Guided ray
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numerical aperture
(nü′mer·i·kəl ′ap·ə·chər)
(optics) A measure of the resolving power of a microscope objective, equal to the product of the refractive index of the medium in front of the objective and the sine of the angle between the outermost ray entering the objective and the optical axis. Abbreviated N.A.
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numerical aperture
The measurement of the acceptance angle of an optical fiber, which is the maximum angle at which the core of the fiber will take in light that will be contained within the core. Taken from the fiber core axis (center of core), the measurement is the square root of the squared refractive index of the core minus the squared refractive index of the cladding. See critical angle and fiber optics glossary.
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Guided ray
A guided ray (also bound ray or trapped ray) is a ray of light in a multi-mode optical fiber, which is confined by the core. For step index fiber, light entering the fiber will be guided if it falls within the acceptance cone of the fiber, that is if it makes an angle with the fiber axis that is less than the acceptance angle,
,
where
- θ is the angle the ray makes with the fiber axis, before entering the fiber,
- no is the refractive index along the central axis of the fiber, and
- nc is the refractive index of the cladding.
This result can be derived from Snell's law by considering the critical angle.
Rays that fall within this angular range are reflected from the core-cladding boundary by total internal reflection, and so are confined by the core. The confinement of light by the fiber can also be described in terms of bound modes or guided modes. This treatment is necessary when considering singlemode fiber, since the ray model does not accurately describe the propagation of light in this type of fiber.
See also
References
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
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