18.1.2 Principles: Pulse and Backscatter of Light
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An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR in its most basic form uses a pulse of optical radiation to interrogate a waveguide, typically an optical fiber.
As the pulse propagates along the fiber, discontinuities formed by tiny density fluctuations (frozen into the fabric of the fiber at the time of manufacture) cause a small fraction of the incident light to scatter. This process is usually referred to as Rayleigh scattering.
This scattered light is then recaptured by the fiber and guided back toward the starting end where it can be detected. By measuring properties of the scattered light as a function of time, properties of the waveguide or fiber as a function of distance can be inferred.
The simplest way of understanding it is as a one-dimensional (1D) RADAR, but at optical wavelengths.
All OTDR systems share the following properties:
- Detection time maps linearly to location along the fiber.
- Properties of the scattered light are a function of the local properties of the waveguide.
By tailoring the properties of the launched pulse and the processing applied, different information about the waveguide/fiber being interrogated can be inferred. In the case of coherent OTDR, the fiber is interrogated by a pulse of coherent light of a finite length.
As stated above, this pulse propagates along the fiber and a small fraction is scattered. The resulting intensity of the scattered light as a function of time and hence distance is determined by the coherent sum of scatter from typically millions of scatter sites. The distribution of the position and magnitude of the scatter sites are random and hence the intensity of the scattered field is also random as a function of distance.
From this point on, different DAS technologies may use different properties of the scattered light to infer the action of an acoustic disturbance. For example, systems may measure the absolute phase of the scattered light or they may measure the differential phase; however, for illustration of how the OTDR principle may form a DAS system and for simplicity, the following example describes an intensity-based, single-pulse DAS system.