Numerical aperture: Encyclopedia II - Numerical aperture - Fiber optics

Numerical aperture - Fiber optics

Multimode optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, θ_max. For step-index multimode fiber, the acceptance angle is determined only by the indexes of refraction:

where n_f is the refractive index of the fiber core, and n_c is the refractive index of the cladding.

This has the same form as the numerical aperture in other optical systems, so it has become common to define the NA of any type of fiber to be

where n_o is the refractive index along the central axis of the fiber. Note that when this definition is used, the connection between the NA and the acceptance angle of the fiber becomes only an approximation. In particular, manufacturers often quote "NA" for single-mode fiber based on this formula, even though the acceptance angle for single-mode fiber is quite different and cannot be determined from the indices of refraction alone.

In multimode fibers, the term equilibrium numerical aperture is sometimes used. This refers to the numerical aperture with respect to the extreme exit angle of a ray emerging from a fiber in which equilibrium mode distribution has been established.

Numerical aperture - Sources

• Federal Standard 1037C
• MIL-STD-188

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