Chromatic Dispersion (CD)

Chromatic dispersion (CD) is a deterministic phenomenon that refers to the broadening effect experienced by optical pulses emitted by a transmitter light source as they travel at different velocities in a propagating dispersive medium. Severe pulse broadening results in inter-symbol interference, which changes a 0 into a 1.

Light Sources

All light sources have a finite non-zero wavelength or frequency spectrum. The wavelengths or frequencies in the spectrum do not propagate at the same velocity from input to output in a transmission link (e.g., optical fiber, optical components, amplifiers) because each wavelength or frequency has a different index of refraction (IOR).

The IOR is what makes source wavelengths or frequencies travel at different velocities (group velocity) and have different arrival times (phase and group delay). Chromatic dispersion is the variation of the IOR as a function of frequency or wavelength. This variation is nonlinear, and consequently, makes the spreading and dispersive effect more severe at some wavelengths than others, thus limiting the transmission bandwidth.

Chirping

The chirp effect occurs when the source wavelength spectrum varies during the pulse, especially at the rise and decay times. The wavelengths at the end of the pulse are delayed with respect to those at the beginning, which causes broadening. However, when the wavelengths at the beginning of the pulse are delayed with respect to those at the end, this causes compression.

Compensation

Chromatic dispersion can be compensated passively and actively.

Passive Techniques

• CD-compensating fiber (DCF)
• Fiber CD mapping
• Fiber Bragg grating (FBG)
• Higher-order-mode (HOM) fiber
• Virtually imaged phased array (VIPA)
• CD-supported transmission such as CD mapping
• Holey fiber

Active Techniques

• Nonlinear effect (NLE) and nonlinear-conditioned transmissions such as soliton transmission
• Pre-conditioning such as pre-chirping
• Electronic signal processing
• Phase correction
• Decision-level feedback
• Forward error correction (FEC)
• Spectral inversion (i.e., inverting of spectral components typically at mid-system range)

Testing Methods

There are four internationally-standardized methods for testing chromatic dispersion:

1. Phase-shift method (PSM)
2. Differential phase-shift method (DPSM)
3. Spectral group delay in the time domain method (OTDR method)
4. Interferometric method (INTY)

Measurement

Since chromatic dispersion is deterministic and can be fitted to a mathematical derivative model, it can be calculated two basic ways. For a small number of fibers, the derivative can be extrapolated from the group delay versus wavelength function. For a mix of fibers, the derivative must be extrapolated locally, using a limited number of delay measurements over a short wavelength span.

Finally, chromatic dispersion can be measured both in the lab and in the field.