112 Gb/s sub-cycle 16-QAM Nyquist-SCM for intra-datacenter connectivity

Datacenter traffic is exploding. Ongoing advancements in network infrastructure that ride on Moore’s law are unable to
keep up, necessitating the introduction of multiplexing and advanced modulation formats for optical interconnects in order
to overcome bandwidth limitations, and scale lane speeds with energy- and cost-efficiency to 100 Gb/s and beyond. While
the jury is still out as to how this will be achieved, schemes relying on intensity modulation with direct detection (IM/DD)
are regarded as particularly attractive, due to their inherent implementation simplicity. Moreover, the scaling-out of
datacenters calls for longer transmission reach exceeding 300 m, requiring single-mode solutions.
In this work we advocate using 16-QAM sub-cycle Nyquist-SCM as a simpler alternative to discrete multitone (DMT),
but which is still more bandwidth-efficient than PAM-4. The proposed optical interconnect is demonstrated at 112 Gb/s,
which, to the best of our knowledge, is the highest rate achieved in a single-polarization implementation of SCM. Off-theshelf
components are used: A DFB laser, a 24.3 GHz electro-absorption modulator (EAM) and a limiting photoreceiver,
combined with equalization through digital signal processing (DSP) at the receiver. The EAM is driven by a low-swing
(<1 V) arbitrary waveform generator (AWG), which produces a 28 Gbaud 16-QAM electrical signal with carrier frequency
at ~15 GHz. Tight spectral shaping is leveraged as a means of maintaining signal fidelity when using low-bandwidth
electro-optic components; matched root-raised-cosine transmit and receive filters with 0.1 excess bandwidth are thus
employed. Performance is assessed through transmission experiments over 1250 m and 2000 m of SMF.