How the OTDR works

As its name suggests, the OTDR’s function is based on the propagation of light in an optical fiber during a specific length of time. Figure is a schematic illustration of an OTDR instrument.

 In a pulse generator, short (e.g 0.1, 1 or 10 us) electric pulses with specific time intervals are generated. The electric pulses are transformed into light pulses in the laser diode. A light pulse passes a directional coupler before it is sent out to the fiber. Because the fiber is not completely transparent a certain amount of light will be returned into the instrument due to Raleigh scattering. The returned light is detected by an avalanche photodiode (ADP) .The signal from the APD is amplified by an amplifier and processed by a microprocessor. The Optical Signal is weak and the process is therefore repeated several times to provide a basis for an average value.

The signal strength is represented by the vertical axis of a coordinate system and time (often transformed into distance) is represented by the horizontal axis. The coordinate system is shown together with other useful information on a video screen. The optical signal from the fiber is shown as a curve sloping from left to right. Normally the video screen image is printed out directly on a built in printer. This printout is then saved for the final documentation of the installation.

 schematic illustrations of the main parts of an OTDR.

 

simplified graph from an OTDR.