What kind of dispersion exists in multimode fiber

Multimode fibers primarily experience modal (intermodal) dispersion, along with chromatic and polarization mode dispersion, which together affect signal quality and bandwidth.

Modal (Intermodal) Dispersion

Modal dispersion occurs because light in a multimode fiber travels along multiple paths or modes, each with a different length and propagation time. Rays entering the fiber at shallow angles travel more directly, while rays at steeper angles reflect multiple times off the core-cladding boundary, arriving later at the receiver. This difference in arrival times spreads the optical pulse, a phenomenon also called pulse broadening or differential mode delay (DMD), and is the dominant factor limiting the bandwidth of multimode fibers . Step-index fibers exhibit higher modal dispersion, whereas graded-index fibers reduce it by gradually varying the refractive index of the core to equalize travel times for different modes .

Chromatic (Intramodal) Dispersion

Chromatic dispersion arises because different wavelengths of light travel at slightly different speeds in the fiber material. This occurs in all fiber types, including multimode fibers, and is influenced by both material properties and waveguide structure. Material dispersion depends on the refractive index variation with wavelength, while waveguide dispersion depends on the fiber geometry relative to the wavelength . In multimode fibers, chromatic dispersion is generally less significant than modal dispersion but still contributes to pulse spreading, especially for broadband sources.

Polarization Mode Dispersion

Although less pronounced in multimode fibers than in single-mode fibers, polarization mode dispersion (PMD) can occur due to asymmetries in the fiber that cause different polarization states to propagate at different speeds. PMD adds another layer of temporal spreading, particularly in high-speed or long-distance multimode links .

Summary

In multimode fibers, modal dispersion is the primary limiting factor, causing significant pulse broadening due to multiple propagation paths. Chromatic dispersion adds wavelength-dependent spreading, and polarization mode dispersion can further affect signal integrity. Using graded-index cores and carefully managing source spectral width can mitigate these effects, improving bandwidth and performance for high-speed optical communication systems .