The test loss of multimode fiber is calculated by comparing the measured insertion loss to a calculated loss budget based on fiber length, wavelength, connectors, and splices.

Step 1: Determine the Loss Budget

A loss budget is an estimate of the total expected loss in a fiber optic link. For multimode fiber, typical attenuation values are:

• 850 nm wavelength: ~3 dB/km (maximum 3.5 dB/km per EIA/TIA 568)
• 1300 nm wavelength: ~1 dB/km (maximum 1.5 dB/km per EIA/TIA 568) Connector and splice losses must also be included:
• Connector loss: typically 0.5 dB per mated connector
• Splice loss: typically 0.1–0.3 dB per fusion splice The total loss budget is calculated as: Loss Budget (dB) = (Fiber Attenuation × Fiber Length) + (Connector Loss × Number of Connectors) + (Splice Loss × Number of Splices) For example, a 100-meter multimode fiber link at 850 nm with 2 connectors and 1 splice would have an estimated loss of:
• Fiber: 0.3 dB (0.3 dB per 100 m)
• Connectors: 1.0 dB (0.5 dB × 2)
• Splice: 0.2 dB (typical) Total Loss Budget ≈ 1.5 dB .

Step 2: Perform Insertion Loss Testing

Use a light source and power meter (LSPM) or an Optical Loss Test Set (OLTS):

1. Set a 0 dB reference using short reference cables and the test equipment.
2. Connect the launch cable to the fiber under test and the receive cable to the power meter.
3. Measure the insertion loss, which includes fiber, connectors, and splices. Ensure the reference method matches the connector types and test equipment compatibility (one-, two-, or three-cable reference methods) to avoid measurement errors .

Step 3: Compare Measured Loss to Loss Budget

• If the measured loss is less than or equal to the calculated loss budget, the fiber link is acceptable.
• If the measured loss exceeds the budget, inspect the link segment-by-segment to identify high-loss components .

Step 4: Consider Wavelength Selection

For modern multimode fiber networks, testing is usually performed at 850 nm, as most LAN systems use VCSELs at this wavelength. Testing at 1300 nm is generally unnecessary unless the system specifically requires it .

Step 5: Optional OTDR Testing

An OTDR can be used to locate splices or faults, but it may underestimate total loss in multimode fibers, especially for short links, so insertion loss testing remains the standard for certification . By following these steps, you can accurately calculate and verify the test loss of multimode fiber optic cables, ensuring the installation meets performance standards.