Optical line terminal and method for transmitting digital information

1. An optical line terminal comprising:

an optical comb generator configured to generate a carrier signal having an optical-frequency-comb spectrum and including N optical tones T k and N optical tones R k , k={1, 2, . . . , N};

N downlink channels D k , each originating from the optical comb generator and being configured to generate a respective downlink signal DS k that includes optical tone T k modulated by downlink data; and

N uplink-photodetectors PD k each configured to receive a respective one of a plurality of modulated uplink signals US k , utilizing optical tone R k .

2. The optical line terminal of claim 1 , each of the N downlink channels D k including a respective slave laser SL k optically coupled to the optical comb generator, the optical comb generator being configured to injection lock slave laser SL k with optical tone T k .

3. The optical line terminal of claim 2 , each slave laser SL k being one of a Fabry-Perot laser and a vertical cavity surface-emitting laser.

4. The optical line terminal of claim 2 , further comprising:

a demultiplexer having (a) an input port optically coupled to the optical comb generator, and (b) N downlink output ports optically coupled to a respective downlink channel D k such that each slave laser SL k is externally injected by optical tone T k .

5. The optical line terminal of claim 2 , each downlink channel D k including a respective signal generator M k configured to generate downlink signal DS k by directly modulating slave laser SL k with the downlink data.

6. The optical line terminal of claim 1 , each of the N downlink channels D k including a respective modulator M k , each modulator M k being a coherent optical modulator configured to modulate optical tone T k per one of a quadrature phase-shift keying modulation scheme and quadrature amplitude modulation scheme.

7. The optical line terminal of claim 6 , the coherent optical modulator including a plurality of Mach-Zehnder modulators.

8. The optical line terminal of claim 1 , the optical comb generator including a slave laser and a tunable laser optically coupled to and configured to injection lock the slave laser, the slave laser being one of a Fabry-Perot laser and a vertical cavity surface-emitting laser.

9. The optical line terminal of claim 1 , further comprising:

a multiplexer having N downlink input ports each optically coupled to a respective downlink channel D k ; and

a demultiplexer having (a) an input port configured to receive the plurality of modulated uplink signals US k , and (b) N output ports each optically coupled to a respective one of the N uplink-photodetectors PD k .

10. A passive optical network comprising:

the optical line terminal of claim 1 ;

N uplink channels U k each optically coupled to the optical comb generator such that optical tone R k propagates in uplink channel U k ; and

N optical network units ONU k , each optical network unit ONU k including a downlink-photodetector optically coupled to downlink channel D k and configured to detect downlink signal DS k ; and

an uplink laser UL k optically coupled to uplink channel U k and configured to be injection locked by optical tone R k .

11. The passive optical network of claim 10 , each optical network unit ONU k further including:

an optical circulator having a first port optically coupled to uplink channel U k and configured to receive optical tone R k , a second port optically coupled to uplink laser UL k , and a third port optically coupled to uplink-photodetector PD k .

12. The passive optical network of claim 10 , the downlink-photodetector being a coherent detector optically coupled to both downlink channel D k and uplink channel U k , and configured to receive both downlink signal DS k and optical tone R k .

13. A method for transmitting digital information comprising:

generating, with a laser of an optical line terminal, a carrier signal having an optical-frequency-comb spectrum that includes a first optical tone and a second optical tone differing in center wavelength from the first optical tone;

generating a downlink signal that includes the first optical tone modulated with downlink data;

injection locking, with the second optical tone, an uplink slave laser of an optical network unit optically coupled to the optical line terminal; and

transmitting the downlink signal to a photodetector of the optical network unit.

14. The method of claim 13 , further comprising:

injection locking, with the first optical tone, a downlink slave laser of the optical line terminal;

wherein generating the downlink signal comprises directly modulating the downlink slave laser with downlink data such that the downlink slave laser emits the downlink signal, a carrier thereof being the first optical tone.

15. The method of claim 13 , further comprising:

directly modulating the uplink slave laser with uplink data such that the uplink slave laser emits an uplink signal, a carrier thereof being the second optical tone; and

transmitting the uplink signal to a photodetector of the optical line terminal.

16. The method of claim 13 ,

in the step of generating the carrier signal, the laser being an injection-locked optical comb generator; and

generating the downlink signal comprising coherently modulating the first optical tone with the downlink data.

17. The method of claim 16 , further comprising demodulating the downlink data by optically heterodyning the downlink signal with the second optical tone.

18. The method of claim 13 , the optical-frequency-comb spectrum including a plurality of additional optical tones, and further comprising, for each optical tone of the plurality of additional optical tones:

generating a respective downlink signal that includes the optical tone modulated with downlink data;

injection locking, with a reference optical tone of the plurality of optical tones, a respective uplink slave laser of a respective optical network unit optically coupled to the optical line terminal, the reference optical tone differing from the optical tone; and

transmitting the respective downlink signal to a respective photodetector of the optical network unit.