How to measure attenuation in multimode fiber

Multimode fiber attenuation is measured using an Optical Loss Test Set (OLTS) or a light source and power meter (LSPM), with careful attention to launch conditions and modal distribution.

Equipment Needed

1. Optical Loss Test Set (OLTS) or Light Source and Power Meter (LSPM) calibrated for the fiber's operating wavelength (typically 850 nm and 1300 nm for multimode) .
2. Launch and receive reference cables of the same fiber type and core size as the fiber under test .
3. Mating adapters compatible with the connectors on the fiber .
4. Cleaning supplies to ensure connectors and adapters are free of debris .

Testing Procedure

1. Warm up the equipment and ensure all connectors are clean .
2. Set a 0 dB reference using the launch and receive reference cables to establish a baseline measurement .
3. Connect the fiber under test between the source and power meter, and measure the optical power loss .
4. Perform unidirectional testing as a minimum; bidirectional testing is recommended for more accurate results .
5. Record the link length from the cable jacket or using the OLTS if it has this capability .

Modal Considerations

• Modal distribution affects attenuation measurements. Higher-order modes in multimode fiber experience more scattering and absorption, leading to higher attenuation .
• Equilibrium Modal Distribution (EMD) is achieved in long fibers, where higher-order modes have decayed, providing a more consistent measurement .
• Launch conditioning may be required, such as using mandrels or EF-conditioned launches, to control modal power distribution and ensure reproducible results .

Optional OTDR Testing

• Optical Time-Domain Reflectometer (OTDR) can be used to locate faults, measure splice and connector loss, and verify fiber length .
• OTDR results complement OLTS/LSPM measurements but do not replace them due to potential variability in modal distribution .

Standards and References

• Follow TIA-568.3-D for recommended loss budgets and testing wavelengths .
• The RS-455 standard provides procedures for restricted launch conditions to approximate long-link attenuation .
• Ensure that the measured attenuation is within the link-loss budget, accounting for fiber, splices, and connectors . By following these procedures and considering modal effects, you can accurately measure the attenuation of multimode fiber and verify that the installation meets performance specifications.