Optical Fiber Interference Devices And Research For Sensing Application Based On Magnetic Fluids

The sensors are tend to simple,sensitive and low cost direction development in recent decades.The traditional optical fiber sensors have attracted extensive interest over the past decades on account of its good anti-interference,low cost,strong adaptability,compact and high sensitivity.With the development of the optical research,magnetic optical properties of magnetic fluid is gradually known by scholars.Magnetic fluid(MF)has many unique magneto-optical effects such as linear dichroism,linear birefringence,Faraday rotation and Faraday ellipticity.Magnetic fluid(MF)is a kind of stable colloidal suspension system consisting of surfactant-coated magnetic nanoparticles with typical sizes of 10 nm dispersed in a suitable liquid carrier.It possesses both the features of magnetic property of solid magnetic materials and fluidity of liquids,which makes it an attractive material.Under no magnetic field,magnetic nanoparticles well-distributed in magnetic fluid and in the static state,magnetic fluids has no magnetic attraction.But under outer magnetic field,with increase of magnetic field,the magnetic particles inside the magnetic fluid will form into a chain structure along the magnetic field direction until relatively high magnetic field and tend to stable which has many unique optical properties.The optical properties including the refractive index of magnetic fluid increase as well as increase of outer magnetic field and tend to stabilization at relatively high magnetic field.These unique characters make magnetic fluid can easy combine with optics device to fabricate new optics sensors.Combination of magnetic fluid and optical fiber structure can be fabricate to high performance optical sensors which often to be used as fiber magnetic field sensors.In our work,we explore “Single-beam Mach-Zehnder micro-interferometer based on magnetic fluids”.Through experiments,we find that the refractive index of magnetic fluid will increase with the increase of outer magnetic field and tend to stabilization at relativity high magnetic field.In addition,the lager adjustable range of the refractive index for higher concentration MF under outer magnetic field.Due to the tunable refractive index of magnetic fluid under external magnetic field,the interference intensity of the Mach-Zehnder interferometer can be adjusted by the magnetic field.Through turning on and turning off the magnetic field alternatively,the repeatability and tunability of the micro-interferometer are verified.The results in this work can be employed to design unique optical devices based on magnetic fluids,such as optical modulator,optical switches.As magnetic sensitive materials,MFs are used as the cladding of particular fiber structures to produce fiber magnetic field sensors.In this paper,we fabricate two fiber structures based on MFs which are magnetic-fluid-clad multimode-singlemode-multimode fiber structure and magnetic-fluid-clad no-core-singlemode-no-core fiber structure.Due to the mode field mismatch,the lead-in multimode and no-core fibers play the role of mode coupler to couple the fundamental core mode of the lead-in multimode and no-core fibers into the core and cladding modes of the sensing SMF.The interference between the core and cladding modes occurs in the lead-out multimode and no-core fibers.In our works,the simple fusion splicing are used to fabricate these fiber structures.For multimode-singlemode-multimode fiber structure,the singlemode fiber(SMF)sandwiched between two sections of multimode fibers(MMFs)is 30 mm.Two interference dips are observed in our spectral range due to different interference mode.We find that with the increase of magnetic field,both of the two interference valleys of 1595 nm and1565nm are tend to longer wavelength.Experimental results indicate that the magnetic field sensing sensitivities of 21.5 pm/Oe and 0.05742 dB/Oe are obtained for interference dip around 1595 nm.For interference dip around 1565 nm,the sensitivities are 6.05 pm/Oe and 0.04821 dB/Oe.Due to the large thermos optical coefficient of MF,the response of temperature is also investigated.The temperature sensitivity for the dip around 1595 nm is obtained to be 9.93 pm/°C.For no-core-singlemode-no-core fiber structure based on MFs,the SMF sandwiched between no-core fiber(NCF)is also 30 mm.Experimental results indicate that the valley wavelengths at around 1462 nm and 1477 nm shift to long wavelength side with the external magnetic field.The achieved sensitives are 67.28 pm/Oe and 49.82 pm/Oe,respectively.The temperature-dependent transmission spectra show that the valley wavelength at around 1462 nm shifts to short wavelength side with the temperature increase and the corresponding sensitivity is 37.8 pm/?.Due to the large thermos optical coefficient of MF,the corresponding sensing structures may be sensitive to the ambient temperature variation simultaneously,which will affect the magnetic field sensing accuracy and performance.That is to say,the issue of magnetic field and temperature cross-sensitivity occurs for the MF-based fiber-optic magnetic field sensors.To resolve the issue of magnetic field and temperature cross-sensitivity,we fabricate long period fiber grating(LPFG)based on MFs.By two-parameter matrix method,the issue of magnetic field and temperature cross-sensitivity can be solved in a simple and easy way.The variation of wavelength and intensity at spectral resonance valley with the applied magnetic field and ambient temperature is utilized to realize the sensing purpose.