Knowledge base · The document
Fluke fiber test results explained
You were sent a LinkWare report for a fibre: the “Test Summary” line, a table with loss and a limit, two instruments with serial numbers. Below is what each part of it means, where the limit comes from, and why a test with a healthy margin can still fail.
Open a .flw database and see the same
The report is printed by LinkWare PC. The figures and the verdict in it were recorded by the instrument.
By Ilya (engineer, 6 years in fibre optics) ·
What to read first
The sheet is laid out the same way for every Fluke instrument, from the old DSP to CertiFiber Pro. It answers three questions.
The test summary
The “Test Summary” line at the top of the sheet: PASS or FAIL for the whole test. The test fails if at least one row fails: the loss at one of the wavelengths, or the length.
The table by wavelength
Each wavelength has its own column: “Result”, “Loss”, “Limit” and “Margin”. The margin is the limit minus the loss. The smaller it is, the closer the link is to failing.
Length
The instrument measures length from the travel time of light (“Propagation Delay”) and compares it with its own limit. The index of refraction it was worked out with is printed next to it.
Where the limit comes from
The limit is worked out by the instrument, not by the program. Before the test the operator picks a “Test Limit” on the tester, usually a cabling standard or a network application, and the instrument derives the allowed loss from it. The name of the limit and the version of the limits table are printed on the left, near the date.
So the same measurement under a different “Test Limit” gives a different limit and a different verdict. A report can only be trusted together with the name of its limit: check it against what the contract asks for.
For limits that depend on what the link is made of, the length and the number of connectors and splices go into the sum. The operator enters those numbers, and a mistake in them changes the limit.
Direction and reference
The label above the table says which way the light went. “Loss (M->R)” means from the main unit to the remote, “Loss (R->M)” means back. A bidirectional test prints both tables, and each direction is judged on its own: the instrument gives no average.
“Reference” is the power recorded when the reference was set, in dBm, and “Reference Date” says when that was. A reference set long ago, or before the test cords were changed, is suspect in itself.
Next to it stands the number of jumpers the reference was made with. It decides how many of the link's connectors count towards the loss, so two reports can only be compared when the method is the same.
Why a test failed on a healthy margin
Sometimes a row shows a loss with a minus sign, a large margin, and FAIL. That is not a bug in the program. A link does not amplify light, and negative loss means the reference was worse than the link itself: a dirty connector when the reference was set, or test cords that were touched after it.
The instrument forgives a small minus and counts a noticeable one as a failure, whatever the margin. The cure is in the reference, not in the link: clean the connectors, set the reference again and retest.
What else is on the sheet
The instruments
The main and remote units, their modules, serial numbers, software version and calibration date. They show whether the right instrument was used and whether its calibration was still valid.
Network applications
The “Compliant Network Standards” block lists the network applications a link with this loss and length is good enough for. It is the instrument's reference note, not a verdict: the test result is set by the “Test Limit”.
The MPO connector
For MultiFiber Pro the sheet has a row for each fibre of the connector, with its own result, and the polarity: type A, B or C.
Tier 1 and Tier 2
A report like this is called Tier 1: the loss of the whole link, its length and polarity. It answers whether the link is fit and does not answer where on it the light is lost.
Tier 2 adds an OTDR trace: it shows every joint and the distance to it. An OTDR report reads differently, and there is a separate page about it.
The same result here
Ask the contractor for the .flw database itself, not just the PDF. It opens here in the browser: the tests become a table, and the selected one shows the same loss, limit, margin and reference power as the LinkWare sheet. The verdict is printed as it is: the instrument recorded it, and we add no judgement of our own.
Our own PDF sheet prints for the selected fibre, with a code and a QR after signing in by email. Copper certification results from the same database do not open.
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