Visual Fault Locator Tutorial: Everything You Need to Know

Updated on Aug 31, 2021 by

Visual Fault locator (VFL) is a basic fiber optic for troubleshooting fiber optic link. It is a must-have test gear in every fiber technician’s tool box. Read on to get information if you are:

  • New to virtual fault locator and willing to know what VFL can be used for;

  • Prepare to purchase visual fault locators but do not know how to choose;

  • Searching for the user guide and maintenance tips of VFL.


What Is Visual Fault Locator?

Visual Fault Locator which can be also called visual fault identifier (VFI), fiber fault locator, fiber fault detector, etc, is a visible laser available in red laser light and designed to inject visible light energy into an optical fiber. It is a cost-effective way to spot defects of fiber glass such as sharp bends, breaks, faulty connectors and other faults which will "leak" red or green light visually. It can also locate the fault of OTDR dead zone and make fiber identification from one end to the other end. No matter for installing new fibers or troubleshooting the existing networks, visual fault identifier is a helpful handy kit.

You can diagnose fiber link problems over fiber optic cables or fiber cable pigtails using visual fault locators. When the fiber fault locator is designed with adapter incompatible with your connector type, using additional adapters is necessary.

Taking FS mini visual fault Locator as an example, with integrated universal 2.5mm SC, ST, FC adapter, this fiber fault locator is used without any other type of additional adapters ,it can locates fault up to 15km in fiber cable,with compact in size, light in weight, red laser output. For detecting LC connectors, optional 1.25mm adapter for LC is needed.


Types of Visual Fault Locators

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.

Pen-Type Visual Fault Locator

Taking FS Pen-Type Visual Fault Locator, 1mW as example, it is small in size and easy to be carried away.

Hand-held Visual Fault Locator

Hand-held VFL general has a larger size, and the weight is higher than pen-type VFL.


How Does Visual Fault Locator Work?

The working principle of fiber optic visual fault locator is as follows: The VFL fills the core of the optical fiber with light from the laser. The light from the laser escapes the optical fiber at a break or macrobend and will typically illuminate the buffer surrounding the optical fiber. Though macrobends are not always visible through the jacket, they are typically visible through the buffer. While breaks may be visible through the jacket of the fiber optic cable depending on jacket color, thickness, number of optical fibers in the cable, and amount of strength member.


Important Parameters of VFL: How to Choose Ideal Visual Fault Locators?

Fiber distance

The fiber distance spec tells the longest fiber length where you can still see the leakage light without difficulty. This spec is determined by the laser light power. The industry standard spec is 10km for multimode fibers and 5km for single mode fibers. FS high power 30mW fiber optic fault locator enables up to 15km dynamic distance.

Output power

Output power is an important spec for visual fault identifiers. Some may ask how many power do I need? The key is: The higher output power is, the longer fiber distance it can support. This spec is usually 0.5mW coupled to single mode fibers and 2mW to free space (which means no fiber coupling, just the light output from the tester's connector adapter).

Fiber mode

All visual fault locators can work with both single mode and multimode fibers. Since multimode fiber has a 50um or 62.5um core, which is much bigger than single mode fiber's 9um core, more light power can be coupled in to multimode fibers. That translates to longer fiber distance for multimode fibers than single mode fibers.

Modulation frequency (CW and 2Hz)

Human eyes are much more sensitive to blinking light than to a steady light. So visual fault locators provides two operating modes: continuous wave and pulse light. In most cases, fiber optic installation technicians prefer the pulse light mode which turns the laser source on and off in 2Hz or 1Hz frequency.


Usually two AAA or two AA alkalescence batteries are used.


Wavelength is not a critical spec. 635nm, 650nm and 670nm will all work just fine.


How to Use Visual Fault Locators?

Step 1: Clean the fiber optic cable connector endface and attach your visual fault locator to the connector.

Clean and inspect the fiber optic cable connector with a microscope. If the endface finish is acceptable, the VFL can be connected to a fiber connector.

Step 2: Connect the fiber visual fault locator to the opti-crimp connector ferrule (with the included patch cord), turn on the VFL.

Notice the red glow in the connector ferrule. If the light comes up directly on the other end, it means there is no leakage.

Once there is a fault, light will leak out from the cladding and the light will not go through the other end of the cable.

Here is a video of FS.com visual fault locator solution, illustrating how to use and select VFL properly.


Precautions of Using Visual Fault Locators

  • Never look into the path of the Laser Beam and never look into the end of a fiber.

  • Keep away from water, dust, static electricity, and environments of extreme temperature.

  • Remove the batteries for prolonged periods of storage.

  • Never launch visual fault locator into active equipment or microscope.

  • Make sure that the end opposite the Visual Fault locator is not connected to any electronics or active equipment during termination.


VFP Maintenance Tips

  • Clean both ends of the VFL after 50 matings or sooner if performance diminishes.

  • Clean ST connector ferrule end face with a lint-free wipe (FWP-C) soaked with alcohol (reagent grade isopropyl alcohol, with a minimum 90% concentration).

  • Clean the split sleeve with an alcohol soaked cleaning swab (FSWB-C). Replace the split sleeve onto fiber ferrule and screw housing into place securely.

  • Clean FVFL output by unscrewing housing and removing split sleeve. Be careful not to lose the small spacer ring around the ferrule.

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