  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.
