Abstract
Concerted rotations of a self-focused varied line-space diffraction grating about its groove axis and surface normal define a new geometric class of monochromator. Defocusing is canceled, while the scanned wavelength is reinforced at fixed conjugate distances and horizontal deviation angle. This enables high spectral resolution over a wide band, and is of particular advantage at grazing reflection angles. A new, rigorous light-path formulation employs non-paraxial reference points to isolate the lateral ray aberrations, with those of power-sum ≤ 3 explicitly expanded for a plane grating. Each of these 14 Fermat equations agrees precisely with the value extracted from numerical raytrace simulations. An example soft X-ray design (6° deviation angle and 2 × 4 mrad aperture) attains a resolving power > 25,000 over a three octave scan range. The proposed rotation scheme is not limited to plane surfaces or monochromators, providing a new degree of freedom in optical design. Grating rotation about its third (meridional) axis may be employed to cancel vertical deflection of the diffracted beam while maintaining the above aberration correction. This enables a simpler (pure rotary) motion for the exit slit and a fixed beam direction both horizontally and vertically.
Highlights
The reflection of light at wavelengths λ À 100 nm is encumbered by losses to absorption and scattering, whose reduction favors few and simple optical surfaces
Concave grating, while the second introduced surface-normal rotation (SNR) and initially employed a constant line-space (CLS) concave grating. These have been the only single-element solutions which remain “in-focus” with scanned wavelength, yet employ slit positions and ray directions fixed in the direction of dispersion
This paper reports the detailed imaging characteristics of the basic configuration, the spectral resolution as a function of aperture and scan range
Summary
The reflection of light at wavelengths λ À 100 nm is encumbered by losses to absorption and scattering, whose reduction favors few and simple (plane or spherical) optical surfaces. Concave grating, while the second introduced surface-normal rotation (SNR) and initially employed a constant line-space (CLS) concave grating To date, these have been the only single-element solutions which remain “in-focus” (no spectral aberration linear versus aperture) with scanned wavelength, yet employ slit positions and ray directions fixed in the direction of dispersion. These have been the only single-element solutions which remain “in-focus” (no spectral aberration linear versus aperture) with scanned wavelength, yet employ slit positions and ray directions fixed in the direction of dispersion The endpoint in this progression towards minimization would be reflection from a single plane grating surface, which can exhibit near-invariance of the focal length with graze angle and provide access to the most accurate fabrication methods.
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