Optical Time Domain Reflectometer (OTDR) is a tool used for fiber optic testing. The OTDR shoots a laser light down the optical cable and the laser emits a pulse of light at a specific wavelength, this pulse of light travels along the fiber being tested, as the pulse moves down the fiber portions of the transmitted light are reflected/refracted or scattered back down the fiber to the photo detector in the OTDR. This tests the integrity of a fiber cable and is used for the building, certifying, maintaining, and troubleshooting fiber optic systems.