Introduction

In a conventional optical system, the refractive index within each optical component is considered to be homogeneous. In the design of such systems, the curvatures, thickness, and refractive index of each component are varied independently to optimize the performance of a lens system. It is possible, on the other hand, to manufacture lens elements whose index of refraction varies in a continuous way as a function of spatial coordinate. Such materials, which are often said to be GRadient INdex (GRIN) materials, have various advantages for use in both focusing and imaging purposes.

There are three types of refractive index gradients depending upon the type of symmetry [1–3]. The first is the axial gradient (a-GRIN), in which the refractive index varies in a continuous way along the optical axis. These gradients are particularly useful for replacing aspheric surfaces in monochromatic systems, e.g. in collimators for laser beams.

The second type of gradient is the radial gradient (r-GRIN). The refractive index n in this case varies perpendicular to and continuously outward from the optical axis. This type of gradient index lens, often called a GRIN rod lens when the diameter is small, exhibits the property that light propagating parallel to the optical axis of symmetry can be focused periodically if n varies as an appropriate function of its radial distance from the axis of symmetry. The main applications of gradient index materials in use today are based only on this type of gradient.