# A Notched Long-Period Fiber Grating Magnetic Field Sensor Based on Nanoparticle Magnetic Fluid
## 1 Introduction  
Magnetic field sensors include many aspects of mechanical and electronic techniques. In terms of the applications for which such sensors are used, there is a clear and continuous trend toward using sensors of smaller size, lower energy consumption, and lower cost to achieve similar performance [1]. Fiber-optic based technology possesses various advantages that can be beneficial in achieving that aim, because optical fiber sensors are light weight, immune to electromagnetic interference, resistant to corrosion and high temperatures, and are electrically passive when operating while also requiring only limited power consumption [2; 3]. Long-period fiber gratings (LPFG) are playing an increasingly important role in the field of optical communication and sensing. LPFG consists of periodic refractive index variations with periods of 100-1000 \(\upmu\)m [4; 5]. LPFG promotes the coupling between a propagating core mode and a cladding mode to provide an attenuation loss band, which has characteristics conducive to sensor and communication applications [6], including gain flattening filtering [7], sensing for strain [8], sensing for temperature [9; 10; 11], and sensing variations in refractive index [12; 13]. Magnetic field sensing techniques have been researched and studied for decades. It is important for sensing magnetic field in many scientific applications included in bioengineering, electric engineering, industries and so on. In 2007, [PERSON] _et al._[14] later proposed a tunable filter based on LPFG. The filter's sensitivity in terms of the center resonant wavelength shift was reported to be 4.45 pm/Oe and that of the dip transmission loss was 0.0382 dB/mT. [PERSON] _et al._[15] presented spectral tuning of long-period fiber grating by utilizing water and oil based magnetic fluid as outcalding overlayers. There were two distinct actuating methods adopted to achieve magnetic tuning of the spectral features of LPFG. [PERSON] _et al._[16] reported a magnetic field sensor by using a water based ferrofluid encapsulated two identical blazed gratings. The cladding ring sensor respondedto magnetic field induced changes through altering the visibility of fringes of its interference spectrum. In 2012, [PERSON] _et al._[17] designed a magnetic sensor utilizing LPFG written by high frequency CO\({}_{2}\) laser pulses in a D-shaped fiber and the magneto-optical effect of magnetic fluid. By immersing the D-shaped LPFG in water based MF within a capillary, the transmission spectra could be measured while the applied magnetic field intensity was altered. The sensor exhibited a sensitivity of about 176.4 pm/mT. In 2015, [PERSON] _et al._[18] demonstrated a magnetic field sensor based on the combination of MF and an optical microfiber mode interferometer. The magnetic field could be measured with a sensitivity of \(-\)293 pm/Oe, while the applied magnetic field strength was in the range of 0 to 220 Oe. An optical fiber magnetic field sensor, which was developed by [PERSON] _et al._[19], using a long-period grating coated with MF. They demonstrated that the proposed sensor can maintain a high sensitivity of \(\sim\)0.154 dB/Gauss at field strength of as low as \(\sim\)7.4 Gauss. The reported magnetic sensor showed advantage of high sensitivity for low field strength measurement. In this study, we managed to use the MF as a sensing material based on a notched long-period fiber grating (NLPFG) to produce a magnetic field sensor. The MF, which is also named as ferrofluid, is a stable colloidal suspension of ferromagnetic nanoparticles in a suitable carrier liquid. The particles are coated with a stabilizing dispersing surfactant which prevents particle agglomerating even when a strong magnetic field gradient is applied to the ferrofluid. Such MF exhibits a diversity of magneto-optical properties because of its magnetically structural patterns.