Which option is NOT a method to reduce modal dispersion in a multimode fiber?

Which option is NOT a method to reduce modal dispersion in a multimode fiber?

• A. Using a graded-index fiber
• B. Using a smaller core
• C. Using a longer wavelength
• D. Using a higher refractive index cladding

Answer: (D)

Modal dispersion comes from different modes traveling at different speeds and along different paths in a multimode fiber, which broadens pulses. To reduce it, you design the fiber so those travel times are more similar. A graded-index core achieves this by smoothly varying the refractive index from the center outward. Higher-order modes spend more of their path in the outer, lower-index region, which speeds them up just enough to compensate for their longer paths, cutting the overall dispersion. A smaller core reduces the number of supported modes, so there are fewer distinct travel times to separate the pulse. With fewer modes, the timing spread decreases. A longer wavelength also helps because the number of guided modes decreases as wavelength increases (the V-number drops with wavelength), leading to less modal dispersion. Using a higher refractive index cladding would prevent proper guiding, since guiding relies on the core having a higher index than the surrounding cladding. If the cladding index were higher, light would not be confined to the core, so this approach does not reduce modal dispersion and is not a viable method.

Modal dispersion comes from different modes traveling at different speeds and along different paths in a multimode fiber, which broadens pulses. To reduce it, you design the fiber so those travel times are more similar.

A graded-index core achieves this by smoothly varying the refractive index from the center outward. Higher-order modes spend more of their path in the outer, lower-index region, which speeds them up just enough to compensate for their longer paths, cutting the overall dispersion.

A smaller core reduces the number of supported modes, so there are fewer distinct travel times to separate the pulse. With fewer modes, the timing spread decreases.

A longer wavelength also helps because the number of guided modes decreases as wavelength increases (the V-number drops with wavelength), leading to less modal dispersion.

Using a higher refractive index cladding would prevent proper guiding, since guiding relies on the core having a higher index than the surrounding cladding. If the cladding index were higher, light would not be confined to the core, so this approach does not reduce modal dispersion and is not a viable method.