Which source type is typically used to test multimode graded-index fiber?

• A. Light Emitting Diode
• B. Laser
• C. Vertical-Cavity Surface-Emitting Laser
• D. Superluminescent Diode

Answer: (A)

When testing multimode graded-index fiber, you want a light source that excites many propagation modes and reflects how the fiber is typically used. A light emitting diode provides a broad spectrum and a wide emission angle, so light is launched into the fiber across a large number of modes. This creates a realistic mix of modal dispersion and losses, which is exactly what multimode fiber tests aim to measure. In contrast, lasers and VCSELs produce narrow-band, highly coherent, directional light that tends to couple into a small set of modes, which can bias the test toward lower modal content. A superluminescent diode sits between LED and laser in brightness and coherence, but it’s not the standard choice for representative multimode fiber testing. So the LED is the typical source for testing multimode graded-index fiber.