Transition-Based Decision Feedback Equalization to Enhance Transmission in VCSEL-Based Short-Reach IM/DD System

This paper presents a transition-based decision feedback equalizer (TB-DFE) designed to mitigate bandwidth limitations and transition-dependent distortions in four-level pulse amplitude modulation (PAM-4) optical interconnects using directly modulated vertical-cavity surface-emitting lasers (VCSELs). In contrast to conventional decision feedback equalizers (DFEs), which apply feedback taps regardless of the transition context, the proposed TB-DFE adapts its feedback coefficient based on detected symbol transitions, thereby capturing the non-stationary impulse response variations observed in skew-prone links. Experimental validation was performed using a 26.5625 GBd PAM-4 VCSEL-based transmission system operating over an OM4 multimode fiber at distances up to 500 meters. Results show that, compared to conventional feedforward equalizer (FFE) and FFE-DFE schemes, FFE with TB-DFE improves bit error rate (BER) by nearly an order of magnitude at longer distances. Although its benefit is modest at 100 m and 300 m, a significant gain is observed under severe bandwidth constraints at 500 m. This work demonstrates a 53 Gb/s transmission per lane over 500 m of OM4 multimode fiber using a commercial multimode VCSEL, achieving a pre-forward error correction (FEC) BER below the Reed-Solomon FEC (RS-FEC) threshold (2.4 × 10⁻⁴), highlighting the potential of the proposed method to extend transmission reach under skew and bandwidth-limited systems. These findings establish TB-DFE as a practical and low-complexity equalization technique to boost the direct-modulation-based transmitter transmission distance for next-generation short-reach and directreach data center applications.