摘要： This paper focuses on upstream signaling in TWDM-PON (one of the two systems speci?ed in the new series of ITU-T G.989.x Recommendations for NG-PON2), for what concerns the discovery phase of a new ONU. The key idea is that a new ONU, that needs to be discovered by the OLT, would send a low bit rate signal that is set to a suf?ciently low-power level to not affect signi?cantly the upstream transmission performance of already active ONUs. We properly dimension the key physical layer parameters of this technique and demonstrate it experimentally. We named this proposal as “photon ranging” due to its ultralow-power transmission and we experimentally demonstrate its feasibility. As a “side effect” of this study, we had to deeply revise the power penalty generated by the optical interferometric crosstalk, the main impairment that impacts the proposed setup, obtaining new results that may be of interest also in areas not directly related to the speci?c framework of TWDM-PON.