Thickness error compensation for digital gradient-index optical coatings

Abstract

A method of fabricating an optical coating includes a layer pair, with a layer of a first optical material having a first refractive index and a first nominal thickness and a layer of a second optical material having a second refractive index and a second nominal thickness. The first layer is deposited, then its actual thickness is measured. A target thickness for the second layer is determined so that the optical properties of the combined actual first layer and target second layer are substantially the same as the optical properties of the combined nominal first layer and nominal second layer. The second layer is then deposited. Where the optical coating further includes a plurality of layer pairs, the steps of depositing, measuring, determining, and depositing are repeated for each of the layer pairs in the pairwise approach. Alternatively, in the layer-to-layer approach, for each layer pair after the initial layer pair the actual thickness of the second layer in the preceding layer pair is measured and a target thickness for the first layer is determined so that the optical properties of the combined actual preceding second layer and target first layer are substantially the same as the optical properties of the combined nominal preceding second layer and nominal first layer. The first layer is then deposited and its actual thickness measured. A target thickness for the second layer is determined so that the optical properties of the combined actual first layer and target second layer are substantially the same as the optical pro