Over the past years, there has been a renewed interest in investigating light-matter interactions in optical Fabry-Pérot cavities. When molecules are put inside such cavities, they interact collectively and resonantly with the cavity electromagnetic field, leading to the formation of hybrid light-matter excitations called polaritons. It was shown experimentally that various physico-chemical properties of cavity-embedded molecules were deeply modified in such a polaritonic regime. While most of the available quantum mechanical theoretical models used to describe such properties consider the cavity optical modes and molecules embedded inside to be a closed system, the computation of experimentally relevant signals like the cavity optical spectra (transmittance, reflectance, or absorbance), fluctuation spectra and higher-order correlation functions of the field necessitate to « open » the cavity. The goal of this M2 internship is to develop and implement an « input-output  theory » for computing optical properties of polaritonic Fabry-Pérot cavities, taking into account both the mirror losses and polarization of light. The application of this theory to chiral mirrors and chiral light-matter interactions for which optically active molecules are embedded inside the cavity constitute an open and interesting problem in this field of research.