Research On Mmulti-Parameter Distributed Sensor Demodulation Instrument

The health of the large-scale infrastructure such as bridges and tunnels,has a direct relationship on human daily life even the lives.Therefore,it is significant to diagnose and monitor the health of large-scale infrastructure.Distributed fiber sensing(DFS)uses a single fiber as the sensing unit and transmission medium,which can obtain the physical parameters distribution and changes over time.It satisfies the demand of global real-time detection of large-scale structures.By now,there have been commercial instruments for fiber distributed sensing.However,most of them can measure only one parameter.In order to locate and diagnose the events in more detail,multi-parameter has become the development trend.Based on the existing research foundation and resource of our research group,we researched the multi-parameter(temperature,vibration and strain)distributed sensor demodulation instrument,which based on phase-sensitive optical time domain reflectometry(?-OTDR)and Brillouin time-domain reflectometry(BOTDR).The main contents of this paper are as follows:(1)The importance of the DFS to large-scale infrastructure is described.Then,the research status and the instrumentation of DFS are presented.In response to the application of fiber distributed sensing,we analyze the physics performance accompany with the faults of the large-scale structure,which lay the groundwork to study the instrument of multi-parameters distributed sensing.(2)Theories on vibration sensing technology based on Rayleigh scattering,temperature and strain sensing technology based on Brillouin scattering are described,which lay the foundation to multi-parameters distributed sensing.Based on the characteristics of the back scattering signal,the working principles of the?-OTDR and the principle of the BOTDR were described.Then,the feasibility of simultaneous sensing of temperature and vibration strain based on the mechanism of Rayleigh scattering and Brillouin scattering were analyzed.(3)The instrument was described from three parts,i.e.,light source,modulation and demodulation.A laser with suitable linewidth was chosen as Brillouin laser.In the modulation part,a semiconductor optical amplification switch(SOA)was used to modulate continuous light into pulse light.The output light spectrum from the erbium-doped optical fiber amplifier(EDFA)with different drive current was also tested.In the demodulation part,moving average is adopted in data progress in ?-OTDR.The signal is processed by Gaussian window short time Fourier transform method to demodulate the Brillouin profile in BOTDR,which improves the real-time performance of the system without the frequency sweeping.Finally,the power supply of laser,SOA,EDFA and polarization scrambler(PS)are controlled by micro controller unit(MCU).Serial communication was used to realize the communication between the PC and toptical modules.