Custom Diffraction Gratings

The objective of this work was to design, develop and manufacture a batch of custom transmission diffraction gratings with a low zero-order optical transmission across the visible and near-infrared waveband. A high laser damage threshold was also desirable. It was shown that neither conventional blazed gratings nor holographically fabricated sinusoidal photoresist gratings were entirely suitable. The preferred approach was to use reactive ion etching to transfer a quasi-sinusoidal groove profile into the surface of a hard substrate such as fused silica. Differential etch rates were exploited to achieve a larger groove amplitude in the etched gratings than in the photoresist mask. Of the batch of ten grating which was supplied, the best grating exhibited a zero-order transmission which did not exceed 6 over the wavelength range 543nm - 875nm. The main conclusion of the work was that a deep grooved surface relief transmission diffraction grating can provide a broad band optical attenuation of around 95 across the visible and near infrared waveband. The use of reactive ion etching techniques can enable the production of such gratings with high laser damage thresholds.