Fabrication And Torsion Characteristics Of Mach-Zehnder Fiber Torsion Sensor Based On Structure Modulation

Based on the research of fiber structure modulation,a Mach-Zehnder fiber structure based on interference arm modulation is proposed in this article.The structure is prepared in two steps:firstly,a fiber fusion splicer is used to splice different optical fibers to obtain a Mach-Zehnder Interferometer（MZI）.Then a high frequency CO₂ laser is used to etch the interference arm of the MZI and introduce a symmetrical polishing plane into the interference arm.This method destroys the circular symmetry of the MZI interference arm structure and enhances its sensitivity to torsion.When the equivalent diameter of the fiber structure decreases,the torsion sensitivity is further enhanced.Based on this method,we have prepared two kinds of highly sensitive fiber torsion sensors.The first one is the highly sensitive torsion sensor（SP-SMS）based on symmetrically polished（SP）single-mode-multi-mode-single-mode fiber structure（SMS）,the second one is a high sensitivity torsion sensor（SP-MSM）based on the multi-mode-single-mode-multi-mode（MSM）fiber structure,which measures temperature and torsion simultaneously.Their sensing and spectral characteristics are simulated and experimentally studied.Firstly,the torsion properties of SP-SMS are studied.SP-SMS is prepared by introducing a symmetrical polishing plane on the SMS-structured multi-mode fiber.Both the polishing depth and polishing length of the multi-mode fiber have an effect on the sensitivity of the structure to torsion.In the experiment,the control variable method is used to obtain the best polishing depth and length.Because the circular symmetry of the interference arm structure is broken and the fiber core diameter is different,the high-order modes in the multimode fiber are excited,which further enhances the torsion sensitivity of SP-SMS.The torsion sensitivity of SP-SMS with a polishing depth of 25μm on the interference arm is shown to be up to 1.032nm/（rad/m）in the measurement range of-10.217 rad/m to 10.217 rad/m.Based on the theory that the torsion sensitivity of Mach-Zehnder fiber structure can be enhanced by the introduction of symmetrical polishing structure on the interference arm.SP-MSM is prepared in the experiment,which is prepared by introducing symmetrical polishing plane on the single-mode fiber with MSM structure.Since the interference fringes of the MSM structure are more uniform and denser,two interference peaks near 1550 nm in SP-MSM are selected for demodulation,achieving simultaneous measurement of torsion and temperature.In addition,the optical fiber forming the interference arm is changed to single-mode fiber,and the polishing parameters of single-mode fiber are adjusted appropriately.The experimental results show that the torsion sensitivities of interference peak A/B of SP-MSM is 0.277 nm/（rad/m）and 0.273 nm/（rad/m）,and the temperature sensitivity of interference peak A/B is 71.09 pm/℃and 51.45 pm/℃.By solving the matrix equation,temperature and torsion can be measured simultaneously.The method of symmetrical polishing plane is introduced into the interference arm of Mach-Zehnder fiber structure,which simply and effectively improves the torsion sensitivity of the structure.The sensor prepared by this method is small in volume,simple in structure,high in sensitivity and good in repeatability,and has a good development prospect in the manufacture of optical fiber torsion sensor.