Development And Application Of Optical Fiber Temperature/Humidity Sensing System

It often needs to measure the environment temperature and humidity in the fields ofindustrial and agricultural production, weather, environmental protection, national defense,scientific research, aerospace, etc. Optical fiber temperature and humidity sensors will be oneof the underlying technologies of Internet of things in the future. This research topic based onthe intensity modulation,uses the structure of double clad single-mode optical fiber sensor as aprobe, aims to develop a set of optical fiber temperature/humidity sensor system used in theactual life.From the optical waveguide theory, this paper used the method of scalar to get thedouble cladding fiber mode field distribution, and further solving the characteristic equation.Then, this paper use of Matlab software to analyze the guide mode and cladding modes in thedouble cladding fiber sensor, and the calculation show that the number of the guide modeassociated with the refractive index of the outsourcing sensitive materials, there are multiplecladding modes in the sensor. This article also analyzed the the power coupling loss betweenthe double cladding fiber and the single cladding fiber in both ends, Matlab calculation showthat when the difference between the radius of outsourcing layer and the radius of fiber core isgreater than5μm, the refractive index of the outsourcing sensitive materials nearly has noeffect on the optical power coupling.In this paper, the corrosion technology of optical fiber, the choice of temperature/humidity sensitive material and the coating methods were studied.It was found that thecoating thickness of the sensitive materials has great influence on the range and linearity ofthe light power output response.Therefore, it needs to control the thickness of the coatinglayer when making sensor, in order to achieve the better modulation depth and responselinearity.This paper used the method of experiment to make out the attenuation characteristics oftwo types of sensors. The detection on the optical fiber temperature sensor show that themaximum attenuation can reach up to40dB, the minimum insertion loss is1.4dB, and thechange of light intensity possess quasi linear characteristic at temperature range of55~105C;The detection for the optical fiber humidity sensor show that it can be obtained a nearly linearresponse in the relative humidity range of30~100%RH, the optical power change can reach37dB.In this paper, from the overall to local,we designed and analyzed the light emissionmodule and the optical receiving module in both the hardware and software aspects, including the laser drive circuit, preamplifier and range automatic conversion circuit,amplifying circuit, A/D conversion circuit, microprocessor and its peripheral circuit, LCDinterface circuit, etc. The test of the circuit board showed that the system has a good stability,precision is high enough,it can meet the requirements of practical application.We have explored the calibration method of sensing system in this paper.According tothe characteristics of the temperature and humidity sensor, we put forward the "table method"for the scale conversion of the sampling data. This paper, by using this method to calibrate theoptical fiber humidity sensing system through experiments. Testing after calibration of thesystem, the test results showed that: The absolute error of relative humidity is less than2%RH, the standard deviation can reach0.027%RH.