Research On The Inscription And Application Of High-Performance Fiber Bragg Gratings Based On Femtosecond Laser

In the field of oil logging,it is great significance that how to safely and reliably obtain well and wellbore environmental parameters to improve the exploration and development efficiency of oil and gas resources.Fiber grating sensors,due to their intrinsic special advantages,they can withstand the extreme conditions of the underground.Therefore,they are widely used in the field of optical fiber logging.In this thesis,fiber Bragg gratings(FBG)have been written on three different types of fiber by phase mask technology based on femtosecond laser.Besides,the three sensing characteristics of temperature,bending and high temperature strain have been experimentally investigated.Using the near-infrared femtosecond laser combined with the phase mask,three different types of FBGs have been written over the single-mode photosensitive fiber core and part of cladding by changing the focal-line position of laser beam.The grating transmission spectra show two separate resonant modes,namely,core mode and cladding mode.Among them,we choose the FBG which the refractive index modulation of fiber cladding is greater than the core for the temperature testing.The experimental results showed that the cladding-FBG has a good temperature resistance as well as the core FBG.Moreover,the temperature response sensitivity was almost equal to that of the core FBG,so the cladding-FBG can also be used as the monitoring of the temperature change in the high temperature environments.Using 800 nm femtosecond laser side-illumination technique,three different types of FBG have been successfully written in multimode fiber(MMF)with hydrogen.Focusing the laser at different locations of the multimode fiber core can trigger different grating patterns.When the laser beam has been located at the center axis of MMF core,resulting in one resonance in transmission.When the focus spot has been deviated about 10?m from the center axis of MMF core,including fundamental mode and multiple high-order modes have been excited.Further moving the spot(offset?20?m),only high-order modes have been excited.A grating with only fundamental mode has been used as a strain test for high-temperature.The experimental results showed,as the temperature increases,the strain sensitivity of the sensor was significantly improved(Up to 5.24pm/??).Therefore,multimode fiber Bragg grating(MFBG)is particularly suitable for high temperature strain monitoring.Using a femtosecond laser combined with a phase mask technique,the FBG was fabricated in a load of hydrogen four-cladding single-mode fiber with a depressed first cladding by focusing the laser beam at the interface between the fiber core and the first cladding.Grating structures have been formed over the fiber core and part of the first cladding.Moreover,the bending and temperature characteristics of the FBG have been experimentally demonstrated.The bending tests result showed that the intensity of the cladding-FBG reflection peak was sensitive to the microbending of the fiber.And because of the asymmetric distribution of the cladding-FBG over the cross section of the fiber,the bending response of the cladding reflection intensity has a high orientation-dependence while its resonance wavelength was insensitive to bending.It is possible to realize the simultaneous measurement of bending and temperature by separately monitoring the energy and the wavelength information of the cladding reflection peak.The successful development of multi-cladding FBG is of great significance to the monitoring of structural health.