Glossary entry

Coherent detection

An optical receiver architecture that recovers both amplitude and phase of the optical signal by interfering it with a local oscillator laser. Enables higher-order modulation formats and digital impairment compensation.

In coherent detection, the incoming optical signal is mixed with a stable local oscillator (LO) laser before photodetection. The interference between signal and LO produces beat terms in the photodetected current at the optical-frequency difference:

i(t) \;\propto\; |E_S + E_{LO}|^2 \;=\; |E_S|^2 + |E_{LO}|^2 + 2 \text{Re}\!\left[E_S \, E_{LO}^* \, e^{i(\omega_S - \omega_{LO}) t}\right].

The middle term ( $|E_{LO}|^2$ ) is a large DC offset; the first ( $|E_S|^2$ ) is the direct-detection signal; the third is the coherent beat term that carries both amplitude and phase information of $E_S$ .

Variants.

Configuration	Description
Homodyne	LO frequency matched to signal: beat is at baseband (DC) — direct recovery of in-phase and quadrature components
Heterodyne	LO offset from signal: beat is at intermediate frequency (IF), then electronically demodulated
Intradyne	LO close to but not locked to signal — DSP tracks frequency offset (standard in modern telecom)

Modern coherent telecom receiver architecture:

Polarization splitter divides signal into two orthogonal polarizations
90° optical hybrid mixes each polarization with LO and produces four optical outputs (I+, I-, Q+, Q-)
Balanced photodetectors convert each I/Q pair into a current difference, suppressing common-mode LO noise
ADCs and DSP sample the four electrical signals and recover the complex optical field $E_x(t)$ and $E_y(t)$ via digital signal processing

DSP performs:

Carrier frequency recovery (compensates LO-signal frequency offset)
Carrier phase recovery (tracks signal phase)
Polarization recovery (recovers original signal polarizations from arbitrary received polarizations)
Chromatic dispersion compensation
PMD compensation
Symbol decoding (QPSK, 16-QAM, 64-QAM, etc.)
Forward error correction

Sensitivity advantage. Coherent detection achieves shot-noise-limited reception even at modest LO power because the LO acts as a gain stage — small signals are amplified by the LO before reaching the thermal noise floor of the electronics. The quantum-limited sensitivity is 0 dB worse than for direct detection (when both account for polarization-diversity) but in practice coherent achieves 3–10 dB better sensitivity due to other advantages.

Practical performance for telecom coherent receivers:

Parameter	Typical value
Symbol rate	30 – 130 GBaud
Modulation format	DP-QPSK, DP-16QAM, DP-64QAM
Net data rate	100 – 1200 Gb/s per wavelength
Required OSNR (BER threshold)	13 – 30 dB depending on format
LO power	$\sim$ +10 dBm
Reach with no per-span optical compensation	up to 5000 km (with DSP CD compensation)

Coherent detection has displaced direct detection in long-haul and increasingly in metro and DCI applications above 100 Gb/s per wavelength. The cost has dropped from $\sim$ $50,000 per port in early deployments to $<$ $1,000 in modern pluggable coherent transceivers.