5 Testing optical paths by using Optical Time Domain Reflectometer
5.1 Optical Time-Domain Reflectometer

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.

Optical Time Domain Reflectometer.

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.

- The OTDR can be configured to produce pulses of either 1550nm or 1310nm central wavelength, and pulse durations of 10ns to 10us.
- There is a trade-off between penetration depth and resolution of the measurement, which is determined by pulse duration.
- Shorter pulses have both a better spatial resolution and a worse penetration depth.
- There is preferred pulse duration for each application, ideally the greatest resolution while maintaining depth data.
- There may exist fibres for which adequate clarity is not achieved at great depths.

A simple solution to this problem is to connect the OTDR at the opposite end of the fibre.

## Interpretation of measured data

The on-screen results obtained with the OTDR are given in a figure.

- OTDR manages to identify network components: connectors and splices due to the shape of the attenuation profile.
- A short, sharp drop indicates a splice.
- A local peak is observed at connectors, with a greater drop-off on the right-hand side of the peak.

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.

On-screen attenuation diagram measured by OTDR.
Connectors and splices on an OTDR diagram.