A transmitter for an optical free-beam communication system includes two light transmitters for the optical transmission of a data signal using one single-sideband modulation, wherein each light transmitter emits a side of the band modulation so that a light signal arriving at a receiver corresponds to a double-sideband modulation.

Carrier suppression (CS-M-PAM) may be applied to M-PAM modulated optical signals to improve transmission reach. Additional rescaling of CS-M-PAM, referred to as adaptive CS-M-PAM, may further improve transmission reach by reducing low level symbol interference.

A radio frequency (RF) multiplexer includes, for example, a common port, a first port for a first frequency band, a second port for a second frequency band, and a third port for a third frequency band. The RF multiplexer also includes, for example, a first quadrature hybrid coupler (QHC), a second QHC and a third QHC. A coupling of the first QHC, a first pair of filters, and the second QHC separates the first frequency band and the second frequency band from the common port to the first port and to the second port, respectively. A coupling of the first QHC, a second pair of filters, and the third QHC separates the first frequency band and the third frequency band from the common port to the first port and to the third port respectively.

A method of RF signal processing comprises receiving an incoming RF signal at each of a plurality of antenna elements that are arranged in a first pattern. The received RF signals from each of the plurality of antenna elements are modulated onto an optical carrier to generate a plurality of modulated signals that each have at least one sideband. The modulated signals are directed along a corresponding plurality of optical channels with outputs arranged in a second pattern corresponding to the first pattern. A composite optical signal is formed using light emanating from the outputs of the plurality of optical channels. Non-spatial information contained in at least one of the received RF signals is extracted from the composite signal.

A microwave photonic notch filter comprising: a modulator to modulate an optical signal with an electrical signal to generate a first sideband and a second sideband; a configurable optical processor to generate a modified optical signal by adjusting the power of the sidebands to achieve a power difference between first sideband and second sideband and by producing an antiphase relationship between light within two sidebands corresponding to the selected frequency band; an optical resonance to adjust the power of the first sideband of the modified optical signal corresponding to the selected frequency band by a resonance power adjustment to generate a resonance output signal; an optical-to-electrical converter to generate a copy of the electrical signal with suppressed frequency components within the selected frequency band; and a control unit to re-configure the configurable optical processor to adjust the power difference between two sidebands towards the resonance power adjustment.

A method of digital communication is described. The method includes generating a first orthogonal frequency division multiplexed (OFDM) signal from a first portion of information bits received at the communication apparatus, generating a second OFDM signal from a second portion of information bits received at the communication apparatus, generating a first input for a modulator using the first OFDM signal and the second OFDM signal, generating a second input for the modulator using the second OFDM signal and the second OFDM signal, and operating the modulator to produce a modulated signal from the first input and the second input.

An orthogonal frequency division multiplexing (OFDM) transmitter is able to communicate simultaneously with a simple direct detection receiver and also with a coherent receiver. The transmitter transmits a polarization multiplexed self-coherent signal by multiplexing a carrier in the polarization state orthogonal to the polarization state of the data signal that is embodied in the sidebands. In accordance with one particular aspect of the disclosure, the direct detection receiver receiving this self-coherent signal utilizes a single polarization analyzer before the photodiode, which simplifies the receiver architecture for direct detection of a polarization multiplexed self-coherent single sideband signal.

Multiple bit values can be encoded on a single photon in a quantum key distribution (QKD) system using a plurality of sidebands of an optical carrier frequency. Computational and conjugate bases can be defined, and photons decoded based on a selected state from either basis. If n sidebands are available, as many as log.sub.2n bits can be encoded on a single photon. Errors in detected bit values due to selection of an incorrect basis state or other errors can be at least partially corrected by bit distillation to identity bit strings for which a transmitter and a receiver record the same values, without insecure transmission of these values.

Multiple bit values can be encoded on a single photon in a quantum key distribution (QKD) system using a plurality of sidebands of an optical carrier frequency. Computational and conjugate bases can be defined, and photons decoded based on a selected state from either basis. If n sidebands are available, as many as log.sub.2 n bits can be encoded on a single photon. Errors in detected bit values due to selection of an incorrect basis state or other errors can be at least partially corrected by bit distillation to identity bit strings for which a transmitter and a receiver record the same values, without insecure transmission of these values.

A microwave photonic notch filter comprising: a modulator to modulate an optical signal with an electrical signal to generate a first sideband and a second sideband; a configurable optical processor to generate a modified optical signal by adjusting the power of the sidebands to achieve a power difference between first sideband and second sideband and by producing an antiphase relationship between light within two sidebands corresponding to the selected frequency band; an optical resonance to adjust the power of the first sideband of the modified optical signal corresponding to the selected frequency band by a resonance power adjustment to generate a resonance output signal; an optical-to-electrical converter to generate a copy of the electrical signal with suppressed frequency components within the selected frequency band; and a control unit to re-configure the configurable optical processor to adjust the power difference between two sidebands towards the resonance power adjustment.