Sammendrag
We present our activity on the characterization and simulation of diffractive optical elements (DOEs) for near infrared spectroscopy. The DOE surface is designed to direct and focus light of a limited set of wavelengths onto a detector. In order to perform this, the surface pattern has a complicated structure and varies significantly over the element. This leads to a variation in the diffraction efficiency if the incident light only illuminates certain parts of the DOE. For the use of a DOE in a spectrometer, it is for some applications important that the local diffraction efficiency of each wavelength component relative to the others is as uniform as possible over the DOE. Otherwise, the spectroscopic measurement can become unreliable. A measurement setup was built at SINTEF to characterize the DOEs. This setup uses a movable aperture in front of the DOE, so that it is possible to measure the diffraction efficiency in different positions on the DOE. The simulations were carried out using the PCGrate software [1], which computes the diffraction efficiency for a periodic diffraction grating, employing full vectorial representation of the optical field. We carried out the simulations for one wavelength and position at a time, modeling the DOE surface by a perfectly periodic diffraction grating, with a period and an orientation designed to yield a surface with the same performance as the fabricated DOE. This yields a simple model that gives variations of local diffraction efficiency of the same order of magnitude as the measurements.