Research On Some Key Technologies For High-speed Optical Fiber Communication

Large-capacity and long-haul dense wavelength-division multiplexed (WDM) transmission technologies have been rapidly developed recently. Up to now, numerous WDM transmission experiments at 40Gb/s per wavelength have been reported, and have demonstrated the achievements of more than 10Tb/s capacity per fiber. Transmission at so high speed must be severely limited by transmission impairments. In high-speed optical fiber communication the impairments include accumulated ASE (Amplified Spontaneous Emission) noise from optical amplifiers, group-velocity dispersion (GVD), polarization mode dispersion (PMD) and fiber nonlinear effects such as self-phase modulation (SPM), cross-phase modulation (XPM), stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and four-wave mixing (FWM). In order to overcome these transmission impairments, several key enabling technologies such as GVD compensation, PMD compensation, distributed Raman amplification (DRA), forward error correction (FEC) and suitable modulation format, should be employed. Some modulation formats may overcome the transmission limitations and improve spectral efficiency.Today WDM systems being installed are mainly point-to-point systems where the switching and processing is made by electronics. The introduction of optical add/drop multiplexer (OADM) and optical cross-connect (OXC) is a natural evolution of WDM technology to include switching in the optical layer. Optical crosstalk imposes a major limit to the practical implementation of WDM optical networks. Two crosstalk mechanisms, interband and intraband crosstalk, caused by nonideal wavelength (de)multiplexer, space switch and optical filter. The intraband crosstalk, where the crosstalk elements fall within the signal wavelength carrier band, is much more deleterious to the network end-end transparency performance.This paper deals with modulation formats for high-speed optical transmission and crosstalk in WDM optical networks, and the main research work and results are listed as follows.1. The modulation formats suitable for 10Gb/s systems, which include optical duo-binary, optical single sideband (SSB) and optical quaternary, are studied. The bandwidth of the three kinds of modulated format signal is approximately one half that of conventional binary (or double sideband: DSB) signal. Simulation results show that optical quaternary transmission is more susceptible to noise and laser's side mode, and results in large penalty. It is shown that transmission distance of theoptical duobinary (or SSB) signal due to chromatic dispersion limitation is about twice that of the conventional binary (or DSB) signal in the perfect extinction. It is discovered that the optical duobinary (or SSB) transmission has characteristic superior to that of the binary (DSB) transmission only if the extinction ratio is more than 25dB(or 20dB).2. Some key enabling technologies for 40Gb/s systems are studied. A 40Gb/s RZ-based standard single-mode fiber (SMF) transmission system is simulated which uses precompensationing dispersion or postcompensationing dispersion separately. It is shown that transmission performance which uses postcompensation is superior to that uses precompensation and it can be concluded that the postcompensation should be used for dispersion compensation in long-haul, high-capacity optical transmission in the case of the nonlinear effects. Four different modulation formats including nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed return-to-zero(CSRZ) and single sideband return-to-zero (SSB-RZ), is introduced, and method of their modulated signal generation is described, and a comparison of their spectra and waveforms is made. 40Gb/s single channel transmission over nonzero dispersion-shifted fiber (NZDSF) is simulated for the four formats. It is shown that RZ. CSRZ or SSB-RZ format is more tolerant than NRZ in nonlinearities, and CSRZ tolerance to nonlinearities is the highest among the four formats. Compared to NRZ, CSRZ is 10.2dBm more tolerant in fiber input power. RZ, CSRZ and...