Title OTDR & Fiber Characterization

Originally, fiber-optic networks underwent a limited number of tests before they were actually put into service. Increasing data rates to several tens of gigabits per second, wavelength multiplexing using CWDM (Coarse Wavelength Division Multiplexing), DWDM (Dense Wavelength Division Multiplexing) or even DWDM over CWDM, and the extension of the transmission spectral window to the O, E, S and L bands (1,260 to 1,625 nm) soon made it imperative to supplement distance and attenuation measurements with more detailed characterization of the infrastructure's performance. Fiber characterization thus encompasses a list of measurements to be carried out to determine the network's ability to meet current and future transmission requirements, and thus to qualify it before it is commissioned or upgraded. The combination of a number of measuring devices today enables us to measure distances, linear fiber attenuation, insertion loss and reflectance, ORL (Optical Return Loss), CD (Chromatic Dispersion), PMD (Polarization Mode Dispersion) and spectral attenuation or attenuation profile.

The extensive capabilities of the Optical Time Domain Reflectometer (OTDR) make it the key measurement device in this family of fiber optic network characterization tools.

In the majority of applications, the optical reflectometer has supplanted the radiometric method, which required the connection of a radiometer and a source on either side of the link. A kind of optical radar, the reflectometer requires access to only one end of the link under test, thus necessitating the presence of a single operator.

In some cases, certain characterization measurements may be considered optional. On the other hand, the distance and attenuation measurements provided by the OTDR are essential.

In the following pages, we focus on the use of optical reflectometry in the characterization of optical fiber networks. Despite some similarities, we will not deal with the case of characterization in fiber or cable production, but will remain focused on the application to networks.

This is followed by a description of reflectometry, the physical phenomena involved and the reflectometer. After describing the principle and the tool, the use of the reflectometer, from its implementation and measurement to the analysis and interpretation of curves and results, is described, ending with a presentation of current trends towards the automation of fiber characterization by reflectometry.

The aim of this article is to provide the technical basis for reflectometry in its application to the characterization of telecommunication networks, whether long-, medium- or short-distance, such as fiber-to-the-home (FTTH) access networks. We...