## 4 State of Art
### Magnetic Force Sensors
#### 4.3.3 Magnetic Fluid  
In 2001, [PERSON] _et al._[75] investigated the physical mechanism of the optical transmission of magnetic fluid films under magnetic fields. Originally dispersed, the magnetic particles agglomerated forming magnetic columns in the presence of the magnetic field. The liquid phase was transparent, whereas the columns were opaque. Therefore, the liquid phase dominates the optical transmission of the magnetic fluid film. When the magnetic field strength was raised more columns were formed leading to a decrease in the optical transmission. Later on, the same group investigated the refractive index of the magnetic fluid film and founded to be magnetically modulated in the presence of an external magnetic field [76, 77].  
In 2007 [PERSON] _et al._[78] reported a magneto optic tunable filter based on a long period fiber grating (LPG) coated with magnetic fluid as environment media. Applying a variable magnetic field, the center wavelength shift of the attenuation band of LPG was about 7.4 nm. The refractive index dependence of magnetic fluid on the external magnetic intensity was measured and the simulation results exhibited a good agreement.  
A fiber optic current sensor based on the magnetic fluid was developed by [PERSON] _et al._[79]. The magnetic fluid was used as the medium in a Fabry-Perot (FP) resonant cavity. The refractive index characteristic of the magnetic fluid suffers variations due to the external magnetic field and the current was measured by the output wavelength of the FP fiber sensor. A signal demodulation method using a FBG wavelength measurement system was proposed. The results indicated a good linearity and, the thickness and initial concentration of the magnetic fluid affected the performance of the sensor.  
[PERSON] _et al._[80] investigated a novel fiber optic sensor based on magnetic fluid and etched gratings. The nanoparticles of Fe\({}_{3}\)O\({}_{4}\) (the magnetic fluid used) were injected into capillaries containing etched FBGs acting as sensing elements. The grating with smaller diameter exhibited a wavelength shift of 86 pm, being the most sensitive. Experimental results showed a reversible response to magnetic field under 16 mT.  
A magnetic field sensor consisting of a photonic crystal fiber and small amount of Fe\({}_{3}\)O\({}_{4}\) magnetic nano-fluid trapped in the cladding holes of a polarization maintaining photonic crystal fiber were reported by [PERSON] _et al._[81]. They demonstrated that magnetic field of few mT can be easily and very well detected with higher sensitivity of 242 pm/mT.  
Later on, a magneto optic modulator with a magnetic fluid film inserted on a Sagnac fiber interferometer was proposed by [PERSON] _et al._[82]. When the magnetic field was applied, the magnetic fluid exhibited a variable birefringence, and this Faraday Effect led to phase and polarization state variations.  
[PERSON] _et al._[83] reported the use of hollow core photonic crystal fiber (HC PCF) in the fabrication of the cavity of the FP sensor, as shown in Figure 16, which exhibits low transmission losses and insensitivity to temperature variations. The core of the HC PCF was filled with magnetic fluid allowing this way its use as a magnetic sensor. The sensitivity achieved was 0.41 pm/(A/m).  
**Figure 16.** Structure diagram of magnetic filled HC PCF FP sensor proposed by [PERSON] _et al_. [83].  
[PERSON] _et al_. [84] proposed a new magnetic field fiber sensor design based on magnetic fluids. The sensor was configured as a Sagnac interferometer with a magnetic fluid film and a section of polarization maintaining fiber inserted in a fiber loop. The wavelength shift sensitivity to magnetic field was 0.21 pm/(A/m) with a resolution of 47.7 A/m. The optical power also varied with the magnetic field strength obtaining a sensitivity of \(5.02\times 10^{-3}\) dB/(A/m).