Multimode graded index glass fiber optic cables are tested with which sources at which wavelengths?

• A. Light Emitting Diode, 650 nm, 1300 nm
• B. Light Emitting Diode, 850 nm, 1300 nm
• C. Laser, 980 nm, 1400 nm
• D. Laser, 1310 nm, 1550 nm

Answer: (B)

The main idea is to test multimode graded-index fiber with light sources that mimic how these fibers are typically used in practice. LED sources are preferred because they emit broad, incoherent light that excites many modes in the fiber, which is essential for accurately assessing modal dispersion and bandwidth in multimode systems. The wavelengths chosen—850 nm and 1300 nm—are standard near the infrared windows for multimode fiber: 850 nm is the traditional short-range, LED-friendly wavelength, and 1300 nm is a common window for longer reach within multimode networks, with suitable LED availability. Using these LED sources at 850 nm and 1300 nm provides a realistic indication of how the fiber will perform in typical deployments. Lasers at other wavelengths (like 980 nm, 1310 nm, or 1550 nm) tend to excite fewer modes or align with single-mode, long-haul applications, which doesn’t reflect common MMF use.