What will happen to the number of modes as the wavelength decreases?

What will happen to the number of modes as the wavelength decreases?

• A. More modes
• B. Fewer modes
• C. No change
• D. Variable

Answer: (A)

Lowering the wavelength increases the normalized frequency of the fiber, which lets more distinct transverse field patterns be guided. In a step‑index fiber, modes exist only if the field can satisfy the boundary conditions with total internal reflection, and the count of these modes is governed by the V-number: V = (2π a / λ) NA, where a is the core radius and NA is the numerical aperture. With a fixed core and NA, making the wavelength smaller raises V, so more modes meet the guiding condition. For larger V, the number of guided modes grows roughly with V^2, so shorter wavelengths substantially increase how many modes the fiber can support. Conversely, longer wavelengths lower V and reduce the number of supported modes. Therefore, as the wavelength decreases, the number of modes increases.

Lowering the wavelength increases the normalized frequency of the fiber, which lets more distinct transverse field patterns be guided. In a step‑index fiber, modes exist only if the field can satisfy the boundary conditions with total internal reflection, and the count of these modes is governed by the V-number: V = (2π a / λ) NA, where a is the core radius and NA is the numerical aperture. With a fixed core and NA, making the wavelength smaller raises V, so more modes meet the guiding condition. For larger V, the number of guided modes grows roughly with V^2, so shorter wavelengths substantially increase how many modes the fiber can support. Conversely, longer wavelengths lower V and reduce the number of supported modes. Therefore, as the wavelength decreases, the number of modes increases.