  We demonstrate that a triangular cavity can operate as an efficient
discriminator for Hermite-Gaussian optical modes. The process is applied to
distinct fields produced by a Spatial Light Modulator, decomposing the
generated paraxial field onto a basis of modes defined by the resonant modes of
the optical cavity. We also present a proposal for the realization of a mode
sorter based on a sequence of these cavities, without the need for optical
isolation. We discuus its possible applications in sensitive measurements and
distribution of information in optical systems. The high purity of the process
should enable the use at the level of quantum information sharing.
