method of claim 1 , wherein the optical modulation is performed by an optical modulator having an operating frequency and the baseband signal has a bandwidth that is substantially half the operating frequency of the optical modulator.

7. The method of claim 6 , wherein the baseband signal has a higher-order modulation format.

8. The method of claim 1 , wherein the receiving comprises receiving an optical signal comprising the second optical communication signal and a reflected portion of the first optical communication signal;

performing coherent optical demodulation of the optical signal to obtain a digital signal;

performing digital filtering on the digital signal to remove a part of the digital signal representing the reflected portion of the first optical communication signal; and

downconverting the filtered digital signal to obtain a baseband signal.

9. An optical transceiver comprising:

a digital signal processor configured to:

receive information for transmission and generate a baseband signal comprising a representation of the information, and perform digital upconversion of the baseband signal to form an upconverted baseband signal;

a digital to analogue converter configured to convert the upconverted baseband signal into a modulation signal;

optical signal generation apparatus configured to receive the modulation signal and to modulate an optical carrier signal with the modulation signal to generate a first optical communication signal for transmission,

wherein the digital signal processor is additionally configured to perform digital filtering in addition to digital upconversion to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot or the optical signal generation apparatus additionally comprises an optical filter to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot; and

coherent optical demodulation apparatus configured to receive a second optical communication signal and configured to perform coherent detection of the second optical communication signal, wherein the second optical communication signal has an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

10. The optical transceiver of claim 9 , wherein the baseband signal is a real baseband signal and the digital signal processor is configured to perform digital filtering in addition to digital upconversion to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

11. The optical transceiver of claim 10 , wherein digital filtering comprises upper-single sideband modulation.

12. The optical transceiver of claim 9 , wherein the coherent optical demodulation apparatus is configured to receive an optical signal comprising the second optical communication signal and a reflected portion of the first optical communication signal and the coherent optical demodulation apparatus is configured to perform coherent detection of the received optical signal to obtain a digital signal, and wherein the digital signal processor is additionally configured to:

perform digital filtering on the digital signal to remove a part of the digital signal representing the reflected portion of the first optical communication signal; and

downconvert the filtered digital signal to obtain a real baseband signal.

13. The optical transceiver of claim 9 , wherein the baseband signal is a complex baseband signal and the optical transceiver further comprises an optical filter configured to filter the optical carrier signal after modulation with the modulation signal.

14. The optical transceiver of claim 9 , wherein the optical signal generation apparatus comprises an optical modulator configured to modulate the optical carrier signal with the modulation signal and configured to operate at a modulation frequency and the baseband signal has a bandwidth that is substantially half the modulation frequency.

15. The optical transceiver of claim 14 , wherein the baseband signal has a higher-order modulation format.

16. The optical transceiver of claim 9 , wherein the optical transceiver comprises a laser configured to generate an optical signal used to provide both the optical carrier signal and a local oscillator signal of the coherent optical demodulation apparatus.