Optical Fiber Sensing Devices Based On Magnetic Fluids

The traditional optical fiber sensing techniques have attracted a great deal of attention and many applications over the past decades duo to their particular characteristics,such as anti-interference,compactness,high-temperature and high-pressure resistance and high sensitivity.On the other hand,magnetic fluids(MF)have versatile optical features,such as tunable refractive index,tunable transmission,magneto-dielectric anisotropy and so on.It possesses both the features of magnetic property of solid magnetic materials and fluidity of liquids,which can be combined with some photonics devices to become new photonics devices.For example,novel magnetic field sensors can be achieved by integrating magnetic fluids with traditional optical fiber sensors.To know about the development of traditional optical fiber sensors in brief firstly,this paper proposes a compact fiber sensor,i.e.a curvature sensor based on the singlemode-multimode-singlemode(SMS)fiber structure.Two sensing structures with different parameters are investigated in this paper,and the lengths of the multimode fiber section are 26 mm and 48 mm,respectively.It was found that the interference valley wavelength of the SMS fiber structure with 26 mm-long MMF is highly sensitive to the bending.The shift of the valley wavelength has a good linear relationship with the curvature in the range of 0.60-1.55 m-1.For this wavelength-shift-type curvature sensor,the obtained sensitivity is-13.17 nm/m-1.The temperature sensitivity is less than 10pm/?,which is negligible.For the SMS fiber structure with 48 mm-long MMF,the valley wavelength is not sensitive to the bending,but the intensity of the interference valley varies with the curvature linearly in the curvature range of 0.22-0.89 m-1.For this intensity-variation-type curvature sensor,the sensitivity is around 24.64 dB/m-1.As magnetic sensitive materials,magnetic fluids are used as the cladding of particular fiber structures to produce fiber magnetic field sensors in this paper.Two fiber magnetic field sensors are presented.The first device is based on taper-like and lateral-offset fusion splicing techniques,the other one is based on up-tapered joints.In the process of investigating the spectral responses,the wavelength shift and intensity variation of interference valleys with magnetic field are monitored.For the fiber magnetic field sensor based on taper-like and lateral-offset fusion splicing techniques,the shift of the valley wavelength has a good linear relationship with magnetic field strength in thewide ranges(38-225 Oe and 250-475 Oe).The corresponding sensitivities at low and high field ranges are 14.1 pm/Oe and 26 pm/Oe,respectively.The intensity at wavelength of 1566.36 nm decreases linearly with magnetic field strength in the range of 250-475 Oe.The sensitivity is obtained to be-0.024 dB/Oe.The previous studies and applications of MF in optics field mainly focus on these features of tunable refractive index,tunable transmission,magneto-dielectric anisotropy.In this paper,we firstly investigate how the external magnetic field affects the volume of MF.According to analyzing the micro-mechanism of this phenomenon,the Scale Reduced Effect(SRE)and the Scale Expanded Effect(SEE)are presented to clarify the physical mechanism of magneto-volume variation.The traditional air-gap fiber FP interferometer is integrated with MF to produce a novel fiber magnetic field senor in this paper,of which high sensitive to the width variation of air-gap.The fiber magnetic field senor is very sensitive to the volume variation of MF column,which is utilized to prove the previous analysis and deduction.In addition,the sensor is applied to magnetic field sensing.In our experiments,the highest sensitivity of magnetic field sensing is achieved to be 11.73 pm/Oe for the interferometer when the sensing head is set at45°azimuthal angle.