An Optical Time-Domain Reflectometer (OTDR) is a measurement device that can be used to characterise optical communication systems. By recording the intensity of Rayleigh back-scattered light from a source (typically a pulsed laser) in the time domain, the OTDR can approximate the distance travelled by the light, given that the refractive index of the fibre is known.
Thereby it is able to identify the attenuation of different lengths of the fibre, and the position of network components, for example splices or connectors. The identification and location of defects within the optical communication system is the primary motivation for using OTDRs.
The OTDR should be connected to the end of an optical fibre in the order of tens of kilometres in length and with a number of splices and connectors along that length.
A simple solution to this problem is to connect the OTDR at the opposite end of the fibre.
The on-screen results obtained with the OTDR are given in a figure.
This is due to the better optical coupling of splices; the local maxima observed at connectors are a consequence of increased back reflection due to poor quality of the joint.
The peak is an artefact of the assumptions made in the system (specifically, the assumption that back reflection intensity can be used as an analogue of attenuation) and therefore the attenuation of the connector is not related to the height of the peak, but rather the power loss from the start to the end of the peak.