How to use a light pen to illuminate fiber optic cables

A light pen, or Visual Fault Locator (VFL), emits a visible red light through fiber optic cables to detect breaks, bends, and connectivity issues.

What a Light Pen Does

A light pen, also called a VFL, is a handheld device that emits a 650nm red laser into a fiber optic cable. The light travels through the fiber, illuminating any faults such as breaks, sharp bends, or poor splices. It can also be used for end-to-end fiber identification and to locate faults in OTDR dead zones, making it an essential tool for fiber maintenance and troubleshooting .

Steps to Use a Light Pen

1. Prepare the Fiber: Remove any dust caps from the fiber connectors and ensure the fiber is clean.
2. Connect the Pen: Insert the fiber optic connector into the VFL's optical interface. Use the appropriate adapter if your connector type differs from the pen's standard interface .
3. Power On: Press and hold the power button to activate the continuous red light. A short press may switch the pen to a flashing mode, which helps quickly locate fault points .
4. Observe the Fiber: Look for red light emerging from the other end of the fiber or leaking through the jacket at fault points. Continuous light indicates connectivity, while flashing light highlights breaks or bends .
5. Identify Faults: Any visible red light escaping from the fiber indicates a microbend, break, or splice issue. Mark these points for repair or further testing.
6. Turn Off and Store: After use, turn off the pen and remove the battery if it will not be used for an extended period to prevent damage .

Tips and Safety

• Always use the pen in a controlled environment to avoid accidental exposure to the laser.
• Do not look directly into the laser beam.
• Ensure the fiber is properly stripped and cleaned to get accurate results.
• The pen can detect faults in both single-mode and multi-mode fibers and is effective for distances ranging from a few meters up to several kilometers depending on the model . Using a light pen is a quick, cost-effective, and reliable method to troubleshoot fiber optic cables, verify connectivity, and locate faults without requiring equipment at the far end of the fiber .