Optical power meters measure absolute optical power in units like dBm, which represents the logarithmic ratio of the measured power to 1 milliwatt. For example, a laser output might be +0 dBm (1 mW) or –20 dBm (0.01 mW) depending on the source and fiber type . When measuring attenuation, the meter compares the input power (reference) to the output power after a fiber, connector, or splice.
Attenuation is calculated as the difference between the reference power and the measured power: Attenuation (dB) = Reference Power (dBm) – Measured Power (dBm) For instance, if the input is –20 dBm and the output is –22.3 dBm: Attenuation = –20 – (–22.3) = 2.3 dB Even though the optical power decreased, the attenuation is expressed as a positive number to indicate the magnitude of loss . This convention makes it easier to interpret fiber losses without dealing with negative numbers.
Some optical loss test sets (OLTS) and power meters are designed to display attenuation directly as a positive value, rather than showing the negative difference in dBm . This approach aligns with IEC standards that define attenuation as a positive quantity, simplifying field measurements and reporting. The positive display does not indicate a gain; it simply represents the amount of power lost in the fiber or component.
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