Angular phase-plate analyzers for measuring the dimensionality of multimode fields

Analyzers comprised of an angular phase plate and a single-mode fiber have recently been introduced to study the angular profile of optical fields. Here, we quantify the number of degrees of freedom, or modes, that such an analyzer can resolve. Its performance is described by means of an angular coherence function and we introduce a dimensionality that gives the effective number of modes that a given analyzer can probe. This quantity can, as we show experimentally, easily be retrieved from a dual analyzer setup.