Abstract: Apparatus and methods are provided for transmitting and/or for receiving data over a free-space optical link. An optical data transmitter generates duplicate digital data streams carrying a same digital data sequence with a relative temporal offset and mixes them onto two different polarization channels according to a preselected linear mixing rule. An optical data receiver demodulates first and second digital streams from the two polarization channels and unmixes the demodulated data, to produce first and second output digital data streams according to a preselected linear unmixing rule. The receiver applies a quality indicator to select between segments of the respective output digital data streams that correspond to the same transmitted data sequence.

Abstract: Proposed is a method of optical data transmission. The method comprises different steps. At least one stream of data bits is received. Sets of data bits are mapped onto successive logical states. The logical states correspond to respective sets of two initial QPSK symbols chosen according to a QPSK mapping and according to a set partitioning rule. The logical states are differentially encoded according to a differential encoding rule. For the differentially encoded logical states respective sets of two resulting QPSK symbols are derived according to the QPSK mapping and according to the set partitioning rule. The encoded sets of two QPSK symbols are transmitted using polarization division multiplexing. The differential encoding rule results in a differential encoding of one of the initial QPSK symbols but not the other of the initial QPSK symbols.

Abstract: The present document relates to optical communication systems. In particular, the present document relates to high efficiency wavelength division multiplexing (WDM) optical communication systems. An optical transmitter (210) adapted to transmit an optical signal on an optical wavelength division multiplexed, referred to as WDM, transmission channel (111) to a corresponding optical receiver (230) is described.

Abstract: Proposed is a method of optical data transmission. The method comprises different steps. At least one stream of data bits is received. Sets of data bits are mapped onto successive logical states. The logical states correspond to respective sets of two initial QPSK symbols chosen according to a QPSK mapping and according to a set partitioning rule. The logical states are differentially encoded according to a differential encoding rule. For the differentially encoded logical states respective sets of two resulting QPSK symbols are derived according to the QPSK mapping and according to the set partitioning rule. The encoded sets of two QPSK symbols are transmitted using polarization division multiplexing. The differential encoding rule results in a differential encoding of one of the initial QPSK symbols but not the other of the initial QPSK symbols.

Abstract: The present document relates to an optical communication system. In particular, the present document relates to the alignment of the laser frequency at a transmitter of the optical communication system and the local oscillator frequency at a coherent receiver of the optical communication system. A coherent optical receiver (222) is described. The receiver (222) comprises a reception unit adapted to convert a received optical signal at a carrier frequency into a digital signal using a local oscillator, referred to as LO, having an LO frequency; a spectral analysis unit adapted to estimate a degree of asymmetry of a spectrum of the received optical signal, based on the digital signal; and a transmission unit adapted to transmit an indication of the degree of asymmetry to a near-end transponder (210) comprising a transmitter (211) of the received optical signal.

Abstract: Proposed is a method of demodulating a phase modulated optical signal received from an optical channel. A time-discrete electrical signal is derived from the phase modulated optical signal, using a local optical signal, which has a frequency that is essentially equal to a carrier frequency of the phase modulated optical signal. A phase error between the local optical signal and the phase modulated optical signal is compensated, by deriving from the local optical signal a phase offset and modifying the derived time-discrete electrical signal by this phase offset. A chromatic dispersion caused by the optical channel is compensated, by filtering the modified time-discrete electrical signal using a digital filter.

Abstract: The present document relates to optical communication systems. In particular, the present document relates to high efficiency wavelength division multiplexing (WDM) optical communication systems. An optical transmitter (210) adapted to transmit an optical signal on an optical wavelength division multiplexed, referred to as WDM, transmission channel (111) to a corresponding optical receiver (230) is described.

Abstract: The present document relates to an optical communication system. In particular, the present document relates to the alignment of the laser frequency at a transmitter of the optical communication system and the local oscillator frequency at a coherent receiver of the optical communication system. A coherent optical receiver (222) is described. The receiver (222) comprises a reception unit adapted to convert a received optical signal at a carrier frequency into a digital signal using a local oscillator, referred to as LO, having an LO frequency; a spectral analysis unit adapted to estimate a degree of asymmetry of a spectrum of the received optical signal, based on the digital signal; and a transmission unit adapted to transmit an indication of the degree of asymmetry to a near-end transponder (210) comprising a transmitter (211) of the received optical signal.

Abstract: The embodiments of the present invention describe a method for optimizing the capacity of an optical communication network that uses wavelength division multiplexing, wherein the spectral distribution of the signals intended to be transmitted over a plurality of channels is done dynamically through the use of a variable spectrum grid whose spectral spacings between two successive channels are determined based on the spectral width of said signals and in which dynamic filtering of said signals is carried out before their transmission in order to adjust their spectral width based on the available spectral space, and thereby reduce crosstalk between adjacent channels when the signals are transmitted.

Abstract: Proposed is a method of demodulating a phase modulated optical signal received from an optical channel. A time-discrete electrical signal is derived from the phase modulated optical signal, using a local optical signal, which has a frequency that is essentially equal to a carrier frequency of the phase modulated optical signal. A phase error between the local optical signal and the phase modulated optical signal is compensated, by deriving from the local optical signal a phase offset and modifying the derived time-discrete electrical signal by this phase offset. A chromatic dispersion caused by the optical channel is compensated, by filtering the modified time-discrete electrical signal using a digital filter.

Abstract: The embodiments of the present invention describe a method for optimizing the capacity of an optical communication network that uses wavelength division multiplexing, wherein the spectral distribution of the signals intended to be transmitted over a plurality of channels is done dynamically through the use of a variable spectrum grid whose spectral spacings between two successive channels are determined based on the spectral width of said signals and in which dynamic filtering of said signals is carried out before their transmission in order to adjust their spectral width based on the available spectral space, and thereby reduce crosstalk between adjacent channels when the signals are transmitted.