The Data Acquisition System Of Optical Time Domain Reflectometry Based On Coding And Digital Accumulation Technology

Distributed optical fiber sensing system based on optical time domain reflection technology can obtain space and time distribution of the measuring object by injecting an optical probe pulse into the sensing fiber and synchronous monitoring its backscattering signal continuously.Compared with the traditional sensor,it is being applied to many new type of sensor field for its advantages of continuous distributed,high sensitivity,electromagnetic tolerant,high temperature and corrosion resistance and so on.The loss of the fiber has been reduced to 0.2dB per kilometer and extend distance of non-relay communication tremendously driven by long-haul fiber communication requirements.However,the loss is still the key factory which limit the dynamic range of sensor for optical time domain reflection technique.Because of the fiber loss,the optical pulse power will exponential decay along the transmission distance in fiber core,and lead to the backscattering signal drops in the same way,the measuring dynamic range of this kind of fiber sensor is limited by this characteristic seriously.The dynamic range can be improved by increasing pulse width partly,but it also deteriorate the special resolution.In order to solve the problem between special resolution and dynamic range,the solution can improve the dynamic range in same special resolution from two aspects of time and special domain.In time domain,the zero mean uncorrelated noise can be suppressed by the results of multiple acquisition with digital accumulation process,thus improving the dynamic range.In special domain,coding the optical pulse and obtain an equivalent single pulse measurement results with corresponding decoding method,it also increase the dynamic range.Currently,the pulse coding based fiber sensing experiment is implemented by pulse pattern generator and oscilloscope with segmented acquisition,this approach is not only expensive but also limited to laboratory research.It require a compact data acquisition system with pulse coding and digital accumulation for instrument coding based fiber sensor.The dedicated digital accumulation data acquisition card can be found on the market,but they lack both software defined pulse coding and synchronized accumulated acquisition.Due to the limitations of traditional accumulation data acquisition card,a data acquisition system with pulse coding and digital accumulation is designed and implemented in this thesis.It can support any pattern of accumulated data collection by software defined coding and accumulation parameters.The data acquisition system can export internal trigger signal through synchronized pulse with no external trigger input needed,it eliminates the need for external synchronous device and also reduces the impact of cumulative data alignment from trigger jitter.In order to verify the performance of the data acquisition system,the first test of single pulse and pulse coding parameters and driving performance of optical pulse which is generate by high extinction ratio pulse modulator is performed,and then,verifying the accumulation function in different accumulating times by input mixed function generator with white noise signal,finally,the 3.6dB dynamic range improvement is obtained from 63bit return to zero simplex code compare with single 1 Ons pulse with 63 times accumulation.The data acquisition system with pulse coding and digital accumulation is quite suitable for improvement of measurement dynamic range in various kinds of optical time domain reflection based fiber sensor,and can also design coding based optical fiber sensor instrument,which has wide application prospects.