The increasing complexity of digital networks on which society depends on renders these networks increasingly vulnerable to attacks. Focusing on physical layer security, we propose to develop single-spatial-mode optical Physical Unclonable Functions (PUFs) as an authentication solution for quantum and classical communication links. These novel PUFs are read out through standard optical fibers or free-space links. Several implementations of single-mode PUFs are proposed exploiting the time / frequency domain for the encoding challenge / response space. Together with the PUFs, we will develop tools to generate challenge-forming few-photon light pulses and to validate PUF responses at the few-photon level and take specific steps toward wide implementation.