Dispersion compensation
Undoing accumulated chromatic dispersion so pulses arrive intact. Historically done with negative-dispersion fiber or chirped gratings; now handled almost entirely in DSP at coherent receivers.
Standard single-mode fiber at 1550 nm disperses at ps/(nm·km): a 10G NRZ signal's ~0.1 nm spectrum spreads ~1.7 ps per kilometer, and after ~60–80 km the spreading rivals the bit period. Every long link needs an answer; three eras of answers coexist.
Dispersion-compensating fiber (DCF). Fiber designed with strongly negative dispersion (−80 to −150 ps/nm/km) spliced in at amplifier sites, sized to cancel the preceding span. Whole-band and format-transparent, but DCF adds loss (demanding amplifier gain, hence noise), has a small effective area that invites nonlinearity, occupies rack space as spools, and only matches the fiber's dispersion slope approximately, leaving residuals at band edges. Dispersion maps (the deliberate schedule of over- and under-compensation along a route to manage nonlinear interaction) were a defining design discipline of the 10G/40G era, and DCF still sits in legacy plant everywhere.
Chirped fiber Bragg gratings. A grating whose period varies along its length reflects each wavelength from a different depth: wavelength-dependent group delay in a palm-sized package with a circulator. Low loss, negligible nonlinearity, available per-channel or multichannel; watch group-delay ripple, the artifact spec.
Electronic compensation. Coherent detection preserves the optical field (amplitude and phase), and chromatic dispersion is a deterministic all-pass filter on that field. A frequency-domain equalizer in the receiver DSP therefore inverts any accumulated dispersion, tens of thousands of ps/nm, adaptively and for free. This single fact reorganized link engineering: modern coherent routes deploy no optical compensation, run dispersion-uncompensated (which even improves nonlinear tolerance), and DCF procurement ended for new coherent builds. Direct-detect links (10G client optics, access, some datacom) still live by the old rules, since without the field, DSP cannot invert the channel.
Residual practice: know a link's total ps/nm, check the transceiver's dispersion tolerance spec, compensate the difference (optically for direct detect, numerically for coherent).