Optical time domain reflectometer (OTDR) is an instrument that analyzes the light loss in an optical fiber in optical network trouble shooting. It is used by organizations to certify the performance of new fiber optics links and detect problems with existing fiber links. This article will help you to have a better understanding of OTDR from the aspects of its working principle, common types and selection.
To start, it is better to know that the biggest factor in optical fiber loss is scattering and the light is scattered in all directions (including back toward the source) in the fiber. The OTDR makes use of the “backscattered light” to complete its measurement.
The OTDR firstly injects a short, intense laser pulse into the optical fiber, and then measures the backscatter and reflection of light as a function of time. At any time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fiber. Assuming that the laser pulse injected by the OTDR is a “virtual source” that is testing all the fiber between itself and the OTDR as it moves down the fiber. Then it is possible to calibrate the speed of the pulse as it passes down the fiber. And the OTDR can correlate what it sees in backscattered light with an actual location in the fiber. Thus, it can create a display of the amount of backscattered light at any point in the fiber.
There are three common types of OTDR: bench top, rack mountable and hand-held. Bench top OTDR is the one of the old OTDRs, which is relatively large and is designed to sit on a desk or atop a lab bench. Rack mountable OTDR can be bolted into racks with other electronic equipment. The hand-held OTDR is popular nowadays for its lightness, smallness and portability (as shown in the following picture).
When selecting a OTDR, it is important to take consideration of cable type. A single-mode OTDR is designed to be used with fiber optic cable allowing only one mode to be transmitted. The core diameter of this fiber optic cable is only approximately 8 µm, which permits signal transmission at extremely high bandwidth and allows very long transmission distances. While a multi-mode OTDR is designed to work with optical fiber that supports the transmission of multiple modes. The core diameter of multi-mode fiber may be 50 to 100 µm with a refractive index that is graded or stepped, allowing the use of inexpensive LED light sources. A single-mode/multi-mode OTDR may be used with both single-mode and multi-mode cables.
There are various OTDR models available in the market, which can meet different test and measurement needs. Fiberstore’s OTDRs are available with a variety of fiber types and wavelengths. All these OTDRs offer customers an overview of the whole system they test and can be used for estimating the fiber length and overall attenuation.
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