Research On Key Technologies Of Distributed Optical Fiber Sensing

When the light transmits in the fiber,some physical quantities such as phase,wavelength,polarization,and the intensity of the backward reflected light will be affected due to the variation of external factors,so we could use optical fiber as a sensor for sensing the variation of the physical quantity outside.Distributed optical fiber sensing is a new technology which use the light as the carrier and the optical fiber as the transmission channel to sense and transmit the external signals.Using distributed optical fiber sensing technology,it is possible to monitor and diagnose the external physical quantity on the whole optical fiber link.In this paper,we mainly study on the OTDR(Optical Time Domain Reflectometer),and propose a relevant coding technology to improve the SNR(signal-to-noise ratio).The basic principle is improving the power of light based on transforming the traditional single pulse into coded pulse to increase the SNR.The main work of this paper are as follows:(1)Firstly,we analyze the traditional operating theory of traditional OTDR and it's performance metrics.Then we present the advantages of Gray code,CCPONS code,Biorthogonal code,S-code over traditional single pulse OTDR.At last,we make a comparison about the different code and gain a conclusion that S-code has some advantages over other codes in coding gain and decoding complexity.(2)In order to obtain the OTDR curve using S-code method experimentally,we need to design a laser which can output coded pulse.Firstly,we investigate the operating principle of optical module,then we design a laser drive circuit using the optical module's laser driver chip combined with the additional circuit.The device has three characters:TTL or PECL input;75mA's max modulation current;80mA's bias current.At the same time,we use the FPGA to generate the code and input it into the device.Then driving the laser diode(PLD-F312)by the device,we can get the output of laser light with the pulse peak power of 11.54mW.(3)Finally we built the experimental platform to test the S-code with different orders.In the experiment,under 50ns pulse width and 12km fiber length,we collect the backward signal of the S-codes with orders of 7,15 and 31 respectively.The collected signal are decoded by MATLAB.Finally,we compare it with the traditional OTDR curve.The decoded curves show that the S-code improves the signal-to-noise ratio obviously,and with the coding order increasing,the performance of SNR improves directly.