Research On Fiber Optic Sensor Of Mach-Zehnder Interferometer Based On Helical Taper

Compared with traditional electrical sensors,fiber-optic sensors have the advantages of small size,light weight,resistance to electromagnetic interference,and corrosion resistance,and have received widespread attention from researchers in recent years.With the development of materials science and advanced processing technology,various fiber sensor fabrication methods have appeared,such as femtosecond laser processing,ion beam etching,and high temperature arc discharge.Among them,Mach-Zehnder interferometers for fiber optics are fabricated by arc discharge,and the sensor has attracted more attention due to its advantages such as simple operation and low cost.The optical fiber helical taper structure based on the principle of Mach-Zehnder interferometer has become a research hotspot as a new type of torsional sensor.In this paper,the Mach-Zehnder interferometer fiber sensor based on the helical taper structure is explored in detail,and its fabrication method and sensing characteristics such as torsion,temperature,strain and refractive index are studied.First,a helical taper structure was fabricated on a panda fiber using the arc discharge method of a fusion splicer.The helical taper structure was fabricated by arc discharge combined with rotating one end of the fiber.The optimal fabrication parameters were obtained by optimizing the parameters such as the discharge time and the rotation angle of the fiber clamp:(second step)the discharge time is 7 s and the rotation angle of the clamp is360~°.The torsion,temperature and strain sensing tests were performed on the fabricated optical fiber helical taper structure.Experimental results show that the structure has excellent torsional characteristics,sensitivity is about an order of magnitude higher than other fiber optic torsional sensors,reaching-3.191 nm/(rad/m),and can distinguish the direction of torsion(clockwise/counterclockwise);further research found its temperature sensitivity is extremely low,and the spectral wavelength drift is only 0.74nm in the temperature range of 21~100?.It can be considered that it is insensitive to temperature and can effectively solve the problem of cross sensitivity in practical applications.In the strain range of 0~784,its strain sensitivity is-29.16 pm/.In order to study different types of fiber helical taper structures and their sensing characteristics,helical taper structures were fabricated on ordinary single-mode fibers,and their temperature,strain,refractive index,and torsional characteristics were studied.The experimental results show that the temperature sensitivity of the single-mode fiber helical taper structure is also relatively low,with a maximum wavelength fluctuation of0.62 nm and an intensity fluctuation of 0.03 dB.In addition,as the applied axial tensile strain increases,the transmission spectral wavelength shifts toward the shortwave directional,and its strain sensitivity is-22.82 pm/;and as the refractive index of the external environment increases,the spectral wavelength drifts toward the longwave direction,and its refractive index sensitivity is 640.34 nm/RIU;the torsional characteristics of this single-mode fiber helical taper is a change in the transmission spectral intensity,with almost no shift in the spectral wavelength,and its maximum torsion sensitivity is-0.458 dB/(rad/m).Finally,inspired by the S-shaped fiber taper and the helical taper,a combination of the these two was used to fabricate an S-shaped helical taper structure on a single-mode fiber.The experimental results show that in terms of strain and refractive index sensing performance,the S-shaped helical taper does not show a particularly large difference,both of which show the characteristics of"blue shift"to the strain and"red shift"to the refractive index.The maximum strain sensitivity is-156.50 pm/,and the maximum refractive index sensitivity is 711.36 nm/RIU.However,the temperature characteristics of the S-shaped helical taper structure are non-linear,and the wavelength and intensity of the spectrum don't change significantly with increasing temperature.In addition,regarding the torsional characteristics of the structure,as the clockwise twist rate is increased,the intensity of the spectral expression changes.The clockwise torsion sensitivity is-0.087 dB/(rad/m);with the increase of the counterclockwise twist rate,it appears more as a shift in the spectral wavelength.The counterclockwise torsion sensitivity is 0.475 nm/(rad/m).