Hello everyone!
Today, I want to explain troubleshooting with a VFL. VFL is an abbreviation from Visual Fault Locator. It is a very simple instrument for optical fibers, but it can be very useful in many situations.
VFL is a red light source (laser at a wavelength of 635-650 nm). It is like a pen, and its output optical power is about 10 mW. This optical power is enough for a distance from 2 to 5 km. This tool, usually, has about min two working modes: continuous and frequently.
VFL is used for testing by attaching to a connector at one end of the fiber/cable (pec cord, pig tail, etc). If the light is visible only at the end of the fiber, everything is fine, the fiber is not damaged. However, if there is particularly noticeable light on any part of the fiber, it is a sign that there is a problem.
The construction of the optical network requires three measurements, these are measurements before the installation of the cable (the cable is on the drum), each segment of the cable and finally the whole link (it is composed of several segments). For testing the cable on the drum, we can use VFL. The target is to test fiber continuity. If we do not see light on the end of optical fiber, there is a problem. For this measurement, we can use the fiber optical tracer, too.
We can use VFL for identification optical fibers during troubleshooting on the ODN. It allows you to find the right fibers very quickly and thus solve problems quickly.
OTDR is the most important instrument in every optical network. But only some OTDR can use for indoor installation, because links are very short. This is a big problem in FTTH networks, in the drop optical segment. In these situations for troubleshooting, we use VFL.
Fig 1. Problem with a patch cord
(https://www.thefoa.org/tech/ref/testing/test/visual.html)
Fig 2. Problem with fiber in the splice cassette
(https://www.thefoa.org/tech/ref/testing/test/visual.html)
VFL is especially useful for indoor installation, where there are white or yellow drop cables, patch cords, pig tails, etc. If there is a problem the light can be seen easily. The problem is only with the black drop optical cable.
Also, VFL is very useful when checking fusion splice in optical cassettes, pig tails in patch panels, optical splitters, etc. These are very short links and the use of OTDR is practically impossible, and the light of this instrument can be seen very easily.
Fig 3. The process of making a mechanical splice
(https://www.thefoa.org/tech/ref/testing/test/visual.html)
This instrument is also used when making or checking mechanical splices. During the construction of a mechanical splice, we need to use this instrument. We adjust the fibers in the groove until the light of the VFL is minimally visible. In this way, we allow maximum transmission and minimum loss.
Finally, let me repeat. With this instrument, we can very simple and easy to see problems at short distances and in many other situations. The most common problems are bent, broken or damaged fiber. After that, we can solve them.
Thank you!
