Coherent DSP
The digital signal processor that makes coherent optical detection practical: it compensates chromatic dispersion, separates the two polarizations, tracks the laser frequency and phase, recovers timing, and decodes the forward error correction, all in the electronic domain after the optical front end.
Coherent optical detection recovers the full field of the received light, amplitude and phase on both polarizations, by beating it against a local-oscillator laser in an optical hybrid and digitizing the four resulting photocurrents. That front end produces a torrent of raw samples that are, on their own, unusable: they carry the accumulated chromatic dispersion of the fiber, a scrambled mixture of the two polarizations, an unknown frequency offset between the two lasers, and the phase noise of both. The coherent DSP is the electronics that turns those samples back into data.
Its work divides into a fixed sequence of operations. Chromatic dispersion compensation comes first, and it is the reason coherent links abandoned dispersion-compensating fiber: dispersion is a known quadratic phase in frequency, so a static digital filter inverts it exactly, undoing hundreds of kilometers of accumulated chromatic dispersion in the receiver. Adaptive equalization follows, a set of filters that continuously separate the two polarizations and track polarization-mode dispersion and polarization rotation, both of which drift on millisecond timescales. Carrier recovery then removes the frequency offset and the phase noise set by the laser linewidths, which is what lets the receiver read the phase of a QAM constellation at all. Timing recovery aligns the sampling clock, and the forward error correction decoder cleans up what remains.
Two facts about the coherent DSP explain the shape of the market around it. It is the dominant power draw of a coherent transceiver, historically large enough that coherent detection was confined to long-haul systems where its reach justified the cost; the achievement of the ZR-class pluggables was shrinking that DSP enough to fit coherent into a QSFP-DD module. And its size scales with what it must correct: a short campus link accumulates little dispersion, tolerates a shorter equalizer, and can relax carrier recovery, which is the basis of the emerging reduced-DSP or coherent-lite designs that trade reach for power on the way down from 400ZR toward the 100ZR and future 1600ZR grades. The DSP is where coherent's cost lives, so the DSP is where every coherent product decision is really made.
References: Savory, "Digital coherent optical receivers: algorithms and subsystems," IEEE J. Sel. Top. Quantum Electron. 16, 1164 (2010); Kikuchi, "Fundamentals of coherent optical fiber communications," J. Lightwave Technol. 34, 157 (2016). The full function-by-function account is in What the coherent DSP does.