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Encyclopedia of Laser Physics and Technology

Optical Intensity

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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 commonly) 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

optical intensity from electric field

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 above equation 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 optical power P and Gaussian beam radius w, the peak intensity (on the beam axis) is

peak intensity of Gaussian beam

which is two times higher than is often assumed.

In a multimode laser beam, generated in a laser where higher-order transverse resonator modes are excited, the shape of the transverse intensity profile can undergo significant changes as the relative optical phases of the modes change with time. The peak intensity can then vary, and may occur at locations at some distance from the beam axis.

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:

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

Category: general optics


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Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.

You may order the print version via Wiley-VCH.

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