Research Of Optical Fiber Sensor Based On Liquid Crystal For VOC Gas Monitoring

Volatile organic compounds(VOC)sensing and monitoring are of great significance in the fields of pollution prevention and chemical production.In recent years,cholesteric liquid crystal provides a simple and efficient choice for VOC optical sensing with the advantages of rapid response to gas and form of visualization.However,the traditional sensor based on liquid crystal membrane can not meet the requirements for accurate detection of gas concentration in disease detection and scientific research.Therefore,this paper proposes a combination of cholesteric liquid crystal and optical fiber sensing,which can sense VOC in real time,accurately and quickly.Because the detection results are easily affected by temperature fluctuation,in order to eliminate the interference of temperature,an optical fiber sensor based on liquid crystal with temperature compensation is fabricated to improve the accuracy of gas detection.The main research contents are as follows:1.A small amount of cholesteric liquid crystal is attached to the end face of an optical fiber to fabricate an optical fiber liquid crystal probe sensitive to VOC.The influence of different concentrations of VOC on the reflection spectrum can be detected at any time by using a multimode optical fiber coupler.The response of four different liquid crystals to acetone gas is studied.Within a certain detection range,the shift of the central wavelengths of their reflection bands is linear with the change of gas concentration,with the maximum sensitivity of-58.87 nm?L/mmol.Compared with chiral nematic liquid crystals,cholesteric ester liquid crystals show better repeatability.The optical fiber liquid crystal probe is also fabricated by welding the quartz capillary to the end face of fiber,which enhances the mechanical stability of the gas sensor.2.The laser emission of dye doped cholesteric liquid crystal is studied and the optical fiber probe based on liquid crystal laser is fabricated.The parallel aligning layer is added into the capillary fused with the optical fiber to form a good resonant microcavity,then the liquid crystal laser of photonic band gap(PBG)mode is inspired,and the wavelength of band edge laser is used for sensing.The effect of thickness of liquid crystal on gas sensing is studied and it is found that when the thickness is 85 μm,the sensitivity of the probe to acetone gas increases to-98.16 nm?L/mmol.A sensor with dual probe for detecting temperature and acetone is made,and its temperature sensitivity and concentration sensitivity are-1.001 nm/℃and-88.49 nm?L/mmol respectively.3.An optical fiber liquid crystal probe with temperature compensation function is designed and fabricated.Two cholesteric liquid crystals with different positions of Bragg reflection band are added into the capillary fused with fiber,in order to measure the temperature and gas concentration at the same time.Combining the ray tracing simulations and the effects of liquid crystal with different thicknesses on the sensitivity of acetone as well as temperature,the appropriate thickness of liquid crystal,inner diameter and length of capillary are finally determined.The temperature sensitivity of the temperature-compensated sensor with single probe is 2.53 nm/℃ in the range of 8 ℃-44 ℃,the detection range for acetone is 0-0.946 mmol/L,and the sensitivity to acetone is 48.46 nm?L/mmol.The temperature compensation ability,repeatability and response time of the sensor are evaluated.It is proved that the acetone gas can be measured accurately and stably under the temperature fluctuation within 10 ℃,and this sensor has great application potential in high-precision and real-time monitoring of VOC.