Abstract: A digital processor (DP) is configured to obtain a temporal sequence of digital phase distortion measurements of a first optical signal received by a coherent optical receiver (COR) from an optical fiber link, where the first optical signal co-propagates with a second, power-modulated, optical signal in different frequency channels. The DP is configured to estimate a cross-correlation between the temporal sequence of digital measurements and a temporal sequence of powers of the second optical signal for a plurality of relative time shifts between the sequences, and to identify a location along the optical fiber link based on a magnitude of the cross-correlation exceeding a threshold for a particular time shift.

Abstract: An apparatus for random key transmission comprising a laser (101) configured to generate a coherent source optical signal; a quantum random number generator, QRNG, (105) configured to generate random bits (R1); an electro-optical modulator (107) configured to modulate the coherent source optical signal to generate a modulated optical signal including Quadrature Phase Shift Keying, QPSK, pilot symbols temporally multiplexed with quantum key distribution, QKD, symbols representing the random bits, wherein quadratures values of the modulated optical signal follow a modulation map, wherein the modulation map is a quadrature amplitude modulation, QAM, based Probabilistic Constellation Shaping, PCS, modulation map defining a mapping function for the random bits.

Abstract: Proposed is a method of equalizing an optical signal that has an overall bandwidth formed on a number of adjacent spectral slots, wherein the signal comprises a set of non-overlapping subcarrier signals. A distribution of the subcarrier signals onto the slots is such, that at least one slot is occupied by more than one subcarrier signal. The signal is received and amplified. Respective power levels are measured for the subcarrier signals. Distribution data is provided, which indicates the distribution of the subcarrier signals onto the spectral slots. Power level data is provided, which indicates for the spectral slots respective desired power levels. For the spectral slots respective attenuation values are derived, using the measured power levels, the distribution data and the power level data. Finally, the optical transmission signal is attenuated within the spectral slots individually, using the derived attenuation values.

Abstract: Proposed is a method of equalizing an optical signal that has an overall bandwidth formed on a number of adjacent spectral slots, wherein the signal comprises a set of non-overlapping subcarrier signals. A distribution of the subcarrier signals onto the slots is such, that at least one slot is occupied by more than one subcarrier signal. The signal is received and amplified. Respective power levels are measured for the subcarrier signals. Distribution data is provided, which indicates the distribution of the subcarrier signals onto the spectral slots. Power level data is provided, which indicates for the spectral slots respective desired power levels. For the spectral slots respective attenuation values are derived, using the measured power levels, the distribution data and the power level data. Finally, the optical transmission signal is attenuated within the spectral slots individually, using the derived attenuation values.

Abstract: An apparatus, a method and a computer program for determining estimates is disclosed. The apparatus determines estimates for transmitted symbols from received symbols of an optical signal affected by cross-Polarization Modulation (XpolM). The apparatus groups two or more subsequent received symbols from the optical signal into a group of received symbols and estimates an optical channel based on the group of received symbols and a hypothesis on the transmitted symbols. The apparatus further determines a reliability measure for a combination of the group of received symbols, the channel estimate, and the hypothesis, and determines the estimates for the transmitted symbols based on reliability measures for multiple hypotheses on the transmitted symbols.

Abstract: An apparatus, a method and a computer program for determining estimates is disclosed. The apparatus determines estimates for transmitted symbols from received symbols of an optical signal affected by cross-Polarization Modulation (XpolM). The apparatus groups two or more subsequent received symbols from the optical signal into a group of received symbols and estimates an optical channel based on the group of received symbols and a hypothesis on the transmitted symbols. The apparatus further determines a reliability measure for a combination of the group of received symbols, the channel estimate, and the hypothesis, and determines the estimates for the transmitted symbols based on reliability measures for multiple hypotheses on the transmitted symbols.

Abstract: An exemplary technique is provided for a coherent optical receiver adapted to receive an optical signal transmitted over an optical transmission channel exhibiting Cross-Polarization Modulation (XPOLM). The received optical signal comprises a first polarization component and a second polarization component.