Splice Loss Measurement
Splice Loss Measurement
This domain tests the difference between splice loss, connector loss and total link loss, and the correct use of dB formulas. It also tests why OTDR splice readings need care.
Core rules and concepts
- Insertion loss in dB = −10 log10(Pout/Pin). Power must be in the same linear units, such as mW or µW, not dBm.
- A splice loss of 0.1 dB means about 97.7% of the power passes; 0.5 dB means about 89%; 3 dB means half the power.
- Measure splice loss by optical time-domain reflectometer (OTDR) or by a light source and power meter. For a single splice, a cutback or a reference cable method can be used in a laboratory.
- OTDR measures backscatter. A splice between fibers with different backscatter can appear as a gain or an exaggerated loss in one direction. Use bidirectional measurements and average the two directions for acceptance.
- Record loss at the specified wavelength; 1310 nm and 1550 nm results are not interchangeable.
Worked example
A source launches 100 µW into a fiber and the power meter reads 90 µW after one splice. Pout/Pin = 0.90. Loss = −10 log10(0.90) = 0.46 dB. If the meter instead reads −10 dBm, that is 100 µW, so the reading must be converted before using the formula. A second splice gives 80 µW: 80/100 = 0.80, loss = 0.97 dB, which is too high for a good fusion splice and needs investigation.
Traps that cost marks
Do not put dBm values directly into the loss formula. Do not accept a negative OTDR splice loss as real loss; it is usually a backscatter artifact. Do not compare a 1310 nm splice result with a 1550 nm limit. Do not ignore connector losses when checking a link.