A method of shaping diffraction-grating grooves to optimize the spectral efficiency, spectral range, and image quality of a spectral imaging instrument is under development. The method is based on the use of an advanced design algorithm to determine the possibly complex shape of grooves needed to obtain a desired efficiency- versus- wavelength response (see figure). Then electron- beam fabrication techniques are used to realize the required groove shape. The method could be used, for example, to make the spectral efficiency of the grating in a given wavelength range proportional to the inverse of the spectral efficiency of a photodetector array so that the overall spectral efficiency of the combination of the grating and the photodetector array would be flat. The method has thus far been applied to one-dimensional gratings only, but in principle, it is also applicable to two-dimensional gratings.

All of the Grooves of a Grating are designed to have the same possibly complex shape, which is chosenin an iterative process to obtain a desired spectral response. This approach offers greater designflexibility than does a prior method of tailoring the spectral response of a grating by dividing the gratinginto two or more zones that contain conventional sawtooth grooves having different blaze angles.
The algorithm involves calculations in the spatial-frequency domain. The spatial- frequency spectrum of a grating is represented as a diffraction-order spectral- peak-width function multiplied by an efficiency function for a single grating groove. This representation affords computational efficiency and accuracy by making it possible to consider only the response from one grating groove (one period of the grating), instead of from the whole grating area, in determining the response from the entire grating. This combination of efficiency and accuracy is crucial for future extensions of the algorithm to two-dimensional designs and to designs in which polarization must also be taken into account.

The algorithm begins with the definition of target values of relative efficiency that represent the desired spectral response of the grating in certain spectral frequencies calculated from the diffraction order and wavelength. The grating period is divided into a number of cells — typically, 100. The phase contribution from each cell is determined from the phase of the incident electromagnetic wave and the height of the grating surface in the cell. The total contribution from all cells to each target value is then calculated. Then a method known to specialists as the optimum- rotation-angle method is used to adjust the height of each cell so that the total response from all cells is optimized. The computation is iterative and continues until the desired response is obtained. In the event that the desired response is unphysical, the algorithm nevertheless strives to generate a grating- grove profile for which the response approximates the desired one as closely as possible.

This work was done by John Backlund, Daniel Wilson, Pantazis Mouroulis, Paul Maker, and Richard Muller of NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to: Innovative Technology Assets Management

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Refer to NPO-40429, volume and number of this NASA Tech Briefs issue, and the page number.

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Shaping Diffraction-Grating Grooves to Optimize Efficiency

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NASA Tech Briefs Magazine

This article first appeared in the March, 2008 issue of NASA Tech Briefs Magazine (Vol. 32 No. 3).

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Overview

The document discusses advancements in the design and optimization of structured-groove diffraction gratings, particularly for applications in spectroscopic instruments. Developed by NASA's Jet Propulsion Laboratory, the new design algorithm enhances the performance of gratings by allowing for the synthesis of arbitrary desired spectral responses, which is a significant improvement over conventional grating designs.

The structured-groove gratings are designed to eliminate the need for multiple blaze zones, which are common in traditional dual-blaze gratings. This simplification allows for uniform treatment of the grating area, enabling designers to optimize imaging performance without the complications of zonal apodization effects that can degrade spectral imaging quality. The document emphasizes that the new gratings can be tailored to match specific spectrometer source illumination and detector responsivity curves, thereby optimizing signal-to-noise ratios and overall imaging performance.

The design process involves dividing the grating period into numerous cells (typically 100), where the phase contribution from each cell is calculated based on the incident wave and the local height of the grating profile. An iterative procedure is employed to adjust the heights of these cells using an optimal rotation angle method until the design specifications are met. This approach allows for the generation of grating profiles that closely fulfill desired performance criteria, even if the initial specifications are unphysical.

The document highlights the potential applications of these structured-groove gratings in various wavelength bands, including the visible-near-infrared (VNIR) and short-wave infrared (SWIR) ranges. For instance, in the VNIR band, a flat efficiency response is desirable, while in the SWIR band, the efficiency should align with the solar blackbody curve. The successful implementation of electron-beam fabrication techniques is also noted as crucial for realizing these structured grating designs.

Overall, the report underscores the proprietary value of the new design algorithm and the structured-groove grating technology, which promises to enhance the efficiency, image quality, and spectral range of spectroscopic instruments, paving the way for improved performance in future aerospace applications.