Visual Fault Locator (VFL)

Visual Fault Locator (VFL)

A Visual Fault Locator (VFL) is a critical tool used to visually locate breaks or in some instances, excessive bends in fiber optic cables. VFL’s use a 650 nm laser diode as the laser light source, which is bright red. They are also used to identify a specific fiber cable within a bundle.

Look for options such as 2.5mm to 1.25mm adapter, constant wavelength or pulse mode, and a dust-proof cover will help ensure that the interface is well protected.

 

Visual fault locator overview

  • Locate breaks within the dead zone of an OTDR.

  • Locate bad splices.

  • Locate potential fiber problems ( imperfection).

  • Can be used as fiber identifier.

  • Locate micro and macro bends.

Visual fault locator, also known as a visual fault identifier, is used to locate faults visually and quickly. But of course, in order to locate the fault technician must be able to see the cable.

What is a VFL?

The VFL is the most commonly used test device to check fiber cable continuity, trace optical fibers and find faults such as brakes, bad splices, and sharp bins in fiber optic cable. The 10 type of VFL is the most popular visual fault locator.

How VFL's work

The transmitted signal over fiber-optic cable is usually 1300 to 1650 nm wavelength which is invisible to the naked eye. The VFL uses a powerful visible light at 360 to  670 nm wavelength. Visible light travels through the fiber core until reaching the break or problem area allowing the light to leak through the plastic coating invisible to the technician. Assuming of course that the ambient light level is low enough.

Visual fault locators radiate in continuous wave (CW) or pulse modes. The glint of the light source in VFL is usually at 1 or 2 Hz, kHz range is also being provided in today’s market. The output power is generally at 1 mW or less. The working distance of a VFL is usually in the range of 2 to 5 km.

Never looked directly into the VFL's output. Cover the VFL's output with a suitable dust cap when not in use.

Tracing Fibers

One of the best uses for these devices is to trace fibers for identification or to determine correct connections. To trace fibers using the fiber optic tracer or VFL, connect the fiber to the output connector of the unit. The light output will be visible to the eye at the other end of the fiber. This allows finding particular fibers in multifiber cables easily for proper connections during installation. 

If a powerful enough visible light ,such as a red HeNe laser or visible diode laser at 635-650 nm is injected into the fiber, high loss points can be made visible. Most applications center around short cables such as used in telco central offices to connect to the fiber optic trunk cables. However, since it covers the range where OTDRs are not useful because of the dead zone of the OTDR, it is complementary to the OTDR in cable troubleshooting. This method will work on buffered fiber and even jacketed single fiber cable if the jacket is not opaque to the visible light. The yellow jacket of singlemode fiber and orange of multimode fiber will usually pass the visible light. Most other colors, especially black and gray, will not work with this technique, nor will most multifiber cables. However, many cable breaks, macrobending losses caused by kinks in the fiber , bad patchcords, spices etc. can be detected visually. Since the loss in the fiber is quite high at visible wavelengths, on the order of 9-15 dB/km, this instrument has a short range, typically 3-5 km. 

VFLs use laser light sources classified as Class II with power levels low enough to prevent harm to the eyes, but with enough power to send down 4 km of singlemode fiber for tracing. 

 

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