Common-Channel Optical Physical-Layer Network Coding

We propose and experimentally demonstrate a coherent optical physical-layer network coding scheme on polarization-multiplexed differential quaternary phase-shift keying (PM-DQPSK) signals, in which the two PM-DQPSK signal components in the network-coded signal occupy the same channel and cannot be separated by conventional means of demultiplexing. In the scheme, network coding at intermediate nodes is realized by combining the two PM-DQPSK signals with a coupler. For network decoding at terminal nodes, one of the signal components is reconstructed and subtracted from the network-coded signal to obtain the other signal component. From its known bit sequence, waveform reconstruction of the signal to be subtracted is implemented by a lookup table. Carrier phase estimation for the reconstructed waveform utilizes the property that DQPSK signals have constant modulus at the sampling point. Experimental results show that, for 12-GBd PM-DQPSK signals, network decoding at a bit error rate of 10^-3 can be achieved, with 2.1-dB optical signal-to-noise ratio penalty relative to back-to-back transmission.