Optical Time Domain Reflectometer (OTDR):

i. A measurement technique which is far more sophisticated and which finds wide application in both the laboratory and the field is the use of optical time domain reflectometry(OTDR).

ii. This technique is often called the backscatter measurement method. It provides measurement of the attenuation on an optical link down its entire length giving information on the length dependence of the link loss.

iii. When the attenuation on the link varies with length, the averaged loss information is inadequate.

iv. OTDR also allows splice and connector losses to be evaluated as well as the rotation of any faults on the link. It relies upon the measurement and analysis of the fraction of light which is reflected back within the fiber’s numerical aperture due to Rayleigh scattering.

v. A block schematic of the backscatter measurement method is shown in Figure 2.13.

vi. A pulsating laser generates optical pulses and using optical circular or directional couplers; these pulses are passing to optical cable.

vii. In the optical cable; due to various reasons, the scattering of light takes place.

viii. The backscattered light is detected using an avalanche photodiode receiver which drives an integrator in order to improve the received signal-to noise ratio.

ix. The signal from the integrator is fed through a logarithmic amplifier and averaged measurements for successive points within the fiber are plotted on a chart recorder.

Advantages:

i. A number of optical time domain reflectometers are commercially available for operation over the entire wavelength range.

ii. These instruments are capable of carrying out tests over single or dual wavelengths for multimode and for single-mode optical fiber links.

iii. Although the OTDR functionality is provided, these instruments are also often capable of performing a number of other optical system and network tests (e.g. optical loss, dispersion measurement etc.).

iv. Such instruments are usually referred as universal or optical network test systems rather than simply optical time domain reflectometers.

Applications:

i. An Optical Time Domain Reflectometer (OTDR) is an important instrument used by organizations to certify the performance of new fiber optics links and detect problems with existing fiber links.

ii. OTDRs are also used for maintaining fiber plant performance.

iii. An OTDR allows to see more detail impacted by the cabling installation. An OTDR maps the cabling and can illustrate termination quality, location of faults.

iv. An OTDR provides advanced diagnostics to isolate a point of failure that may hinder network performance.

v. An OTDR allows discovery of issues along the length of a channel that may affect long term reliability.

vi. OTDRs characterize features such as attenuation uniformity and attenuation rate, segment length, location and insertion loss of connectors and splices, and other events such as sharp bends that may have been incurred during cable installation.