The ratio between the optical power of a one and a zero — how decisively a transmitter turns the light off.
What Extinction Ratio Is
Extinction ratio is the optical power of a logical one divided by the optical power of a logical zero, quoted in decibels. A laser does not switch fully off between symbols — driven below threshold it would take too long to come back — so it idles at some low level, and the extinction ratio says how low.
The reason it matters is that average optical power on its own tells you almost nothing. Two transmitters can launch the same average power and present the receiver with completely different eye openings, because it is the difference between the levels, not their mean, that carries the information.
The Penalty Is Arithmetic, Not Opinion
Optical modulation amplitude is the difference between the one and zero levels. Expressed against average power, OMA divided by average power equals 2(r-1)/(r+1), where r is the linear extinction ratio. Turn that around and a receiver specified on average power suffers a penalty of 10 log10 of (r+1)/(r-1).
Put numbers in it and the shape of the problem appears. At 10 dB the penalty is about 0.9 dB — annoying but survivable. At 4 dB it is about 3.7 dB, and at 3.5 dB about 4.2 dB. The curve is steepest precisely in the region where the standards place their minimums, so a transmitter half a decibel below spec is not marginally worse, it is meaningfully worse. The dB converter makes the ratios concrete if decibels are not intuitive to you.
Why It Is Not Simply Maximised
Pushing the zero level lower widens the eye, but the laser then takes longer to recover, which costs rise time and adds chirp — a wavelength excursion at every transition that chromatic dispersion converts into pulse spreading. Keeping the zero high gives a fast, clean-spectrum transmitter with a small eye.
Directly modulated DFB lasers therefore tend to sit close to the specified minimum, trading eye opening for spectral purity. An externally modulated EML does not face the same trade at all, because the laser runs at constant current and only the modulator switches, which is why EMLs routinely reach 8 to 13 dB.
Where You Meet It
On the transmitter section of every single-mode datasheet, immediately below launch power, and it should always be read together with it — a strong average power with a poor extinction ratio is a worse transmitter than a modest average power with a good one. Our optical power converter handles the dBm and milliwatt conversions when a datasheet mixes them.
On PAM4 parts it is no longer the headline transmitter test, because TDECQ subsumes it — but it is still specified in its own right, at 3.5 dB minimum for 400GBASE-DR4, 400GBASE-FR4 and 800GBASE-DR8 and 3 dB for the multimode SR classes. It also survives as a diagnostic: extinction ratio degrades as a laser ages and its threshold current creeps up, so a part whose ER has dropped over years of DDM history is approaching end of life even while it still passes traffic.
Extinction ratio reference
| Definition | 10 log10 (optical power of a one / optical power of a zero) |
|---|---|
| 10GBASE-LR / 10GBASE-ER minimum | 3.5 dB / 3 dB |
| 100GBASE-LR4 minimum | 4 dB |
| 400GBASE-DR4, FR4 and 800GBASE-DR8 minimum | 3.5 dB |
| Typical directly modulated DFB | 4 to 6 dB |
| Typical EML | 8 to 13 dB |
| Relation to OMA | OMA divided by average power = 2(r-1)/(r+1), with r the linear ratio |
| Penalty at 3.5 dB ER | about 4.2 dB against an ideal source |
| Penalty at 10 dB ER | about 0.9 dB |