It is a device containing an electro-optic crystal and using the Pockels effect. A voltage applied across the crystal generates birefringence, causing plane-polarized light propagating through the crystal to be resolved into two orthogonal vectors. The change in retardation between the two vectors (ellipticity) is proportional to the magnitude of the electrical field. A crossed polarizer analyzes the output beam, resulting in intensity modulation. Response time can be in picoseconds.