How to use a visual fault locator
A visual fault locator shines visible red laser light into a fiber. Wherever the fiber is broken, cracked or bent too tightly, the light escapes and glows. It is the fastest way to find a fault you can get your hands on.
What a VFL is
A VFL is a pen or small box with a red laser (around 650 nm) behind a universal connector adapter. Plug a fiber in and the light travels down it. Light that stays in the glass is invisible from outside; light that leaks out at a fault shows up as a red glow you can see through buffer, tight-buffered jackets and splice sleeves.
What it finds
- Breaks and cracks in jumpers, pigtails and fibers in a tray: a bright red spot.
- Macrobends: a fiber wrapped too tightly in a tray or pinched by a lid glows at the bend.
- Bad splices and connectors: a poor mechanical splice or a cracked ferrule glows.
- Continuity and fiber identification: shine it in at one end and look for the light at the other to confirm which fiber is which, or that a fiber is unbroken.
Worked example: the OTDR and the VFL together
An OTDR shows a 0.9 dB event at 1,240 m, which matches a splice case. At the case, connect the VFL to the fiber through a pigtail or the panel and open the tray. The fiber glowing red at the edge of the tray is the one pinched under the lid. The OTDR told you where; the VFL tells you exactly which fiber and which spot.
How to use it
- Clean the fiber's connector, and the VFL's adapter. Dirt scatters the light and makes the whole connector glow.
- Connect the VFL directly, or through an adapter: the common universal adapter fits 2.5 mm ferrules (SC, ST, FC); LC needs a 1.25 mm adapter.
- Switch it on. Use pulse (blink) mode where it has one, typically 1 to 3 flashes a second: a blinking glow is easier to spot in daylight.
- Look along the fiber, in the tray, at each connector and at the far end. Dim the work light if you can.
Limits
- It cannot see through black or heavily jacketed outside plant cable. It works on bare fiber, buffer tubes opened in a tray, jumpers and light-colored jackets.
- Its reach is limited. Makers claim about 3 to 5 km for low-power pens and up to around 10 km for high-power units, but the glow you can actually see fades well before that, because red light is lost much faster in fiber than the infrared light networks use.
- It does not measure loss or distance. For a fault beyond arm's reach, use an OTDR to find the distance first.
Laser safety. Never look into the VFL's output or the end of a fiber it is connected to. Low-power pens (around 1 mW) are usually Class 2; the 10 to 30 mW units sold for longer reach are Class 3B and need more care. Check the class on the label and treat every lit fiber as live.