What are the three ways to reduce modal dispersion in a multimode system?

What are the three ways to reduce modal dispersion in a multimode system?

• A. Graded index fiber, smaller core, longer wavelength
• B. Graded index fiber, larger core, shorter wavelength
• C. Step-index fiber, smaller core, longer wavelength
• D. Single-mode fiber, smaller core, longer wavelength

Answer: (A)

Modal dispersion in a multimode fiber comes from different modes following different paths and speeds, causing a pulse to spread as it travels. Using a graded-index fiber helps because its refractive index decreases gradually from the center outward, which makes higher-order modes travel in faster regions and lower-order modes in slower regions. This compensation keeps arrival times closer together, reducing the spread. A smaller core limits the number of propagating modes, so there are fewer distinct travel times and less time broadening. Operating at a longer wavelength lowers the number of supported modes for a given core and numerical aperture (the V-number decreases with wavelength), further reducing modal dispersion. Together, these approaches minimize the time spread of a signal in multimode systems.

Modal dispersion in a multimode fiber comes from different modes following different paths and speeds, causing a pulse to spread as it travels. Using a graded-index fiber helps because its refractive index decreases gradually from the center outward, which makes higher-order modes travel in faster regions and lower-order modes in slower regions. This compensation keeps arrival times closer together, reducing the spread. A smaller core limits the number of propagating modes, so there are fewer distinct travel times and less time broadening. Operating at a longer wavelength lowers the number of supported modes for a given core and numerical aperture (the V-number decreases with wavelength), further reducing modal dispersion. Together, these approaches minimize the time spread of a signal in multimode systems.