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The Functionality and Working Principles of OTDR Optical Time Domain Reflectometer

Date: 30, May, 2023
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The English full name of OTDR is Optical Time Domain Reflectometer, which means optical time domain reflectometer in Chinese. OTDR is a precision optoelectronic integrated instrument made by using the backscattering caused by Rayleigh scattering and Fresnel reflection during the propagation of light in optical fibers. It is widely used in the maintenance and construction of optical cable lines, and can measure the length of optical fibers, transmission attenuation of optical fibers, joint attenuation, and fault location.


Functions of OTDR


Powerful software application


Using powerful software to quickly track data and perform offline analysis, it forms an intuitive graphical interface, helping users improve work efficiency.


Intelligent trace analysis


The built-in intelligent trace analysis module can quickly and accurately analyze the event points and location information in the test curve, and display them in the form of an event table.


Ultra-short event blind zone


OTDR has an ultra-short event blind zone and is particularly suitable for testing ultra-short optical fiber links or optical fiber jumpers.


Convenient VFL function


The visible red light fault function can quickly and conveniently identify the location of the interruption point or loss point in the short-distance optical fiber link, so that maintenance personnel can take timely measures.


Multiple interfaces, flexible connection


Perfect interface types: RJ-45, USB, power interface, etc., flexible connection, USB port can export test data directly through data cable.


Humanized touch interface


Transmissive color LCD display, can observe the test results clearly even under the sun, with simple button design, the operation is simple and flexible.


Working principle of OTDR


The basic principle of OTDR optical time domain reflectometry technology is to use the method of analyzing the backward scattering light or forward scattering light in the optical fiber to measure the transmission loss of the optical fiber caused by scattering, absorption and other reasons, and the structural loss caused by various structural defects. When a certain point of the optical fiber is affected by temperature or stress, the scattering characteristics of that point will change, so detecting the disturbance information of the external signal distributed on the sensing optical fiber by displaying the correspondence between the loss and the length of the optical fiber.


OTDR optical time domain reflectometer testing is performed by emitting light pulses into the optical fiber and then receiving the information returned at the OTDR port. When the light pulse is transmitted in the optical fiber, scattering and reflection will be produced due to the properties of the optical fiber, connector, joint point, bending or similar events. Part of the scattering and reflection will be returned to the OTDR. The useful information returned is measured by the detector of the OTDR, and they are represented as time or curve fragments at different positions in the optical fiber. The distance can be calculated by determining the time from the transmission signal to the return signal and then determining the speed of light in the glass material. The following formula explains how OTDR measures distance.


d=(c×t)/2, in this formula, c is the speed of light in a vacuum, and t is the total time (round trip) from the signal transmission to receiving the signal. Because light travels slower in glass than in a vacuum, in order to measure the distance accurately, the refractive index of the fiber being tested must be specified.


The narrow light pulse of the OTDR is injected into the fiber end face as a detection signal. As the light pulse propagates along the optical fiber, the backscatter part of the Rayleigh scattering at each point will continuously return to the fiber input end. When the light signal encounters a crack, Fresnel reflection will occur, and its backscattered light will also return to the fiber input end. By detecting the size and arrival time of the backscattered light at the input end through appropriate optical coupling and high-speed responsive photodetectors, the transmission characteristics, length, and fault points of the optical fiber can be quantitatively measured.


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