Input-Output Relations in Optical Cavities: a Simple Point of View

In this work we present a very simple approach to input-output relations in optical cavities, limiting ourselves to oneand two-photon states of the field. After field quantization, we derive the non-unitary transformation between Inside and Outside annihilation and creation operators. Then we express the most general twophoton state generated by Inside creation operators, through base states generated by Outside creation operators. After renormalization of coefficients of inside twophoton state, we calculate the outside photon-number probability distribution in a general case. Finally we treat with some detail the single mode and symmetrical cavity case.