An OTDR sends short light pulses into a fiber and analyses the signal returning to the instrument. A trace can show distance, reflective events and changes in signal level.
Correct interpretation depends on settings suited to the link and measurement conditions. Pulse width, range, refractive index and the use of launch and receive fibers all matter.
Clean and inspect connectors before testing. Contamination or a poor connection can affect results. For acceptance work, follow the procedure specified for the project.