Research On Asymmetric Structure Optical Fiber MZ Sensing Technolog

Fiber optic Mach Zehnder（M-Z）sensors have the advantages of simple fabrication,compact structure,high sensitivity,and fast response speed,and are widely used.Although it can determine the magnitude of bending when conducting curvature sensing,it cannot identify the direction of bending.When conducting torsion sensing,there is a problem of low torsion sensitivity and inability to determine the direction of torsion.The reason why the bending direction cannot be identified is that the fiber is symmetrically distributed along the neutral axis.When bent in any direction,the fiber is always compressed on one side and stretched on the other side.The effective refractive index difference caused by bending in different directions is the same,so the bending direction cannot be identified;The reason why the twisting direction cannot be recognized is that the fiber is cylindrical symmetric,and the effect of clockwise and counterclockwise twisting on the fiber is the same.The variation trend of the effective refractive index difference between interference modes is the same,and the interference wavelength moves in the same direction,so the twisting direction cannot be recognized.At the same time,the variation of the effective refractive index difference between modes is small,and the wavelength shift caused by twisting is small,so the twisting sensitivity is low.In order to realize bending and torsion direction identification and torsion sensitivity enhancement,it is necessary to destroy the cylindrical symmetry of the optical fiber.Based on this,this paper designs two asymmetric structure optical fiber M-Z bending vector sensors and one asymmetric refractive index modulated optical fiber M-Z torsion vector sensor.The experimental results show that,The sensor proposed in this paper can effectively solve the problems of identifying the bending direction during the curvature sensing process,determining the direction during the torsion sensing process,and enhancing the torsion sensitivity,and has superior performance for multiple sensing parameters.（1）In order to solve the problem that the fiber M-Z sensor has difficulty in identifying the bending direction,a new type of asymmetric structure fiber M-Z bending vector sensor is proposed.The structure first constructs a multi-mode single-mode multi-mode（MSM）fiber heterocore structure,and then uses a CO₂ laser to process equidistant zigzag structures on the single-mode fiber cladding in the middle section.Because part of the cladding on the single-mode fiber is removed by zigzag,At this point,the fiber core is no longer in the neutral position of the fiber optic.When the sensor is bent in the 0°or 180°direction,it will cause an increase or decrease in the optical path between the fiber core mode and the cladding mode,resulting in a red or blue shift in the interference wavelength,thus achieving bending direction recognition.（2）In order to solve the problem of temperature and strain interference in measuring bending of the asymmetric core cladding interference type M-Z bending vector sensor,a bending direction recognition theory based on internal mode interference of the fiber core is proposed.Based on this theory,a new type of asymmetric structure inter mode interference sensor for D-type multimode optical fibers is proposed.The sensor consists of a single mode D type multimode single mode configuration.Due to interference occurring within the D-type multimode fiber core,different fiber core modes have similar responses to temperature and strain.When temperature and strain change,the effective refractive index difference changes less,resulting in a smaller shift in the interference wavelength.Therefore,the sensitivity to temperature and strain is lower.At the same time,the design of the D-type asymmetric structure allows it to recognize the bending direction while sensing curvature.（3）In order to solve the problems of difficulty in identifying the direction of torsion and low sensitivity to torsion in fiber optic M-Z sensors,a new type of dumbbell fiber structure is proposed.The dumbbell fiber structure is designed based on the Filon model principle,which increases the stress level in the fiber core,thereby increasing the torsion sensitivity.Then,a hydrogen oxygen flame taper machine is used to perform spiral machining on the dumbbell fiber,further increasing the torsion sensitivity,And under asymmetric refractive index modulation,when subjected to torsion in different directions,the optical path difference between the core and cladding modes increases or decreases,and the interference wavelength undergoes a red and blue shift,thereby achieving torsion direction recognition.