If an optical time domain reflectometer (OTDR) trace shows one optical splice when there are known to be two, what is the probable cause?

The reasoning behind selecting that the pulse width is set too long as the probable cause for the OTDR trace showing one optical splice instead of the expected two lies in the fundamental operations of OTDRs. An OTDR sends a pulse of light down the fiber, and when it encounters an optical splice or any other change in the fiber characteristics, a portion of that light is reflected back to the OTDR, allowing it to detect different events along the fiber.

When the pulse width is set too long, the OTDR may not be able to distinguish between closely spaced events. The reflection from the first splice can overlap with the reflection from the second splice, causing these events to merge on the trace. This results in the OTDR displaying a single splice rather than two distinct ones.

In essence, the pulse width determines the time resolution of the OTDR. A longer pulse width reduces the ability to resolve closely spaced features, such as multiple splices, in the fiber, leading to inaccurate interpretation of the fiber's condition.