Selection Guide to OTDR

An OTDR uses the effects of Rayleigh scattering and Fresnel reflection to measure the characteristics of an optical fiber. It locates defects and faults, and determines the amount of signal loss at any point in an optical fiber, and it can also measure overall fiber optical return loss and the reflectance of a connector. How to choose an OTDR? The following are a few key criteria that you need to keep in mind when selecting an OTDR. The picture below shows an OTDR.

Dead Zones

Dead zones affect the OTDR's ability to accurately measure attenuation on shorter links and differentiate closely spaced events, such as connectors in patch panels, etc. There are two kinds of dead zone, event dead zone and attenuation dead zone. The event dead zone is the distance after a reflective event starts (point A) until another reflection can be detected, which according to the Telcordia definition, is the location where the falling edge of the first reflection is 1.5 dB down from the top of the first reflection (point B). The attenuation dead zone is the distance after a reflective event starts (point A) until the backscatter level can be measured which, according to the Telcordia definition, is the location (point C) where the signal is within 0.5 dB above or below the backscatter line that follows the first pulse. The attenuation dead zone specification is always larger than the event dead zone specification.

Dynamic Range

The dynamic range of an OTDR determines how long of a fiber can be measured. The total optical loss that an OTDR can analyze is mainly determined by the dynamic range. The dynamic range affects the accuracy of the link loss, attenuation and far-end connector losses. Thus, having sufficient dynamic range is really important. The manufacturers specify dynamic range in different ways. The higher the dynamic range, the longer the distance an OTDR can analyze. An OTDR whose dynamic range is 5 to 8 dB higher than the maximum loss you will encounter is recommended.

Pass/Fail Thresholds

This is an important feature because a great deal of time can be saved in the analysis of OTDR traces if the user is able to set pass/fail thresholds for parameters of interest (such as splice loss or connector reflection). These thresholds highlight parameters that have exceeded a warning or fail limit set by the user and, when used in conjunction with reporting software, it can rapidly provide re-work sheets for installation/commissioning engineers. Thus, pass/fail thresholds is also an crucial factor.

Sampling Resolution

Sampling resolution is the minimum distance between two consecutive sampling points acquired by the OTDR. It is an important parameter for the reason that it defines the ultimate distance accuracy and fault-finding capability of the OTDR.

When selecting an OTDR, you're supposed to take all the above factors into consideration. Only when you know clearly about the applications can you choose the proper OTDR for every specific application. Fiberstore supplies a wide range of OTDRs. They are available with various fiber types and wavelengths, including single-mode fiber and multi-mode fiber.

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