Signal transmission over optical fibers relies on preserving the waveform from transmitter to receiver. But when light goes through optical fibers, the timing of the pulses can be smeared by fiber dispersion. Chromatic dispersion (CD) delays the signal depending on the wavelength of the light and polarization mode dispersion (PMD) makes the delay dependent on the polarization of the light. Because a signal is distributed over a range of wavelength and polarization, the pulses become dispersed. Although this group delay (GD) actually varies by small amounts, the short duration of the pulses, or bits, at today’s high data rates makes this significant. For example, the bit duration of a 10 Gb/s signal is only 100 ps.