Optical Fiber Humidity Sensor Based On Water Absorption Peak At 2 Micron Waveband

At present,the application of laser in 2 micron waveband in laser medical surgery is very common compared to lasers in other wavebands.The laser in 2 micron waveband has a more obvious advantage: water molecules have strong absorption peaks near the 2 micron waveband.This characteristic,the moisture in the tissue can instantly absorb the energy emitted by the laser in the waveband,resulting in a strong vaporization cutting effect.In addition,with the advent of low-loss fiber and erbium-doped fiber amplifiers in the 2 micron waveband,this waveband has also been gradually applied to research in the field of optical communication.At present,the application of this waveband laser in the field of sensing is relatively small.This paper proposes to apply the 2 micron waveband laser to the field of humidity sensing to achieve the measurement of humidity.Based on the strong absorption peak of water in the near-infrared 2 micron waveband,the characteristics of fiber absorption loss in the humidity environment of this waveband are studied.The water absorption peak in the 2 micron waveband is experimentally verified,and the humidity sensor based on the absorption peak is realized.We first investigated the position of each absorption peak of water in the near-infrared region,analyzed the possibility of each absorption peak for humidity measurement,and finally determined the feasibility of humidity detection based on the 2 micron waveband.According to the development trend of miniaturization of optical systems,we use micro fiber structure as a dielectric waveguide of 2 micron laser to prepare the sensor.Then the basic model of the tapered fiber is established.Based on the principle of evanescent wave,the transmission characteristics of the tapered fiber used as the humidity sensor are discussed.In addition,according to the structure and propagation principle of the micro fiber knot resonator,the output spectrum formula is derived.We calculated the free spectral range,quality factor,fringe fineness and extinction ratio of the micro fiber knot resonator.The four parameters that characterize the spectral characteristics of the micro fiber knot resonator are analyzed.The different refractive indices and different cavity perimeters are analyzed.And the change of the output spectrum of the micro fiber knot resonator under the condition of different absorption coefficient changes and the influence of these four parameters on the humidity measurement results.The above lays a theoretical foundation for real-time measurement of humidity.Then the paper introduces the preparation method of tapered fiber and micro fiber knot resonator.Under the experimental conditions and environmental requirements required in the preparation process,two ideal micro fiber sensors are obtained,and these two are realized.Zero coating of the moisture sensitive material on the sensor-like surface simplifies the experimental steps and saves material costs.Finally,we designed and built a complete humidity generator,and placed the above two successfully prepared sensors into an environment with adjustable humidity.Analyze the output optical power of each humidity sensor at room temperature,the change of output optical power during continuous measurement under certain humidity conditions,the stability of the sensor under long-term measurement conditions and the response time of the sensor.In addition,the experimental test of the output optical power changes with humidity of the two sensors under different temperature conditions,eliminating the influence of temperature on the humidity measurement results.