Research On A Tapered Optical Fiber Refractive Index Sensor Based On Intensity Demodulation

Fiber-optic mode interferometers not only have the characteristics of corrosion resistance,anti-electromagnetic interference,but also have the advantages of high sensitivity,easy preparation,and compact structure.They have broad development prospects in the fields of building health monitoring and high-precision physical quantity detection.In this paper,in order to reduce the use cost and temperature crosstalk,and improve the detection sensitivity,a tapered fiber interferometer based on intensity demodulation is proposed by the arc discharge technology.Furthermore,the refractive index and temperature sensing characteristics are studied separately.The main contents are as follows:(1)The structure and optical transmission principle of tapered single-mode fiber are theoretically studied,and the principle of mode interference based on Mach-Zehnder interferometers and Michelson interferometers is analyzed.By beam propagation method,the excitation and coupling of guided modes are simulated in tapered single-mode fiber.The refractive index and temperature sensing principle of the fiber mode interferometer and the response of interference fringes in terms of wavelength and intensity are studied.(2)The refractive index sensor of tapered fiber under the condition of primary discharge is studied.First of all,the refractive index and temperature sensing characteristics are studied for typical Mach-Zehnder interferometers based on double tapers cascade structure.In order to reduce the use cost and eliminate the temperature crosstalk,an Mach-Zehnder interferometers based on the tapered single mode-multi mode-double cladding-multi mode fiber structure with the intensity demodulation is proposed,and the refractive index and temperature sensing tests are investigated.The experimental results show that double cladding fiber has RI-free characteristics.The interferometer has a linear intensity response to the refractive index and a linear wavelength-intensity response to temperature.Highly distinguishable refractive index and temperature simultaneous measurement is realized,thereby temperature crosstalk is eliminated.(3)In order to improve the intensity sensitivity of the refractive index of the interferometer,a novel tapered fiber fabrication method called secondary discharge is proposed.The experimental results show that the refractive index intensity sensitivity of the interferometer reaches 520 d B/RIU,which is better than that of the primary discharge structure.Furthermore,to further enhance the sensitivity and miniaturization,a reflective structure based on Michelson interferometers is designed and completed.The experimental results show that the refractive index sensitivity of the reflective structure is twice that of the transmissive structure,and the size is half that of the transmissive structure,which effectively improves the practicability of the refractive index sensor.