Dispersion in multimode fiber is mainly due to…

The primary cause of dispersion in multimode fibers is modal dispersion, with additional contributions from material and waveguide dispersion.Modal DispersionModal dispersion occurs because multimode fibers support multiple light paths (modes) that travel different distances through the fiber. Light traveling along straight, low-order modes reaches the receiver faster than light following zigzag, high-order modes. This difference in arrival times causes pulse broadening, limiting the fiber's bandwidth and transmission distance. Step-index multimode fibers typically have lower bandwidth (~33 MHz·km), while graded-index fibers reduce modal dispersion by gradually varying the refractive index in the core (~500 MHz·km) to equalize mode travel times .Material DispersionMaterial dispersion arises from the wavelength-dependent speed of light in the fiber material. Shorter wavelengths (blue light) travel slower than longer wavelengths (red light) due to the refractive index variation of silica. This effect is more pronounced in LED-based systems with broad spectral output and can lead to pulse spreading over distance. Material dispersion is minimized by using narrow-spectrum lasers or operating at wavelengths around 1550 nm, where dispersion is lower .Waveguide DispersionWaveguide dispersion results from the distribution of light between the fiber core and cladding. Different propagation paths within the waveguide cause slight variations in speed. While more significant in single-mode fibers, waveguide dispersion also contributes to chromatic dispersion in multimode fibers. Engineers can adjust core and cladding dimensions to optimize performance and reduce pulse broadening .Chromatic DispersionChromatic dispersion is the combined effect of material and waveguide dispersion. In multimode fibers, it is generally less dominant than modal dispersion but still contributes to pulse spreading, especially when multiple wavelengths are transmitted simultaneously .SummaryIn multimode fibers, modal dispersion is the dominant factor, causing the largest pulse broadening due to multiple light paths. Material and waveguide dispersion add smaller contributions, particularly in systems using broad-spectrum light sources. Techniques such as graded-index cores, narrow-spectrum lasers, and careful fiber design help mitigate these effects and improve bandwidth and signal integrity .