Which statement describes the relationship between wavelength and mode count in a fiber?

Which statement describes the relationship between wavelength and mode count in a fiber?

• A. More modes as wavelength decreases
• B. Fewer modes as wavelength decreases
• C. No change with wavelength
• D. Mode count depends only on fiber type

Answer: (A)

In a fiber, how many distinct propagating patterns, or modes, can fit in the core depends on the normalized frequency V. This V-number grows as the wavelength gets shorter (while the core size and numerical aperture stay the same). When V increases, more transverse field patterns can exist inside the core, so the fiber supports more modes. That’s why shorter wavelengths yield a higher mode count. A handy way to think about it is that the mode count increases roughly with the square of V, so shortening the wavelength pushes V up and opens room for additional modes. If the wavelength becomes long enough, V drops and the fiber can support fewer modes, potentially down to a single mode in the single-mode regime. So the statement that describes this relationship is that there are more modes as the wavelength decreases. The other ideas don’t fit because changing wavelength does affect mode count, and the count depends on the core size and NA in addition to the fiber type, not exclusively on the fiber type, and it does not stay unchanged or decrease when the wavelength gets shorter.

In a fiber, how many distinct propagating patterns, or modes, can fit in the core depends on the normalized frequency V. This V-number grows as the wavelength gets shorter (while the core size and numerical aperture stay the same). When V increases, more transverse field patterns can exist inside the core, so the fiber supports more modes. That’s why shorter wavelengths yield a higher mode count.

A handy way to think about it is that the mode count increases roughly with the square of V, so shortening the wavelength pushes V up and opens room for additional modes. If the wavelength becomes long enough, V drops and the fiber can support fewer modes, potentially down to a single mode in the single-mode regime.

So the statement that describes this relationship is that there are more modes as the wavelength decreases. The other ideas don’t fit because changing wavelength does affect mode count, and the count depends on the core size and NA in addition to the fiber type, not exclusively on the fiber type, and it does not stay unchanged or decrease when the wavelength gets shorter.