Impact of Chromatic Dispersion on Oversampled Digital Clock Recovery in Direct-Detection Systems: Analysis and Solutions

Cloud-based services such as the training of artificial intelligence (AI)/machine learning (ML) models in large-scale data centers as well as smart city applications, such as the Internet of things (IoT), connected driverless vehicles, etc., are key drivers for ever higher data rates, especially in short-range fiber-optic links. Such optical transceivers employ intensity-modulation and direct-detection (IM/DD) to reduce the costs and power consumption and, therefore, chromatic dispersion (CD) is a nonlinear channel effect with respect to the received signal. With net data rates currently going up to 100 Gbit/s for passive optical networks (PONs) and 400 Gbit/s for Ethernet connections, CD is a main limitation in transmission performance at such high data rates. To reconstruct the signal impaired by CD, research has mainly discussed nonlinear equalizers and ML models, while, to the best of the authors knowledge, no research has been conducted on the influence of CD on non-data-aided, digital clock recovery in direct-detection systems. In this work, we investigate analytically and in simulation the effect of CD on oversampled digital clock recovery and find that for some dispersion values, clock recovery fails entirely. Based on our findings, we present two dispersion-tolerant clock recovery algorithms that work for non-return-to-zero (NRZ) and digital pulse-shaped signals as well as faster-than-Nyquist (FTN) signals. Finally, we validate our findings and algorithms in a 34-GBd four-level pulse amplitude modulation (PAM4) transmission experiment for NRZ and root-raised cosine (RRC) pulse-shaped signals and different accumulated dispersion values.