The reading capabilities of an OTDR are limited by the event dead zone: a brief saturation period after a strong reflection that is overwhelmed by loss events immediately following it. Only by understanding where the blind spots come from can we clearly read densely connected links.

The blind zone is determined by the pulse width and is converted based on experience.

The event dead zone corresponds roughly to twice the pulse width fiber length. A pulse width of 10 ns corresponds to about 1 m on G.652 optical fiber, and the blind zone is about 2 m; a pulse width of 30 ns corresponds to about 3 m, and the blind zone is about 6 m. In the scenario of adding jumpers and splice trays in the cabinet, the two events are often only 1 to 2 m apart. At this time, the shortest available pulse width must be cut, otherwise the number will not be counted.

Setting sequence: short pulse width positioning, long pulse width measurement loss

• First use a short pulse width of 10 ns or 30 ns to confirm the number and location of events. The refractive index is set according to the nominal value of the optical cable manufacturer, which is about 1.4680 for conventional single mode.
• Then use a medium-long pulse width of 100 ns to 1 μs and an averaging time of more than 30 s to stabilize the attenuation step.
• The distance gear increases by one gear according to the total link length. If the gear is too large, the near-end event will be covered up. If the gear is too small, the far end will be cut off.

Two-way testing and curve comparison

The unidirectional reading is affected by the directivity of the mode field diameters of the two fiber sections. The same joint may be 0.02 dB measured from end A and 0.08 dB measured from end B, or even upward steps. For acceptance, it is recommended to measure once in both directions and take the arithmetic average, and save the two trajectories for comparison. Separate ghost images from real events when interpreting: Ghost images often appear near the total length and have no loss steps. The position will change after changing the test jumper or changing the pulse width. High-speed Internet links between buildings also produce reports according to the same rules, please refer to100G optical fiber interconnection upgrade between buildings in a large data center in East China.

An executable principle: use short pulse width at the near end and long pulse width at the far end. The conclusion is based on two-way average and does not use any single curve as the basis for judgment. If you are not sure about the settings, you can send the trajectory file to EB-LINK technical support for review. Call 0755-83179002 on working days.