Pulse Spreading: The Dispersion Concept

In a perfect vacuum, light travels as a precise pulse. In an optical fiber, different components of that light travel at slightly different speeds. This causes the pulse to "smear" out as it travels downstream. If it smears too much, the receiver cannot distinguish between a binary '1' and '0'.

1. Chromatic Dispersion (CD)

Chromatic Dispersion occurs because the refractive index of glass is wavelength-dependent. Even a "single-color" laser has a finite spectral width (it emits a narrow range of wavelengths).

• Material Dispersion: Different colors (wavelengths) travel at different speeds through the silica.
• Waveguide Dispersion: Some light travels in the core, while some travels in the cladding. The ratio depends on the wavelength.

2. Polarization Mode Dispersion (PMD)

While CD is predictable and constant, PMD is stochastic and dynamic. It arises from the fact that an optical fiber is not a perfect cylinder. Physical stress, bending, and manufacturing imperfections make the fiber birefringent.

Light travels in two orthogonal polarization modes (Vertical and Horizontal). Birefringence causes one mode to travel faster than the other, resulting in Differential Group Delay (DGD).

Mitigation & Compensation

To overcome these limits, engineers use several strategies:

1. Dispersion Compensating Fiber (DCF): A spool of specialized fiber with a "negative" dispersion value that cancels out the positive dispersion of the transmission fiber.
2. Coherent Detection (Digital): Modern 100G+ transceivers use Digital Signal Processing (DSP) to mathematically "un-smear" the signal at the receiver end.
3. Zero-Dispersion Fiber (G.653/G.655): Fiber types manufactured to have minimum dispersion at 1550nm.

Summary

Chromatic dispersion is the primary limit for distance, while PMD is the primary limit for bit-rates. As we push toward Terabit networking, the ability of glass to maintain pulse integrity is just as important as the quality of the laser.