Glossary entry

Direct detection

An optical receiver architecture in which the photodetector responds to the optical power (intensity) of the received signal. The simplest and most widely deployed detection scheme in optical communication.

In direct detection, an incoming optical signal illuminates a photodetector that produces electrical current proportional to optical power:

i_\text{photo}(t) \;=\; \mathcal{R} \, |E_\text{signal}(t)|^2 \;=\; \mathcal{R} \, P_\text{signal}(t),

where $\mathcal{R}$ is the photodetector responsivity. The detector responds only to intensity — phase information of the optical signal is lost.

Modulation formats compatible with direct detection.

Format	Encoding	Receiver
NRZ on-off keying (OOK)	Amplitude (presence/absence)	Single photodiode + decision threshold
Pulse amplitude modulation (PAM-N)	Multilevel amplitude	Single photodiode + multilevel decoder
Subcarrier-multiplexed	RF subcarriers on optical carrier	Photodiode + RF demodulation
Duobinary	Optical filtering produces correlated waveform	Single photodiode

Noise sources in direct detection:

Noise type	Source	Scaling
Shot noise	Quantum nature of photodetection	$\propto \sqrt{P_\text{signal}}$
Thermal (Johnson) noise	Receiver electronics	Constant, independent of signal
Relative intensity noise (RIN)	Laser source intensity fluctuations	$\propto P_\text{signal}$
Dark current shot noise	Detector reverse-bias dark current	Constant

Sensitivity. At the quantum limit (shot-noise-limited reception), direct detection requires $\sim$ 10 photons per bit for BER $= 10^{-9}$ in NRZ-OOK. Practical receivers, limited by thermal noise, typically require 100 – 10,000 photons per bit.

Strengths.

Simple — single photodiode, no local oscillator
No requirement for phase or polarization stability
Low-cost — workhorse of datacenter and short-reach telecom
Suitable for $\le$ 100 Gb/s short-reach links

Limitations.

No phase modulation formats possible — limits spectral efficiency
Cannot recover from chromatic dispersion or PMD in electrical domain (must be optically pre-compensated)
3 dB sensitivity disadvantage vs coherent detection in shot-noise limit (since direct detection only sees one polarization unless polarization diverse)
Difficult to scale to $> 100$ Gb/s per wavelength

Direct detection dominates the 100 Gb/s and below datacenter market. Coherent detection (with full optical field recovery, including phase and polarization) has displaced direct detection in $\ge$ 100 Gb/s long-haul transmission and is increasingly common in metro and access networks above 100 Gb/s per wavelength.

The contrast between direct detection (intensity only) and coherent detection (full optical field recovery via interference with a local oscillator) is the major architectural distinction in optical communication receivers.