| Photonic crystal fiber(PCF)has become a research hotspot of the fiber sensing field owe to its excellent light control capability and flexible design freedom.Different sensors with high sensitivity can be realized by selectively filling the PCF cladding structure with functional materials which are sensitive to the external environment.However,external factors such as temperature and magnetic field will easily interfere with the optical characteristics of the PCF and cause deviation during the measurement process.If the simultaneous measurement of multiple parameters can be realized in the same device,the integration of the sensing device can be improved and the cross-sensitivity problem can be solved.Therefore,the novel design ideas of highly sensitive,easily prepared and integrated sensors are provided by investigating the multi-parameter sensing mechanism of infiltrated PCF.Three types of infiltrated PCF-based sensors are optimized and designed in this paper to achieve multi-parameter sensing.(1)A highly sensitive temperature sensor is proposed by etching long period grating(LPG)in the infiltrated PCF and utilizing its dual resonance characteristics.By selectively infiltrating the cladding air-holes with magnetic fluid(MF)material,the coupling between cladding mode and core mode induce dual resonance peaks in the transmission spectrum.The temperature sensor with high-sensitivity can be realized by monitoring the spacing of two resonance peaks.In addition,in order to expand the detection range of temperature,the air holes of the inner layer are filled with ethanol and the other holes are filled with MF.The temperature sensitivity of modified PCF sensor is 2.99nm/°C in the range of 0°C to 60°C.(2)The dual-parameter measurement of temperature and magnetic field is realized based on the sensing mechanisms of modal interference(MI)and directional coupling(DC).The specific air-holes in the PCF cladding structure are selected and infiltrated with MF material to introduce the defect channel.The freedom degree of sensing can be added by utilizing the coupling between the high-order core mode and the defect mode.The drift characteristics of interference spectrum and loss spectrum with temperature and magnetic field are investigated.Moreover,the cross-sensitivity problem can be solved by combining with dual-parameter matrix method.(3)A multi-parameter sensor that can simultaneously detect curvature,bending direction,temperature and magnetic field is designed based on the defect coupling mechanism in selectively infiltrated PCF.Four air-holes of PCF cladding are selected and filled with MF material to introduce two symmetric defect channels in the vertical direction and the horizontal direction,respectively.Based on the coupling between core mode and defect modes,two distinct peaks are achieved in the core mode loss spectrum.The studies show that when PCF is bent in X or Y direction,the corresponding loss peak splits while the other loss peak remains unchanged.The simultaneous measurement of curvature and bending direction can be achieved by monitoring the shift characteristics of loss splitting peaks in X and Y directions with the varying bending conditions.Interestingly,the bending measurement is not affected by the external temperature or magnetic field.On the other hand,the center-wavelength mean value of two split peaks shifts with temperature and magnetic field.By utilizing the drift characteristics,simultaneous sensing of temperature and magnetic field can be achieved without bending interference. |
