We propose the ultrahigh-speed demultiplexing of Nyquist OTDM signals using an optical Nyquist pulse as both a signal and a sampling pulse in an all-optical nonlinear switch. The narrow spectral width of the Nyquist pulses means that the spectral overlap between data and control pulses is greatly reduced, and the control pulse itself can be made more tolerant to dispersion and nonlinear distortions inside the nonlinear switch. We apply the Nyquist control pulse to the 640 to 40 Gbaud demultiplexing of DPSK and DQPSK signals using a nonlinear optical loop mirror (NOLM), and demonstrate a large performance improvement compared with conventional Gaussian control pulses. We also show that the optimum spectral profile of the Nyquist control pulse depends on the walk-off property of the NOLM.
We report a substantial increase in PMD tolerance in a single-channel ultrahigh-speed transmission using optical Nyquist pulses. We demonstrate both analytically and experimentally a large reduction in depolarization-induced crosstalk with optical Nyquist pulses, which is one of the major obstacles facing polarization-multiplexed ultrashort pulse transmission. By taking advantage of the high PMD tolerance, a low-penalty 1.28 Tbit/s/ch optical Nyquist TDM transmission at 640 Gbaud was achieved over 525 km.
We evaluate the dependence of system performance on the roll-off factor, α, of a Nyquist pulse in a single-channel 1.28 Tbit/s-525 km transmission both experimentally and analytically. Low α values are preferable in terms of spectral efficiency and tolerance to chromatic dispersion and polarization-mode dispersion, while a strong overlap with neighboring symbols results in larger nonlinear impairments. On the other hand, a Nyquist pulse with high α values also suffers from nonlinearity due to higher peak power. As a result, we found experimentally that the optimum α value is 0.4~0.6, which agrees well with the analysis.
We propose a simple and high-performance scheme for demultiplexing coherent Nyquist TDM signals by photo-mixing on a photo-detector with Nyquist LO pulses. This scheme takes advantage of the time-domain orthogonality of Nyquist pulses, which enables high-SNR demultiplexing and homodyne detection simultaneously in spite of a strong overlap with adjacent pulses in the time domain. The feasibility of this scheme is demonstrated through a demultiplexing experiment employing 80 Gbaud, 64 QAM Nyquist pulse OTDM signals. This scheme exhibits excellent demultiplexing performance with a much simpler configuration than a conventional ultrafast all-optical sampling scheme.
We demonstrate a 2.56 Tbit/s/ch polarization-multiplexed single-carrier transmission over 300 km using subpicosecond DQPSK signals. We adopted an ultrafast time-domain optical Fourier transformation technique to reduce waveform distortions. For such an ultrashort optical pulse, depolarization components resulting from second-order polarization-mode dispersion (PMD) become a dominant factor as regards signal distortion because of the broad signal bandwidth. The influence of inter-polarization crosstalk induced by second-order PMD, is presented in detail.
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