Erwin G. Loewen

Grating efficiency theory as it applies to blazed and holographic gratings

Recently developed rigorous theories have been used to investigate the diffraction efficiency behavior of both blazed and holographic gratings. In order to assist designers of spectrometric systems we have covered a complete range of blaze angles for triangular grooves and modulations for sinusoidal groove shape in first and second orders. Several types of mountings are included together with the role played by finite conductivity of aluminum. Useful classifications of both types of gratings are given, as they apply from the near uv to ir regions. Comparisons showing the close agreement between theory and experiment are presented.

Dielectric coated gratings: a curious property

Theoretical calculations predict that under certain special conditions a shallow fine pitch diffraction grating, given a precise dielectric overcoating, acts as a complete absorber of incident light of a specific wavelength and polarized in the P plane. Despite lack of a physical explanation, the phenomenon was confirmed by experiment.

Simple selection rules for VUV and XUV diffraction gratings

The bulk of diffraction gratings used in the VUV and XUV regions are used at small angles of incidence and have shallow groove depths. Using rigorous electromagnetic theory, it is shown that such gratings behave in a scalar manner. It is then possible to derive universal efficiency curves from which one can obtain with simple rules the efficiencies of gratings with any metallic surface. The role of standard dielectric overcoatings is investigated and is found to be small. Some comparisons with experiments are given.

Efficiency optimization of rectangular groove gratings for use in the visible and IR regions

Rigorous electromagnetic theory is used to investigate the low order diffraction efficiency behavior of perfectly conducting rectangular groove plane diffraction gratings in the lambda/d domain of 0.25-1.8 of interest to spectrometry. We show how to optimize performance and make comparisons with blazed and holographic gratings. The role of finite conductivity and departure from Littrow conditions is also investigated. Comparison with experiment confirms the accuracy of the theory.

Echelles: scalar, electromagnetic, and real-groove properties

For lack of alternatives, echelle-grating diffraction behavior has in the past been modeled on scalar theory, despite observations that indicate significant deviations. To resolve this difficulty a detailed experimental, theoretical, and numerical study is performed for several echelles that work at low (18-13), medium (35-55), high (84-140), and very-high (to 660) diffraction orders. Noticeable deviations from the scalar model were detected both experimentally and numerically, on the basis of electromagnetic theory: (1) the shift of the observed blaze position was shown to decrease with the wavelength-to-period ratio, and it tends to zero more rapidly than the decrease of the maximum width, so that the TE- and TM-plane responses tend to merge into each other; (2) cut-off effects (Rayleigh anomalies) were found to play a significant role for high groove angles, where passing-off orders are close to the blaze order. A possibility for evaluation of the blaze angle from angular, rather than from spectral, measure nts is discussed.Several reasons for the differences between real and ideal echelles (material-index deviations, profile deformations, and groove-angle errors) are analyzed, and their effects on the performance of echelles is studied.

Correlation between Efficiency of Diffraction Gratings and Theoretical Calculations over a Wide Range

Results described here show that infinite conductivity theory matches experimental grating efficiencies very closely when wavelengths exceed 4 µm, in fact often serves adequately above 1 µm. At shorter wavelengths, i.e. the visible region, the influence of finite conductivity on the theory must be included, in which case once again excellent agreement with experiment can be demonstrated. The nature of anomalies, especially as affected by different metal surfaces, is a highly sensitive test for theory. In the vacuum ultraviolet region correspondence between theory and experiment could still be observed, although some new practical limitations intervene.

On an asymptotic theory of diffraction gratings used in the scalar domain

Starting from the electromagnetic theory, we derive an asymptotic formalism to investigate the behavior of perfectly conducting gratings used at small wavelengths to groove pitch ratios and near normal incidence. The theory is applied to study three classical types of profiles: sinusoidal, lamellar, and blazed gratings. Results are given for both −1 and −2 Littrow (or near Littrow) mounts. The accuracy of the theory and the limits of the domain where it applies are studied by the use of rigorous electromagnetic computations. The role of a finite conductivity of the surface is also investigated.

Echelle gratings: their testing and improvement.

Tests are described on new master echelles of very large size and on others having unusually high blaze angles, produced on interferometrically controlled ruling engines. Defects arising from the gravitational distortion of large grating blanks during ruling were reduced by the use of calibrated edge-supporting springs to diminish sag. The sixth echelle in a series of the largest yet produced (400 x 600 mm, or 16 x 24 in.), ruled on a fused silica blank 127 mm thick with 79 grooves/mm blazed at 63.4 degrees , gives high speed at resolutions well in excess of 10(6). A series of 254-mm echelles ruled at blaze angles whose tangents range from 2 to 8 were compared. Because an echelle blazed at 63 degrees can theoretically give 90% of the maximum resolving power available at any blaze angle, little resolution is gained by using steeper angles, and optical defects are likely to increase with groove asymmetry. Very high blaze angles are useful for raising intrinsic dispersion, however, making possible construction of echelle spectrometers of extreme compactness.

Total absorption of light by a sinusoidal grating near grazing incidence.

A heretofore unknown total absorption phenomenon on a diffraction grating has been demonstrated both theoretically and experimentally. It occurs only in a series of six special conditions, namely, groove shape, angle of incidence, groove frequency, depth modulation, polarization, and metal surface. None of the classical explanations for grating anomalous behavior seems to correspond to this case.

Brewster effects for deep metallic gratings

Total absorption of light by highly modulated metallic gratings is demonstrated both theoretically and experimentally. This phenomenon occurs when only the zeroth-order propagates and is linked with the excitation of surface plasmons.