Dead Zones Influence on OTDR

Dead zones originate from reflective events (connectors, mechanical splices, etc.) along the link, and they affect the OTDR’s ability to accurately measure attenuation on shorter links and differentiate closely spaced events, such as connectors in patch panels, etc. When the strong optical reflection from such an event reaches the OTDR, its detection circuit becomes saturated for a specific amount of time (converted to distance in the OTDR) until it recovers and can once again measure backscattering accurately. As a result of this saturation, there is a part of the fiber link following the reflective event that can’t be “seen” by the OTDR, hence the term dead zone.

When specifying OTDR performance, analyzing the dead zone is very important to ensure the whole link is measured. Two types of dead zones are usually specified:

Pocket Visual Fault Locator

1. Event dead zone: This refers to the minimum distance required for consecutive reflective events to be “resolved”1, i.e., to be differentiated from each other. If a reflective event is within the event dead zone of the preceding event, it will not be detected and measured correctly. Industry standard values range from 0.8 m to 5 m for this specification.

2. Attenuation dead zone: This refers to the minimum distance required, after a reflective event, for the OTDR to measure a reflective or non-reflective event loss. To measure short links and to characterize or find faults in patch cords and leads, the shortest possible attenuation dead zone is best. Industry standard values range from 3 m to 10 m for this specification.

Sampling Resolution

Sampling resolution is defined as the minimum distance between two consecutive sampling points acquired by the instrument. This parameter is crucial, as it defines the ultimate distance accuracy and fault-finding capability of the OTDR. Depending on the selected pulse width and distance range, this value could vary from 4 cm to 5 m for EXFO’s FTB-7000D series.

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 (e.g., 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.

Report Generation

Report generation is another major timesaver, as post-processing time can be reduced by up to 90% if the OTDR has specialized post-processing software allowing fast and easy generation of OTDR reports; these can also include powerful batch processing, bidirectional analyses of OTDR traces, live templating for OTDR testing, various report templates for ORL/OTDR/CD/PMD, and summary reports for high-fiber-count cables.

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