Optical Intensity | previous | next | feedback |
Definition: optical power per unit area
The optical intensity I e.g. of a laser beam is the optical power per unit area, which is transmitted through an imagined surface perpendicular to the propagation direction. The units of the optical intensity (or light intensity) are W/m2 or (more common) W/cm2.
For a monochromatic propagating wave, such as a plane wave or a Gaussian beam, the local intensity is related to the amplitude E of the electric field via
where vp is the phase velocity, c is the vacuum velocity of light, and n is the refractive index. For non-monochromatic waves, the intensity contributions of different spectral components can simply be added, if beat notes are not of interest.
Note that the formula as give above does not hold for arbitrary electromagnetic fields. For example, an evanescent wave may have a finite electrical amplitude while not transferring any power. The intensity should then be defined as the magnitude of the Poynting vector.
For a laser beam with a top-hat intensity profile (i.e., with a constant intensity over some area, and zero intensity outside), the intensity is simply the optical power P divided by the beam area. For a Gaussian beam with the optical power P and the Gaussian beam radius w, the peak intensity (on the beam axis) is
which is 2 times higher than is often assumed.
The term intensity is often used in a non-quantitative or not very precise way, and not clearly distinguished from the optical power. For example, the intensity noise normally refers to noise (fluctuations) of the optical power, rather than the intensity.
Optical intensities are relevant in various situations:
- In conjunction with cross sections, intensities govern the rates of optical transitions, e.g. in laser gain medium. Strong saturation of an optical transition in the steady state occurs when the intensity exceeds the saturation intensity.
- The refractive index change via the Kerr effect in a transparent medium is the nonlinear index times the local intensity.
- Optical damage of a medium may occur for intensities above a certain damage threshold, which however can usually only be reached with optical pulses, and then depends on the pulse duration.
- Extremely high peak intensities can be achieved with amplified ultrashort pulses. For intensities of e.g. 1014 W/cm2 or higher in a gas, high harmonic generation can occur.
Beam profilers can be used for measuring the shape of the intensity profile of a laser beam.
See also: Gaussian beams, laser beams, brightness, intensity noise
