3.2 Optical Signal Dispersion Effects
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Fig. 3.7 Spectral emission pattern of a representative Ga 1–x Al x As LED with a peak emission at
850 nm and a half-power width of 36 nm
Eq. (2.15) the velocity of light depends on the value of the refractive index.
Thus, pulse spreading occurs even when different wavelengths simultaneously
follow the same path, because each wavelength within a pulse travels at a slightly
different velocity.
2. Waveguide dispersion causes pulse spreading because only part of the optical
power propagation along a fiber is confined to the core. Within a single propagating mode, the cross-sectional distribution of light in the optical fiber varies
for different wave-lengths. Shorter wavelengths are more completely confined
to the fiber core, whereas a larger portion of the optical power at longer wavelengths propagates in the cladding, as shown in Fig. 3.8. The refractive index is
lower in the cladding than in the core, so the fraction of light power propagating
Fig. 3.8 Shorter wavelengths are confined closer to the center of a fiber core than longer wavelengths