Scaling Laws for Unamplified Coherent Transmission in Next-Generation Short-Reach and Access Networks

International standardization bodies (IEEE and ITU-T) working on the evolution of transmission technologies are still considering traditional direct detection solutions for the most relevant short-reach optical link applications, that are Passive Optical Networks (PON) and intra-data center interconnects. Anyway, future jumps towards even higher bit rates per wavelength will require a complete paradigm shift, moving towards coherent technologies. In this paper, we thus study both analytically and experimentally the scaling laws of unamplified coherent transmission in the short-reach communications ecosystems. We believe that, given the extremely tight techno-economic constraints, such a revolutionary transition towards coherent in short-reach first requires a very detailed study of its intrinsic capabilities in largely extending the limitation currently imposed by direct detection systems. To this end, this paper focuses on the ultimate physical layer limitations of unamplified coherent systems in terms of bit rate and power budget. The main parameters of our performance estimation model are extracted through fitting with a set of experimental characterizations and later used as the starting point of a scaling laws study regarding local oscillator power, modulator-induced attenuation, bit rate, and maximum achievable power budget. The analytically predicted performance is then verified through transmission experiments, including a demonstration on a 37-km installed metropolitan dark fiber in the city of Turin. Our findings show that coherent detection without optical pre-amplification and using PM-QPSK can tolerate optical power budget (OPB) well above, for instance, the 29 dB imposed by the current PON standards even at extremely high raw bit rates up to 800
$Gbps$
. PM-16QAM, on the other hand, can provide up to 190
$Gbps$
at 29 dB OPB only if combined with soft FEC algorithms. Even higher bit rates are also shown for the less demanding power budget needed in intra-data centers links.