Research On Demodulation Technology And System Of Identical Weak Fiber Bragg Grating Sensing Network

Fiber Bragg grating with its high sensitivity,electromagnetic immunity,excellent multiplexing capability and fine long-term working stability has become an important and popular research subject in sensing field.Nowadays,fiber Bragg grating sensing technology based on wavelength division multiplexing has grown into maturity and been widely used.However,usually thousands of sensors will be needed in monitoring for many large scale constructions and structures.Because of limitation of the bandwidth of light source,wavelength division multiplexing technology can not achieve a large multiplexing number.The identical weak reflection fiber Bragg grating sensing network based on time division multiplexing is able to reduce multiplexing number limitations caused by the bandwidth of the light source and lower the crosstalk effect among gratings,thus making it easy to carry out a large number and long distance measurement.This paper chooses the demodulation technology and system of identical weak reflection fiber Bragg grating sensing network as the major study object,uses identical weak reflection fiber Bragg gratings to constitute a sensing network by time division multiplexing and researches on the demodulation system,the demodulation algorithm and its optimization.The main research works of this paper are as follows.(1)The mathematical model for temperature sensing of fiber Bragg grating is derived through the analysis of coupled mode theory.Analysis are made to show the relationship between multiplexing capability and fiber Bragg grating's reflection,Rayleigh scattering,reflection crosstalk and optical fiber transmission loss in time division multiplex mode.(2)A demodulation system for identical weak reflection fiber Bragg grating sensing network is proposed and implemented,whose working structure is given and major functions of each module are analyzed.Using Gaussian fitting algorithm to calculate center wavelength and its feasibility is proved by experimental results.Then a complete process of the demodulation algorithm is designed and each step is analyzed.A set of PC software for the demodulation system is also designed and implemented.(3)In order to improve the slow demodulation speed caused by complex Gaussian fitting process,an optimization solution is proposed which includes the Gaussian fitting algorithm linearization,extraction of the fixed coefficients for advance calculation and replacing logarithmic operation with look-up table methods.On the other side,there exist large demodulation errors when the wavelength sweeping range is much wider then the fiber Bragg grating's reflection spectrum's width.The demodulation errors get much reduced by removing the side-lobe data and keeping the main-lobe data.(4)Temperature sensing properties of the identical weak fiber Bragg grating are calibrated.Then a temperature demodulation experiment is carried out using equipment such as a constant temperature box in order to test and check the demodulation system's demodulation errors and long time working stability.Analyses of the experiment's data show that,the time 10000 times Gaussian fitting processes cost changes from 0.2 seconds to 0.008 seconds by the algorithm optimization.In the temperature range of 0? to 60?,the system's temperature demodulation errors are within ±0.4?,demodulation linearity is up to 0.99,12 hours' demodulation fluctuation does not exceed ±0.3?,which prove that the demodulation system designed in this paper has a good working stability.