An optical relay apparatus includes a first signal converting unit that converts a received optical signal into an electric signal and demodulates and outputs the electric signal and modulates an input electric signal in a first modulation scheme, converts the electric signal into an optical signal, and transmits the optical signal and a second signal converting unit that modulates the electric signal output from the first signal converting unit in a second modulation scheme, converts the electric signal into an optical signal, and transmits the optical signal and converts a received optical signal into an electric signal, demodulates the electric signal, and outputs the electric signal as an electric signal input to the first signal converting unit.

An improved coherent communication scheme is provided. The coherent communication scheme encodes both classical and quantum information simultaneously using isolated groups of states: classical information is represented by different groups and can be decoded deterministically; and quantum information is represented by highly overlapped states within the same group, thus guaranteeing security. Decoding includes projecting the detection results at the receiver to one of the distinguishable encoding groups first, which allows the classical information to be read out, and then generating a quantum key from the residual randomness. This communications scheme enables simultaneous classical communication and QKD over the same communication channel using the same transmitter and receiver, opening the door to operate QKD in the background of classical communication and at negligible costs.

Apparatus and method for digital signal constellation transformation are provided herein. In certain configurations, an integrated circuit includes an analog front-end that converts an analog signal vector representing an optical signal into a digital signal vector, and a digital signal processing circuit that processes the digital signal vector to recover data from the optical signal. The digital signal processing circuit generates signal data representing a signal constellation of the digital signal vector. The digital signal processing circuit includes an adaptive gain equalizer that compensates the signal data for distortion of the signal constellation arising from biasing errors of optical modulators used to transmit the optical signal.

In a coherent optical receiver device, the dynamic range considerably decreases in the case of selectively receiving the optical multiplexed signals by means of the wavelength of the local oscillator light, therefore, a coherent optical receiver device according to an exemplary aspect of the invention includes a coherent optical receiver receiving optical multiplexed signals in a lump in which signal light is multiplexed; a variable optical attenuator; a local oscillator connected to the coherent optical receiver; and a first controller controlling the variable optical attenuator by means of a first control signal based on an output signal of the coherent optical receiver; wherein the coherent optical receiver includes a 90-degree hybrid circuit, a photoelectric converter, and an impedance conversion amplifier, and selectively detects the signal light interfering with local oscillation light output by the local oscillator out of the optical multiplexed signals; and the variable optical attenuator is disposed in the optical path of the optical multiplexed signals in a stage preceding the photoelectric converter, inputs the optical multiplexed signals, and outputs them to the coherent optical receiver controlling the intensity of the optical multiplexed signals based on the first control signal.

Methods and apparatus to realize high spectral efficiency in optical signals transmitted over long distances.

Aspects of the present disclosure are directed to improved systems, methods, and structures providing coherent detection of DAS. In sharp contrast to the prior art, systems, methods, and structures according to aspects of the present disclosure advantageously reduce the beating diversity terms such that required memory and bandwidth are reduced over the art.

An optical coherent receiver includes a 90-degree optical hybrid configured to receive an input signal and a reference signal, and mix the input signal with four quadrature states associated with the reference signal to generate four output signals. The 90-degree optical hybrid includes a plurality of 3-dB couplers; and a plurality of optical waveguides, wherein each optical waveguide of the plurality of optical waveguides couples two respective 3-dB couplers of the plurality of 3-dB couplers, and wherein each optical waveguide of the plurality of optical waveguides has a same optical path length. Each optical waveguide of the plurality of optical waveguides is dimensioned according to a figure of merit (FoM) to reduce a phase error.

An optical modulator includes first and second waveguides; a first phase shifter provided in at least one of the first and second waveguides and configured to control a phase of the laser beam; a first optical element configured to combine the laser beam propagating through the first waveguide and the laser beam propagating through the second waveguide and separate the combined laser beam into two laser beams; a third (fourth) waveguide on which one (the other) of the laser beams separated by the first optical element is incident; a second phase shifter provided in at least one of the third and fourth waveguides and configured to control a phase of the laser beam; and a second optical element configured to combine the laser beam propagating through the third waveguide and the laser beam propagating through the fourth waveguide and emit the laser beam in the superposition state.

A network or system in which a hub or primary node may communicate with a plurality of leaf or secondary nodes. The hub node may operate or have a capacity greater than that of the leaf nodes. Accordingly, relatively inexpensive leaf nodes may be deployed to receive data carrying optical signals from, and supply data carrying optical signals to, the hub node. One or more connections may couple each leaf node to the hub node, whereby each connection may include one or more spans or segments of optical fibers, optical amplifiers, optical splitters/combiners, and optical add/drop multiplexer, for example. Optical subcarriers may be transmitted over such connections, each carrying a data stream. The subcarriers may be generated by a combination of a laser and a modulator, such that multiple lasers and modulators are not required, and costs may be reduced. As the bandwidth or capacity requirements of the leaf nodes change, the number of subcarriers, and thus the amount of data provided to each node, may be changed accordingly. Each subcarrier within a dedicated group of subcarriers may carry OAM or control channel information to a corresponding leaf node, and such information may be used by the leaf node to configure the leaf node to have a desired bandwidth or capacity.

An optical coherent receiver includes a 90-degree optical hybrid configured to receive an input signal and a reference signal, and mix the input signal with four quadrature states associated with the reference signal to generate four output signals. The 90-degree optical hybrid includes a plurality of 3-dB couplers; and a plurality of optical waveguides, wherein each optical waveguide of the plurality of optical waveguides couples two respective 3-dB couplers of the plurality of 3-dB couplers, and wherein each optical waveguide of the plurality of optical waveguides has a same optical path length. Each optical waveguide of the plurality of optical waveguides is dimensioned according to a figure of merit (FoM) to reduce a phase error.