Methods and apparatus for transmission of low photon density optical signals
An optical transmitter and a method of producing a low-photon-density modulated optical signal are disclosed. The optical transmitter and method include an optical source configured to emit a continuous optical carrier waveform, a dilation module configured to apply a spreading code to a data payload to spread each of the plurality of symbols in time to expand the symbol duration by a dilation factor and produce a corresponding plurality of time-dilated symbols, the plurality of time-dilated symbols having a lower photon density than the plurality of symbols; a mapping module configured to map the plurality of time-dilated symbols to a modulation scheme; and a modulator configured to modulate the optical carrier waveform with the plurality of time-dilated symbols according to the modulation scheme to produce the low-photon-density modulated optical signal encoded with the plurality of time-dilated symbols corresponding to the data payload.
1. An optical transmitter configured to produce a low-photon-density modulated optical signal, the optical transmitter comprising:
an input configured to receive a data payload including a plurality of symbols each having a constant symbol duration;
an optical source configured to emit a continuous optical carrier waveform;
a dilation module configured to apply a spreading code to the data payload to spread each of the plurality of symbols in time to expand the symbol duration by a dilation factor and produce a corresponding plurality of time-dilated symbols, the plurality of time-dilated symbols having a lower photon density than the plurality of symbols;
a mapping module configured to map the plurality of time-dilated symbols to a modulation scheme; and
a modulator configured to modulate the optical carrier waveform with the plurality of time-dilated symbols according to the modulation scheme to produce the low-photon-density modulated optical signal encoded with the plurality of time-dilated symbols corresponding to the data payload.
2. The optical transmitter of claim 1 , wherein the optical source is a laser.
3. The optical transmitter of claim 1 , wherein the modulator is an electro-optic modulator.
4. The optical transmitter of claim 3 , wherein the modulator includes the optical source.
5. The optical transmitter of claim 1 , wherein the modulation scheme is an intensity modulation scheme.
6. The optical transmitter of claim 1 , wherein the modulation scheme is a phase modulation scheme.
7. The optical transmitter of claim 1 , wherein the modulation scheme includes a combination of intensity modulation and phase modulation.
8. The optical transmitter of claim 1 , further comprising a pulse-shaping filter coupled to the mapping module and to the modulator, and configured to receive the plurality of time-dilated symbols output from the mapping module and to control the modulator to impose the modulation scheme on the optical carrier waveform to generate the low-photon-density modulated optical signal.
9. The optical transmitter of claim 1 , further comprising an optical assembly configured to output the low-photon-density modulated optical signal.
10. The optical transmitter of claim 9 , wherein the optical assembly includes at least one mirror or lens.
11. The optical transmitter of claim 1 , further comprising a forward error correction module configured to implement forward error correction by adding redundancy to the data payload.
12. The optical transmitter of claim 11 , wherein the forward error correction module is configured to implement the forward error correction using one of a block code and a convolution code.
13. A method of producing a low-photon-density modulated optical signal, the method comprising:
receiving a data payload including a plurality of symbols each having a constant symbol duration;
emitting a continuous optical carrier waveform;
applying a spreading code to the data payload to spread each of the plurality of symbols in time to expand the symbol duration by a dilation factor and produce a corresponding plurality of time-dilated symbols, the plurality of time-dilated symbols having a lower photon density than the plurality of symbols;
mapping the plurality of time-dilated symbols to a modulation scheme; and
modulating the optical carrier waveform with the plurality of time-dilated symbols according to the modulation scheme to produce the low-photon-density modulated optical signal encoded with the plurality of time-dilated symbols corresponding to the data payload.
14. The method of claim 13 , wherein modulating the optical carrier waveform with the plurality of time-dilated symbols includes using intensity modulation.
15. The method of claim 13 , wherein modulating the optical carrier waveform with the plurality of time-dilated symbols includes using phase modulation.
16. The method of claim 13 , wherein modulating the optical carrier waveform with the plurality of time-dilated symbols includes using a combination of intensity modulation and phase modulation.
17. The method of claim 13 , further comprising transmitting the low-photon- density modulated optical signal using an optical assembly that includes at least one mirror or lens.
18. The method of claim 13 , further comprising applying forward error correction by adding redundancy to the data payload.
19. The method of claim 18 , wherein the applying the forward error correction includes using one of a block code and a convolution code.
20. The method of claim 13 , further comprising imposing the modulation scheme on the optical carrier waveform to generate the low-photon-density modulated optical signal.