Input-output relations for a three-port grating coupled Fabry-Perot cavity.

We analyze an optical three-port reflection grating by means of a scattering matrix formalism. Amplitude and phase relations among the three ports, i.e., the three orders of diffraction, are derived. Such a grating can be used as an all-reflective, low-loss coupler to Fabry-Perot cavities. We derive the input-output relations of a three-port grating coupled cavity and find distinct properties that are not present in two-port coupled cavities. The cavity relations further reveal that the three-port coupler can be designed such that the additional cavity port interferes destructively. In this case the all-reflective, low-loss, single-end Fabry-Perot cavity becomes equivalent to a standard transmissive, two-port coupled cavity.