Advanced digital modulation formats for optical coherent transmission systems

Abstract

Field modulation with coherent detection leverages the use of optical channel bandwidth and allows mitigating transmission impairments by exploiting the two quadrature components of the optical carrier and providing a linear electrical-to-optical (EO) and optical-to-electrical (OE) conversion. In addition, polarization multiplexing represents an effective means to double the information rate of single-polarization systems. The advent of high-speed digital signal processing (DSP) engines and high-speed data converters has enabled the reliable transmission of bit rate in excess of 400 Gbit/s at achievable symbol rate using spectrally efficient dual-polarization (DP) M-ary quadrature amplitude modulation (DP-MQAM) and forward error correction (FEC) schemes. The evolution of such systems towards higher capacity depends on designing signaling schemes which increase the information rate in the presence of linear and nonlinear impairments. QAM constellation shaping has emerged as a practical approach for tailoring modulation formats to be more power efficient and more tolerant to fiber nonlinearities.

The major contribution of this thesis is to extensively analyze the role of constellation shaping in boosting the transmission capacity of modern coherent systems. This includes describing a simplified approach to estimate the system performance of arbitrary modulation formats following nonlinear transmission. The approach employs well-established models for intra- and inter- nonlinear effects and hence requires less computational effort compared to the more accurate split-step Fourier simulation. Additionally, I proposed a novel framework for optimized constellation design. The framework uses genetic algorithms to optimize multidimensional QAM constellation subset selection and has the potential to provide a methodical alternative to existing non-trivial constellation design approaches. Using this approach, I could obtain optimized 4D and 8D constellations. Finally, I thoroughly investigated the system performance of probabilistically shaped (PS) and uniform DP-64QAM constellations in the presence of polarization-dependent loss (PDL). An accurate simulation model was employed to evaluate the fluctuations of the bit-wise (BW) achievable information rate (AIR) due to PDL. Moreover, extensive laboratory experiments were conducted to demonstrate the statistical behaviour of the BW AIR. A single-span loop setup with partially automated equipment allowed accurate emulation of distributed link PDL as well as capturing a sufficient number of PDL instances.