| Optical fiber sensors can transform the measured signal into the optical signal which transmitted in the integrated optical fiber,it has the advantages of small size,light weight,high precision,anti-electromagnetic interference,etc.It is widely used in structural health monitoring,aerospace,ocean detection and other fields.With the large-scale application of optical fiber sensor,conventional optical fiber sensors are faced with the problems of insufficient sensitivity,mechanical performance,compactness and other indicators,which are difficult to meet the requirements of the measurement in complex environments.Therefore,it is necessary to propose high performance optical fiber sensors to meet the needs of complex and changeable real working conditions.In this dissertation,optical fiber sensor with fiber core deformation is proposed and prepared,and its preparation method and sensing characteristics are studied.In order to prepare the sensor,a laser is used to process and modify the fiber surface morphology,at the same time,the fused cone technology or arc discharge technology is used to modulate the laser processed optical fiber.This technology can change the spatial position state of fiber core after fiber formation,the fiber sensor can achieve excellent sensing characteristics.So that the optical fiber sensor to obtain excellent sensing characteristics.At the same time,the formation mechanism,transmission spectrum characteristics and measurement principle of deformed core fiber sensing structure are deeply studied.On this basis,three kinds of fiber optic sensors with deformed core are prepared,and their processing parameters are optimized through experiments,and their refractive index,torsion and strain sensing characteristics are measured.The experimental results show that the sensor has high sensitivity,compact size and strong mechanical properties.The main research contents of this dissertation are as follows:1.A novel approach of combining laser polishing technology with melting tapering technology is proposed to fabricate a deformed core long-period fiber grating(LPFG)sensing structure.The single-mode fiber is first polished using a high-frequency carbon dioxide laser to create a preheating structure with grooves,which is then modulated using a high-temperature heat source.The fiber is heated to the melting point,and the surface tension causes the grooves on the surface to transform into deformations in the fiber core.This deformation results in refractive index modulation.The formation mechanism of the deformed core LPFG is theoretically analyzed and attributed to residual stress release,melting deformation,and fiber core micro-bending.The transmission spectrum characteristics of the LPFG are investigated through simulation and experimental results,optimizing the structure’s fabrication parameters for designing the subsequent deformed core fiber sensing structure.2.Two deformed core fiber refractive index sensors are proposed: a sinusoidal-core LPFG sensor and a fiber Mach-Zehnder interferometer(MZI)based on an arched-core fiber,and their sensing characteristics are measured.The transmission spectra and sensing properties of the two structures are simulated using optical simulation software,and the simulation results are in good agreement with the experimental results.By optimizing the degree of fiber core bending,the distance between the fiber core and the measured solution can be reduced,and high-order cladding modes can be excited to participate in the sensing process,thereby improving the sensitivity of the fiber refractive index sensor.In the measurement range of 1.33 to 1.42 refractive index units(RIU),the highest sensitivity of the sinusoidal-core LPFG sensor reaches620 nm/RIU,and the highest sensitivity of the arched-core MZI sensor reaches 401 nm/RIU.3.A compact fiber torsion sensor based on spiral core LPFG structure is proposed.Because the spiral core has a high refractive index modulation,it has a strong coupling effect on the light entering the fiber,so that the structure can form an obvious resonant peak under the condition of relatively few cycles,and the size of the sensor is effectively reduced to 1.75 mm.When the external torsion acts on the structure,the helical fiber core is deformed,which changes the refractive index modulation of the structure and effectively increases the torsion sensitivity of the structure to 0.37 nm/rad/m.On this basis,an MZI sensor based on helical core fiber structure is also proposed.The structure has a maximum torsion sensitivity of 2.48 nm/(rad/m),and the torsion and temperature can be measured simultaneously by demodulation of the movement of the interference peak,and the torsion resolution can reach 0.0045 rad/m.The temperature resolution is 1.22 °C.4.An LPFG strain sensor based on M-shaped fiber core is proposed.When the structure is subjected to axial stress,the fiber core will bend and deform,which makes the sensor have good strain sensing performance and can reach-19.05 pm /με in the measurement range of 0 μεto 500 με.It is-5.82 pm/με in the measurement range of 500 με to 1000 με.To improve the strain sensitivity for the sensor,a core bending LPFG structure was fabricated on the sheetshaped fiber substrate to achieve highly sensitive axial strain measurement.Due to the reduction of the equivalent diameter of the cladding and the stress concentration effect,the sensitivity of the structure was effectively improved.Through simulation and reasonable optimization of preparation parameters,the maximum strain sensitivity of the sensor can reach-0.846 nm/με in the measurement range,which is two orders of magnitude higher than the existing conventional LPFG structure.At the same time,an MZI sensor based on S-shaped fiber core was fabricated on the fiber V-groove substrate by arc discharge method,which can realize the simultaneous measurement of strain and temperature.In the axial strain range of 0 με to 350 με,the strain sensitivity is-66.5 pm/με and-40.1 pm/με,respectively.By calculating the measurement matrix simultaneously,the temperature resolution of the sensor is 0.44 °C,and the strain resolution is0.085 με.This dissertation uses laser polishing technology as the basis,combined with fiber fusion tapering technology and arc discharge technology,to design and develop a variety of deformed core fiber-optic sensors for measuring refractive index,twist,strain,and other parameters in specific application scenarios.The sensor has enhanced sensitivity,compact sensing unit,and good mechanical properties,providing research ideas and specific methods for the design and fabrication of fiber-optic sensors. |
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