How much optical power a component removes from the link — measured in dB, and the currency every link budget is spent in.
What Is Insertion Loss?
Insertion loss is the optical power removed by inserting a component into an otherwise continuous path, expressed in decibels. Every mated connector pair, every splice, every splitter and every kilometre of fibre contributes some, and the total has to fit inside what the transceiver's power budget allows.
Because decibels are logarithmic they simply add: four connector pairs at 0.3 dB is 1.2 dB, no arithmetic required. That additivity is the whole reason optical engineering works in dB rather than percentages. If percentages are what you need, the dB to percentage converter does the translation — 0.25 dB is 5.6% of the light, and 3 dB is very nearly half — exactly half is 3.01 dB.
Where the Loss Comes From
At a mated pair, four mechanisms account for almost everything: lateral offset between the two cores, angular misalignment, an end separation that leaves an air gap, and contamination. The first three are geometry, fixed when the connector was made, and are what you are paying for when you buy factory-terminated assemblies with interferometer-verified apex offset and fibre height.
The fourth is housekeeping, and it is by far the most frequent cause of an unexpectedly lossy link. A single particle on a core is worth more decibels than every geometric error combined — see end-face inspection.
How Tight the Budgets Really Are
100GBASE-SR4 allows 1.9 dB for the entire channel over 100 m of OM4, and 400GBASE-SR8 allows the same. The fibre itself takes 0.35 dB at the specification maximum, leaving about 1.55 dB for connectors. At 0.35 dB a mated pair that is four pairs, and a structured design with two cross-connects reaches four pairs quickly.
This is why low-loss and extra-low-loss connector grades exist, and why IEC 61753-1 grades matter when you buy. Grade B is 0.12 dB mean against grade C at 0.25 dB — over four pairs that is half a decibel of margin bought for a modest premium. Model the channel in the link budget calculator before the design is committed.
Where You Meet It
On every assembly specification, every acceptance test and every fault you chase. A link that was fine and is now marginal has almost always gained loss rather than lost transmitter power — the link-down diagnostic guide works through the order to check things in. We measure insertion loss on every fibre of every assembly and ship the report with the cable.
Insertion loss reference
| Definition | 10 log10 (power in / power out), in dB |
|---|---|
| 0.25 dB | 5.6% of the power gone |
| 3.0 dB | very nearly half the power gone — exactly half is 3.01 dB |
| IEC 61753-1 grade B | ≤ 0.12 dB mean, ≤ 0.25 dB for 97% of matings |
| IEC 61753-1 grade C | ≤ 0.25 dB mean, ≤ 0.50 dB for 97% of matings |
| IEC 61753-1 grade D | ≤ 0.50 dB mean, ≤ 1.00 dB for 97% of matings |
| Fusion splice | about 0.1 dB, often 0.02 to 0.05 dB |
| 100GBASE-SR4 channel | 1.8 dB over 70 m OM3, 1.9 dB over 100 m OM4 |
| OM4 fibre | ≤ 3.5 dB/km at 850 nm — 0.35 dB over 100 m |
Related Terms
return loss, end-face inspection, Tier 1 and Tier 2 testing, APC vs UPC, pigtail.