When installing a fiber optic system or cable plant the system must be tested to verify performance. This is carried out to ensure that light will pass through the system properly. Three types of optical testing are carried out routinely in the field: continuity, power and Optical Time Domain Reflectometer (OTDR) testing. A distinct advantage of a fiber optic system is its electrical immunity. Because fiber cable is nonmetallic, it cannot emit or pick up electromagnetic interference (EMI) or radio frequency interference (RFI). Both EMI and RFI can cause significant problems with metallic conductors.
An Optical Time Domain Reflectometer is essentially an optical radar: it sends out a flash of bright light, and measures the intensity of echo or reflections. This weak signal is averaged to reduce detection noise, and computation is used to display a trace and make a number of mathematical deductions.
This instrument is really good for measuring points loss on installed systems, where it is used to find faults and measure point losses such as caused by splicing. However to do this accurately is more complicated and time consuming than is commonly supposed, since a measurement should be taken from both ends of the system, and then averaged. If this is not done, spurious excess losses and “gainers” may be recorded where different fibers are joined, resulting in wasted splicing effort while non-existent faults are “repaired”. This is a particular issue when measuring the fusion splice joints, where the loss is small, and the adjacent sections may have fibers with different intrinsic backscatter characteristics.
OTDRs can be used for return loss measurements, although quoted accuracy is not very high.
This is most commonly used during installation acceptance and maintenance of outside plant cables. In this role, it is likely to be used to identify point losses, the length of various cables, and to measure return loss.