Optimal Design And Experiments Of Chemical Sensor Based On Long-period Fiber Grating

Biosensors have a multitude of important applications in basic research,environmental monitoring,and medicine.Fiber optic sensing technology has been broadly used for chemical and biological sensing and detection.Aiming at performance optimization of the chemical sensor based on long-period fiber grating(LPFG),the sensitivity of chemical sensing of coated LPFG and the refractive index sensing of LPFG in photonic crystal fiber(PCF)are deeply studied;The thickness of each layer of the self-assembled film is several nanometers,which can realize the nanoscale control of the thickness of the film.Using the LPFG coated with optimal thickness of self-assembled PAH/PAA film,the high sensitive sensing of humidity,p H value and surrounding refractive index(SRI)is realized.In this paper,the main contents and results are as follows:??Optimization design of coated LPFG chemical sensorsThe sensitivity of resonant wavelength of the coated LPFG to SRI and the thin film parameters is analyzed.The optimum parameters of thin film for high-sensitive sensing different SRI and film refractive index are determinded.The chemical sensing properties of LPFG coated with the absorbent film are studied,and the function of the amplitude attenuation peak with the film absorption coefficient and the gating coupling strength is obtained.It is pointed out that the grating coupling strength kL is greater than before coating,this is called over-coupling,can reduce the decrease of the loss peak caused by the absorption of the coating film.? / 2The dual-peak resonance induced by the coupling between the core mode and the higher order cladding mode is analyzed.The way for improving the sensitivity of coated dual-resonant fiber gratings to the film parameters and the SRI is given.That is making LPFG working in mode transition region by choosing optimal film thickness firstly,and then making the interval of dual-peak of LPFG small after coating.Usually,the interval of dual-peak will increase after coating,This problem is solved by etching optical fiber before coating and the method for finding optimal fiber radius is given.The sensitivity of coated ?-phase shift LPFG to film refractive index is investigated.The sensitivity of the sensor is defined by the change rate of the transmission at specific wavelength with respect to the film refractive index.The qualitative and numerical analyses show that to get a high sensitivity,the optimal coupling strength of each long-period fiber grating is ?2? / 4.With this coupling strength,the longer the grating is,the higher sensitivity the sensor has.Meanwhile selecting the optimum film parameters,making the cladding mode be located in mode transition region,the sensitivity of sensor will be improved further.??The chemical sensing experiment using coated LPFGThe thin film of PAH/PAA is coated onto the outside of LPFG by self-assembly method.The mode transition caused by the increasing of film thickness is observed.In the vicinity of mode transition region,the resonance wavelength of LPFG shifts 0.6 nm for1 nm increase in film thickness.Surrounding refractive index sensing experiment was carried out over the range of 1.33-1.45 using the coated LPFG.It was found that the LPFG coated with 25-layers film is more sensitive to high refractive index,and the LPFG coated with 45-layers film is more sensitive to low refractive index.This is because the environment refractive index is higher,the film thickness for mode transition is smaller.The sensitivity of the LPFG coated with 25-layers film to SRI in the range of 1.4-1.42 is 479 nm/RIU.The sensitivity of the LPFG coated with 45-layers film to SRI in the range of 1.35-1.37 is 950 nm/RIU.The humidity sensing experiment was carried out using the LPFG coated with PAH/PAA film.It was found that the amplitude of attenuation peak increases with the increasing of environmental humidity and the sensitivity is 0.08 d B/%RH.Results show that the loss of thin film layer decreases with the increasing of humidity,so the humidity detection based on intensity modulation can be realized.The multilayers of PAH/PAA was assembled outside the LPFG which has been corroded by hydrofuoric acid solution.The coated LPFG was made to work in the vicinity of mode transition region and the phase matching turning point.The resonance wavelength of LPFG shifts 1.25 nm for 1 nm increase in film thickness.The sensitivity of the LPFG to SRI is as high as 3985 nm/RIU in the 1.333–1.372 index range,which is at leading level in coated LPFG RI sensors that have been reported.The humidity sensing experiment was carried out by optimzed dual peak LPFG coated with PAH/PAA film.The sensitivity of resonant wavelength to humidity reaches 0.23 nm/% RH.The dual peak LPFG coated with PAH/PAA film was put in the acidic solution of different p H,the sensitivity of the interval of dual peak to p H is 4.2 nm/p H.??The optimization of refractive index sensor using LPFG based on photonic crystal fiberThe mode characteristics of the refractive index guide type PCF is analyzed using the finite element method.The influences of the optical fiber radius to the cladding mode are analyzed,it was found that,with the increase of cladding radius,the changes of cladding mode are similar to the mode transition phenomena in coated LPFG.For the LPFG based on PCF,selecting the appropriate fiber radius,making the cladding mode located in transition zone,the sensitivity of resonance wavelength to the SRI can be as high as 2660 nm/RIU.This sensitivity is greater than that of LPFG based on common bare fiber.It is even competitive with that of long-period grating in PCF in response to changes in refractive indices in the cladding air channels.The leakage properties of PCF cladding mode are analyzed by applying the perfectly matched layer boundary condition.Expanding the air hole of the outermost layers of the PCF,we found the depth of attenuation band in the transmitted spectra and the total transmitted power is sensitive to the RI in the larger holes.The simulation calculation shows that the refractive index detection can be achieved by detecting the transmission power of the grating in this structure,the intensity modulated refractive index sensor can be realized.