25 and 50 Gb/s/${{\lambda}}$ PAM-4 Transmission Over 43 and 21 km Using a Simplified Coherent Receiver on SOI

Abstract

We demonstrate 25 and 50 Gb/s four-level pulse amplitude modulation (PAM-4) transmission over 43- and 21-km standard single-mode fiber (SMF), respectively, using a silicon photonic 120° hybrid-based integrated simplified coherent receiver in the $C$ -band. The integrated receiver is composed of an edge coupler, a vertical grating coupler, a polarization splitter–rotator, a $3\times 3$ multimode interference coupler-based 120° hybrid, and three germanium p-i-n photodetectors. For 25 Gb/s PAM-4 transmission, we achieved the receiver sensitivities of −18, −17.5, −16.6, −14.5, and −9.8 dBm in back-to-back (B2B) and after 10.5, 21, 31, and 43 km, respectively, at a bit error rate (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., $3.8\times 10^{-3}$ ) without using any receiver equalizer, while for the 50 Gb/s PAM-4 transmission, the receiver sensitivities of −13.6 dBm in B2B, and −13.5 and −11.6 dBm after 10.5 and 21 km are obtained using a 31-tap linear feed-forward equalizer (FFE), respectively. Furthermore, we achieved up to 44 Gb/s receiver equalizer-free PAM-4 transmission over 10.5 km below the HD-FEC. The characterization of the receiver at different wavelengths within the $C$ -band is provided as well.