Free-space optical communication system and method in scattering environments

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.

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.