Numerical aperture: Encyclopedia II - Numerical aperture - Laser physics

Numerical aperture - Laser physics

In laser physics, the numerical aperture is defined slightly differently. Laser beams spread out as they propagate, but slowly. Far away from the narrowest part of the beam, the spread is roughly linear with distance—the laser beam forms a cone of light in the "far field". The same relation gives the NA,

but θ is defined differently. Laser beams typically do not have sharp edges like the cone of light that passes through the aperture of a lens does. Instead, the irradiance falls off gradually away from the center of the beam. It is very common for the beam to have a Gaussian profile. Laser physicists typically choose to make θ the divergence of the beam: the far-field angle between the propagation direction and the angle for which the irradiance drops to 1/e² times the peak irradiance. The NA of a Gaussian laser beam is then related to its minimum spot size by

where λ is the wavelength of the light, and D is the diameter of the beam at its narrowest spot, measured between the 1/e² irradiance points ("Full width at e^-2 maximum"). Note that this means that a laser beam that is focused to a small spot will spread out quickly as it moves away from the focus, while a large-diameter laser beam can stay roughly the same size over a very long distance.

Adapted from the Wikipedia article "Laser physics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki