Can you see red light from multimode fiber

Yes, you can see red light in a multimode fiber if a visible light source, such as a red LED or red laser, is used.

Multimode fiber (MMF) is designed to carry multiple light modes through a relatively large core, typically 50 or 62.5 microns, which allows light from low-cost sources like LEDs or VCSELs to propagate efficiently . While standard communication systems use infrared light at 850 nm or 1300 nm, which is invisible to the human eye, visible red light at around 635–650 nm can be injected into the fiber for testing purposes . This is commonly done using fiber tracers or visual fault locators (VFLs), which are low-power devices that emit red light into the fiber. The light can be seen at the fiber ends or through the fiber jacket if it is not opaque, allowing technicians to trace fiber paths, check continuity, and locate breaks or splices . Red HeNe lasers or diode lasers are often used for longer cables or higher-loss points, while simple LEDs suffice for short links . It is important to note that the red light used for visual inspection is separate from the infrared signals used for data transmission. Standard multimode communication LEDs or lasers operate at wavelengths outside the visible spectrum, so you would not see the actual data-carrying light without specialized equipment . In summary, injecting a visible red light source into a multimode fiber allows you to see the light for testing and troubleshooting, but normal operational signals in multimode fiber are typically invisible infrared light.