Crosstalk And Crosstalk Mitigation Techniques In WDM Optical Network

Optical fiber is the best choice to satisfy high-bitrate and high-capacity transmission for its wide bandwidth and low loss. With the widespread use of EDFA and the advent of WDM, several Tbit/s information can be transmitted on one optical fiber. However, with the ever-increasing demand for data transmission capacity in optical networks, some transmission impairment cannot be avoided. With the decreasing of channel spacing and the increasing of power in optical fiber, fiber nonlinearity impairs system performance. On the other hand, in OXC nodes, WDM signals of the same wavelength are going to be routed in a switch matrix, where partial failure of switches can generate interferometer effects of optical crosstalk that corrupts a desired signal, thus affecting system performance of optical nodes. In the dissertation, crosstalk and crosstalk mitigation techniques in WDM systems is presented theoretically and experimentally. The main contents are as follows:The impact of nonlinear crosstalk on WDM systems and crosstalk mitigation techniques is investigated. FWM in a high nonlinear fiber is studied experimentally. It is shown that 1.5dB larger penalty is introduced in the case of equal channel spacing than in unequal channel spacing.Optical amplifiers are key components for large capacity long haul transmission and optical processing. Semiconductor optical amplifiers (SOAs) have been extensively studied, since they have several desirable features such as wide gain spectrum, compactness and possibility of integration with other components. Theoretical study on channel crosstalk due to gain saturation in SOAs is presented. Crosstalk mitigation techniques in SOAs are studied. We have demonstrated experimentally that a CW light propagating with signal can mitigate crosstalk in SOAs at channel rates of 40Gb/s. Reduction of crosstalk in SOA by amplifying dispersed WDM signals is confirmed experimentally.An improved dilated Benes(IDB) network and a generally improved dilated Benes(GIDB) network composed of optical switches and SOA gates are introduced to improve the crosstalk and optical path loss characteristics of the dilated Benes(DB) network. The signal-to-noise ratio (SNR) characteristic of the GIDB network and IDB network are compared with the DB network when crosstalk coefficient of switching elements is–20dB. It can be seen that the IDB network improves SNR about 3dB. When the switching capacity N is small, the GIDB network increases SNR about 5dB. With the increase of capacity N, the GIDB network improves the SNR characteristic notably more and more.The crosstalk sources in the share-per-node OXCs and share-per-link OXCs are identified and the power penalty impose by crosstalk are calculated. Results show that coherent crosstalk is found to be the most important limitation in OXCs, especially when the delay between the signal and crosstalk less than bit duration T. At the same time, the impact of wavelength conversion on crosstalk in OXCs is considered. It can be found that the impact of crosstalk will be reduced compared with the case without wavelength converter.Intraband and interband crosstalk are studied experimentally. It is shown that interband crosstalk can be removed with narrow-band filter and have no impact on WDM system. Coherent crosstalk and incoherent crosstalk influence are compared experimentally. Coherent crosstalk damages system performance more than incoherent crosstalk. When polarization states of signal and crosstalk align, crosstalk cause higher noise power than when they misalign.BER produced by crosstalk is experimentally investigated and compared in 4×4 Benes networks, DB networks, IDB networks and GIDB networks. Experimental results show that the GIDB network can effectively eliminate coherent crosstalk influence even though the crosstalk coefficient of the switching elements is–20 dB.