Characterization Of The Liquid Level Sensor Based On Tilted Fiber Bragg Grating

Fiber grating sensors have the advantages of small size,light weight,corrosion resistance,anti-electromagnetic interference,etc.And can be widely used in chemical,food,automation industrial production and potentially hazardous environments.The tilted fiber Bragg grating(TFBG)is one of the short-period fiber gratings,and its grating plane has a certain tilt angle with the fiber axis,which greatly enhances the coupling strength of the cladding mode,is highly sensitive to the external environment,and the liquid measurement can maintain the integrity of the optical fiber structure and mechanical strength,and has low cross-sensitivity to temperature.This paper first introduces the common methods of liquid level measurement and the development of tilted fiber gratings,describes the coupled-mode theory of TFBG,and theoretically analyzes the sensing characteristics of TFBG from the three aspects of stress,temperature and external environment refractive index.The Opti Grating software was used to simulate the grating tilt,grating length,grating period and refractive index modulation depth in the grating structure,and the influence of various parameters on the TFBG transmission spectrum was analyzed.At the same time,the external refractive index and temperature are simulated.The results show that when the external refractive index is 1.408~1.444,the transmission spectrum amplitude of the TFBG transmission spectrum cladding mode gradually decreases with the increase of the external refractive index,and is basically not affected by the temperature.However,the peak wavelength shifts to the long-wavelength as the temperature increases,so the cross-sensitivity of the temperature and the external refractive index can be eliminated.An experimental device for measuring the temperature,ambient refractive index,and temperature was constructed.The experimental results show that the center wavelength of the resonance peaks of the various cladding modes and the core mode of the TFBG increase with the temperature rising from 20°C to 50°C,the center wavelength of the resonant peak of the core mode is most sensitive to temperature changes and has a good correlation.When the external refractive index changes from 1.408 to 1.436,the center wavelength of the resonance peak of the TFBG cladding mode shifts to the long wave direction,the center wavelength of the resonance peak of the core mode does not change significantly,and the high order mode of the cladding mode gradually disappears,and the normalized area disappears.As the temperature and ambient refractive index change at the same time,the influence of temperature on the peak of the resonance peak of the cladding mode of the TFBG can be neglected,and the temperature change is monitored by using the shift of the center wavelength of the TFBG core mode.The variation of the normalized area of the cladding modes of each order monitors the change of the refractive index of the outside world and realizes the simultaneous measurement of the temperature and the outside refractive index.Then,five levels of five glycerol solutions were used to measure the liquid level,and the area normalized by the cladding mold normalized area was used as the liquid level measurement measure to increase the sensitivity.It was confirmed that as the liquid level increased,the normalized area gradually decreased,and The closer the solution refractive index is to the refractive index of the clad,the higher the liquid level measurement sensitivity.At the same time,the temperature and liquid level can be measured simultaneously by using the drift of the center wavelength of the TFBG Bragg resonance peak and the normalized area of the cladding mode.