GRIN-V: progress in gradient-index imaging

Donald B. Keck

The Fifth International Meeting on Gradient-Index Optical Imaging Systems was held in Monterey, Califor-nia on 19-20 April 1984. Progress in the field is reviewed.

For more than a decade a reasonable fraction ofresearch in optical imaging has concentrated in thearea of gradient refractive index (GRIN) lenses. Thistechnology holds the promise of impacting optical sys-tem design with regard to size, weight and economics inaddition to overall optical quality. Some systems canbe designed using GRIN lenses which would be virtual-ly impossible to configure in any other way.

As demonstrated by the papers in this issue, steadyprogress continues to be made in this technology. Thefield continues to move toward microoptics with gradi-ent lenses made with ion exchange techniques by Nip-pon Sheet Glass Corporation being used commerciallyin photocopier lens arrays, medical endoscopes and awide variety of devices for splitting, and coupling lightin optical fiber telecommunication applications. Thebasic underlying imaging theory and radiometric pro-peries of these lenses are well understood. Lens per-formance characterization techniques for these appli-cations are also in hand. Papers such as "Radiometricanalysis of gradient-index lens arrays for reduction-/enlargement" by W. Lama and J. Durbin, "Computa-tion of optical path length in gradient-index media: afast and accurate method" by A. Sharma, and "Mea-surement of the differential thermal expansion andtemperature dependence of refractive index in gradi-ent-index glass" by P. McLaughlin and D. T. Moore inthis issue attest to the status of our understanding.

The new exploration in this field is still centered inthe microoptics area. The emphasis has been on ex-panding the performance characteristics. Theoreticalwork aimed at describing higher order aberrations con-tinues, such as the paper "Fifth-order analysis ofGRIN lenses" by E. Marchand in this issue. Fabrica-tion of lenses with larger gradient constants, that is,

The author is with Corning Glass Works, Research & Develop-ment Division, Corning, New York 14831.

Received 22 October 1985.0003-6935/85/244287-01$02.00/0.© 1985 Optical Society of America.

numerical apertures greater than 0.4, is having goodsuccess and offers an increased range of applicationpossibilities. An increasingly interesting researcharea deals with the combination of gradient-index ma-terials with simple spherical lenses. These designsprovide aberration correction with the gradient whileusing the simple sphere to gain lens power.

To date large diameter, radial gradient lenses havenot been fabricated with much success. However, newfabrication techniques are still being identified to in-crease the lens size. Lenses 5 mm in diameter madewith a porous glass process have been described at arecent conference.

Photochemical processes in both plastic and glassare being studied as another alternative to ion ex-change. A considerable effort on gradient lens arraysfor use in telecommunications is underway at TokyoInstitute of Technology. Using stacked arrays, nu-merical apertures in the 0.5-0.6 range have beenachieved. A wavefront aberration of less than 0.25 Xwith a 0.17 N.A. has been shown possible.

Fabrication techniques continue to represent thelimiting factor in this technology. Independent of thisdesigners have moved forward so that GRIN lenseshave been designed for more than a dozen applications.In addition to those mentioned above, designs for bothwide-angle and 50-mm f/12 photographic objectives,Huygens, Ramsden and Kellner eyepieces, binocularand biocular objectives (the latter in this issue), andspecial lenses for aerodynamics have been defined.Design at this point leads most other aspects of thistechnology.

The work continues by a small but dedicated groupof scientists and in fact the sixth Topical Meeting onGradient-Index Optical Imaging Systems was just heldin Palermo, Sicily. New gradient materials for mid-IRand UV were described as well as further improve-ments in performance for most other gradient lensprocesses. Details of these papers will be reported in asubsequent issue of Applied Optics. In summary, theexcellent work reported in the papers of this issue iscontinuing in this fascinating and important area ofoptics.

15 December 1985 / Vol. 24, No. 24 / APPLIED OPTICS 4287