Optical power meters measure light power in units of watts (W), milliwatts (mW), or decibels relative to 1 milliwatt (dBm), with relative measurements often expressed in decibels (dB).

Absolute Units

Optical power meters can display absolute optical power, which quantifies the actual energy delivered per unit time by a light source. The most common absolute units are:

• Watts (W) – the SI unit of power, representing joules per second.
• Milliwatts (mW) – commonly used in fiber optics and laser systems because optical powers are typically very low, often in the range of 0.001 W or less . For example, a laser output of 1 mW corresponds to 0 dBm, while 10 mW corresponds to +10 dBm .

Relative Units

Optical power meters also use relative units to express power changes or losses:

• Decibels (dB) – a logarithmic unit representing the ratio of two power levels. It is commonly used to measure optical loss or gain in fiber optic systems. A negative dB value indicates a loss, while a positive value indicates a gain .
• Decibels-milliwatts (dBm) – an absolute power measurement expressed in decibels relative to 1 mW. For instance, 0 dBm = 1 mW, -10 dBm = 0.1 mW, and +10 dBm = 10 mW .

Practical Use

When using an optical power meter, the user typically selects the appropriate unit based on the application:

• dBm is standard for fiber optic communications and laser testing because it allows easy comparison of power levels and losses.
• dB is used for relative measurements, such as comparing input and output power to determine attenuation.
• Watts or milliwatts are used when absolute power values are required, such as in laboratory experiments or high-power laser systems . Modern optical power meters often allow switching between these units on the display, providing flexibility for both absolute and relative measurements.