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What Is Visual Fault Locator and How to Use It

The Fibre Fault Locator (VFL) is an essential tool for every Fibre Termination Kit. It is like the continuity tester. The VFL is not one of the least expensive tools in your tool kit. It will allow you to quickly identify breaks or macrobends in the optical fibre, and identify a poor fusion splice in multimode or single-mode optical fibre.


The big difference between the VFL and the continuity tester is the light source and optical output power of the light source. The VFL typically uses a red (635-650nm) laser light source. The optical output power of the laser is typically 1mW or less. Because of the high optical output power, you should never view the output of the VFL directly.

The Visual Fault Locator is available in different shapes and sizes. Some may look like a pen. Others may be built into an optical time domain reflectometer (OTDR), and some may look like a small test equipment box. There are two types of VFLs: contact and non-contact. With a contact VFL, the optical fibre under test will make contact with the VFL. However, with a non-contact VFL the optical fibre under test will not touch the VFL.

Unlike the continuity tester, the VFL is not limited to testing multimode optical fibres 2km or less in length. The VFL can be used to verify continuity of multimode or single-mode optical fibre longer than 2km. Due to attenuation of the 635–650nm laser light source by the optical fibre, macrobends may not be detectable beyond 1km in multimode optical fibre and 500 metres in single-mode optical fibre. The same holds true for finding breaks in the optical fibre through the jacket of the fibre-optic cable.

How to Use Visual Fault Locator

As with the continuity tester, the first thing you will need to do is clean the connector endface and inspect it with a microscope. If the endface finish is acceptable, the VFL can be connected to a Optical Fibre Connectors should not be viewed directly during this testing.

The VFL fills the core of the optical fibre with light from the laser. The light from the laser escapes the optical fibre at a break or macrobend. The light escaping from the optical fibre will typically illuminate the buffer surrounding the optical fibre. Macrobends are not always visible through the jacket but are typically visible through the buffer. Breaks may be visible through the jacket of the fibre optic cable depending on jacket color, thickness, number of optical fibres in the cable, and amount of strength member.

The VFL and the fibre OTDR work hand in hand with each other when it comes to locating breaks in an optical fibre. The OTDR can provide the operator with the distance the break. The VFL allows the operator to see the break in the optical fibre.


Fibre optic cables are not the only place where the optical fibre may break. The optical fibre may break inside the connector or connector ferrule. Unless the optical fibre is broken at the endface of the connector, it is not visible with a microscope.

Usually, students connect cables that look great when viewed with the microscope but fail continuity testing. When this happens, the hardest part is determining which connector contains the break in the optical fibre. Without a VFL in the classroom, students would have to cut the cable in half and use the continuity tester to identify the bad connection.

The VFL will often identify the bad termination or connector.  Looking at the photograph, you can see VFL illuminating the break in the optical fibre. The output of the VFL is so powerful that it penetrates the ceramic ferrule.

The visual fault locator can be used to test the continuity of an optical fibre in the same manner. The first step when using the continuity tester is to clean and visually inspect the end face of the connector before inserting it into the continuity tester. After the connector has been cleaned and inspected, you need to verify that the continuity tester is operating properly. Turn the continuity tester on and verify that it is emitting light.

The visual fault locator also can be used to locate a macrobend in an optical fibre. However, macrobends do not allow nearly as much light to penetrate the buffer and jacket as does as break in the optical fibre. Locating a macrobend with the VFL may require darkening the room.

Macrobends and high loss fusion splices appear the same on an OTDR trace. The VFL allows the identification of a high-loss fusion splice.