Knowledge base · Measurement
Launch cable: what it is for and what goes unmeasured without it
Without a launch cable an OTDR can measure neither the first connector of a link nor the last. Not «measures them imprecisely» — does not measure them at all: it has nothing to compare against. Connector loss is the difference between the backscatter levels before and after it, and on one side of an end connector there is no fibre.
This is the only reason two spools travel to an acceptance test, and the commonest reason an acceptance record carries no rows for the end connectors.
What it does
Three things, each solving its own problem.
It gives a reference before the link
With the instrument connected straight to the link there is no «before the first connector» level: what stands there is the OTDR's own connector and its dead zone. A launch cable puts hundreds of metres of fibre between them — the «before» level appears, and with it a measurable connector loss.
A receive cable does the same at the far end
Without one, the last connector sits exactly at the end of the trace with noise behind it, and again there is no difference in levels to take. With one, the end of the fibre moves past the connector, and the connector becomes an ordinary event with both a loss and a reflectance.
It moves the front-end dead zone out
The dead zone of the instrument's own connector now falls inside the spool rather than inside your link, and the first metres of the link stop being blind. On multimode there is one more gain: over hundreds of metres the mode distribution settles, and the measurement becomes steadier.
How long, and which fibre
The length follows the dead zone, and the dead zone follows the pulse width: the longer the pulse, the longer the spool has to be. Common practice is hundreds of metres for multimode, one to two kilometres for singlemode, and up to four kilometres on long-haul spans. The fibre in the spool must match the fibre under test, and the connectors must be good ones: a poor connector on the spool is what you will measure instead of your own.
And the thing that is easy to forget: a two- or three-metre test cord is not a launch cable. It only brings the instrument's connector outboard so that the port is not worn out. It moves no dead zone and saves no first connector.
How this shows up here
The file does not record that its first metres are not your link — only the person who measured knows that. What it does record is where the operator put the near marker. If that marker sits at the end of the spool, the trace ends up with two different lengths, and we show both side by side: «Optical length», between the markers, which is your span, and «End of fibre», where the fibre physically ran out.
In part of our corpus these two figures differ, and on the traces of one manufacturer the difference is exactly 504 metres — the length of the spool they were shot with. In such a file the loss carries the note «Loss measured from …, the near end is not included», which means precisely what this guide is about: the end connector did not make it into the measurement.
We do not subtract the spool ourselves and do not guess where it ended: the file does not say, and substituting a plausible number would pass a guess off as a measurement. Removing it from the curve is done by hand, with Crop: it cuts the head off, the kept samples are unchanged, and the distance origin moves to the new first sample along with the events.
Next
A launch cable does not improve the link or reduce its loss. It only gives the instrument somewhere to look at the end connectors from: without it they are not worse — they are simply unmeasured.