How to measure the optical decay value of a pigtail fiber

The optical decay (attenuation) of a pigtail fiber is typically measured using an OTDR or an optical loss test set, with bidirectional testing recommended for accurate results.

Using an OTDR

An Optical Time Domain Reflectometer (OTDR) is the most common tool for measuring the optical decay of a fiber pigtail. The OTDR works by sending a light pulse down the fiber and measuring the backscattered light to calculate loss along the fiber length. To measure attenuation:

1. Connect the pigtail to the OTDR using appropriate launch and receive cables. If the pigtail is short (e.g., 3–4 meters), the OTDR may only measure combined connector and splice loss, so a longer launch fiber (around 10 meters) is recommended for accurate readings .
2. Set markers at the start and end of the fiber section you want to measure. The OTDR calculates the difference in power levels between these points.
3. Calculate attenuation: The OTDR divides the measured loss by the fiber length to give the attenuation coefficient in dB/km, which represents the optical decay .
4. Bidirectional measurement: For higher accuracy, measure the fiber in both directions and average the results. This helps negate backscatter coefficient mismatches between fibers, especially if using different fiber types .

Using an Optical Loss Test Set (OLTS)

Alternatively, you can use a light source and power meter (OLTS) to measure insertion loss:

1. Connect the light source to one end of the pigtail and the power meter to the other.
2. Measure the transmitted power and compare it to the reference power from a known good fiber.
3. Calculate the loss in dB, which represents the optical decay over the pigtail length .

Best Practices

• Visual inspection: Always inspect connectors for contamination or damage before testing, as defects can affect measurements .
• Avoid short sections: Short fibers can produce higher measurement uncertainty; longer sections provide more reliable attenuation values .
• Use proper wavelengths: Single-mode fibers are typically tested at 1310 nm and 1550 nm, while multimode fibers are tested at 850 nm and 1300 nm .
• Document results: Record OTDR traces or OLTS readings for future reference and troubleshooting. By following these procedures, you can accurately determine the optical decay value of a pigtail fiber, ensuring it meets design specifications and performance standards.