Research On Dense Fbg Demodulation System With Large Capacity

Fiber Bragg Grating sensors are widely used in many engineering fields due to their fast real-time monitoring,low production cost,and easy implementation to large capacity.However,with the continuous development of application,higher requirements are imposed of fiber Bragg gratings sensing system on the longer monitoring distance,denser spatial distribution,more large capacity,and lower cost.In this paper,based on the time division and wavelength division multiplexing of FBGs demodulation system,the idea of section demodulation is proposed.By using the accumulation of the optical time domain reflectometry of all gratings in the section as a section spectrum,the section spectrum can be regarded as the envelope of the spectrum of gratings in this section.It is possible to demodulate the changes of environmental parameters by the partition spectrum.The right edge detection algorithm for section spectrum is studied to realize fast,accurate and real-time demodulation for ultra-long-distance and ultra-dense grating arrays.The main research contents of the article are as follows:(1)Firstly,the sensing principle of fiber grating is derived and a mathematical model is established.Considering the influence of Rayleigh scattering,multipath reflection and different reflectivity on the capacity of identical weak grating arrays.To reduce transmission loss,suppress multiple reflections and crosstalk between FBGs,the random micro-variation of central wavelength and spacing between gratings is proposed.Combining the idea of section demodulation to analyze the feasibility of large-capacity dense fiber gratings sensing.(2)To study the characteristics for section spectrum of the dense gratings with temperature variation,the section spectral model of dense gratings is deduced and simulated.And then the demodulation method of section spectrum is designed.According to the characteristics of section spectrum at different temperature,a new algorithm that suitable for section spectrum is used to demodulate wavelength for dense gratings,by detecting the right edge position of section spectrum.(3)Design the optical circuit of the large-capacity dense fiber gratings section demodulation system based on time division and wavelength division system.To reduces the interference between dense gratings,the tunable narrow-band laser with a bandwidth of 100 pm is adopted.The low-noise and high-sensitivity photoelectric conversion circuit provides effective multi-stage amplification for the system.And the high-speed and high-precision AD sampling circuit is used to sample the weak signal.(4)Research the system performance optimization method.Design a comprehensive experiment for section spectrum demodulation of dense fiber gratings.That verify the accuracy and stability of the system.And proves that the system can accurate demodulate 10 cm interval dense gratings at any position of 10 km longdistance fiber gratings.(5)The reality application test of the system proves that the system can achieve centimeter temperature perception,meter-level spatial positioning and kilometer-level monitoring range,with superior application advantages in early fire warning and rapid fire direction.