Focusing Diffraction Grating Element With Aberration Control

The device has application in spectrometers, optical processors, and remote sensors.

Diffraction gratings are optical components with regular patterns of grooves, which angularly disperse incoming light by wavelength in a single plane, called dispersion plane. Traditional gratings on flat substrates do not perform wavefront transformation in the plane perpendicular to the dispersion plane. The device proposed here exhibits regular diffraction grating behavior, dispersing light. In addition, it performs wavelength transformation (focusing or defocusing) of diffracted light in a direction perpendicular to the dispersion plane (called sagittal plane).

The device is composed of a diffraction grating with the grooves in the form of equidistant arcs. It may be formed by defining a single arc or an arc approximation, then translating it along a certain direction by a distance equal to a multiple of a fixed distance (“grating period”) to obtain other groove positions. Such groove layout is nearly impossible to obtain using traditional ruling methods, such as mechanical ruling or holographic scribing, but is trivial for lithographically scribed gratings. Lithographic scribing is the newly developed method first commercially introduced by LightSmyth Technologies, which produces gratings with the highest performance and arbitrary groove shape/spacing for advanced aberration control. Unlike other types of focusing gratings, the grating is formed on a flat substrate. In a plane perpendicular to the substrate and parallel to the translation direction, the period of the grating and, therefore, the projection of its k-vector onto the plane is the same for any location on the grating surface. In that plane, no waveform transformation by the grating k-vector occurs, except of simple redirection.

Therefore, diffracted light experiences no wavelength transformation in the dispersion plane. It is redirected in much the same way as with a flat mirror. However, in the sagittal plane light gets redirected in a manner similar to a cylindrical mirror. It exhibits focusing with the focal length only defined by the curvature of the grooves, the incident wavelength, and the grating period.

Because of this, one single diffraction grating can exhibit two dispersal patterns. It disperses light just like a regular flat diffraction grating, while at the same time focusing (or defocusing) diffracted light onto a perpendicular plane. This focusing generates less aberrations than a mirror would. Non-trivial property of this device is that its focal length for a fixed wavelength does not depend on the incident angle, even if the angle is extremely non-paraxial.

This work was done by Dmitri Iazikov, Thomas W. Mossberg, and Christoph M. Greiner of Goddard Space Flight Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. GSC-15680-1

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