FIS Glossary

A single instrument that measures numerous network parameters

In today’s environment, network providers are optimizing their networks to increase the quality of service, while also attempting to reduce costs. Unfortunately these goals can sometimes be contradictory. Optimization requires information about the quality of the network, which is determined using test-and-measurement equipment to analyze the characteristics of the optical fibre and system components. Such testing can be costly in terms of labour and equipment. Equipment capable of performing several types of measurements and analyzing various different characteristics of the network can reduce these test-related costs.

The optical time-domain reflectometer (OTDR) has evolved into an instrument that can help optimize a network for high-quality service while also reducing the labour and equipment costs. It is capable of measuring total link loss, optical return loss, splice loss, and fault identification, as well as qualifying a fibre for Raman amplification and measuring chromatic dispersion. Furthermore, the OTDR can monitor an installed network for faults, reducing the mean time to repair, which then increases the network’s quality of service.

• Averaging - Accumulating multiple samples of data and dividing the data by the number of samples taken averages the data and reduces the random noise content of the measurement signal.
• Backscatter coefficient - The ratio of the backscattered power to the energy launched into a fiber.
• Deadzone - The distance immediately following a fiber event (splice or connector), which can partially or completely inhibit the ability to perform accurate measurements on a closely spaced subsequent event.
• Dynamic range - Maximum optical-power loss that a signal can experience before it is indistinguishable. For an OTDR, it is the maximum optical-power loss before fiber is not displayed by the OTDR, generally expressed in dB. Also can indicate the difference between the highest backscatter signal and one dB above the root-mean-square value of the noise floor.
• Pulsewidth - The duration (length) of a test pulse of light, which is generally expressed in nanoseconds. The pulse width is directly proportional to both the dynamic range and the deadzone of the OTDR. A longer pulse width results in an effectively higher-power signal, increasing the range but also increasing the deadzone. A shorter pulse width provides a shorter dead zone, but also less dynamic range.
• Resolution - The spacing between adjacent data-sampling points. The lower the resolution, the more accurate the graphing of actual fiber characteristics.
• Gainer - A splice that shows an apparent increase in signal on an OTDR display after the splice. This is caused by differences in scattering properties between the two fiber segments being joined caused by different mode-field diameters or fiber properties. Bidirectional averaging is needed to accurately measure the loss at such splices.