If you're working with single-mode and multimode fibres, testing them with an Optical Time Domain Reflectometer (OTDR) is essential for ensuring your network is up to standard. Testing both types is possible, though there are some significant differences and considerations to remember. The OTDR. Whether you're a professional or a DIY enthusiast, knowing how to test fiber optic cables is crucial. As the components like fiber, connectors, splices, LED or laser sources, detectors and receivers are being developed, testing confirms their performance specifications and helps. This Applications Engineering Note (AEN 135) explains and recommends standard measurement methods for characterizing optical fiber system performance.