Category Archives: Visual Fault Locator

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.

WDM PON and TWDM PON Technology

After the 10Gigabit PON(Passive Optical Network), WDM(Wavelength-division multiplexing) technology entered into traditional TDM PON fields. In April 2012, standard organization FSAN(Full Service Access Network) determined the time and wavelength division multiplexed passive optical network (TWDM PON) technology became the preferred solution for next-generation passive optical network stage-2 (NG-PON2) architecture after10G PON. To better understanding WDM PON, I list the WDM technology below.

What Is WDM?

WDM is a method of combining multiple signals on laser beams at various infared wavelengths for transmission along fiber optic media. WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart.

WDM systems are divided according to wavelength categories, generally course WDM (CWDM) and dense WDM (DWDM). CWDM operates with 8 channels (i.e., 8 fiber optic cables) in what is known as the C-Band or erbium window with wavelengths about 1550 nm (nanometers or billionths of a meter, i.e. 1550 x 10-9 meters). DWDM also operates in the C-Band but with 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing.

CWDM multiplexer module allows multiple optical signals at different wavelengths to pass through a single optical fiber strand. The common configuration of CWDM mux/demux module is 2CH, 4CH, 8CH, 16CH, 18CH CWDM mux/demux module.

DWDM Mux/Demux Modules – DWDM Mux and DWDM DeMux are designed to multiplex DWDM channels into one or two fibers. 50G DWDM Mux Demux is used to provide 50G transport solution for DWDM networking system. The common configuration is 4, 8, 16 and 32 channels, and also has 40, 44 channels. These DWDM modules passively multiplex the optical signal outputs from 4 or more electronic devices, send them over a single optical fiber and then de-multiplex the signals into separate, distinct signals for input into electronic devices at the other end of the fiber optic link.


WDM PON uses multiple different wavelengths over a physical point-to-multipoint fiber infrastructure that contains no active components (PON). Each provides a dedicated wavelength channel at the rate of 1Gbps to each optical network unit (ONU). The use of different wavelengths allows for traffic separation within the same physical fiber. The result is a network that provides logical point-to-point connections over a physical point-to-multipoint network topology. WDM-PON allows operators to deliver high bandwidth to multiple endpoints over long distances.


For simple network deployment and inventory management purposes, the ONUs use colorless tunable transmitters and receivers. The transmitter is tunable to any of the upstream wavelengths, while the receiver can tune to any of the downstream ones. Optical Amplifiers are employed at the OLT side to boost the downstream signals as well as to pre-amplify the upstream signals. ODN remains passive since both the optical amplifier and WDM Mux/Demux are placed at the OLT side.

This type of TWDM PON system is valuable in a market where multiple operators share one physical network infrastructure. Coexistence with previous PON generations in the legacy ODN depends on the TWDM PON wavelength plan, reuse the XG-PON wavelength bands, redefine the C-band enhancement band to contain both the upstream and downstream wavelengths and mixture of both of these plans.

TWDM-PON standards are expected to complete in 2013, and a finished commercial system should see the light of day in 2014.

Pen Style Handheld Visual Fault Locator

Toronto, Canada- GAO Fiber Optics recommends its optical visual fault locator used in quickly verifying continuity, checking the validity of patch cables and locating faults in the continuity of both single mode (SM) and multimode (MM) optical fibers.

This handheld optical visual fault locator, is a basic tool for the maintenance of fiber optic networks, optical systems including LAN, FDDI and ATM, telecommunications and CATV networks, for example, CATV Optical Amplifier. It has a 650 nm visible laser source that transmits continuous light or a 1 Hz adjustable strobe light into a fiber allowing the user to visually detect a fiber fault over distances of up to 9 km. The fault detector operates for up to 80 hours with modulated light and up to 50 hours with continuous light using two 1.5 V AA alkaline batteries.

It is easy to use optical visual fault locator belongs to GAO’s family of Visual Fault Locators. Two other featured products in this line are Rugged Handheld Visual Fault Locator which detects breaks and micro-bends through jacketed fiber and performs an end-to-end fiber identification easily, and Handheld Visible Laser Source which overcomes the usual limitation of the dead zone of an OTDR testers to accurately detect fiber fault positions making it an ideal instrument for the installation and maintenance of fiber optic networks.