What is the light source of a fiber optic red light pen

A fiber optic red light pen uses a 650 nm semiconductor laser as its light source.Fiber optic red light pens, also known as visual fault locator (VFL) pens, emit a stable red laser light at a wavelength of approximately 650 nm, which is easily visible to the human eye . The light is generated by a semiconductor laser diode driven by a constant current source, ensuring consistent output for detecting fiber faults . This laser light is coupled into the optical fiber through a standard optical interface, allowing technicians to trace fiber strands, identify breaks, bends, or poor connections, and verify connectivity . The power output of these pens typically ranges from a few hundred microwatts to around 10 mW, sufficient to illuminate fibers over distances up to 10 km depending on fiber type and mode . The red light is highly visible because any scattering at damaged points or breaks in the fiber allows the fault to be easily located externally . Some pens also offer continuous and flashing modes, which help in quickly identifying faults or verifying fiber continuity . In summary, the 650 nm semiconductor laser is the core light source of a fiber optic red light pen, providing a reliable, visible, and focused beam for fiber inspection and maintenance tasks.