## 4 State of Art
### All-Fiber Sensors  
All-fiber sensors use very simple configurations because the fiber can be simply coiled around the electric conductor to be measured. Also, the sensor sensibility can be changed by simply changing the number of turns of the optical fiber around the conductor [3]. Since these kind of devices usually uses several meters of fiber, the sensors are more vulnerable to pressure and temperature gradients, mechanical vibrations and other environmental noises than smaller devices (such as bulk-optic). When operating at high currents, the magnetic crosstalk between different conductors can also be a relevant problem. Schemes to reduce the magnetic crosstalk in three-phase electric systems have been proposed [6]. However, if the detection scheme forms a closed circuit around the electrical conductor, as most of them do, it will not be sensitive to external magnetic fields. Typically, standard silica fibers have a low Verdet constant when compared with bulk-optic glasses. Many studies have been done in order to increase the Verdet constant of the fibers. Materials such as flint glass have been tested and showed a Verdet constant six times higher than fused silica and much lower photo-elastic coefficient (780 times) [3,7,8]. A detailed review of current transducers using flint glass fibers can also be found in the literature [9]. The study provides specific information of the sensors (setup configurations, temperature dependence, accuracy, _etc._) and thefibers used (Verdet constants, photo-elastic constants, wavelength dependence, _etc._) as well as results of field tests performed. In Figure 7, a typical current sensor with flint fiber is represented. A comparison between the performances of flint glass fiber and twisted single-mode fiber (usually used before the flint glass) as a Faraday element was presented [10], and showed that the system with the flint glass fiber is more stable than the one with twisted fiber. In 2009, a new optical glass for optical fibers with high refractive index for applications in optical current sensors was reported. The results showed that these fiber glasses could have ten times higher Verdet constant and small internal mechanical stress [11]. Recently, a terbium-doped-core phosphate optical fiber with a Verdet constant six times larger than standard optical fibers was presented [12]. The same authors also presented in 2010 a high concentration of terbium-doped fiber with a record Verdet constant of -32 rad/(T m), which is 27 times larger than standard optical fibers and corresponds to 83% of the Verdet constant of commercially available crystals used in bulk optics-based isolators [13]. Another problem of using the fiber as the transducer is the effect of the linear birefringence that is induced by mechanical stress (when the fiber is bent for example), thermal stress, manufacture imperfections and other effects. The presence of linear birefringence significantly reduces the sensor sensitivity due to the polarization state degeneration. The linear birefringence can be neglected if the circular birefringence is high enough as we can see from Equation (5). The use of a twisted single mode fiber to impose a circular birefringence in the fiber has been demonstrated [14]. A similar approach is to use spun high birefringence fibers [15]. Linear birefringence can also be reduced by fiber annealing process [16, 17]. Theoretical models and experimental measurements for the Verdet constant dispersion in annealed fibers have been presented [18]. Some methods for linear birefringence compensation using reflected light propagation have been presented, such as Faraday rotating mirrors [19] and fiber polarization rotators [20] which also doubles the sensitivity for the same fiber length. In Figure 8, a Faraday rotator mirror is shown. This component leads to a polarization shift of 90\({}^{\circ}\) and light that propagates in one axis is coupled to the other axis, and vice-versa.