ENGINEERING RESEARCH INSTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR REPORT NO. 5 MICROWAVE MEASUREMENTS OF THE INTENSITY DISTRIBUTION OF ECHELETTE DIFFRACTION GRATINGS Period August, 1955 - February, 1954 C. W. PETERS R. H. HUNT W. K. PURSLEY T. F.. ZIPF Project 2085 U. S. ARMY, CORPS OF ENGINEERS ENGINEER RESEARCH AND DEVELOPMENT LABORATORIES CONTRACT DA-44-009-ENG - 14l0, PROJECT 8-25-02-009 August, 1954

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN MICROWAVE MEASUREMENTS OF THE INTENSITY DISTRIBUTION OF ECHELETTE DIFFRACTION GRATINGS SUMMARY AND CONCLUSIONS Microwave measurements of the intensity distribution in the spectral orders of echelette diffraction gratings have been made for a set of gratings of blaze angles 25~, 550, and 45~ through the range of the ratio of wavelength to grating space of 0.5 to 1.5 and as a function of the angle of incidence of the beam of radiation onto the grating for both directions of polarization. The results show a manifold of strange effects and forcibly demonstrate the complexities of diffraction phenomena. The results can be of considerable value in selecting a grating for a spectrograph, predicting its performance for different wavelengths, and estimating the quality of an actual grating. OUTLINE OF THE WORK These measurements are an extension of the work covered in Report No. 2 of this project in which the energy distributions from a 150 and a 25~ blaze-angle grating were reported. The idea behind the present program was to obtain a background of experimental data which could be used to predict the performance of echelette diffraction gratings. The following set of gratings was selected for this purpose, and are listed in the table below according to blaze angle and the ratio of the grating space to the wavelength [D/x]. Blaze Angle 25 35" 45|.7.7.7.94.85 1.0 o 1,2 1.2 1.41 1067 1.7 2.4 1 e

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN The apparatus, method of constructing the gratings, procedure in the measurements, etco were the same as detailed in Report No. 2. RESULTS Graphs of the measurements are arranged as listed in the previous table, with the polarization having the electric field parallel to the grating grooves first, followed by the perpendicular component. Little can be said in general that cannot be gleaned from a cursory inspection of the curves. On the other hand, there is a wealth of details; as for example the anti-echelette action of the perpendicular component on a 355 grating at normal incidence [i = 0] in Fig. 18, or the symmetry of the curves for the parallel polarization for positive and negative angles of incidence when the grating space is small. Again in Fig. 16, the peaks in the 0th order intensity correspond to the angle of disappearance of other spectral orders -- the condition for the "Wood Anomalies," and evidently an example of the same. And, as more general conclusions, the parallel polarization has its greatest concentration into a single order not at the blaze, but where the wavelength is such that the 1st order spectrum occurs at a smaller angle than the blaze angle; also, efficiencies approach 100 percent for the perpendicular polarization at the blaze angle and larger angles. These curves constitute a survey of the performance of echelette gratings and may be used as a guide to the selection of the most suitable grating for a spectrometer, or may serve to indicate how far any actual grating deviates from the behavior of a more or less perfect model. 2.

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