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A Must-Have Tool for Fiber Maintenance: Visual Fault Locator

(Last Updated On: April 24, 2023)

Fiber optic communication has become an indispensable part of our modern world, providing high-speed and reliable data transmission. However, faults and breaks in fiber optics can cause significant disruptions in communication. Visual Fault Locators (VFL) play a crucial role in detecting and locating faults in fiber optics. In this blog post, the following information will be available:

* New to virtual fault locator, want to know the purpose of VFL;

* Ready to buy a visual fault locator but don’t know how to choose;

* Search for VFL usage guides and maintenance tips.

What is a Visual Fault Locator?

A Visual Fault Locator (VFL) is a handheld device that detects faults or breaks in optical fibers. It operates by emitting a bright and visible red laser light into the fiber and detecting the location of faults by observing the light leaking out of the fiber. It is also possible to locate faults in OTDR dead zones and perform fiber identification from one end to the other. VFLs are essential tools in optical fiber communication as they make it easier to identify and repair faults in optical fibers.

Take VFL-HTO9V30 as an example, it is suitable for FC, SC, and ST connectors, and it can also be used with LC connectors when used with the LC Female to FC Male hybrid adapter. Compact structure, small size, lightweight, red laser output.

What is a Visual Fault Locator

How do Visual Fault Locators Work?

The basic principle of a VFL is simple. A red laser light is emitted into the optical fiber, and any light leaks from the fiber indicate faults or breaks in the fiber. The red laser light is bright enough to be seen even in well-lit environments, making it easy to locate faults. When the VFL detects a fault, it emits a signal that alerts the user to the location of the fault. This signal can be a beep, a flashing light, or a combination of both.

Important Parameters of VFL: Help You Choose the Ideal VFL

1. Fiber Distance

The fiber optic distance specification tells you the longest fiber length you can still see leaking light without trouble. This specification is determined by the laser power. Industry-standard specifications are 10km for multimode fiber and 5km for single-mode fiber.

2. Output Power

Output power is an important indicator for visual fault identifiers. What is the most suitable power? The key is: the higher the output power, the longer the supported fiber distance. This specification is typically 0.5mW coupled into single-mode fiber and 2mW coupled into free space (meaning no fiber coupling, just light output from the tester connector adapter).

3. Fiber Mode

All visual fault locators can be used with single mode and multimode fiber. Since multimode fiber has a 50um or 62.5um core, which is much larger than the 9um core of the single-mode fiber, more optical power can be coupled into multimode fiber. This means that multimode fiber has a longer fiber distance than single-mode fiber.

4. Modulation Frequency (CW and 2Hz)

The human eye is much more sensitive to flickering light than to steady light. Therefore, the visual fault locator provides two working modes: continuous wave and pulsed light. In most cases, fiber optic installation technicians prefer a pulsed light mode that turns the laser source on and off at 2Hz or 1Hz.

5. Battery

Typically, two AAA or two AA alkaline batteries are used.

6. Wavelength

Wavelength is not a critical specification. 635nm, 650nm, and 670nm all work fine.

7. Cost

Consider the cost of the VFL and compare it to your budget. High-end VFLs may have more features and better performance, but they may also be more expensive.

By considering these factors, you can choose the ideal VFL for your needs and ensure that you are able to locate faults quickly and accurately.

How to Use the Visual Fault Locator?

Step 1: Clean the fiber optic cable connector end face, if the end face finish is acceptable, you can attach the VFL to the fiber optic connector. Turn on VFL.

How to use the Visual Fault Locator 1

Step 2: Note the red light in the connector ferrule.

①If the light comes on directly from the other end, there is no leak.

How to use the Visual Fault Locator 2

②Once a fault occurs, light leaks out of the cladding, making it impossible for light to travel through the other end of the cable.

How to use the Visual Fault Locator 3

Considerations for Using the Visual Fault Locator

  1. Never look directly into the path of the laser beam or into the end of the fiber optic. The laser light emitted by the VFL can be harmful to the eyes, so it’s important to wear protective glasses.
  2. Keep away from water, dust, static electricity, and extreme temperature environment.
  3. When the VFL is not in use, cover the output end of the VFL with a dust cap.
  4. Not recommended for dark-colored or armored cables.
  5. Please take out the battery when storing it for a long time.
  6. Never fire the visual fault locator into powered equipment or microscopes.
  7. Make sure that the end opposite the visual fault locator is not connected to any electrical or active equipment during termination.

The Importance of Visual Fault Locators in Optical Fiber Communications:

VFLs are essential tools in optical fiber communications. They help to identify faults in optical fibers quickly and accurately, reducing the time and cost associated with repairing faults. This means that optical fiber communications can be maintained at optimal performance levels, ensuring that data is transmitted reliably and quickly.

In addition, VFLs are portable and easy to use, making them ideal for use in the field. This means that faults can be detected and repaired even in remote locations, reducing the downtime associated with faults in optical fibers.


In conclusion, VFLs are essential tools for identifying faults in optical fibers. By emitting a red laser light into the fiber and observing any light leaks, VFL makes it easier to locate faults and repair them quickly. With this understanding of VFLs and how to use them, you can ensure that your optical fiber communications are functioning at their best.

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