## 4 State of Art
### Magnetic Force Sensors
#### 4.3.1 Magnetocristive Sensors
A technique for the measurement of weak DC and low frequency AC magnetic fields using an all fiber SMF magnetometer was described by [PERSON] _et al._[57]. For the first time, the experimental scheme used, consisted in a Michelson interferometer where the two cleaved fibers ends were coated with silver film to form the mirrors. Detection sensitivities of \(\approx\)1 \(\times\) 10\({}^{-7}\) T/m at 20 Hz was achieved by the bulk nickel magnetostrictive element. Figure 12 represents the experimental configuration used. A fiber Mach-Zehnder interferometer passively stabilized using a 3 \(\times\) 3 fiber coupler was used by [PERSON] _et al._[58]. The system response was examined by subjecting the metallic glass to a DC and AC magnetic fields. The sensor sensitivity was in the order of \(7.96\times 10^{-5}\) A/m\({}^{2}\) for a fiber length of 1 m at 1 Hz bandwidth. Figure 12: Scheme of the all fiber magnetometer by [PERSON] _et al._[57]. The first demonstration of a closed-loop fiber optic magnetometer with dynamic magnetostrictive response was reported by [PERSON] _et al_. [59]. The sensor was capable of detecting low frequency and DC changes of magnetic field with a sensitivity of \(\approx\)2 nT at frequencies below 2 Hz. One year later, the same group proposed a similar configuration, where the main difference in the magnetometer was that they kept at constant value the total local field (bias+ambient) [46]. By active-bias field stabilization they ensure that the magnetostrictive sensing element was maintained in a zero state zero magnetization. This method eliminated any problems arrived due to magnetic hysteresis of the metallic glass materials used (Metglass and Vitrovac). A minimum detectable of \(8\times 10^{-5}\) A/m was observed over the frequency range from DC to 20 Hz using a small length of optical fiber (\(\approx\)0.5 m). The effects of external perturbations, such as, mechanical stress and vibrations on the operation of fiber optic magnetometers were investigated by [PERSON] _et al_. [47]. These perturbations were controlled by maintaining the DC magnetic field at prescribed levels. The analysis performed has shown that the method used by the authors could be applicable in all interferometric fiber sensors employing nonlinear transducing mechanisms. In 1989, the first demonstration of the detection of magnetic fields at frequencies above 50 kHz using a fiber optic magnetometer was reported by [PERSON] _et al_. [60]. A Mach-Zehnder interferometer with a thick cylindrical metallic glass transducer in one of the fiber arms was used. Mixing high frequencies signals in the magnetostrictive sensor allowed a heterodyne detection scheme, where the transducer acted as both the receiving and nonlinear mixing element. The same group reported a minimum detectable AC magnetic field of 70 fT/\(\backslash\)Hz at 34.2 kHz [61]. Optical fibers coated with magnetostrictive ceramic films were tested by [PERSON] _et al_. [49], by using a modified Mach-Zehnder interferometer operating in an open-loop configuration. The sensors exhibited excellent linearity and good sensitivity. The materials used were magnetite, Fe\({}_{2}\)O\({}_{3}\), nickel ferrite and cobalt doped nickel ferrite (NCF\({}_{2}\)) jackets, being the last one with the best response to magnetic field. They achieved a minimum detectable magnetic field of \(3.2\times 10^{-3}\) A/m for optical fibers jacketed with 2 \(\upmu\)m thick and 1 m long NCF\({}_{2}\) material. A fiber optic sensor for measuring DC magnetic fields based on the extrinsic Fabry-Perot interferometer (EFPI) was proposed by [PERSON] _et al_. [62]. A SMF and a Metglass wire magnetostrictive transducer constituted the input-and-output and reflector arms of the extrinsic Fabry-Perot interferometer (EFPI) sensor. Due to the sensor optical geometry design, it showed a low vibration sensitivity and high thermal induced compensation (better than 99%). [PERSON] _et al_. [63] demonstrated a new approach for measuring electric current on high voltage systems.