gradedindex

Graded index, often written as GRIN, refers to an optical material whose refractive index varies continuously with position. In contrast to uniform media, graded-index materials have a spatially varying refractive index n that is designed to control the path of light. The gradient can be radial, axial, or azimuthal and is achieved by compositional or structural variation within the material.

In graded-index optical fibers, the index is highest at the core axis and decreases with distance from the axis, typically following a parabolic profile. This causes light rays to bend gradually as they propagate, guiding light through refraction rather than through a sharp core–cladding boundary. Parabolic-index fibers can reduce modal dispersion and offer more uniform transit times compared with step-index fibers.

Manufacturing methods include diffusion-based ion exchange in glass, graded-doping during glass fabrication, and gradual layering in polymers or crystals. GRIN lenses are common components that use a built-in refractive index gradient to focus light without curved surfaces, enabling compact imaging elements and integrated optics.

Applications span fiber-optic communications, endoscopy, and compact imaging systems. Advantages include reduced aberrations, the possibility of flat or short focal-length optics, and simplified assembly. Challenges involve fabrication tolerances, material losses in some gradients, and limited availability of suitable gradient profiles for certain wavelengths.

See also: gradient-index optics, parabolic-index fiber, optical waveguide.