The Internet has become one of the essential pillars to sustain life, especially in the pandemic era, and its backbone is fiber optic communication links. Increased demand for capacity in optical communication systems is met with coherent communication technology coupled with advanced digital signal processing algorithms. Next-generation fiber optic communication systems target a single carrier transmission operating well beyond 100 Gbps, for both long-haul and for short-distance access and data-center networks. These high bit rates are realised with higher-order modulation at large symbol rates, made possible with high-speed modulators and coherent receivers. The tolerance to device imperfections such as - the nonlinear operation of electronic amplifiers, path mismatch, and phase imbalance between the in-phase (I) and quadrature (Q) signals - become smaller at high signalling rates and for higher-order modulation. This, coupled with laser phase noise and frequency offset in the heterodyne detection pose critical challenges to the performance of high-speed single-carrier transmission systems. Efficient digital signal processing algorithms need to be developed to ensure compensation of a combination of these impairments with minimal complexity and latency. Emulation of signal propagation through long links in a laboratory environment is also critical to study the effectiveness of the developed algorithms.

The first part of our talk will focus on describing the digital signal processing algorithms that work for polarisation multiplexed 16 and 64QAM modulation at 32 GBd, focussing on the compensation of transceiver IQ imbalance and skew in the single carrier transmission system. We use the widely linear filtering approach that allows the joint compensation of IQ gain and phase imbalances, IQ skew, laser phase noise and frequency offset, and analyse its performance benefits. We will present the experimental results for the transmission of data modulated with 16 QAM and 64 QAM, to validate the benefits. The later part of the talk will focus on the design of an optical recirculating loop to study the performance of long-haul transmission in the laboratory using a single span of fiber and its experimental demonstration. We demonstrate the error-free transmission (SD-FEC bounded) of polarisation multiplexed QPSK and 16QAM modulated data with a 25kHz linewidth laser over 492 km and 246 km respectively using a single fiber span of length 49.2 km.