Likai Zhu

Impact of XPM and FWM on the digital implementation of impairment compensation for WDM transmission using backward propagation.

The impact of cross-phase modulation (XPM) and four-wave mixing (FWM) on electronic impairment compensation via backward propagation is analyzed. XPM and XPM+FWM compensation are compared by solving, respectively, the backward coupled Nonlinear Schrödinger Equation (NLSE) system and the total-field NLSE. The DSP implementations as well as the computational requirements are evaluated for each post-compensation system. A 12 x 100 Gb/s 16-QAM transmission system has been used to evaluate the efficiency of both approaches. The results show that XPM post-compensation removes most of the relevant source of nonlinear distortion. While DSP implementation of the total-field NLSE can ultimately lead to more precise compensation, DSP implementation sing the coupled NLSE system can maintain high accuracy with better computation efficiency and low system latency.

Folded digital backward propagation for dispersion-managed fiber-optic transmission

In periodically dispersion managed long-haul transmission systems, waveform distortion is dominated by chromatic dispersion. As a result of the periodic waveform evolution, the nonlinear behavior also repeats itself in every dispersion period. It is shown that, under the weakly nonlinear assumption, nonlinear effects accumulated in a large number (K) of spans can be approximated by nonlinear effects accumulated in a single span with the same dispersion map and K times the nonlinearity. Thus, significant savings in computational load can be achieved in digital compensation of fiber nonlinearity using folded digital backward propagation (DBP). Simulation results show that the required computation for DBP of dispersion managed transoceanic transmission systems can be reduced by up to 2 orders of magnitude with negligible penalty using folded DBP.

Nonlinearity compensation using dispersion-folded digital backward propagation

A computationally efficient dispersion-folded (D-folded) digital backward propagation (DBP) method for nonlinearity compensation of dispersion-managed fiber links is proposed. At the optimum power level of long-haul fiber transmission, the optical waveform evolution along the fiber is dominated by the chromatic dispersion. The optical waveform and, consequently, the nonlinear behavior of the optical signal repeat at locations of identical accumulated dispersion. Hence the DBP steps can be folded according to the accumulated dispersion. Experimental results show that for 6,084 km single channel transmission, the D-folded DBP method reduces the computation by a factor of 43 with negligible penalty in performance. Simulation of inter-channel nonlinearity compensation for 13,000 km wavelength-division multiplexing (WDM) transmission shows that the D-folded DBP method can reduce the computation by a factor of 37.

Multi-channel nonlinearity compensation of PDM-QPSK signals in dispersion-managed transmission using dispersion-folded digital backward propagation.

We demonstrate nonlinearity compensation of 37.5-GHz-spaced 128-Gb/s PDM-QPSK signals using dispersion-folded digital-backward-propagation and a spectrally-sliced receiver that simultaneously receives three WDM signals, showing mitigation of intra-channel and interchannel nonlinear effects in a 2560-km dispersion-managed TWRS-fiber link.

Compensation of nonlinear effects using digital coherent receivers

Nonlinearity compensation in digital coherent receivers is feasible from a fundamental point of view. This paper shows that it is possible to reduce the computational load required for nonlinearity compensation to practical level.

Complementary FIR Filter Pair for Distributed Impairment Compensation of WDM Fiber Transmission

A method of designing a complementary filter pair is proposed to reduce error accumulation in the split-step backward propagation for distributed impairment compensation of wavelength-division-multiplexing transmission. The complementary filter pair is designed so that the individual errors of the two filters cancel each other. A 12 times 100 Gb/s 16-ary quadrature amplitude modulation transmission system in nonzero dispersion-shifted fiber is simulated using such a filter pair. The required filter length is halved by the complementary filter pair design for a transmission distance of 1200 km.

Coherent optical transmission for digital and analog applications

This paper reviews recent work on receiver-side compensation of nonlinear impairments in optical fiber transmission. Intuitively, both linear and nonlinear impairments can be compensated since they are deterministic processes. In particular, fiber transmission can be modeled using the nonlinear Schrodinger equation that can be solved by the well-known split-step Fourier transform method and fiber impairment can be compensated by propagating the received signal backward in the digital domain. The first publication on digital backward propagation (DBP) also showed that the computation complexity can be met by the advances in DSP technology even if the split-step finite-impulse filtering method, which may be more suitable for real-time implementation.