An ?-OTDR Technology With Broadband Quantitative Detection Capability

Phase-sensitive time-domain reflectometer(?-OTDR)can detect weak vibration events with high sensitivity,fast response and distribution sensing,which has a big application value in many applications,such as large-scale structural health monitoring,intruder detection,power line monitoring and optical fiber communication monitoring.The traditional amplitude-discrimination ?-OTDR can locate the vibration events by monitoring the viariation of amplitude.Due to the uncertain relationship between the intensity of vibration events and the variation of optical intensity,most amplitude-discrimination ?-OTDR systems identify the vibration event by spectrum analysis.By calculating the phase difference variation between both ends of the vibration position,the phase-discrimination ?-OTDR system demodulates the phase of the Rayleigh scattering signal light and realizes the quantitative measurement of the fiber length variation.The fiber length variation is linearly related to the intensity of the vibration event,so the phase discrimination system can identify the vibration events in the time domain and frequency domain.Thus,compared with the amplitude-discrimination system,phase-discrimination system has a greater potential application value in the field of event classification.However,there are some shortcomings in the phase discrimination ?-OTDR system.On the one hand,when the repetition frequency of the probe light pulse remains constant,the highness of the frequency and amplitude of the vibration would cause the phase result distorting.Thus,the system will not work normally.Although there are many schemes for quantitative measurement,the influence of vibration frequency,amplitude and sampling rate on demodulation performance has not been studied.A unified demodulation model is needed to analyze the performance of the system under different signal-to-noise ratio.On the other hand,in the quantitative measurement system with Rayleigh scattered light,the scattered signal light at both ends of the vibration position is not a single ideal signal light.The scattered signal light is a superimposed light of the scattered light in the region,and an external vibration event will cause the superposition of light intensity fluctuations.Meanwhile,due to the low scattering coefficient and the dead zone,the signal-to-noise ratio of the scattered light signal is low.The light intensity fluctuations and low signal to noise ratio will seriously decrease the performance of system.To solve the problems above,this paper firstly proposes a demodulation method based on phase matching.By analyzing the phase limitation conditions and the signal distortion caused by insufficient sampling rate,the relationship between the vibration frequency,amplitude and sampling rate with signal-to-noise ratio is given.The phase matching method can figure out the extreme operating conditions of system,which is conducive to make full use of-OTDR system.Secondly,this paper presents a high-performance ?-OTDR quantitative measurement system based on fixed reflection point.Benefiting from the Mach-Zehnder interferometer structure based on 3×3 coupler and phase matching method,the phase change between adjacent two reflected light is obtained to realize the high-fidelity quantitative measurement of vibration events.The generating method of fixed reflection point,the light suppression of source frequency drifting and other key technologies improve the sensitivity,linear intensity response and signal-to-noise ratio of system without reducing the spatial resolution and sensing distance.The ?-OTDR system based on with ultra weak reflectivity fiber Bragg gratings(UWFBGs)is carried out.The experimental results show that the system can measure the vibration events at the n? level on a 2.5km long fiber.The sensitivity of is up to 117pm/(Hz)1/2 and the correlation coefficient of linear response is 0.99856.The structure could effectively detect the vibration events in the frequency range of 50Hz-2075Hz,and the maximum SNR of the demodulation results is 65.65dB.The successful demodulation of the triangular amplitude modulated signal and sine amplitude modulated signal demonstrates that the system have the ability to demodulate complex signals.The experiments above show that the proposed high performance ?-OTDR system can rebuild the weak vibration event with high fidelity.With the advantages of high sensitivity,high linear intensity response,broadband and high signal to noise ratio,the system is expected to expand ?-OTDR technology applications.