## 1 Introduction  
Due to the massive growth in the use of electric equipment in developed countries and the worldwide increase of the electrical distribution/consumption, the interest in sensors for electrical current metering, particularly in high voltage levels, has increased significantly over the last decade. Although optical current sensors are still too expensive for low voltage applications (such as, in residential areas), they are extremely attractive for high voltage applications (such as, in electrical high voltage (EHV) substations) when compared with conventional sensors, due to their intrinsic properties, such as:  
\(\bullet\) Immunity against electromagnetic interferences (EMI);  
\(\bullet\) Electrical isolation (the optical sensors are made of dielectric materials);  
\(\bullet\) Possibility for measuring AC and DC;  
\(\bullet\) Absence of saturation effects;  
\(\bullet\) Low power consumption;  
\(\bullet\) Small size, lightweight, and relatively low cost.  
Furthermore they are usually connected to optical fibers that have large communication bandwidths, and due to their very low absorption loss allow remote detection, high multiplexing capability and data transmission over long distances. Also, optical current sensors measure the magnetic field generated by the electric current rather than the current itself, thus avoiding the electric hazards that the high voltage measurements imply.  
There are basically two linear effects by which the magnetic field can be measured by optical sensors: magneto-optic effect (or Faraday effect) and magnetic force (or Lorentz force).  
In the case of sensors that use the Faraday effect, a rotation in the angle of polarization of the light propagating in magneto-optic material is induced by an external magnetic field. A short explanation regarding the Faraday effect, as well as the different interrogation techniques that can be used to interrogate the polarization rotation angle, are presented. Then, two different groups of optical current sensors using this effect (all-fiber optic sensors and bulk-optic based sensors) are analyzed. A key aspect of the concept of all-fiber optic sensors is that the optical fiber acts both as the sensing element and communication channel, which allows for simple solutions and reduces the losses in the fiber connections. By winding the optical fiber around the electric conductor, immunity to external currents or magnetic fields [1], as well as, tunable sensitivity can be easily achieved. As for bulk-optic sensors, they usually present higher sensitivity and robustness, which are very important aspects in real applications. In both cases, saturation effects such as the ones that occur in ferromagnetic-based sensors are avoided allowing higher measurement ranges.  
As for the magnetic force sensors, they measure the force applied by the magnetic field on the sensing element. The possibility of bonding or jacketing optical fibers with magnetostrictive materials to measure the intensity of the magnetic field is also discussed, with special emphasis to Terfenol-D, which has the highest magnetostriction of any alloy. With these types of sensors, multi-point magnetic field measurements are possible with a single optical fiber (optical multiplexing). Other solutions using magnetic fluids are also discussed.  
Most optical sensors described can be installed without the interruption of the current in the conductor, which is a very advantageous property when dealing with high voltage distribution systems.  
Lastly, the possibility of hybrid solutions for current measurement, using both conventional technology and new (optical) technology, are also discussed. Besides some operational advantages, these hybrid sensors may also be very important in the first step of implementing optical current sensors in the industry, before developing fully optical current measuring systems.