# Power Spectral Density

Optical power or Noise power per unit Frequency interval.

In optics, power spectral densities occur basically in two different forms. One form is as Optical Power spectral densities which are defined as the optical power per optical frequency interval. The other form is as noise power densities which are defined as the power spectral density of the fluctuations of a quantity such as an optical power or phase where the frequency argument refers to a noise frequency.