Research On Modeling Of Quasi-distributed Fiber Optic Sensor Based On TDM And Related Experiments

With the growing emphasis on gas safety in production and life of our government and society,flammable and explosive dangerous gas sensors and measurements with high sensitivity,fast response,safe use,easy maintenance and access to networking have been attractted attention.In this paper,for the distributed(especially along the roadway,pipe distribution point)gas detection needs,intrinsically safe fiber optic sensors and fiber optic sensor network is proposed,the proposed solution is satisfied for measuring multi-point long-range gas concentration.Based on the theory of absorption spectra of gas molecules,the principle of absorption of light and gas molecules at specific wavelengths can be used to detect the concentration of gas.When the light passes through the gas,if the light source spectrum is partially coincident with the absorption spectrum of the gas,The light intensity will be absorbed by the measured gas,the output light intensity will be weakened,but the absorption spectrum of the gas is narrow,the absorption is weak,a wavelength modulation spectral model of the laser output wavelength is designed to be stable in the required range in this paper.The effects of different modulation depths on the second harmonic intensity can be obtained by analyzing the laser light source drive based on the WMS design.The wavelength modulation model of the tunable diode laser absorption spectrum is completed to the quasi-distributed fiber optic sensor.A new fiber-optic network model is proposed for the traditional network scheme which detect the optical fiber distance twice the detection path and the optical power attenuation of the coupler.According to the model,the related devices are analyzed and designed to meet this improved networking method.In this paper,a gas detection model based on OTDR and time-division multiplexing(TDM)is proposed and developed.A quasi-distributed fiber-optic absorption gas sensor network is designed.A single fiber-optic bus is combined with an open-chamber was able to complete the measurement of the gas concentration of the multiple chambers and completed the modeling according to the protocol.According to the optical path of the optical path,the optimization of the splitting ratio of the circulator and the splitter and the power loss of the optical signal are analyzed,and theoretically maximum measurable number of points and their distance are analyzed in order to achieve the maximum range of gas concentration detection.Aiming at the requirement of optical time domain reflection distributed detection,a model of second harmonic detection based on wavelength modulation spectrum is proposed.Harmonic detection can detect weak photodetection signals while overcoming noise interference effectively.Based on the high frequency modulation and harmonic detection technology,the detection model and simulation of the second harmonic multi-point fiber gas sensor with narrowband light source are established.After the model design and simulation verification,a kind of multi-point weak signal acquisition and locking amplification methodare established to achieve a low concentration of gas accurate and rapid measurement.Depending on the modulation depth,the transmission distance of the distributed system and the number of measurement points can be increased.A normalized method of the second harmonic is designed,and the absolute value of the signal is measured by the normalization of the signal and comparison with the simulation result.Compared with the traditional normalization,suitable for long-distance,low concentration of gas detection.Based on the analysis of the above theory and simulation,the design of hardware circuit based on wavelength modulation spectrum and laser temperature control is completed with methane gas as the research object.According to the design of the hardware circuit and optical path,the experimental platform to the light source output wavelength experiment is set up,multi-point chamber attenuation test is completed,WMS-2f normalization experiment is completed,spectral absorption input and output experiments are completed,quasi-distributed network sensitivity is completed,linear and accuracy experiment is completed.The experimental results show that the analysis of the modulation depth,the design of the reflector chamber,and the single-fiber quasi-distributed structure meet the requirements of multi-point long-range detection.