What is the impact of chromatic dispersion in fiber optics?

Chromatic dispersion (CD) is the phenomenon where different wavelengths travel at slightly different speeds in fiber, causing pulse broadening over distance.

Why It Matters

• Broadens pulses, causing intersymbol interference
• Limits maximum data rate for given distance
• Becomes critical at high speeds (10G+) and long distances

Standard Fiber Dispersion Values

Dispersion Limit

Rule of thumb: CD limit for uncompensated links

• 10G at 1550nm: ~60-80 km on standard SMF
• 40G: ~10-20 km
• 100G: 4-6 km without coherent detection

Compensation Methods

1. Dispersion Compensation Fiber (DCF)

• Special fiber with opposite dispersion sign
• Deployed as spool at amplifier sites
• Introduces additional loss (~10 dB per 100km compensation)

2. Fiber Bragg Grating (FBG)

• Reflective grating creates delay for specific wavelengths
• Lower loss than DCF
• More expensive

3. Electronic Dispersion Compensation (EDC)

• DSP in coherent transceivers
• Compensates ANY dispersion amount
• Enabled 400G+ long-haul
• Now standard in coherent modules

PMD (Polarization Mode Dispersion)

• Different polarizations travel at different speeds
• Bigger issue in older fiber
• Statistical — varies with temperature
• Also compensated by coherent DSP

Fiber Choices for Long Distance

• Old links: G.652 SMF widely deployed
• Modern deployments: G.655 NZDSF for DWDM systems
• Ultra long haul: Coherent with G.652 works well

Practical Impact

• Older 10G modules limited to ~80km without compensation
• Modern coherent optics essentially eliminate CD as an issue
• Design considerations shifting to power and noise