## 4 State of Art
### Bulk-Optic Current Sensors  
As seen before, all-fiber current sensors are easy to implement and to increase the sensitivity, but suffer from high linear birefringence and have relative low Verdet constants. In this context, bulk optic sensors rapidly imposed and are the most industrial implemented technology [1,27]. When compared to all-fiber devices, bulk optic sensor presents some obvious advantages. These are usually smaller devices and mechanically stiffer. Therefore, mechanical and thermal gradients, vibrations and other external noises in the bulk optic material are very small. Also their Verdet constants are typically, 2 times higher than the ones found in optical fibers, and due to their low photo-elastic coefficients, the intrinsic linear birefringence is also very small, which allows for sensors with high sensitivities. Another big advantage is that the sensor around the conductor does not have to be a single piece of material. This allows an easy installation without the need of interrupting the current in the conductor. The main disadvantages of these sensors are related to the fact that reflections need to occur inside the bulk optic material for light to go around the conductor. Since the reflection angle will be  
Figure 8: Faraday rotator mirror. dependent on the refractive index of the crystal and outside surrounding medium (such as air), the sensor will be affected by a number of external factors such as humidity. This can be solved by isolating the sensor from external factors but will add higher cost to installation and maintenance operation of the sensor [28]. Also, every internal reflection of the light that occurs will introduce an optical phase difference between the two polarizations. In a polarimetric or interferometric scheme, this leads to an unavoidable error source [29]. The problem can be solved with the use of double reflections in each corner of the bulk optic material, where the optical phase difference introduced in the first reflection is compensated by the second reflection. A patent describing this idea was registered in 1986 [30] and it is shown in Figure 9.  
However, this idea presents another problem: in the optical path between the first and second reflections, the polarization state is elliptical and the signal will be non-linearly affected by the magnetic field to be measured. This will cause the system to be affected by external magnetic fields. This problem takes special relevance when measuring current in three phase systems, since the three conductors are usually close to each other. Studies have also been developed to minimize the crosstalk between different conductors [6]. Another possible solution is using a triangular shaped bulk optic material in which the light is always reflected at the critical angle [31], as shown in Figure 10.  
Figure 10: Bulk optic triangular shape current sensor. Figure 9: Bulk optic current sensor with double reflection. This configuration is simpler than the previous one. However this requires a precise reflection angle cut and a near perfect bulk optic material well isolated from external noise sources. Several schemes have been proposed to further increase the sensitivities of these sensors, including field concentrators (which consist in an air-gap core surrounding the conductor) and extending the light optical path inside the crystal (by using total internal reflections before the light exits from the sensing element) [32]. For this purpose, a circular sensor head was developed, where light is injected through a prism and at specific angles, light travels five times before exiting the prism. This scheme enhances sensibility and it is not sensitive to external fields [33]. Other sensors are available in a square configuration where light passes around the conductor several times [34; 35]. Also, in order to increase the sensitivity of the sensor, the compensation of the linear birefringence is very important.