Broadband AR for a Cone of Light

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Here we consider a coating on glass that reflects only 0.5% of a cone of light for a range of wavelengths from 420 to 680 nm. The cone axis is normal to the glass. The cone has an F-number of 0.778, which means the half-angle is 40 degrees. An uncoated glass surface reflects 4.4% of this cone of light. We use a feature new to version 3.5: cone-angle targets. As optimization targets we use

R < 0.5% for wavelengths 420, 425, 430,..., 680 and for a 40-degree cone

In TFCalc, these targets are easy to enter by using the "Generate Cone Targets" command on the Options menu of the "Targets - Cone Angle" window. We use TFCalc's needle/tunneling optimization to design the coating from "scratch". That is, we begin the design process with a single thin layer of TiO2 and allow TFCalc to increase the thickness and number of layers in the design. TFCalc finds a 6-layer design in a few minutes. The performance of this design is shown below. The little circles indicate the optimization targets.

Plot of AR design

Here is the design, starting with the layer closest to glass, and with thicknesses given in nm:

TIO2	  11.04
SIO2	  44.43
TIO2	  34.99
SIO2	  28.25
TIO2	  30.46
SIO2	 104.92
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