A device for measuring an electromagnetic field and the temperature in an analysis zone includes a light source for sending a light beam into a polarization-maintaining optical fiber. The light beam is directed along one axis of the fiber. An anisotropic electrooptic material is placed in the zone for receiving the beam from optical fiber and for sending the beam into the fiber. The device further includes an arrangement for analyzing the intensity of the linear wave and a mechanism for determining the variations in the orientation of the linear wave output by phase-shifting means. A quarter-wave plate is inserted between the optical fiber and the crystal, wherein the plate has its axes oriented at substantially 45° to the axes of the optical fiber.