|Title:||Monitoring and transmission strategies for long-haul digital coherent communication systems|
|Advisors:||Lau, Pak Tao Alan (EE)|
|Subject:||Optical data processing.|
Optical fiber communication.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||x, 78 pages : color illustrations|
|Abstract:||Over the last decade, fiber-optic communications have evolved from early non-coherent systems to digital coherent systems in order to comply with the exponential growth of bandwidth demand. Advanced optical modulation formats offering high spectral efficiencies have been successfully employed in conjunction with coherent receivers and digital signal processing (DSP). Additionally, complex network architectures utilizing reconfigurable optical add-drop multiplexers (ROADMs), flexible grid, modulation format and bandwidth have been incorporated in order to promote dynamicity, flexibility and better utilization of available transmission capacity. The fundamental technology shift brought about by digital coherent systems also impacted the roles, functionalities and research direction of optical performance monitoring (OPM) and long-haul transmission strategies: 1) Since all the linear impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD) can be estimated and fully compensated by numerous DSP algorithms in the digital coherent receiver, system performance is largely determined by the optical-signal-to-noise ratio (OSNR) and hence OSNR monitoring is most vital for long-haul coherent communication systems. 2) Because of Kerr nonlinearities, current fiber-optic transmission technologies designed for linear channels will not be capable of meeting the customer bandwidth demand even equipped with nonlinear compensation techniques such as digital back propagation (DBP). New transmission strategies fundamentally compatible with fiber nonlinearity is expected in the next few years. Therefore, this thesis will focus on the development of OSNR monitoring techniques and nonlinearity-compatible transmission strategies for long-haul digital coherent systems. In particular, a nonlinearity-insensitive OSNR monitoring technique is proposed as most fiber-optic systems are operating in the weakly nonlinear regime where nonlinear distortions are indistinguishable from ASE noise and thus traditional OSNR monitoring techniques cannot work accurately in such circumstance. It characterize fiber nonlinearity induced amplitude noise correlation among neighboring symbols as a quantitative measure of nonlinear distortions to the signal. By incorporating/calibrating such amplitude noise correlations into an error vector magnitude (EVM)-based OSNR estimator, nonlinearity-insensitive OSNR monitoring can be achieved. Experimental as well as simulation results demonstrate an accurate OSNR monitoring for both 112 Gb/s polarization-multiplexed (PM)-quadrature phase-shift keying (QPSK) systems and 224 Gb/s PM-16-Quadrature amplitude modulation (QAM) systems where the launched power can go up to 4 dBm.|
In addition, OSNR monitoring is still needed ubiquitously across the network including intermediate nodes. However, using full digital coherent receivers with symbol-rate bandwidth is simply too costly and impractical for this purpose. A low-cost and filtering-effect-insensitive OSNR monitor for distributed monitoring of optical networks is proposed. It utilizes reduced-complexity coherent receptions, electrical filtering and radio frequency (RF) power measurements. By measuring the RF power of three different frequency components of the coherently received baseband signals, the proposed technique is also insensitive to spectral narrowing by wavelength selective switches (WSSs). We experimentally demonstrate accurate (<0.7 dB error) OSNR monitoring for various modulation formats and transmission distances, hence different number of WSSs. We also study the robustness of the proposed technique to the fiber nonlinearity, laser effects in a five-channel wavelength-division-multiplexing (WDM) system at 50-GHz spacing and other practical considerations. Finally, nonlinear frequency division multiplexing (NFDM), a new transmission strategy that incorporates soliton theory with communication theory and fundamentally compatible with fiber nonlinearity is proposed and experimentally verified for the first time. It is based on the framework of nonlinear Fourier transform (NFT) and resembles the commonly-known orthogonal frequency division multiplexing (OFDM) philosophy in that independent information streams are encoded in parallel sub-carriers (eigenvalues) that are analytically shown to be independent of each other upon propagation in an ideal noiseless and lossless fiber channel. In principle, NFDM is also largely immune to XPM effects and thus it can potentially be a fundamental paradigm shift in long-haul WDM optical communications. On-off keying (OOK) modulated NDFM systems are investigated by simultaneous and independent modulation of 3-eigenvalue multi-soliton NFDM signals. The signal set consists of 1-, 2-, 3-solitons as well as their various nonlinear combinations. Error-free transmission over an 1800 km link of standard single-mode fiber (SSMF) with Raman amplification and coherent detection is demonstrated. Extensions to 4-eigenvalue transmissions are also experimentally investigated. Additionally, simulations are performed to study the impact of frequency offset and laser phase noise on such systems and the results show that they are robust to the frequency offset and laser phase noise in a certain range.
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