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 indi cate significant deviations. To resolve this difficulty a detailed exp erimental, theoretical, and numerical study is performed for several e chelles that work at low (8-13), medium (35-55), high (84-140), and ve ry-high (to 660) diffraction orders. Noticeable deviations from the sc alar model were detected both experimentally and numerically, on the b asis of electromagnetic theory: (1) the shift of the observed blaze po sition 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 signific ant 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, measurements is discussed. Severa l reasons for the differences between real and ideal echelles (materia l-index deviations, profile deformations, and groove-angle errors) are analyzed, and their effects on the performance of echelles is studied .