Gradient Index Lenses

Gradient index lenses or GRIN lenses are used to focus and collimate light within a variety of fiber optic components.  GRIN lenses, short for gradient index, focus light through a precisely controlled radial variation of the lens material’s index of refraction from the optical axis to the edge of the lens. This allows a GRIN lens with flat or angle polished surfaces to collimate light emitted from an optical fiber or to focus an incident beam into an optical fiber.  End faces can be provided with an anti-reflection coating to avoid unwanted back reflection.

Gradient index lenses or GRIN lenses offer an alternative to the often-painstaking craft of polishing curvatures onto glass lenses. By gradually varying the index of refraction within the lens material, light rays can be smoothly and continually redirected towards a point of focus. The internal structure of this index "gradient" can dramatically reduce the need for tightly controlled surface curvatures and results in simple, compact lens geometry. 

Gradient index lenses or GRIN lenses are used in a wide array of products that require either passive or active electrical components.  Passive component manufacturers use GRIN lenses in wavelength division multiplexers (WDM), optical switches, and attenuators.  In active components, GRIN lenses are used in fiber-to-detector and laser-to-fiber coupling.  GRIN lenses are well suited to coupling the output of diode lasers into optical fibers because they can achieve aberration correction without complex multi-element systems or aspheric lenses; and because real images can be formed at the lens surface.  In the field of fiber coupling or in creating beam patterns, GRIN lenses are an economical, special alternative to conventional lenses.

When determining which of the available gradient index lenses or GRIN lenses is best for the application at hand, the following specifications are of greatest import: effective focal length, lens diameter, radius of the curvature, and the thickness of the edge and the center.  Focal length is the distance from the lens the light converges. Focal lengths are positive for a converging system and negative for a diverging system.  Diameter refers to the lens when viewed straight on. This could also be thought of as the height.  The radius of curvature can be determined if the lens were extrapolated into a sphere, then the radius of that sphere would be the radius of curvature.  Center thickness of GRIN lenses is simply the thickness of the lens at its center, while edge thickness is the thickness of the lens at its edge.

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