Solutions for 100/400-Gb/s Ethernet Systems Based on Multimode Photonic Technologies

In this paper, we experimentally demonstrate the transmission of 112 Gb/s four-level pulse amplitude modulation over 100-m OM4 multimode fiber employing a multimode 850-nm vertical-cavity surface-emitting laser (VCSEL) at the transmitter side and equalization techniques at the receiver's digital signal processing (DSP). The penalties imposed by the strong bandwidth limitations due to the optical components as well as the low modal bandwidth of the fiber are compensated by three variant DSP schemes at the receiver, i.e., 1) a finite-impulse response (FIR) filter, 2) a maximum likelihood sequence estimation equalizer (MLSE), and 3) an FIR filter followed by an MLSE equalizer (FIR/MLSE) in a cascaded form. We evaluate all three aforementioned equalization schemes under two different transmitter implementations, i.e., employing a 30-GHz arbitrary waveform generator and a lower bandwidth 15-GHz commercially available digital-to-analog converter and we infer about the applicability of each DSP scheme under these implementations. We show that the hybrid implementation of an FIR followed by a 16-state MLSE can enable the 100-m transmission below the 7% hard decision (HD) forward error correction (FEC) threshold limit and outperforms its other two counterparts for the back-to-back case as well as after 100-m transmission for the high-bandwidth transmitter implementation. On the other hand, lower bandwidth DAC implementations, i.e., 15 GHz, require an increased state MLSE without the need for a preceding FIR filter to bring the bit error rate (BER) below the HD-FEC limit after 100-m OM4 fiber transmission. DSP complexity versus BER performance is assessed for all the aforementioned scenarios evaluating the impact of the transmitter's bandwidth on the overall system's performance. Our proposed solutions show that 112 Gb/s 100-m OM4 multimode links based on VCSELs and standard OM4 fiber can enable next generation 100 and 400 Gb/s wavelength division multiplexed optical interconnects.