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.

Fiber & telecom Lab practice

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.