An optical time domain reflectometer (OTDR) is a precision instrument used to locate events or faults along a fiber link, typically within an optical communications network. The OTDR launches a series of high speed optical pulses into the fiber to be measured. Various events on the fiber generates a Rayleigh back scatter that returns to the OTDR and the strength of the return pulses are measured and integrated as a function of time, and plotted as a function of fiber length. The horizontal axis is the distance and the vertical axis is the loss.