Optical power meters measure light power in absolute units like milliwatts (mW) or in logarithmic units like dBm, while relative changes are expressed in decibels (dB).

Absolute Power: Watts and Milliwatts

Optical power meters can display absolute optical power in Watts (W) or more commonly millwatts (mW), where 1 mW equals 0.001 W . This linear scale shows the actual energy delivered per unit time by the light signal. For example, a typical fiber optic laser might output 2–4 mW, which corresponds to +3 to +6 dBm . Absolute power readings are important for verifying transmitter output or receiver input levels.

Logarithmic Power: dBm

The dBm unit is a logarithmic measure of power relative to 1 milliwatt. A reading of 0 dBm equals 1 mW, +10 dBm equals 10 mW, and -10 dBm equals 0.1 mW . Using dBm simplifies calculations over wide power ranges and is standard in telecommunications. Positive dBm values indicate power above 1 mW, while negative values indicate power below 1 mW.

Relative Power: Decibels (dB)

Decibels (dB) are used to express relative power differences, such as optical loss or gain between two points in a fiber link. For instance, if a signal drops from +3 dBm to -2 dBm, the loss is 5 dB . This unit is convenient for calculating link budgets and assessing attenuation without needing absolute power values.

Practical Considerations

• Wavelength Setting: Optical power meters are wavelength-dependent; the meter must match the signal wavelength for accurate readings .
• Calibration: Meters are calibrated against national standards (e.g., NIST) to ensure consistent measurements across devices .
• Measurement Context: Absolute power (dBm or mW) is used to check transmitter or receiver levels, while relative power (dB) is used to measure losses across connectors, splices, or fiber segments . In summary, mW and dBm provide absolute power readings, while dB expresses relative changes. Understanding these units allows technicians to interpret optical power meter readings correctly, ensuring proper fiber optic network performance and troubleshooting.