| Optical fiber is applied to communication systems in a wide range due to its advantages such as wide communication frequency bandwidth,high anti-interference ability,and low transmission loss.When a fault occurs in optical fiber transmission,it is of great importance to be able to accurately and rapidly locate optical fiber breakpoints.Optical time-domain reflectometer can be used to detect optical fiber breakpoints non-destructively,making it a monopoly in the market.In this thesis,the development history and current research status of fiber optics detection technique were introduced.The operating principle of conventional optical time-domain reflectometer was introduced.Design indexes in the thesis came from the project demand for optical fiber breakpoint detector.First,the feasibility of the position accuracy in the design index was analyzed from two aspects,timing error and refractive index error.The feasibility of measurement range in the design index was analyzed from four aspects,range of measurement time,laser wavelength,laser power and relation between Fresnel reflected power and Rayleigh scattered power.The design of this thesis is based on laser ranging technique and principle of Fresnel reflection at optical fiber breakpoint.The new design in combination with MCU and TDC-GP21 replaced the conventional design in combination with high-speed A/D and FPGA.In the condition of short distance measurement,the new design can improve low positioning accuracy and long response time of conventional optical time-domain reflectometer.In addition,the new design has low cost superiority.The signal detection and amplifying module circuit was designed on the basis of the requirements for measurement distance,low noise and the power range of the Fresnel reflection at optical fiber breakpoint.Whether the programmable gain amplifier will produce timing error when the gain was changed was tested by designed experiments.The constant fraction discriminator was designed to settle deficiency that the discrimination accuracy of the leading edge discriminator formed by the fast comparator in the actual design circuit is easily affected by the signal amplitude.And it was verified through simulation.Time measurement module based on propagation delay technique was used to measure the time accurately.The MCU was used to fulfill operation procedure control and communication with host computer.At last,the optical fiber breakpoint detector measured different distances optical breakpoints.The timing error of detector was analyzed from more than one aspect.Theexperiment result and error analysis show the detector meets the design indexes and demand of the project. |
