Research On Torsional Fiber-optic Devices And Its Sensing Characteristics

In recent years,in the field of aerospace,the demand of modern aircraft with multiple aerodynamic shapes has been put forward.Flexible skin has been widely concerned at home and abroad because it can change the shape of aircraft.The required aerodynamic characteristics can be obtained by changing the shape of the flexible skin of the aircraft,so as to improve the cost performance of the aircraft.The acquisition and feedback of deformation and flight environment monitoring information is one of the bottlenecks in the process of intelligent flexible wing continuous variation.This thesis designs and makes a variety of fiber-optic sensors based on the single helix and multi helix structure to realize the measurement of directional torsion,vector bending,pressure,humidity and other sensing quantities.On the basis of different optical fiber structures,several novel functional applications are studied through sensitization,and the application of twisted fiber-optic sensors are expanded.The specific work includes the following aspects:Firstly,to solve the problem of low efficiency in the fabrication of multi-core fiber grating array,the method of defocusing phase mask is used to fabricate fiber Bragg grating(FBG)on multi-core fiber.The FBG array is fabricated on the triangular four core fiber(TFCF).In this thesis,the curvature,temperature,strain,and torsion characteristics of TFCF FBG array are studied.In order to enhance the torsional sensing ability of TFCF FBG,a large pitch single screw fiber structure was fabricated on TFCF,and FBG was fabricated in the torsional region.The physical parameters such as torsion,vector bending,temperature,and strain were measured.Secondly,based on the off-axis twisted deformation(OATD)as the beam splitter and combiner,a Mach-Zehnder interferometer(MZI)is constructed based on a pair of OATDs,and the simultaneous measurement of torsion and strain in the resolvable direction is realized.The experimental results showed that the resolution of torsion and strain of OATDs-MZI is very good,and the resolution of torsion and strain were ±0.0611 rad/m and ±0.8232 ??,respectively.The air microchannel etched by femtosecond laser is used for pressure sensing on the cladding of single mode fiber(SMF)between two deformation points of OATDs-MZI.The air pressure sensitivity of OATDs-MZI was-5.183 nm/MPa,and the temperature cross-sensitivity was 13.3 kPa/?.In the SMF core between two deformation points of OATDs-MZI,a femtosecond laser etched microcavity was used for relative humidity(RH)sensing.Without using any humidity sensitive materials,the humidity sensitivity of the sample was-0.144 nm/%RH,and it can achieve an ultra fast humidity response of about 68 ms.Thirdly,an off-axis spiral long period fiber grating(OAS-LPFG)is proposed,and the polarization dependence of the coupling of OAS-LPFG modes were analyzed.The sensing characteristics of OAS-LPFG were studied.Based on the off-axis helix structure,a chirped long period fiber grating based on variable pitch off-axis spiral structure(VPOASS)is proposed.A kind of optical fiber SPR sensor for ethanol vapor concentration is prepared by depositing tungsten disulfide(WS2)on the surface of VPOASS with gold film.By monitoring the loss intensity of multiple resonance dips,the ethanol vapor concentration,humidity,and temperature can be measured at the same time,with resolutions of ±0.030 mg/L,±0.035%RH and ±0.010?,respectively.Finally,to solve the problem that most of the interference torsion sensors are not sensitive and can not distinguish the torsion direction,a high sensitivity fiber-optic torsion sensor based on the hybrid cascade structure is proposed.The high torsional sensitivity is due to the spatial frequency difference between the multi-mode interferometer(MMI)and the helix structure assisted MZI.The envelope spectra were recovered by inverse fast Fourier transform(IFFT)The hybrid cascaded structure based on SMF,multi-mode fiber(MMF)and TFCF is very difficult in single end application.In order to solve the above problems,we propose a Michelson interferometer(MI)based on SMF,MMF,and TFCF.Based on the above MI,a kind of helix structure coated with gelatin was proposed.On the basis of SMF-MMF-TFCF-MMF-SMF(SMTMS)hybrid cascade structure,a humidity sensor based on helix structure assistanted and GO coating is proposed and verified by experiments.In this thesis,based on the torsion structure,a variety of optical fiber structures are proposed to realize the deformation sensing and flight environment monitoring of intelligent flexible skin,which provides technical reserves for the realization of intelligent flexible skin.