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

We demonstrate 25 and 50 Gb/s four-level pulse amplitude modulation (PAM-4) transmission over 43and 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 χ 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 ×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.