Modeling Of Metallization Long Period Fiber Grating Sensing Mechanism And Intensity Wavelength Division Multiplexing

Long period fibre grating?LPFG?is fiber grating with a period of ten microns to several hundred microns.Due to its unique structure,the LPFG uses the energy of the core mode coupled with the cladding mode in the same direction,LPFGs have undergone rapid development owing to their advantages over traditional sensors including not interfering with light source,low insertion loss,and low back reflection.Given those advantages,LPFGs have been widely considered for use in the field of sensing and communication.However,LPFGs?mainly composed of SiO₂?are extremely fragile and can be easily bent and even destroyed under nonuniform stress,strain,thermal loads and difficult to locate in practical applications,and the temperature and strain sensitivity of the bare gratings are low.Therefore,LPFG should be sensitized and protected before application.In practical applications,single point fiber grating sensors can not meet the requirements of the application of structural health monitoring.The multiplexing technology makes it possible for optical fiber sensors to carry out multi point and distributed measurement.The intensity wavelength division multiplexing?WDM?is the optical fiber sensor that uses intensity to code to distinguish the distributed measurement line.In this paper,the metallized packaging of LPFG,the temperature sensing model of long period grating grating,intensity wavelength division multiplexing?I-WDM?,intensity WDM signal detection are studied.The main contents are as follows:1)LPFG's metallized packaging.LPFG is metallized by electroless plating.The experimental results show that the surface of LPFG electroless plating Ni and Cu layer is smooth,continuous,uniform and compact,and there is no obvious cracking and peeling defects.The method of electroless plating can protect the LPFG very well.2)A mathematical model for analyzing the temperature sensing of metallized LPFG is proposed and established.The influence factors of metallized LPFG temperature characteristics were simulated by Matlab 7 software,and the simulated values and experimental values were verified.The simulation results showed that temperature sensitivity increased with an increasing elastic modulus,Poisson's ratio,and thermal expansion coefficient of the coating metal.Both the experimental and simulated results showed that the thickness of the metal coating had a significant impact on the associated temperature sensing properties.The calculated temperature sensitivities of the LPFGs with Ni-coating thicknesses of 6.5?m,5.2?m,and 7.2?m were 57.27 pm/°C,60.91 pm/°C,and 66.55 pm/°C,respectively;and with Cu-coating thicknesses of 3.8?m,4.9?m,and 5.4?m,the calculated temperature sensitivities were 58.17 pm/°C,59.58 pm/°C,and 61.18 pm/°C,respectively.Compared to the experimental results,the relative errors in the simulation were less than 9.75%,indicating the feasibility of the new temperature sensing model.3)In this paper,we demonstrate a multipoint fiber Bragg grating?FBG?-based sensor system by using intensity and wavelength-division-multiplexing?I-WDM?technique to enhance the sensing capacity and capability.In the proposed multipoint sensor system,a three output port optical splitter?OS?with different output ratios of50,35 and 15%is proposed to connect each intensity coding FBG sensor.Different output ratios of connected ports can produce different intensity-coding for I-WDM application.The design of the I-WDM system,due to the use of different intensity differentiating,under the light wavelength range of certain conditions,the overall resolution of the system can be multiplied.The proposed FBG sensor system not only can sense the strain and temperature simultaneously,but also can increase the capacity and capability.4)The feasibility of the design system is verified.The I-WDM experiments were carried out by metallized LPFG and FBG,including the temperature and bending sensing experiments of fiber Bragg grating sensors in the system,and the difference of intensity and wavelength was used to distinguish the sensor signals.The fiber optic tension machine is used to bend the sensing experiment of the fiber grating sensor affixed to the aluminum foil,and the temperature sensor experiment is carried out by the drying heater.In the experiment,the speed of the drawing machine dropped was 1mm/min per time,1mm decreased,the residence time was 2min,the total dropped 5mm and then in the same way.The temperature sensor experiments were carried out by dry heating method.The center wavelength of the sensor was recorded every 10 degrees by using the spectrometer,and it was fitted by origin.The flexural sensitivity is 171.865pm/C.When a certain amount of curvature is reached,the transmission summit becomes two peaks and the difference between the center wavelength of the two wavelengths and the bending loss are related to the bending amount.The distance between the two wave lengths decreases with the increase of the bending amount and the loss amplitude decreases with the decrease of the bending amount.