Research On Sensing Properties Of Long-period Fiber Gratings Prepared By Femtosecond Laser Direct Writing

Optic Fiber grating sensors have gained people’s favor in the field of sensing involving extremely harsh environment such as defense industries and aerospace due to their advantages of small size,anti-electromagnetic interference and corrosion resistance.As one of the important members of fiber grating sensors family,the period of long-period fiber grating(LPFG)is from tens to hundreds of micrometers,performs the coupling between the core mode and cladding modes in the same direction.The resonant wavelength is more sensitive to the changes of external environmental parameters,has a higher sensitivity of temperature,strain,and refractive index(RI).At the same time,it also has the advantages of small insertion loss,wide bandwidth,low reflection,etc.Therefore,it has been widely used in fiber communication and sensing.The fiber gratings inscribed by traditional ultraviolet(UV)laser have excellent spectral performance.However,it requires the sensitization treatment of the optical fiber in advance and at a high temperature above 400℃,the RI modulation will be erased.The femtosecond laser technology is a huge breakthrough in the grating fabrication process in recent years.It has no photosensitivity requirements for the written optical fiber,and could withstand high temperature environment(1000°C)and increase the application value of fiber grating.We prepared LPFG in a single-mode silica optical fiber by infrared femtosecond laser scanning direct writing.The main contents are as follows:1.The mechanism of femtosecond(fs)laser-induced refractive index modulation in transparent media has been explained.Since femtosecond lasers have ultrashort laser pulses and extremely high pulse energy,femtosecond pulses energy concentrated in the laser focusing area in a shorter time than the lattice thermal diffusion time(ps order),and extra nuclear electrons absorbed more photons,excited and instantaneously transitioned to a higher energy level in the conduction band.There are two main forms named "photoionization" and "avalanche ionization" in the interaction of femtosecond lasers with quartz dielectric materials.Non-linear photoionization can been further divided into multiphoton ionization and tunnel ionization.The transiently high energy deposition in the focal region of the dielectric material induced by these highly nonlinear effects caused the breaking of the chemical bond even the partial region shrunk or formed a defect and the refractive index of the part of the material got changed.The coupled-mode theory of fiber gratings also has been discussed,and we analyzed the mode coupling characteristics in LPFG.2.We used a 60 x oil-immersed Olympus objective to fabricate the LPFGs with grating period from 200μm to 500μm.The spectrums of LPFGs fabricated by Fs LDW performed well.Then,we discussed the influence of FsLDW processing parameters such as grating periods,the number of cycles and single pulse energy on the spectral properties of LPFG and the temperature and surrounding refractive index response of LPFG had been tested.3.A small-period long-period fiber grating(SP-LPG)has been proposed.The grating period was set from 10μm to 100μm.The spectrum of SP-LPG can be simultaneously observed a series of Bragg reflection peaks and long-period grating loss band.As a single grating structure,it combines the properties of FBGs and LPFGs.The resonant wavelength performs a good response to temperature,strain and surrounding refractive index,and a sensitivity matrix method has been proposed to compensate the error of cross-sensitivity in temperature and strain measurement in practical applications.Realtime monitoring under high temperature and high pressure conditions,such as aerospace,oil and gas pipeline transportation has good application prospects.