Spectral uniformity of two- and four-level diffractive optical elements for spectroscopy

Abstract

We present simulations and characterization of gold coated diffractive optical elements (DOEs) that have been designed and fabricated in silicon for an industrial application of near-infrared spectroscopy. The DOE design is focusing and reflecting, and two-level and four-level binary designs were studied. Our application requires the spectral response of the DOE to be uniform over the DOE surface. Thus the variation in the spectral response over the surface was measured, and studied in simulations. Measurements as well as simulations show that the uniformity of the spectral response is much better for the four-level design than for the two-level design. Finally, simulations and measurements show that the four-level design meets the requirements of spectral uniformity from the industrial application, whereas the simulations show that the physical properties of diffraction gratings in general make the simpler tw level design unsuitable.

Wood’s anomalies and spectral uniformity of focusing diffractive optical elements

“…We showed in [3] that the DOEs designed with four levels have a much better spectral uniformity than the corresponding DOEs with two levels. A better spectral uniformity means that the quality of the spectroscopic measurement is much less dependent on the uniformity of the illumination of the element.…”

“…Several such systems have been realized, see for instance [1] for a review of integrated microspectrometers. In [2] and [3] a DOE was described which acts as a focusing, beam splitting and dispersive element in a spectroscopy application. The task of the spectrometer is to characterize different polymer materials by studying the absorption of several different wavelengths in the infrared.…”

“…However, there are advantages and disadvantages with both the two-and fourlevel DOEs. The two-level DOE yields a lower diffraction efficiency, and also the effect of Wood's anomalies are much stronger than for the four-level DOE, as shown in [3]. On the other hand, the fabrication of four-level DOEs is more demanding since it requires the realization of smaller linewidths, and also two photolithography steps with the alignment of a second mask to the pattern realized by the first mask.…”

“…Thus, for a given set of design wavelengths, the system geometry can in principle be designed such that the Wood's anomalies are avoided. In light of this, the simulation method presented in [3] is a valuable tool to test a given design before fabrication.…”

“…In [3] the main reason for the large variations in spectral uniformity for the two-level DOE was attributed to the Wood's anomalies. The excitation of the anomalies depends amongst other parameters on the incidence angle and the groove width on the DOE.…”