Signal Processing Technologies In Polarization Division Multiplexing Systems

With the rapid development of internet-driven service, especially the explosive increases of the internet-based video and peer-to-peer interactive applications, the bandwidth requirement for the backbone communcaiton network has been increasing dramtically. Currently depoyed DWDM system is not able to meet this ever-growing bandwidth requirement, and signifcant improvent of the transmission capacity for current DWDM opitcal communication system is required imperatively. PDM technique which utilizes the polarization dimension of light, carries two independent data at the same wavelength with orthognal states of polarizaion. It can double the system capacity and spectral efficiency directly. Meanwhile, it can combine with the advanced modulation format and coherent detection techniques, which not only can increase the system capacity and spectral efficiency further, but also can achieve polarizaion demultiplexing and compensate for transmission impairments by employing the DPS techniques in digital domain neatly. PDM technique has been emerging as one of the key techniques for next optical communication networks. This work focuses our researches on the signal processing techniques for PDM system, both all-optical and digital signal processing techniques. The aims are to develop some novel signal processing techniques, and hence, to provide some potential and alternative approaches for next generation optical communication networks in terms of enhancement for system reconfigurability, transpareny, capacity and spectral efficiency.In this dissertation, signal processing tehniques for PMD system are extensively studied theoretically and experimentally, and the main contents include the following parts. Firstly, we propose a novel all-opitcal automatically polarization demultiplexing scheme for direct detetion PDM system which uses the tag light transmitted in a different but close wavelength to the data signal as the feedback control. The effectiveness of this scheme is demonstrated experimentally. Then, we extensively study some fiber-nolinearity-based all-optical signal processing techniques and propose a serial all-optical signal processing schemes for direct detection PDM system, including all-optcal regeneration, wavelength conversion and format conversion. We experimentally achieve these functions for two tributaries of a PDM signal simultanously. Finally, we concentrate our researches on DSP techniques and some typical algorithmes for coherent detection PDM system, and combine these techniques with60-GHz mm-wave RoF communication techniques. The main acheivement of this dissertation are listed as following:(1) We propose a novel all-opitcal atumatically polarization demultiplexing scheme for direct detetion PDM system which uses the tag light transmitted in a different but close wavelength to the data signal as the feedback control. The effectiveness of this scheme is demonstrated experimentally in2×10-Gb/s NRZ-OOK-PDM and RZ-OOK-PDM system. We also study the affect of wavelength gap between signal and tag light and signal power on the demultiplexing performance. Experimental resutls show that tag light will degrade the signal performance when the two wavelengthes are too close and the affect of sigal power is neglectable in the normal transmission power range (e.g. under6-dBm).(2) We demonstrate simultaneous10-Gb/s NRZ-to-RZ format conversion and wavelength multicasting from one to many data channels in a highly nonlinear fiber with only a single pump. Functionalities are achieved based on various nonlinear effects, such as FWM, XPM, and so on, between the pump channel and the NRZ data channel. Up to ten out of nine and twelve out of eleven converted data channels are error-free (10-9BER) as the wavelength spacing between the pump and the NRZ signal is set to0.4-nm and0.8-nm, respectively.(3) We propose and experimentally demosnstrate two SPM-based2R all-optical regeneration schemes for PDM signal, including bidirectional configuration and polarization nonlinear loop mirror configuration. By utilizing the bidirectional configuration, mitigation of inter-channel nonlinearities is achieved through a bidirectional configuration, and rejection of backward stimulated Brillouin scattering noise is obtained by signal re-polarizing before the offset filter and putting the center wavelength of filter at the short wavelength side of the signal. The power penalty improvement up to2.0dB for two PDM signals at10-9BER is achieved. Comparing to bidirectional configuration, polarization nonlinear loop mirror simplifies the regenerator configuration with only one offset optical filter at the output. Experimental results show that PDM signals with orthogonal polarization states are regenerated simultaneously and reassembled automatically. Up to3.0-dB eye-diagram-based signal-to-noise-ratio (SNR) improvement is achieved for both channels with the input SNR of6.7-dB.(4) We propose and experimentall demonstrate a wavelength conversion and format conversion for RZ-OOK signals in PDM systems based on XPM in highly nonlinear fiber using a polarization nonlinear loop mirror configuration. The affect of back scattering noise, FWM, and SPM on the performance of the wavelength or format convertor are invesitigated. For the wavelength convertor, eye-diagram-based SNR and BER results indicate that successful wavelength conversion can be achieved over the whole C-band. As for the format convertor, less thanl-dB power penalty is achieved, and its potential application in high-speed PDM system are proved. (5) DSP functionalities and corresponding algorithms for coherent detection PDM system reciever are extensively investigated. Then, we combine these algorithms in high-speed optical communcation with60-GHz mm-wave RoF system and two main results are obtained. For one thing, up to～±1.3×10-3frequency offset times symbol rate product (△f-Ts) is well compensated at the given bit-error-ratio (BER) value of～10-3in differentially coded square16-QAM60-GHz mm-wave RoF system by utilizing the D-D carrier phase recovery algorithm. For another, optical and wireless tranmission link crosstalks are mitigated simultaneously by applying the CMA-based butterfly filter structure equalizer in PDM60-GHz mm-wave RoF MIMO system.Signal processing techniques will face more and more challenges in continously developing high-speed optical communication system which exploites advanced modulation format and novel transimssion techniques. Our ongoing researches will focus on signal processing techniques in such systems.