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Frequently Asked Questions about Optical Time Domain Reflectometry (OTDR)

Frequently Asked Questions about Optical Time Domain Reflectometry (OTDR)

2022-04-09

1. What is the test principle of Optical Time Domain Reflectometer (OTDR)? What is the function?

Answer: OTDR is made based on the principle of light backscattering and Fresnel reflection. It uses the backscattered light generated when light propagates in the fiber to obtain attenuation information, which can be used to measure fiber attenuation, splice loss, fiber fault location, and Knowing the loss distribution of optical fibers along the length is an essential tool in the construction, maintenance and monitoring of optical cables. Its main index parameters include: dynamic range, sensitivity, resolution, measurement time and dead zone.


2. What is the blind spot of OTDR? What will be the impact on testing? How to deal with the blind spot in the actual test?

A: Usually, a series of "blind spots" caused by the saturation of the OTDR receiving end caused by reflection from characteristic points such as movable connectors and mechanical joints are called blind zones.


The blind zone in the optical fiber is divided into two types: the event blind zone and the attenuation blind zone: the reflection peak caused by the intervention of the active connector, the length distance from the starting point of the reflection peak to the receiver saturation peak, is called the event blind zone; Intervention of the movable connector causes a reflection peak, and the distance from the onset of the reflection peak to the point at which other events can be identified is called the attenuation dead zone.

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For OTDRs, the smaller the blind zone, the better. The blind area will increase with the increase of the width of the pulse broadening. Although increasing the pulse width increases the measurement length, it also increases the measurement blind area. Therefore, when testing the optical fiber, the measurement of the optical fiber of the OTDR accessory and the adjacent event points Use narrow pulses and wide pulses when making measurements at the far end of the fiber.


3. Can the OTDR measure different types of fibers?

Answer: If a single-mode OTDR module is used to measure multi-mode fibers, or a multi-mode OTDR module is used to measure single-mode fibers such as 62.5mm core diameter, the fiber length measurement results will not be affected, but factors such as fiber loss , optical connector loss, and return loss results are incorrect. Therefore, when measuring the optical fiber, be sure to select the OTDR that matches the measured optical fiber for measurement, so as to obtain the correct results of all performance indicators.


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