Research On The Optical Fiber Torsion Sensor With Intensity Modulation And Distinguishable Direction

Torsion is one of the key physical parameters in mechanical measurement.It is widely used in the health monitoring of torsional buckling of building structures and the measurement of robot joints rotating,and the torsion sensor with the ability of distinguishing the direction is particularly important.Compared with the traditional electromagnetic or mechanical torsion sensor,optical fiber torsion sensor has been become a research hotspot due to the unique advantages of small size,low cost,high sensitivity,and no electromagnetic interference.However,most of the reported optical fiber torsion sensors are based on the principle of the wavelength modulation,which require the expensive optical spectrum analyzer.Compared with the spectrum detection,the intensity detection has the advantages of fast response and low cost,but most of the intensity modulated torsion sensors can not distinguish the torsion direction except for few gratings or polarization devices.In this dissertation,we proposed an intensity modulated directional torsion sensor.The main research contents are as follows:(1)An intensity modulated optical fiber torsion sensor based on the interference of non-circular symmetric high-order mode and core fundamental mode is proposed.The theories of both the mode interference and the torsion sensing are established.The non-circular symmetric perturbation is an effective method to excite the non-circular symmetric high-order mode in the fiber,and an initial angle between the two perturbations is the basic principle to realize the intensity measurement of the torsion.It is clarified that the non-circular symmetrical mode which has an electric field distribution with a phase difference of?is the key to cause the phase jump and the reversion of the interference spectrum and hence to distinguish the torsion direction.(2)We proposed a pre-twisted optical fiber torsion sensor fabricated by CO₂ laser.The non-circular symmetric perturbations were fabricated by side exposing the single mode fiber to CO₂ laser.The excited non-circular symmetric cladding mode at the first perturbation interferes with the core mode at the second perturbation.An initial angle between the two perturbations is generated by the pre-twisted single mode fiber,which ensured that the sensor has the ability to distinguish the torsion direction.The spectral peak/dip turns to be the dip/peak when the fiber is twisted from the counter-clockwise to the clockwise direction.Benefiting from the reversal between peak and dip,an intensity-modulated directional torsion sensor is realized in the range from-50 to 50rad/m with a sensitivity of 45.3%/(rad/cm).(3)In view of the uncontrollable initial angle and cladding modes in the torsion sensor fabricated by CO₂ laser,we proposed a two mode fiber interferometer for directional torsion sensor based on the asymmetrical core-offset fusion splicing method.The mode interference between LP₀₁ and LP₁₁ modes in the two mode fiber is realized by core-offset fusion splicing,and the relative direction between two core-offset splicing points is designed to ensure the ability of the torsion direction discrimination.By optimizing the offset distance,the core-offset direction and fusion discharge parameters,we fabricated a two mode fiber interferometer with a high visibility and a low insertion loss.By tuning the relative direction between two core-offset splicing points,the reversal point of the spectral peak/dip is successfully adjusted.After the torsion sensing with difference interference lengths,it is concluded that the increase of interference length can improve the sensitivity.Finally,we realized an intensity-modulated directional torsion sensor with the sensitivity of 122%/(rad/cm)in the range from-52.38 rad/m to 52.38 rad/m by using the device with 3 cm interference length and the intensity difference detecting method.