A modified 160 Gbit/s OTDM system based on QPSK Nyquist pulses and coherent detection

In this paper, we design and implement a 160 Gbit/s orthogonal time division multiplexing (OTDM) transmission system based on quadrature phase shift keying (QPSK) Nyquist pulses and coherent detection. We proposed an optimized Nyquist pulses generation scheme, which produce ultra-flat frequency combs with near-perfect rectangular spectrum and high suppression of out-of-band components by two cascaded Lithium Niobate Mach Zehnder modulators (LN-MZMs). Divided by variable length of optical delay line (Odl), 10GBaud QPSK Nyquist pulses are multiplexed to 80GBaud in a single wavelength. A set of training sequence (TS) which are inserted in front of data are employed to realize the inclusion of optical phase-locking and estimate channel transmission function. In this scheme, we make use of the TS overhead to achieve homodyne detection synchronization between local advanced receiver and Nyquist-pulses generator which is controlled by loop digital signal processing (DSP) module via an optical phase-locking loop. Meanwhile, for the advantage of the TS scheme and DSP, the Nyquist system compensates the residual physical dispersion and improves the transmission quality after 150 km transmission. •A beyond 150 km modified OTDM system based on QPSK Nyquist pulses and coherent detection is proposed.•The ultra-flat frequency comb with the flatness round 0.10 dbm produced by two cascaded LN-MZMs is used as the carrier.•The Nyquist pulses are modulated and multiplexed by optical delay line in to 8 channels.•The training sequences which take responsibility for synchronization and compensation is involved.•The impairment compensation algorithm based on channel transfer matrix.