The most common methods for testing fiber optic splices are optical time-domain reflectometry (OTDR) and optical loss test set (OLTS). Splicing is essential for repairing, extending, or modifying fiber optic networks. However, splicing can also introduce losses, defects, or misalignments that can degrade the performance. This Applications Engineering Note (AEN 135) explains and recommends standard measurement methods for characterizing optical fiber system performance. This note also provides background information on system link configurations, test equipment and system component considerations that influence. Fiber Optic Testing Testing is used to evaluate the performance of fiber optic components, cable plants and systems. The procedures apply to both single optical.