Knowledge base · Measuring
Optical return loss (ORL): what the number is, and why everyone's differs
ORL is how much of the light sent into a link comes back to the source, all of it at once. One number for the whole trace — and it is arrived at in different ways: two instruments will put different values on the same file, and both will be right on their own terms.
Check the ORL in your own file
The return loss of a link is not the reflectance of a single event: reflectance belongs to one connector or splice and sits in a row of the table, while ORL sums everything that came back, the fiber's own scattering included.
Where the returning light comes from
Three sources, and ORL counts all three.
Reflection off end faces
Fresnel reflection wherever light crosses between media: connectors with an air gap, mechanical splices, an open fiber end, a crack. This is the largest part and the one that can be fixed — it is why angle-polished connectors go into links that care.
Scattering in the fiber
Rayleigh scattering is a property of the fiber itself, not a defect. A fraction of the scattered light heads back towards the source and lands in the ORL, along the whole length of the link.
The length of the link
Hence the consequence that keeps the figures apart: the longer the fibre, the more scattered light has come back. ORL is a property of the whole link rather than a datasheet value of a component, and it does not carry from one span to another.
Why the number depends on what measured it
Return loss is measured by two different instruments. An OTDR sorts the returning light by time and can tell where it came from. An ORL meter (OCWR, often part of an optical loss test set) sends a continuous signal and measures only the total — but measures it directly, while being unable to tell reflection from scattering and needing the far end of the link terminated. These are two different measurements of one quantity, and they are not obliged to agree.
On the OTDR itself, the number is derived from the curve rather than measured. Into that derivation go the fiber's backscatter coefficient, taken from the instrument's settings, the stretch it was computed over, and the point it is referenced to. The file records none of the three — only the result. So different manufacturers land on different values for one span: what differs is the convention about what counts as the link, not the arithmetic.
The practical conclusion is simple. It makes sense to compare an ORL figure with one obtained the same way: reflectometric with reflectometric, test-set with test-set. A discrepancy between the two methods on one link is not an error of either. And it makes sense to compare links of comparable length: on a short one the dominant contribution is the reflection at the far end, but with length it fades on the way back, and beyond roughly forty kilometres ORL is set by scattering alone — whether the link is terminated or not barely matters.
What we show
Where the instrument wrote an ORL into the file, the totals carry its number, credited to the instrument. Where it wrote none, the experimental mode shows our own figure from the curve, marked as a calculation: it is our quantity and not a substitute for theirs, and the two never stand under one label. The calculation needs the backscatter coefficient out of the file; where the file states none, we stay silent rather than filling in a plausible value for silica.
There are no threshold values in the service — not for ORL, not for anything else. What an installation requires of its return loss is set by the design and the type of system, and it comes from the site documentation, not from us.
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The sign: formulas usually write ORL as a negative number, while instruments and reports print it positive, as "return loss 45 dB". It is the same quantity — the larger the number, the less light came back and the better the link.