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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. [ABSTRACT FROM AUTHOR]

We propose optimum 3D signal constellations for short reach intra-datacenter interconnects using stokes vector receivers. These optimum 3D alphabets maximize the so-called constellation figure of merit, which is the ratio between the squared minimum Euclidean distance and the average energy of the constellation points. Compared to dual-polarization 4-ary and 8-ary pulse amplitude modulation formats, the optimized 16-point and 64-point 3D signal constellations theoretically achieve 4 and 6 dB improved receiver sensitivity at low symbol error rates of ~10−5, while delivering similar spectral efficiencies of 4 and 6 bits per 3D symbol, respectively. Next, we experimentally verified the significant improvement in receiver sensitivity at symbol rates of 44 and 64 Gbaud that result from using the proposed optimized 3D signal alphabets. [ABSTRACT FROM AUTHOR]