Optical Current Transformers are current sensors which work on the magneto-optic effect. These sensors work by measuring the angle of rotation of a polarized beam of light.
The magneto-optic effect was discovered by Sir Michael Faraday. The Faraday Effect states that the 
rotation of the plane of polarization of a beam of light which passes through a magnetic field is proportional to the component of magnetic field in the direction of the light.
Thus, by measuring the rotation of the plane of polarization of light it is possible to measure the magnetic field and consequently the current producing the field.
Optical Current Transformers are also known as Fibre Optic Current Sensors (FCOS). They are widely used in substations and power systems. The advantage of these sensors is that they do not need calibration.
Fibre Optic Current Sensors are usually integrated with devices such as breakers.