1、 1 中文 4940 字 出处： Optical Communication Theory and Techniques. Springer US, 2005: 137-150 Characterization of Intrachannel Nonlinear Distortion in Ultra-high Bit-rate Transmission Systems Invited Paper Robert I. Killey, Vitaly Mikhailov, Shamil Appathurai, and Polina Bayvel Optical Networks Group, De
2、partment of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK r.killeyee.ucl.ac.uk Abstract: Signal distortion due to intrachannel cross-phase modulation and four-wavemixing limits transmission distances in ultra-high bit-rate optical communicati
3、ons.To gain an understanding of the effects of nonlinear pulse interactions and to quantify the effectiveness of new methods to suppress them, accurate characterization techniques are required to isolate the effects of fibre nonlinearity from the other impairments which occur in transmission. In thi
4、s paper, we discuss two techniques: firstly, the direct measurements of the signal waveform distortion (pulse timing jitter, amplitude fluctuations, and FWM-induced ghostpulse power) and, secondly, measurements of the BER-dependence on optical signal launch power. We describe the use of these charac
5、terization methods to investigate the suppression of nonlinear distortion through the use of optimized dispersion maps, alternate-polarization and alternate-phase return-to-zero signal formats. Key words: dispersion management； optical fiber nonlinearity； four wave mixing； cross phase modulation. 1.
6、 INTRODUCTION Limits to the transmission distances achievable in high bit-rate systems are imposed by nonlinear refraction. The intensity modulation of the signals induces optical phase shifts due to the intensity-dependence of the refractive index of the transmission fibres. In WDM systems with nar
7、row channel spacing, the nonlinear refraction leads to interactions between channels through cross-phase modulation (XPM) and four-wave-mixing (FWM). As the channel bit-rate and channel wavelength spacing is increased, the impact of these interchannel effects is reduced, due to the walk-off between
8、the channels caused by the fibre dispersion, and intrachannel nonlinear effects become increasingly dominant. These effects result from the dispersion-induced pulse broadening and the consequent nonlinear interaction between overlapping pulses within the same channel. As in the multi-channel case, t
9、he pulse overlap leads to cross-phase modulation and four-wave-mixing. These effects were investigated experimentally and numerically in 110 and a number of methods have been proposed and 2 demonstrated aimed at suppressing intrachannel nonlinear distortion, through optimization of the dispersion ma
10、p and pre-compensation 1116, the use of optimized signal formats 17, in particular the use of phase modulated returnto- zero formats 1826, alternate-polarization RZ 2729, subchannel multiplexing30, polarization mode dispersion-supported transmission 31 and channel precoding 32. In this paper, we des
11、cribe experimental and numerical techniques developed to characterize intrachannel cross-phase modulation and four-wave-mixing in long-haul 40 Gbit/s systems. We explain how these techniques can be used to quantify the improvements in performance achievable using new methods, including optimized dis
12、persion maps and novel modulation formats. 2. CHARACTERIZING INTRACHANNEL NONLINEAR DISTORTION One challenge faced in characterizing any physical effect in optical signal transmission is that of isolating it from the numerous other effects which occur simultaneously. The goal of our research was to
13、isolate and quantify the impact on the system performance of intrachannel XPM and FWM, separating it from other effects including amplifier noise, chromatic dispersion, polarization mode dispersion and transmitter and receiver patterning. Methods of measuring the nonlinear distortion include direct
14、BER and Qfactor measurements, and measurements of the signal eye diagrams and signal waveforms using sampling oscilloscopes, for a range of signal launch powers and transmission distances. In the following sections, simulations and experimental measurements of nonlinear pulse interactions in 40 Gbit
15、/s transmission,carried out with the groups recirculating fibre loop testbed, are described 2.1 Signal waveform distortion An effective method of characterizing the effects of intrachannel nonlinear distortion is the direct measurement or calculation of the signal waveform distortion after transmiss
16、ion. Three measures of distortion can be used: pulse timing jitter, resulting from XPM, pulse amplitude fluctuations arising from both FWM and XPM, and ghost pulse power, transferred from the pulses to the zero bit slots through FWM. The effectiveness of the use of novel signal formats and optimized
17、 dispersion maps in reducing nonlinear pulse interactions can be demonstrated by showing the reduction of these three effects. 2.1.1 Optimization of pre-compensation In our initial studies, timing jitter and ghost pulse power were calculated for typical system parameters using the split-step Fourier
18、 algorithm for a single channel 40 Gbit/s link employing standard single-mode fibre (SMF), with dispersion D = 17 ps/(nmkm),fibre nonlinear coefficient=1.2（ W.km)-1and loss=0.21dB/km.Each span comprised 60 km of SMF with dispersion compensating fibre following each span (Fig. 1) 8. 3 Figure 1. Trans
19、mission system with N spans, pre-compensation dispersion Dpre and residual dispersion per span Dres Unchirped RZ pulses, which are more robust than NRZ 17, with 9 ps FWHM were transmitted, encoded with 256-bit random bit sequences. Ideal transmitter and receiver characteristics and the assumption of
20、 noiseless amplifiers allowed the investigation of distortion due solely to the fibre nonlinearity. The system considered was a 12 span link, with each span exactly post-compensated by DCF and the optical launch power was 6 dBm.An effective technique to suppress intrachannel nonlinear effects is the
21、 optimization of the signal waveform at the input to the link. In particular, the use of simple pre-compensation can have a significant effect on the quality of the received signal 2,8,13,14,16. In this study, the effect of using pre-compensation at the transmitter was explored by carrying out multi
22、ple simulations with a range of pre-compensation values from 0 to -600 ps/nm. In each case, additional dispersion was added at the receiver to ensure the total link cumulative dispersion was zero. It can be seen from Fig. 2 that the timing jitter varied between values of 0.4 and 1.4 ps, and was minimized at one particular value of precompensation,approximately -200 ps/nm. Figure 2. Intrachannel cross-phase modulation (IXPM) timing jitter and intrachannel
