Why is the bandwidth of multimode fiber small

The bandwidth of multimode fiber is limited primarily due to modal dispersion, where multiple light paths travel at different speeds, causing pulse broadening and reducing data transmission capacity.

Core Reason: Modal Dispersion

Multimode fiber (MMF) has a large core diameter, typically 50 or 62.5 micrometers, which allows multiple light rays, or "modes," to propagate simultaneously . Each mode follows a slightly different path: some travel straight down the center, while others reflect off the core-cladding boundary multiple times. Because these paths differ in length, light pulses arrive at the receiver at slightly different times, a phenomenon called pulse broadening. This time spread, known as Differential Mode Delay (DMD), limits the maximum data rate and effective bandwidth of the fiber .

Additional Factors

1. Chromatic Dispersion: Light sources like LEDs emit a range of wavelengths, and each wavelength travels at a slightly different speed, further spreading the pulse and reducing bandwidth .
2. Step-Index vs. Graded-Index Profiles: Step-index fibers have abrupt refractive index changes, causing more severe modal dispersion. Graded-index fibers gradually change the refractive index, which reduces dispersion but does not eliminate it entirely .
3. Distance Dependence: Bandwidth decreases as the transmission distance increases because the time difference between the fastest and slowest modes accumulates over longer lengths .

Modern Improvements

Laser-optimized multimode fibers (OM3, OM4) use VCSELs and tighter manufacturing tolerances to excite fewer modes and reduce modal dispersion. This increases the Effective Modal Bandwidth (EMB), allowing higher data rates over longer distances (e.g., OM4 supports 10GbE up to 400 meters), . Despite these improvements, MMF bandwidth remains fundamentally lower than single-mode fiber, which supports only one propagation mode and therefore avoids modal dispersion entirely .

Summary

In essence, the small bandwidth of multimode fiber is a direct consequence of its large core allowing multiple light modes, each traveling at different speeds. This causes pulse spreading, limits data rates, and makes MMF suitable mainly for short-distance applications like data centers and local area networks, whereas single-mode fiber is preferred for long-distance, high-bandwidth communication .