Fiber optic cables are tested using visible light sources for continuity checks and infrared optical light sources for precise signal loss measurements.

Visible Light Sources

For quick fault detection and continuity testing, visible light sources such as visual fault locators (VFLs) or fiber optic flashlights are commonly used. These devices emit a red laser light that travels through the fiber, allowing technicians to visually identify breaks, bends, or faults along the cable. The glowing red light can often be seen through the cable jacket, making it ideal for short cables or near connectors where other instruments like OTDRs may not detect faults. Safety precautions are important: never look directly into the fiber end, as the laser can damage the eyes .

Infrared Optical Light Sources

For more precise testing, especially to measure insertion loss or end-to-end signal attenuation, optical light sources operating at standard wavelengths (850 nm and 1300 nm for multimode fibers) are used. These devices are part of an Optical Loss Test Set (OLTS) or Light Source and Power Meter (LSPM) setup. They provide a stable infrared signal that is invisible to the naked eye and can be connected to various fiber connectors (ST, SC, FC, or LC with adapters). The light source can operate in continuous wave or modulated modes to simulate real signal transmission, ensuring accurate measurement of fiber performance .

Summary

• Visible light sources (VFLs): Quick continuity checks, fault location, short-distance testing, red light visible to the eye.
• Infrared optical light sources (850/1300 nm): Accurate measurement of signal loss, insertion loss testing, used with power meters or OLTS, suitable for field and laboratory testing. Using the appropriate light source depends on the testing goal: VFLs for visual fault detection and infrared light sources for quantitative performance testing.