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Fiber Splice Loss Worksheet (Bidirectional Average)

A single-direction OTDR reading can be wrong about splice loss, sometimes even showing a gain. This worksheet captures both directions, averages them and shows the margin to your limit, so every splice has a clean record you can hand over with the job.

Download:

Closure / location	Fiber #	Splice type	Loss A to B (dB)	Loss B to A (dB)	Bidirectional average (dB)	Acceptance limit (dB)	Margin to limit (dB)	Technician	Date
					0.05		0.05
					0.06		0.04
					0.13		-0.03
					0.06		0.04
					0.2		0.1
					0.04		0.06

Average splice loss (dB)

0.09

Worst splice average (dB)

0.2

Smallest margin to limit (dB)

-0.03

Splices logged

Spread between best and worst (dB)

0.16

A negative margin means that the splice average exceeds your limit and should be redone or reviewed. A negative single-direction OTDR reading (gainer) is normal when fibers differ in backscatter, which is why the average of both directions is the reportable value.

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How to use the worksheet

Enter one row per splice. Record the closure, fiber number and splice type, then type the loss read from each OTDR direction. The sheet computes the bidirectional average and the margin to the acceptance limit, and the totals show the average, the worst splice and the smallest margin. Replace the example rows with your data, add rows as you work, and export to Excel or PDF for the handover package. Set the limit per row, since a fusion splice and a mechanical splice usually have different limits.

Shoot from both ends of the span or use a loop-back method
Use the same pulse width and IOR for both directions
Set the limit from the contract or the project specification
Re-splice any joint above the limit before closing the enclosure

Why average both directions

OTDR splice loss depends on backscatter, which varies with mode field diameter and fiber manufacturing differences. When light goes from a fiber with a larger mode field into a smaller one, the trace may show a gain, and the reverse direction shows an exaggerated loss. The true splice loss is the arithmetic mean of the two directions, which cancels the backscatter difference. That is why standards and most contracts require bidirectional averaging for acceptance.

Typical limits

A well-made single-mode fusion splice is usually under 0.05 dB and a common acceptance limit is 0.1 dB average, with some projects allowing 0.15 or 0.3 dB. Mechanical splices are commonly accepted up to 0.3 dB. Multimode fusion splices are often limited to 0.1 or 0.15 dB. These are project values and not universal rules, so confirm the specification you are bound by. The limit column in the worksheet lets you keep different thresholds in the same log.

Common mistakes

Teams often accept a splice because one direction looks good. Other errors include comparing readings taken with different pulse widths, reading a splice that sits inside the OTDR dead zone, and not cleaning connectors before the test, which hides the real splice. If an average is near the limit, check the cleaver angle and the electrode condition on the fusion splicer, and see the splicer maintenance log for pattern problems.

Frequently Asked Questions

What is an acceptable fusion splice loss?

Many specifications accept 0.1 dB bidirectional average for single-mode fusion splices, with a typical good result under 0.05 dB. Some projects allow 0.15 dB or 0.3 dB, so use the figure in your contract.

Why does my OTDR show a negative splice loss?

A gainer appears when the second fiber has a lower backscatter coefficient or smaller mode field diameter than the first. It is an artifact of the measurement. Averaging both directions gives the real loss.

Do I need to test every splice with an OTDR?

Most acceptance specifications require OTDR traces for every fiber in both directions. In-process checks use the fusion splicer estimate, but that estimate is not accepted as a final record by most contracts.

Can I export this to Excel?

Yes. Use the export buttons on the tool to download the logged rows with the computed averages and margins as an Excel file, or as PDF or CSV for the handover documents.

If you track field work and as-built cable data in an ERP or field service system, we can help connect these records to the asset. Ask us about a private setup.

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