Research On A Novel Optical Fiber Curvature Sensor Based On SPR

Curvature measurement technology plays an important role in building structure health monitoring,geophysics,geological disaster monitoring and other fields.At present,there are many kinds of fiber curvature sensors,such as fiber Bragg grating curvature sensor,long-period fiber grating curvature sensor,Mach-Zehnder interferometer fiber curvature sensor and so on.These optical fiber curvature sensors have their own advantages and disadvantages.The first optical fiber curvature sensor has high resolution,but low sensitivity;The sensitivity of the second optical fiber curvature sensor is higher than that of the former,but it is sensitive to the refractive index of the external environment;The third kind of optical fiber curvature sensor has high sensitivity,but it is relatively difficult to manufacture.In addition,the above three kinds of optical fiber curvature sensors have the problem of cross sensitivity.Surface plasmon resonance(SPR),as a new sensing technology in recent years,has been widely used in refractive index measurement of biological and chemical analytes,but its application in measurement of other physical quantities remains to be studied.Based on this,the paper proposes to apply the surface plasmon resonance technology to curvature measurement,so as to realize the optical fiber curvature sensor which has simple structure and can detect a single physical quantity.In this paper,aiming at the problems of low bending sensitivity and bending direction recognition of optical fiber SPR curvature sensor,the following studies are carried out on the optical fiber curvature sensor based on SPR:1.By establishing the mathematical model of curved fiber,the propagation path of light beam in curved fiber and the mathematical relationship between total reflection angle and bending curvature are theoretically deduced.At the same time,combined with mathematical software,the change law of total reflection angle of light beam entering curved fiber at different angles is calculated and analyzed,and it is concluded that with the increase of bending curvature of fiber,the change law of total reflection angle of light beam with different incident angles in curved fiber is consistent.2.A fiber core diameter mismatch type fiber cladding SPR curvature sensor is proposed.Separately,the single-mode optical fiber is subjected to core-offset welding with the single-mode optical fiber and multimode optical fiber without coating layer,and a gold film with a thickness of 50 nm is plated on the clad surfaces of the core-offset welded single-mode optical fiber and multimode optical fiber,and an ultraviolet curing adhesive is coated to form an SPR sensing area.At the same time,the classical heterogeneous core optical fiber structure formed by clamping and welding a single-mode optical fiber between two multimode optical fibers was fabricated,and compared with the two optical fiber structures formed by core-offset fusion technology.The experimental results show that the average wavelength sensitivity,average resonance intensity sensitivity and resolution of the core-offset fused optical fiber structure are higher than those of the heterogeneous core optical fiber sensing structure,and the heterogeneous core optical fiber structure is stable and easy to package due to the direct fusion.In addition,silver film is also used to modify the stable heterogeneous core fiber structure to improve its sensing performance.3.On the basis of the second research,a bending-induced fiber cladding SPR curvature sensor is proposed,in which the single-mode fiber with the coating removed is directly welded to the step multimode fiber for receiving transmission spectrum,and a gold film with a thickness of 50 nm is plated on the surface of the single-mode fiber cladding and coated with UV curable adhesive to form the SPR sensing area.Due to the bending of the fiber due to external force,the transmitted light in the core of the single-mode fiber leaks into the fiber cladding,and total reflection occurs at the interface between the cladding and air,resulting in evanescent field,and then SPR effect occurs.Experimental results show that,compared with the fiber core diameter mismatched fiber structure,the bending-induced sensing structure has more advantages in resonance wavelength sensitivity,resonance intensity sensitivity and resolution.In addition,the influence of the cut-off wavelength of single-mode fiber on the sensitivity of the sensor is also studied,and the measurement idle distance problem of the sensor is solved.4.The optical fiber SPR curvature sensor with direction recognition is proposed.On the basis of the first research,it is found that the total reflection angle of light beam has an opposite change trend with curvature on the convex side and concave side of bent fiber,and the change trend of total reflection angle is related to the shift direction of SPR resonance peak.According to the characteristic of total reflection angle of light beam,a gold film with a thickness of 50 nm is plated on one side surface of the core of the plastic clad step index multimode fiber with excellent structural stability,and the surface of the gold film is coated with ultraviolet curing adhesive in rotation pattern to form an asymmetric optical fiber SPR sensing region.Experimental results show that the sensor can recognize the bending direction through deviation direction of SPR resonance peak.