The invention relates to a device which is used for the non-invasive detection of an electric potential or field, of the spatial and/or the temporal derivatives thereof, in a medium with a linear or quadratic electrooptical effect. The inventive device comprises: an optical source which is used to illuminate at least one zone of the medium that is to be probed with a light beam, the path of which defines an optical axis; and means for mapping the phase shift of the beam in the zone to be probed. Measuring means which are used to map the light beam phase shift comprise a confocal microscope in which the zone to be probed is placed in order to form an image of a plane of said zone.