Analysis and Demonstration of Low-Complexity Joint Optical-Electrical Feedforward Equalization (OE-FFE) For Dispersion-Limited High-Speed IM/DD Transmission

High-speed intensity-modulation direct-detection (IM/DD) fiber optical transmission system is of great interest to the applications of datacenter interconnects (DCI) as well as edge/access networks. However, when the fiber reach is beyond 40 km such as ER or ZR, the power fading induced by chromatic dispersion (CD) in C-band, S-band and even edge-wavelengths of O-band becomes a severe issue with the ever-increasing signal data rate. In this work, we focus on an ultra-low complexity joint optical-electrical feedforward equalization (OE-FFE) scheme to mitigate the CD distortion. We establish a comprehensive theory to jointly model the IM/DD channel, optical circuit and the digital equalizer, which offers analytical bit-error ratio (BER) expressions of pulse amplitude modulation (PAM)-2 and PAM4 formats and guides co-optimization of optical and electrical parts. As proofs of concept, we experimentally demonstrate C-band 60 Gb/s PAM2 over 85.8 km single-mode fiber (SMF) and up to 112 Gb/s PAM4 over 50 km or 61.3 km SMF, enabled by OE-FFE with only a 1-tap optical delay line and lean digital equalizers. For 112 Gb/s PAM4 over 50 km, BER satisfying 7% hard-decision FEC limit is achieved using a symbol-spaced simplified polynomial FFE with only 39 multiplications per PAM-4 symbol. For 60 Gb/s PAM-2, we achieved 85.8 km transmission with symbol-spaced 25-tap linear FFE and KP4 FEC-compliant BER. In addition, investigations on the parameters of the optical delay line are also carried out. The analytical and experimental results show the great potential of proposed OE-FFE approach for CD-constrained beyond-100G/λ ER and beyond-50G/λ ZR IM/DD systems.