How an OTDR Works
The Optical Time Domain Reflectometer (OTDR) uses the effects of Rayleigh scattering and Fresnel reflection to measure the characteristics of an optical fiber. By sending a pulse of light (the “optical” in OTDR) into a fiber and measuring the travel time (“time domain”) and strength of its reflections (“reflectometer”) from points inside the fiber, it produces a characteristic trace, or profile, of the length vs. returned signal level on a display screen. The trace can be analyzed on the spot, printed out immediately for documentation of the system, or saved to a computer disk for later analysis and comparisons. A trained operator can accurately locate the end of the fiber, the location and loss of splices, and the overall loss of the fiber. Most newer OTDRs provide for automatic analysis of the raw trace data, thereby eliminating the need for extensive operator training.
When a pulse of light is sent down a fiber, part of the pulse runs into microscopic particles (called dopants) in the glass and gets scattered in all directions. This is called Rayleigh (pronounced RAY-lay) scattering. Some of the light — about 0.0001% — is scattered back in the opposite direction of the pulse and is called the backscatter. Since dopants in optical fiber are uniformly distributed throughout the fiber due to the manufacturing process, this scattering effect occurs along its entire length.
Rayleigh scattering is the major loss factor in fiber. Longer wavelengths of light exhibit less scattering than shorter wavelengths. For example, light at 1550nm loses 0.2 to 0.3 dB per kilometer (dB/Km) of fiber length due to Rayleigh scattering, whereas light at 850nm loses 4.0 to 6.0 dB/Km from scattering. A higher density of dopants in a fiber will also create more scattering and thus higher levels of attenuation per kilometer. An OTDR
can measure the levels of backscattering very accurately, and uses it to detect small variations in the characteristics of fiber at any point along its length. The Rayleigh scattering effect is like shining a flashlight in a fog at night: the light beam gets diffused — or scattered — by the particles of moisture. A thick fog will scatter more of the light because there are more particles to obstruct it. You see the fog because the particles of moisture scatter small
amounts of the light back at you. The light beam may travel a long way if the fog is not very thick, but in a dense fog, the light gets attenuated quickly due to this scattering effect. The dopant particles in fiber act like the moisture particles of the fog, returning small amounts of light back towards the source as the light hits them.
Whenever light traveling in a material (such as an optical fiber) encounters a different density material (such as air), some of the light — up to 4% —is reflected back towards the light source while the rest continues out of the material. These sudden changes in density occur at ends of fibers, at fiber breaks, and sometimes at splice points. The amount of the reflection depends on the magnitude of change in material density (described by the Index of Refraction (IOR) — larger IORs mean higher densities) and the angle that the light strikes the interface between the two materials. This type of returned light is called a Fresnel (pronounced freh-NELL) Reflection. It is used by the OTDR to precisely determine the location of fiber breaks.
A Fresnel reflection is like shining a flashlight at a window. Most of the light passes through the window, but some of it reflects back at you. The angle that the light beam hits the window determines whether or not the reflection will bounce back into the flashlight, your eyes, or the ceiling.
Backscatter Level vs. Transmission Loss
Although the OTDR measures only the backscatter level and NOT the level of the transmitted light, there is a very close correlation between the backscatter level and the transmitted pulse level: the backscatter is a fixed percentage of the transmitted light. The ratio of backscattered light to transmitted light is also known as the “backscatter coefficient.” If the amount of transmitted light drops suddenly from Point A to Point B (caused by a tight bend, a splice between two fibers, or by a defect), then the corresponding backscatter from Point A to Point B will drop by the same amount. The same loss factors that reduce the levels of a transmitted pulse will show up as a reduced backscatter level from the pulse.