A brief summary of gradient-index (GRIN) optical principles is given, followed by a state-of-the-art review. Much of the important recent work has been in the area of GRIN materials, specifically: new glasses for ion exchange, new ion exchange techniques, GRIN sol-gel glasses and plastic GRIN. New metrology techniques and fabricated GRIN imaging systems are also discussed.

The widespread use of arrays of gradient-index rod lenses in the office is described. The important imaging and radiometric characteristics of these arrays are provided along with how these properties change with certain lens parameters. A table of commercially available lens arrays is provided.

The optical design and construction of flexible, gradient-index, and conventional endoscopes are described in this paper. Lens and prism components used in medical endoscopic instrumentation are discussed. Emphasis is given to the practical considerations of manufacturing these systems that are unique to their small size and use in a hospital environment.

This paper provides an overview of the optical subsystems used in optical recording, their present status, and future trends. Using reflectivity-sensing and polarization-sensing optical heads as examples, the many requirements that must be integrated into one assembly are described along with the individual technology alternatives. The state-of-the-art of conventional free-space optical, holographic, and hybrid elements is also described along with an integrated head assembly using waveguide optics. Future directions and requirements for these technologies are indicated.

As lightwave systems evolve and mature, and the range of applications for them broadens, there is increasing need for a wide variety of optical components to exploit fully the capacity and capability of the fibers. Many of the components that have been developed for these purposes utilize miniature optical elements, and many of these use gradient-index lenses. Such components are commonly referred to as microoptic components. In this paper we review various component technologies currently available or under development for optical fiber communications systems, with particular emphasis on microoptic components. We consider components for passive optical functions, such as mixing, splitting, wavelength-division multiplexing, or isolation; for low-speed active functions, such as circuit switching or variable attenuation; and for high-speed active functions, such as modulation or time-division multiplexing.

The use of gradient properties in a wide range of physical processes are discussed with emphasis on optical applications of interest to the military.

Gradient-index devices used in integrated optics are reviewed. Gradient-index waveguides, the use of external gradient-index (GRIN) optics in integrated optics and integrated devices and structures are discussed, with strongest emphasis on 'the latter. Both longitudinal and transverse gradients are useful in this technology, and examples of both kinds are given.