Which wavelengths are associated with LED-based testing of multimode graded-index fiber?

• A. 650 nm and 1300 nm
• B. 850 nm and 1300 nm
• C. 980 nm and 1400 nm
• D. 1310 nm and 1550 nm

Answer: (B)

LED-based testing of multimode graded-index fiber uses two practical wavelengths: 850 nm and 1300 nm. The 850 nm region matches common LED emitters and couples well into the many modes of a graded-index MMF, making it a convenient and representative choice for measuring modal distribution and bandwidth in typical test setups. The 1300 nm region sits in the silica low-loss window around 1.3 μm, providing a relevant test point for attenuation and dispersion in the telecom-relevant window, with LED sources available for this range as well. Other options rely on wavelengths tied to other applications—650 nm is visible and not representative for MMF testing; 980 nm is mainly a pump or laser line; 1400 nm is less common for LED-based MMF tests; 1310 nm and 1550 nm are standard telecom windows usually with laser diodes for single-mode fibers rather than typical LED-based MMF testing.