Absolute power in optical fiber is specified as:

Absolute power in optical fiber is typically specified in terms of decibels referenced to one milliwatt (dBm). This unit provides a logarithmic scale that allows for easier comparison of absolute optical power levels, especially in the context of communication systems where signal strengths can vary significantly.

Using dBm for measurement is advantageous because it expresses power in a way that reflects the logarithmic nature of human perception and the operational characteristics of fiber optic systems. For instance, when dealing with the loss of signal over distance or calculating power budgets, having a clear reference point like 1 milliwatt facilitates these calculations.

The other units listed, while relevant in different contexts, do not serve as absolute power measurements in optical fibers. The decibel (dB) is a unit of measurement for the ratio of two power levels, and does not provide an absolute value like dBm does. Amplitude refers to the magnitude of a signal wave but is not a direct indicator of power as expressed in fiber optics. EMP, or electromagnetic pulse, has no direct relation to measuring power in fiber optics but rather refers to a burst of electromagnetic energy that can affect electronic devices. Therefore, dBm is the proper and most relevant measure for absolute optical power in this context.