Free-space optical communication system and method in scattering environments

1. A free-space optical communication system, comprising:

a transmitter configured to optically transmit data in a plurality of subcarrier signals over a free-space scattering medium, and configured to optically transmit the data by selecting m out of n number of the plural subcarrier signals over the free-space scattering medium, where m is less than n, and m, n are positive integer values; and

a coherent receiver configured to receive the plurality of subcarrier signals and extract the transmitted data from the plurality of subcarrier signals after traversal over the free-space scattering medium,

wherein a digital signal processor of the transmitter is configured to select only m of the plural subcarrier signals for transmitting at a time, and is configured to change between the plurality of subcarrier signals as a function of time to transmit information based upon a time-hopping code.

2. The free-space optical communication system of claim 1 , wherein the coherent receiver includes a local oscillator laser, the local oscillator laser having an output frequency offset outside a transmission band of the transmitter.

3. The free-space optical communication system of claim 2 , wherein the coherent receiver includes:

a beam splitter optically coupled to the local oscillator laser upon which an output signal form the local oscillator laser is incident along with the plurality of subcarrier signals,

a first photodetector optically coupled to the beam splitter and configured to output a first electrical signal upon receiving a first optical signal transmitted through the beam splitter; and

a second photodetector optically coupled to the beam splitter and configured to output a second electrical signal upon receiving a second optical signal reflected from the beam splitter.

4. The free-space optical communication system of claim 3 , wherein the coherent receiver includes:

a receiver digital signal processor to receive a difference signal corresponding to a difference between the first electrical signal and the second electrical signal to obtain the data in each of the plurality of subcarrier signals.

5. The free-space optical communication system of claim 1 , wherein the transmitter includes:

a transmitter digital signal processor configured to allocate the data to the plurality of subcarrier signals; and

a transmit laser having a narrow-linewidth output carrier optical signal modulated by the plurality of subcarrier signals prior to a transmission from the transmitter.

6. The free-space optical communication system of claim 5 , wherein a narrow-linewidth output carrier optical signal is modulated by the plurality of subcarrier signals after conversion to an analog signal by a digital to analog converter.

7. The free-space optical communication system of claim 6 , wherein the modulation of the narrow-linewidth output carrier optical signal produces a single-sideband suppressed carrier optical signal.

8. The free-space optical communication system of claim 1 , wherein the coherent receiver is a balanced coherent receiver and the transmitter includes a modulator for modulating a transmit laser with the plurality of subcarrier signals using on-off keying.

9. The free-space optical communication system of claim 1 , wherein the digital signal processor is configured to:

apply the data individually to each subcarrier signal in the plurality of subcarrier signals,

modulate said each subcarrier signal every 1 μs with the data, and

mix the modulated plurality of subcarrier signals with a narrow-linewidth output carrier optical signal from a transmit laser.

10. A free-space optical communication method, comprising:

selecting, at a transmitter, m out of n number of plural subcarrier signals for transmission over a free-space scattering medium, where m is less than n, and m, n are positive integer values;

transmitting optically, from the transmitter, data on the selected m number of subcarriers in the plurality of subcarrier signals over the free-space scattering medium; and

receiving, at a coherent receiver, the plurality of subcarrier signals, wherein said receiving includes extracting, at the coherent receiver, the transmitted data from the plurality of subcarrier signals after traversal through the free-space scattering medium,

wherein the selecting step includes selecting, at a transmitter digital signal processor, only m of the plurality of subcarrier signals for the transmitting step at a time, and changing between the plurality of subcarrier signals as a function of time to transmit information based upon a time-hopping code, and

wherein the transmitting step includes:

applying, at the transmitter digital signal processor, the data individually to each subcarrier signal in the plurality of subcarrier signals,

modulating, at the transmitter digital signal processor, said each subcarrier signal every 1 μs with the data, and

mixing, at an optical modulator, the modulated plurality of subcarrier signals with a narrow-linewidth output carrier optical signal from a transmit laser.

11. The free-space optical communication method of claim 10 , further comprising:

splitting, at a transmitter digital signal processor of the transmitter, the data into the plurality of subcarrier signals;

modulating, at an optical modulator of the transmitter, a narrow-linewidth output carrier optical signal from a transmit laser with the plurality of subcarrier signals including the data; and

amplifying, at an optical amplifier of the transmitter, the modulated narrow-linewidth output carrier optical signal for said transmitting over the free-space scattering medium.

12. The free-space optical communication method of claim 11 , wherein prior to said transmitting, the transmitter digital signal processor carries out an interleaving and/or a forward error correction of the data.

13. The free-space optical communication method of claim 11 , wherein the transmitter digital signal processor is further configured to dynamically allocate the data to the plurality of subcarrier signals, said dynamic allocation being based on a transmission metric of a communication link between the transmitter and the coherent receiver.

14. The free-space optical communication method of claim 10 , wherein the receiving comprises:

mixing, at a beam-splitter of the coherent receiver, the received plurality of subcarrier signals with an output signal from a local oscillator laser, the output signal having a frequency offset outside a transmission band of the transmitter.

15. The free-space optical communication method of claim 14 , wherein the receiving comprises:

detecting, at a first photodetector of the coherent receiver, a first optical signal transmitted through the beam splitter,

outputting, from the first photodetector, a first electrical signal corresponding to the first optical signal,

detecting, at a second photodetector of the coherent receiver, a second optical signal reflected from the beam splitter, and

outputting, from the second photodetector, a second electrical signal corresponding to the second optical signal.

16. The free-space optical communication method of claim 15 , further comprising:

obtaining, at a receiver digital signal processor, the data in each of the plurality of subcarrier signals from a difference signal of the first electrical signal and the second electrical signal, said obtaining including:

correcting for a phase difference and a frequency difference between the local oscillator laser and a transmit laser of the transmitter,

performing a frequency transformation on the difference signal, and

decoding the difference signal, after said correcting and said performing the frequency transformation, for interleaving and forward error correction.

17. The free-space optical communication method of claim 10 , wherein the transmitting comprises:

selecting, at the transmitter digital signal processor, at least two of the plurality of subcarrier signals for said transmitting, said selecting being programmable at the transmitter digital signal processor.

18. The free-space optical communication method of claim 10 , wherein the modulating comprises:

selecting one of an On/Off Keying (OOK), a Quadrature Phase Shift Keying (QPSK), a Frequency-Shift Keying (FSK), a Quadrature-Amplitude Modulation (QAM), and a Pulse Amplitude Modulation (PAM) as a modulation technique for said each of the plurality of subcarrier signals.

19. An underwater optical detection method comprising:

receiving, at a coherent receiver in an underwater environment, m out of n number of a plurality of subcarrier signals optically transmitted from a transmitter in the underwater environment, where m is less than n, and m, n are positive integer values; and

extracting, at the coherent receiver, transmitted data in the m number of the plurality of subcarrier signals after traversal through the underwater environment,

wherein the plurality of received subcarrier signals are based on a digital signal processor of the transmitter that selects only m of the plurality of subcarrier signals for transmitting at a time, and changes between the plurality of subcarrier signals as a function of time to transmit information based upon a time-hopping code.

20. The underwater optical detection method of claim 19 , wherein the plurality of received subcarrier signals are based on the digital signal processor performing the step of:

applying the data individually to each subcarrier signal in the plurality of subcarrier signals,

modulating said each subcarrier signal every 1 μs with the data, and

mixing the modulated plurality of subcarrier signals with a narrow-linewidth output carrier optical signal from a transmit laser.