Optical Time-domain Reflectometer

An optical time-domain reflectometer, or OTDR, is an instrument used to analyse optical fibre. It sends a series of light pulses into the fibre under test and analyses the light that is scattered and reflected back. These reflections are caused by faults such as breaks, splices, connectors, and adapters along the length of the fibre. The OTDR is able to estimate the overall length, attenuation or loss, and distance to faults. It’s also able to “see” past many of these “events” and display the results. The user is then able to see all the events along the length of the fibre run.

Weakness
However, OTDRs do have a weakness — a blind spot that prevents them from seeing faults in the beginning of the fibre cable under test. To compensate for this, fibre launch boxes are used. Launch boxes come in predetermined lengths and connector types. These lengths of fibre enable you to compensate for this blind spot and analyse the length of fibre without missing any faults that may be in the first 10–30 metres of the cable.

OBL - a simplified version
An optical break locator, or OBL, is a simplified version of an OTDR. It’s able to detect high-loss events in the fibre such as breaks and determine the distance to the break. OBLs are much simpler to use than an OTDR and require no special training. However, there are limitations. They can only see to the first fault or event and do not display information on the portion of fibre after this event.