Calculation of optical cable used for optical terminal testing in line section

The optical cable required for testing an optical terminal is determined by the total fiber length under test plus additional lead-in fibers to account for OTDR dead zones and connector interfaces.

Key Considerations

1. Lead-in Fiber (Launch Cable): A lead-in fiber, also called a launch cable, is used to connect the OTDR to the fiber under test. It typically ranges from several hundred meters depending on the OTDR and connector type. This ensures accurate measurement of the initial portion of the fiber and eliminates the dead zone effect caused by the OTDR interface connector . 2. Fiber Under Test: The total length of the optical cable in the line section must be measured, including all intermediate splices and connectors. This is the main portion of the fiber that will be tested for attenuation, insertion loss, and continuity . 3. Tail or Receive Fiber (Optional): A tail fiber, or receive cable, may be used at the far end of the fiber under test to improve measurement accuracy for the final connector and end-of-line reflections. This is especially useful for long-haul or high-precision testing .

Calculation Steps

1. Determine the total installed fiber length in the line section, including all splices and patch panels.
2. Add the lead-in fiber length at the OTDR side (typically 100–500 meters depending on OTDR specifications).
3. Add a tail fiber length if required for accurate end-of-line measurement (optional, usually 50–100 meters).
4. Include extra length for handling and routing, such as loops or slack, to prevent tension or bending during testing. Formula Example:

Total Test Cable Length = Lead-in Fiber + Fiber Under Test + Tail Fiber + Handling Slack

For instance, if the line section is 2 km, the lead-in fiber is 300 m, the tail fiber is 100 m, and handling slack is 50 m, the total optical cable required for testing would be:

2,000 m + 300 m + 100 m + 50 m = 2,450 m

Practical Tips

• OTDR Selection: Ensure the OTDR dynamic range is sufficient for the total fiber length. Longer fibers may require higher dynamic range OTDRs.
• Connector Matching: The lead-in fiber connectors must match the OTDR port and the fiber under test to avoid measurement errors.
• Safety: Always follow laser safety precautions, as OTDR lasers are invisible and can damage eyes .
• Documentation: Record all fiber lengths, splice locations, and measured losses for certification and troubleshooting purposes . By following these steps, you can accurately calculate the optical cable required for testing an optical terminal in any line section, ensuring reliable OTDR measurements and proper certification of the fiber link.