Glossary entry

Extinction ratio (ER)

The ratio of the maximum to minimum transmitted optical power of a device or signal, in dB. Quantifies on/off contrast for modulators, filters, and switches.

The extinction ratio in decibels is

\text{ER} \;=\; 10 \log_{10}\!\left( \frac{P_\text{max}}{P_\text{min}} \right).

For a modulator or switch, ER measures the contrast between fully-on and fully-off states. Higher ER produces sharper switching and lower crosstalk in WDM systems.

For a ring resonator filter, ER is the extinction depth at the resonance wavelength. The depth depends on the coupling regime:

Coupling regime	Extinction at resonance
Critical coupling ( $Q_i = Q_c$ )	Full extinction (in theory, $\infty$ dB)
Under-coupled ( $Q_i < Q_c$ )	Partial extinction
Over-coupled ( $Q_i > Q_c$ )	Partial extinction

Real ring resonators reach 20–40 dB ER at critical coupling; achieving $> 50$ dB requires careful control of coupling gap, propagation loss, and absence of stray feedback.

Typical ER values:

Device	Typical ER
Mach–Zehnder modulator (telecom)	20 – 30 dB
Electro-absorption modulator	12 – 20 dB
Ring resonator filter	15 – 30 dB
Optical switch (MEMS)	40 – 60 dB
Polarization-maintaining components	25 – 30 dB (between principal axes)
Critically-coupled high- $Q$ ring	$> 40$ dB

For digital optical communications, ER also describes the modulated signal: $\text{ER} = P_\text{one} / P_\text{zero}$ . Standard telecom NRZ-OOK transmitters typically specify ER $> 8.2$ dB; higher ER improves receiver sensitivity but reduces average optical power.

Distinguishable from insertion loss: IL is the loss in the "on" state relative to input, while ER is the on-to-off contrast.