Calibration of an optical power meter requires comparing its readings to a NIST-traceable reference standard and adjusting for detector responsivity at specific wavelengths.

Understanding Calibration

An optical power meter measures light via a photodiode, converting the resulting current into a power reading in dBm or mW. The conversion depends on the detector's responsivity (amps per watt) at each wavelength, which is stored in the meter's internal calibration table. Over time, photodiodes drift due to aging, temperature cycling, humidity, and connector wear, which can cause deviations of 0.05–0.3 dB per year, making calibration essential for accurate measurements .

Laboratory Calibration Procedure

1. Reference Standards: True calibration requires a working standard calibrated against a national standard, such as NIST in the U.S., or other recognized metrology institutes internationally .
2. Measurement Comparison: The meter under test (DUT) is exposed to known optical power levels at multiple wavelengths. The DUT readings are compared to the reference standard.
3. Adjustment: If deviations exceed specifications, the meter's internal calibration tables are adjusted by the lab, and the meter is re-tested.
4. Certification: Once within tolerance, the lab issues a calibration certificate and marks the meter with the next calibration due date, typically every 12 months .

Field Verification and Practical Tips

While full calibration requires a lab, you can perform field-level verification:

• Zeroing: Cover the sensor and press the zero or null button to ensure the meter reads zero when no light is present .
• Wavelength Selection: Set the meter to the wavelength of the fiber system under test.
• Reference Power Setting (REF): Use a known-good reference source to check the meter's readings and adjust the reference if necessary .
• Connector Cleaning: Clean fiber connectors and the sensor surface to prevent contamination affecting current readings.
• Cross-Check: Compare readings with another calibrated meter to detect drift or anomalies.

Key Considerations

• Annual Calibration: Standard practice is to send meters for calibration every 12 months, or sooner if heavily used or exposed to extreme conditions .
• Detector Type: Calibration procedures may vary depending on whether the photodiode is Si, Ge, or InGaAs, as each has different spectral responsivity characteristics .
• Traceability: Ensure the calibration is NIST-traceable or equivalent to maintain measurement credibility and compliance with ISO 9001 or TIA standards . By following these procedures, you ensure that the photodiode current accurately reflects optical power, maintaining reliable and traceable measurements for fiber optic testing and certification.