The Research Of Novel Highly Sensitive Optical Fiber Torsion Sensors

Recently,with the development of the modern smart structure monitoring,torsion sensors have been receiving extensive interest because torsion is an important mechanical parameter that reflects the stress state and internal injury of the monitored structure.In principle,conventional torsion sensors are divisible into two classes:the sensors based on electrical methods,and the sensors based on electromagnetic phenomena.However,the former are sensitively affected by the electrical noise and temperature,the latter are complicated manufacturing,bulky heavy structure,and usually difficult to be embedded into the monitored structures.In comparison,optical fiber torsion sensors,as a new kind of torsion sensor,overcome the shortage of the conventional torsion sensors and have been widely used in the area of modern smart structure monitoring.In this thesis,we proposed and demonstrated three kinds of highly sensitive optical fiber torsion sensor.At the same time,theoretical analysis and experiment researches are carried out to confirm the viability of the proposed optical fiber torsion sensors.The specific research results are as follows:?1?We demonstrate the fabrication of phase-shifted fiber Bragg gratings?PSFBGs?realized by the line-by-line?LbL?technique.By accurately controlling the gap??between two LbL inscribed uniform FBGs,the fourth order PSFBGs with the phase-shift values of?/2,?and 3?/2 are realized.The polarization-induced spectral shifts between the narrow transmission bands measured in orthogonal polarization states are 210 pm,252 pm for the two?-PSFBGs,corresponding to the birefringences of 2.0×10^-4,2.4×10^-4,respectively.The LbL technique offers individual phase control,without the need of any postprocess to adjust the phase shift,allowing for a fast inscription process of PSFBGs.?2?A novel temperature-and strain-independent optical fiber torsion sensor based on a PSFBG inscribed by the LbL technique in a standard single-mode fiber with a femtosecond laser has been proposed and experimentally demonstrated.The strong birefringence created by the LbL inscription technique leads to the significant polarization splitting of the transmission peak of the PSFBG.By simply monitoring the variation of the amplitude difference between the two polarization-peaks,the fiber torsion angle and the fiber torsion direction can be simultaneously deduced without temperature and strain confusion.The torsion sensor exhibits a high torsion sensitivity of up to 1032.71 dB/?rad/mm?,with the distinct advantages of low manufacture cost,extremely robust and simple structure and small dimension?just¹.72mm?,which make it very attractive for practical applications.?3?We proposed and experimentally demonstrated highly sensitive optical fiber torsion sensors based on a transmission Lyot filter and a reflective Lyot filter,respectively.Based on the intensity modulation,the torsion sensitivities of the two proposed sensors are 15.586dB/rad and 20.336 dB/rad.At the same time,they are temperature-and strain-insensitive.Moreover,we perform the theoretical simulation of the proposed torsion sensor,and the simulation result obtained agrees well with the experiment results,vividly confirming the viability of the fiber Lyot filter based torsion sensor.?4?A highly sensitive optical fiber torsion sensor with femtosecond laser-induced low birefringence SMF-based Sagnac interferometer?SI?is proposed and experimentally demonstrated in this thesis.A straight-line waveguide positioned parallel with respect to the fiber core is inscribed by the femtosecond laser in the cladding of the SMF,which leads to the asymmetry stress distribution in the SMF,and then gives rise to the low birefringence in the SMF.Compared with most of the previous reported SI based torsion sensors,there is no splicing joint in the femtosecond laser-induced low birefringence SMF-based SI,which lowers the transmission loss and makes the SI based torsion sensor more robust simultaneously.The experiment result shows that the proposed torsion sensor exhibits a torsion sensitivity of up to 3.2562 nm/degree,with the high torsion resolution of 0.003 degree.In contrast,the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low to-0.000055 degree/°C and 0.000013 degree/??,respectively,overcoming the cross-sensitivity problem resulting from temperature and strain.