| As the rapid development of technology,temperature and magnetic field sensing technology has been widely used in many fields such as medicine,national defense,aerospace,marine surveying and so on.Magnetic fluid（MF）has the characteristics of magnetically controlled refractive index.As magnetic field sensitive medium,MF is often combined with optical fiber sensing structure to design optical fiber magnetic field sensor which can measure the intensity of external magnetic field.Both LPFG and FBG have unique light transmission characteristics and is often designed as an optical fiber temperature sensor to measure the external temperature.However,when the optical fiber sensor measures the temperature and magnetic field strength at the same time,there is a phenomenon of cross sensitivity,which leads to the inaccurate measurement results.In this paper,the magnetic fluid LPFG-FBG dual fiber gratings thermomagnetic sensor is designed.The specific research results are as follows:（1）The sensing mechanism of LPFG-FBG is briefly analyzed.Then,the Optigrating software was used to analyze LPFG-FBG about grating period,length,core modulation depth,cascade ordinary fiber length and fiber cladding thickness.At last the optimal LPFG-FBG sensor was obtained.Structural parameters:LPFG period550μm,length 2 cm,FBG period 535 nm,length 1cm and cascade ordinary fiber length 2 cm.Transmission spectrum of LPFG-FBG has two resonance wavelengths,the resonance wavelength of LPFG and FBG together isλ1,and the other resonance wavelength isλ2.（2）A 244 nm frequency-doubled argon ion laser was used to prepare the LPFG-FBG sensor by the phase mask method.Transparent plastic tubes,quartz tubes and medical pipettes were compared as packaging containers,and finally the 5mm quartz tube was selected.Using COMSOL simulation software to simulate the magnetic field of cuboid permanent magnets.By changing the spatial relative positions of the cuboid permanent magnets,the uniform magnetic field were found.According to the simulation results,a magnetic field generator was designed and built.Designing and building the temperature and magnetic field experiment system to experiment on the LPFG-FBG sensor.And the experimental results were obtained:when the temperature range is 35℃to 85℃,the resonant wavelengthλ1has a temperature sensitivity of 6.7 pm/℃,the resonant wavelengthλ2has a temperature sensitivity of 96.2 pm/℃.In the range of magnetic field strength from 0to 24 m T,the resonance wavelengthλ1hardly drifted,but the loss changed,the magnetic field sensitivity is-0.1249 d B/m T.The resonance wavelengthλ2has wavelength drift,and the magnetic field sensitivity is 34.4 pm/m T.The stability of the magnetic field measurement was tested.The stability of magnetic field measurement is tested.The experimental results show that the structure has good stability of magnetic field measurement.In the LPFG-FBG optical fiber structure,FBG is used as a temperature self compensation structure to compensate the measurement of magnetic field strength;LPFG can modulate the intensity of the reflected light of FBG.（3）Designing optimization experiments according to theoretical derivation and simulation results.In order to better corrode the optical fiber cladding,HF solutions with different concentrations were designed to corrode the fiber cladding.HF solution is volatile and corrosive to Si O2.A corrosion vessel made of Acrylic material was designed to hold HF and experiment.A corrosion experiment system was built to experiment SMF fiber cladding corrosion.The corrosion rate of HF with 15%concentration was 0.2215μm/min and the corrosion rate of HF with 30%concentration was 0.7065μm/min.Then the diameter of clad of the LPFG-FBG sensor was corroded to 100μm.The temperature and magnetic field sensing experiments were carried out,and the experimental results were finally obtained:in the temperature range from 35°C to 85°C,the temperature sensitivity of the thin-clad LPFG-FBG resonance wavelengthλ1was 8 pm/°C,the temperature sensitivity of resonance wavelengthλ2was 85.7 pm/°C.When the magnetic field intensity was from 4 m T to 20 m T,the resonance wavelengthλ1still did not change significantly,the magnetic field sensitivity is 0.But the resonance wavelengthλ2changed,and the magnetic field sensitivity is 65 pm/m T.Comparing the temperature and magnetic sensitivity of the sensor before and after the corrosion cladding,the resonant wavelengthλ1andλ2temperature sensitivity did not change significantly before and after the corrosion cladding,but the resonant wavelengthλ2magnetic field sensitivity of the sensor after corroded cladding became the sensor without corroded cladding 1.88 times.The magnetic field sensitivity is effectively improved... |
