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High Resolution Optics Obtained by the Application of Phase Correction Patterns

IP.com Disclosure Number: IPCOM000050924D
Original Publication Date: 1982-Dec-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Spiller, EA: AUTHOR

Abstract

An aspheric optical element (mirror or lens) is corrected by depositing concentric grating rings on the optical element surface. The grating rings are of such a thickness that each ring causes a phase shift of 180 degrees. Optical element correction is accomplished by making the spacings and widths of the rings such that the entire optical element contributes to an image point with substantially equal phase.

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High Resolution Optics Obtained by the Application of Phase Correction Patterns

An aspheric optical element (mirror or lens) is corrected by depositing concentric grating rings on the optical element surface. The grating rings are of such a thickness that each ring causes a phase shift of 180 degrees. Optical element correction is accomplished by making the spacings and widths of the rings such that the entire optical element contributes to an image point with substantially equal phase.

Perfect imaging of an object point into an image point can be obtained by a single aspheric surface, for example, by a mirror in the form of an ellipsoid (or a paraboloid for an object at infinity).

However, it is much more difficult to fabricate aspheric surfaces with high precision than it is to manufacture a spherical surface. Numerous attempts to produce high quality aspherical surfaces have been reported. Correction of spherical surfaces by the addition of evaporated films is reviewed by J. Kurdock and F. Austin in Physics of Thin Films, Vol. 10, pp 261-308, eds. G. Hass and M. Francombe (Academic Press, 1978). These attempts have failed mainly because it is very difficult to produce high quality correcting films of sufficiently large thickness.

A correction method is reported here where the thickness of the correction film is only Lambda/4 for a mirror (Lambda=wavelength of radiation used). The correction procedure consists of the following steps: 1. The height difference between the desired surface and the starting surface is calculated or measured as a function of

the coordinates (x,y) on the (mirror) surface. 2. A deposition or etching mask is fabricated with openings at coordinates where the height difference obtained in step 1

corresponds to a phase shift of an odd multiple of 180

degrees. The width of each opening corresponds to a +/- 90

degrees phase shift around this value. 3. In the areas defined by the mask a film is deposited having a thickness which corresponds to a 1800 phase shift (Lambda/4

for a mirror at normal incidence). Grooves of the same

thickness may be etched out instead.

In the resulting optical element the entire surface contributes with equal phase (+ 90 degrees) to the desired image point and a resolution close to the diffraction limit is obtained. EXAMPLE: Soft X-ray telescope mirror, f=499.5 cm, Lambda=50 Angstroms normal incidence. 1. The drawing gives the calculated phase difference between a spherical and an ideal paraboloidal mirror and the

corresponding distance from the center of the mirror. The

first six distances, where the p...