Improving the Performance of Brillouin Optical Time Domain Reflection System Using Local Stimulated Scattering
As low-cost and compact Brillouin optical time domain reflectometers are increasingly being used in the field of general engineering structural monitoring,improving the performance of these reflectometers is essential,which is beneficial for their functionality and accuracy in monitoring and therefore is conducive to their large-scale application.In this paper,the Brillouin optical time domain reflection structure based on local excitation is used to improve the performance of the ultra-low cost Brillouin optical time domain reflectometry without increasing the operation time and other redundant optoelectronic devices to maintain the ultra-low cost structure.The front end of the experimental device is composed of laser,semiconductor optical amplifier,Erbium-Doped Fiber Amplifier(EDFA)and three couplers.A beam of continuous light output by the laser is divided into 90%and 10%by the coupler1.90%of the light enters the upper branch and is modulated to a width of 100 ns and a period of 40 μs after passing through the Semiconductor Optical Amplifier(SOA).10%of the light enters the middle layer and the lower layer branch,the middle layer is used as the continuous pump light of pulsed light,and the lower layer is used as the reference light path.Probe light and pump light enter 90%port and 10%port of 10∶90 coupler3 respectively,and the two beams are fused and injected into 3 km Fiber Under Test(FUT).Stimulated Brillouin scattering Stokes light generated in the optical fiber to be tested enters EDFA for amplification through the 3rd ports of the circulator,and is filtered out of Amplifier Spontaneous Emission(ASE)noise by the Dense Wavelength Division Multiplexer(DWDM)and converted into 10.8 GHz Radio Frequency(RF)signal by the Photo Detector(PD).After amplification and filtering,the RF signal is down converted to a signal of about 600 MHz by Voltage Controlled Oscillator(VCO),and finally collected by a data acquisition card with a sampling rate of 5 GSa/S.In the Brillouin optical time domain reflectometry system,the sensing distance and temperature measurement accuracy are directly related to the signal-to-noise ratio.The higher the signal-to-noise ratio is,the longer the sensing distance is,and the higher the temperature measurement accuracy is.Therefore,in order to verify the signal-to-noise ratio of the locally excited system,we first carried out the normal temperature experiment to detect the temperature measurement accuracy and detection distance of the terminal.At room temperature,a roll of optical fiber to be measured is measured by using the traditional Brillouin optical time domain reflectometry structure and the locally stimulated Brillouin optical time domain reflectometry structure.Connect port 2 of the circulator to 3 km of corning bare fiber to be tested.The pulse power modulated by the traditional structure SOA is 1.98 mW,the light intensity injected into the fiber to be measured from the circulator 2 port is 1.801 mW,and the reference light intensity is 764 μW.The light intensity of the middle continuous light of the new structure is 163 μW.The SOA modulation pulse power is 1.97 mW,and the reference light intensity is 680 μW.The light intensity of the fused light injected into the fiber to be tested through the two ports of the circulator is 1.803 mW,and the power of the two injected fibers is basically the same,so the difference in the results is not caused by the difference in the input power.The comparison results of Brillouin Frequency Shift(BFS)along the optical fiber are obtained after Short Time Fourier Transforming(STFT)of two groups of time domain data collected.The experimental results show that the data signal-to-noise ratio measured by the Brillouin optical time domain reflectometry system with the new structure is significantly better than that of the traditional coherent detection BOTDR system.The signal fluctuation of Brillouin optical time domain reflectometry system with traditional structure becomes larger at 1 900 m of the optical fiber to be tested,which indicates that the signal-to-noise ratio has deteriorated.The Root Mean Square Error(RMSE)of the BFS measured by the traditional structure is 2.61 MHz from 200 m to 1 950 m,and 8.16 MHz from 1 950 m to 2 350 m.The RMSE of the new structure is 2.36 MHz from 200 m to 1 950 m,and 3.01 MHz from 1 950 m to 2 350 m.It can be seen that the results of the first 1 950 m are basically similar because the pulse loss is not large and the energy is enough to support a more accurate measurement.After 1 950 m,the pulse light energy gradually decays,leading to the continuous increase of RMSE.After using the locally stimulated Brillouin structure,the pulse energy is supplemented,which can be used for longer distance measurement.In order to explore whether the middle path light intensity will affect the locally stimulated Brillouin system,we increased the continuous pump light in the middle layer to about 1 mW,and then detected at room temperature again.The results show that compared with no middle path light,increasing the middle path light can also significantly increase the signal-to-noise ratio of the system.We carried out an experimental study on the effect of the down path light on the structure.Since multiple couplers are used for light splitting,the intensity of the reference light inevitably decreases.Therefore,we magnify the reference light(lower light)of the new structure to the same as the old structure,which is 764 μW.Measure again.The results show that compared with the new structure without increasing any light intensity,increasing the lower light can also slightly improve the signal-to-noise ratio,but the effect is slightly worse than increasing the middle light.It can be seen that increasing the light intensity of each layer can help to improve the signal-to-noise ratio,but the signal-to-noise ratio cannot be improved indefinitely.In addition,we conducted temperature experiments using 2.7 km of Changfei optical fiber.The first 2 250 m of optical fiber is set at normal temperature,250 m of optical fiber is heated in 50℃water bath,and 200 m of optical fiber is reserved at the end to prevent reflection.The experimental results show that the RMSE of the traditional structure after 2 250 m is 4.41 MHz,that is,the temperature fluctuation is±3.39℃(temperature coefficient 1.3 MHz/℃),which is greatly limited in practical application.After using the local stimulated Brillouin structure,the temperature fluctuation is reduced to±1.27℃,which can meet the actual demand.
万方数据
维普
