Fluke Fiber Optic Testing and OTDR Principles

OTDR, whose Chinese name is: Optical Time Domain Reflectometer, is a commonly used fiber optic testing tool. Here O stands for Optical, TD stands for Time Domain, and R stands for Reflectometer. We know that most optical fibers are made of high-purity quartz glass, and glass molecules are lattice structures. The lattice structure is actually uneven. When photons carrying signal energy encounter them, there will be a small change in direction (scattering). In addition, there are some impurities, bubbles and micro-bend structures in the optical fiber. When photons encounter them, they will also change direction and scatter in all directions. In fact, the OTDR instrument uses these characteristics to emit very short light pulses into the optical fiber, and then use optical detection devices to observe very weak reflections. After analysis, it can identify losses, reflections and other events.

Schematic diagram of Rayleigh scattering

OTDR is like a radar in the optical fiber world. If we use an optical signal generator to inject a short-duration optical pulse into the optical fiber link, when the optical energy of this pulse encounters an uneven lattice structure and impurity particles during the forward transmission, there will be extremely weak energy scattered in all directions. This scattering is also called Rayleigh scattering. A part of it will be completely reflected back in the direction of the origin, which is called backscattering. At the same time, if the optical pulse encounters a connector during forward transmission, the photon encounters a medium mutation here, and many photons will be reflected back at the mutation interface, which is also called Fresnel reflection. The maximum reflected energy can reach 8% of the forward transmission light energy. Therefore, we can design a device to receive the returned optical signal in the signal injection port of the optical fiber at the same time, sample and record the backscattered and reflected optical signals, and then analyze them.

Schematic diagram of Rayleigh scattering

Next, let's take a closer look at Rayleigh scattering. In daily life, when we turn on a flashlight at night, we see a beam of light, but why do we see a beam of light? It is because the dust and fog in the air cause the light to scatter, and some of the light enters our eyes, so we see it. The scattering of all the particles forms a beam of light. And when the fog is thick, the beam of light will appear dense, otherwise it will be sparse. If it is in a vacuum, such a beam of light will not be seen.

The working principle of OTDR is similar to that of a radar. It first sends a signal to the optical fiber, and then observes what information is returned from a certain point. This process is repeated, and then the results are averaged and displayed in the form of a track. This track depicts the strength of the signal (or the status of the optical fiber) in the entire section of the optical fiber.

Fluke OptiFiber® Pro OTDR is designed for enterprise, data center, outside plant and PON fiber. As fiber networks grow, the need to test in more places is also increasing.

Now the following fields require OTDR:

• FTTx

• enterprise

• OSP

• PON

• POLAN

Many OTDRs designed for fiber troubleshooting are carrier-oriented and include cumbersome and complex features. The OptiFiber Pro OTDR Series is a first-class OTDR device that is versatile and easy to use, targeting both enterprise network engineers and cable installers working in both enterprise and OSP environments.