Spectrometers consisting of a diffractive lens and a concave diffraction grating

Abstract

The concave diffraction grating is used in many spectrometers and monochromators. Since it has both dispersion and focusing properties, one does not need a collimator or camera. In a nearly symmetric setup with a small deviation angle, the second- and third-order aberrations of the spherical concave diffraction grating are less then the respective aberrations of the setup with a plane diffraction grating and spherical mirrors. However, the residual defocusing limits the applicability of concave gratings in high resolution devices. This aberration can only be partially compensated by a grating with nonequidistant grooves. The focal distance of the diffractive lens has a linear dependence on the wavelength. This property is used to design hybrid diffractive-refractive lens systems for a wide spectral region. It is both tempting and promising to compensate the residual defocusing of a concave diffraction grating by means of a diffractive lens. The present paper analyses, both analytically and numerically, the effect of such compensation. The use of the diffractive lenses in the concave grating spectrometers is shown for the flat-field spectrometer and the double monochromator.