How Does an OTDR Work? Comparing with those sources and power meters which measure the loss of the fiber optic cable plant directly, the OTDR works indirectly. By duplicating the transmitter and receiver of the fiber optic transmission link, the source and meter make the measurement correlate well with actual system loss. However, the OTDR uses a unique optical phenomena "backscattered light" to make measurements along with reflected light from connectors or cleaved fiber ends, thus to measure loss indirectly. During the process of OTDR testing, the instrument injects a higher power laser or fiber optic light source pulse into a fiber from one end of the fiber cable, with the OTDR port to receive the returning information. As the optical pulse is transmitted through the fiber, part of the scattered reflection will return to the OTDR. Only useful information returned could be measured by the OTDR detector which acts as the time or curve segments of fibers at different positions. By recording the time for signals from transmission to returning and the speed of transmission in fibers, the distance thus can be calculated. The following picture shows exactly how an OTDR works for fiber optic testing.