Hello,
Today I will share with you the principle of Optical Time Domain Reflectometer "OTDR"
1- Working principle of optical time domain reflectometer
Optical Time Domain Reflectometer (Optical Time Domain Reflectometer) is currently the most commonly used instrument in the measurement of optical cable lines. It is a precision photoelectric integrated instrument made by Rayleigh scattering and backscattering caused by Fresnel reflection when the light is transmitted in the optical fiber. This instrument is mainly used to test the attenuation of the entire optical fiber link and provide attenuation details related to length. The specific performance is to detect, locate and measure events on any position on the optical fiber link (event refers to the fusion splicing and connector in the optical fiber link , Bending, and other defects, the change in light transmission characteristics can be measured). The OTDR test's non-destructive, one-end access and intuitive and fast advantages make it widely used in the maintenance and construction of optical cable lines, which can measure fiber length, fiber transmission attenuation, splice attenuation and fault location.
Optical time domain reflectometers use the following phenomena: microscopic changes in the refractive index and tiny cracks in optical fibers can cause light to reflect; its working principle is similar to that of radar. It first sends a signal to the fiber, and then observes what information is returned from a certain point. This process will be repeated, and then the results will be averaged and displayed in the form of a t***, this t*** depicts the strength of the signal (or the state of the fiber) within the entire length of fiber.
How OTDR uses the principle of Rayleigh scattering: a narrow pulse is injected at the input end surface of the optical fiber. As the optical pulse propagates along the fiber, the Rayleigh scattering part of each point continuously returns to the input end of the optical fiber. The optical fiber receiving detection circuit detects the returned light, and after digital processing to improve the signal-to-noise ratio, the electrical signal corresponding to the reflected power is connected to the oscilloscope. After processing, the corresponding logarithmic attenuation curve can be displayed on the screen.
How does OTDR use the Fresnel reflection principle: According to the Fresnel reflection principle, as long as the light reflected from the back is detected at the sending end, you can see the echo pulses on the front and rear faces, and the time interval between them is the light travel After two times the length of the fiber, the length of the fiber can be measured accordingly. Similarly, if a break or defect occurs in the fiber, an echo pulse will also be detected at the input end of the fiber, and the fault can be located according to the echo pulse.
2- How does OTDR measure distance
The OTDR test is carried out by transmitting optical pulses into the optical fiber and then receiving the returned information at the OTDR port. When the light pulse is transmitted in the optical fiber, it will be scattered and reflected due to the nature of the fiber itself, connectors, splices, bending or other similar events. Part of the scattering and reflection will return to the OTDR. The returned useful information is measured by the OTDR detector, and they are used as time or curve segments at different positions in the fiber. From the time it takes to transmit the signal to return the signal, and then determine the speed of light in the glass material, you can calculate the distance. d=(c×t)/2(IOR) In this formula, c is the speed of light in vacuum, and t is the total time after the signal is transmitted to the received signal (two-way) (the two values are multiplied by 2 The latter is the one-way distance). Because the speed of light in glass is slower than in vacuum, in order to accurately measure the distance, the measured fiber must indicate the refractive index (IOR). IOR is indicated by optical fiber manufacturers. The resulting t*** is a downward curve, which shows that the power of backscattering is continuously decreasing. This is because the transmitted and backscattered signals are lost after a distance of transmission.
3- Some basic terms of optical time domain reflectometer
Backscattering: Optical signals are attenuated by Rayleigh scattering during transmission along the fiber. This Rayleigh scattering is caused by small differences in the refractive index of reflection in the fiber core. Scattering will affect the entire fiber.
Rayleigh scattering: Scatters light signals in all directions. We refer to the backscattering as the backscattering.
Non-reflective events: fusion splices and microbends in the fiber will cause losses; but will not cause reflections. In the OTDR measurement result curve, these two kinds of events will be manifested in the form of adding a sudden descending step to the backscatter level. Then the change on the vertical axis is the loss of an event. This event is called a non-reflective event.
Figure 2 Non-reflective events
Reflective events: Moving connectors, mechanical splices, and breaks in optical fibers can all cause loss and reflection. The size of the loss is also determined by the amount of change in the back level value. The reflection value (usually expressed in the form of return loss) is determined by the amplitude of the reflection peak on the backscatter.
Figure 3 Reflection events
Fiber end: The first case: if the end of the fiber is a flat end or a movable connector (flat, polished) is connected to the end. At the end of the fiber there will be Fresnel reflection with a reflectivity of 4%.
The second case is: if the end of the fiber is a broken end face. The irregularity of the end face will diffuse the light without causing reflection. In this case, the display signal curve at the end of the fiber is simply reduced from the back reflection level to the OTDR noise floor level. Although cracked tails may also cause reflections, their reflection peaks will not be as large as those caused by flat tails or movable connectors.
Figure 4 Two fiber ends
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