Glossary entry

Eye diagram

The overlay of many consecutive bit periods of a received optical signal, displayed on a fast sampling oscilloscope. Visualizes signal quality, jitter, and noise simultaneously.

An eye diagram is constructed by overlaying many bit-period-long traces of the received electrical signal (after photodetection) on a single time axis. With sufficient traces, the diagram reveals the statistical envelope of all possible bit transitions occurring in the data stream.

The center of the eye — the open region used for decision making — has horizontal opening (timing margin) and vertical opening (amplitude margin). A clean, wide-open eye indicates good signal-to-noise ratio, low jitter, and well-controlled bandwidth. A closed or distorted eye indicates degraded transmission performance.

Extractable parameters:

Parameter	What it measures	Typical specification
Eye opening (vertical)	SNR at decision point	$>$ 50 % of full amplitude
Eye opening (horizontal)	Timing margin	$>$ 0.6 unit interval (UI) at center
Rise / fall time	Bandwidth	20 – 30 % of UI
Crossing percentage	Symmetry between one and zero	$\approx$ 50 % for balanced signal
One level $\mu_1$ , sigma $\sigma_1$	Noise on logical-one	system-dependent
Zero level $\mu_0$ , sigma $\sigma_0$	Noise on logical-zero	system-dependent
Extinction ratio	$\mu_1 / \mu_0$	$>$ 8.2 dB telecom
Eye amplitude	$\mu_1 - \mu_0$	proportional to optical modulation amplitude
Receiver Q-factor	$(\mu_1 - \mu_0) / (\sigma_1 + \sigma_0)$	$>$ 6 for BER $< 10^{-9}$
Jitter	Horizontal noise at crossing	$<$ 0.2 UI peak-to-peak (typical)

Common eye impairments and their causes:

Symptom	Likely cause
Closed at top	One-level noise (RIN, amplifier ASE)
Closed at bottom	Zero-level noise, finite extinction
Asymmetric rise/fall	Driver nonlinearity, modulator chirp
Tilted crossings	DC drift, AC coupling effects
Pattern-dependent shapes	Inter-symbol interference (dispersion or bandwidth limit)
Multiple traces	Discrete jitter sources (clock contamination)
Slow center filling	Random jitter accumulating

Eye diagrams are useful only for direct-detection on-off-keying and simple PAM signals. Coherent modulation formats (QPSK, 16-QAM, etc.) are characterized by constellation diagrams and error-vector magnitude (EVM) rather than eye diagrams; the optical waveform itself does not have meaningful eye structure.

A pseudo-random binary sequence (PRBS) is used to drive the link during eye measurement to ensure all bit transitions and pattern-dependent effects are excited. PRBS lengths of $2^7 - 1$ , $2^{15} - 1$ , $2^{23} - 1$ , and $2^{31} - 1$ are common; longer sequences reveal slower pattern-dependent effects.