Encrypted Thin Films

Introduction

FRED's encrypted file coating type allows the modeling of optical systems with complex coatings whose prescription is complete for the purposes of raytracing and yet whose detail is hidden from the user. In this article we will take a three layer Broadband Anti-reflection (BBAR) coating and create an encrypted coating version of this coating as an .fec file. This file can be shared with other FRED users without revealing the underlying prescription.

Download the FRED file: BBAR_Coating.frd

Example

In order to create an encrypted coating to share with other FRED users, the initial prescription must be known to the author of the encrypted file and be specified as a "Thin Film Layered Coating" within FRED. This coating type requires the materials to also be defined.

Broadband Anti Reflection (BBAR) Coatings consist of several layers

In this case the materials are already defined within the Custom material catalogue within FRED and just needed to be added to the material folder. Note that in the FRED file this step has already been done. The way we add catalogue materials to the folder is by right clicking on the Materials folder of the object tree and choose "Add Glass Catalog Material" (glass is a misnomer, many non-glasses are defined in the custom catalogue).

Also defined in the coating folder of the example file is the Thin Film Multi-layer coating prescription containing the prescription of the coating:

Material definitions and the BBAR coating that uses them defined as a "Thin Film Layered Coating"

To turn this into an encrypted coating we simply right-click in the table and choose the option "Export to Encrypted File..."

In the dialog that opens choose the encrypted file name and save location.

To check the encrypted coating performance is the same as the original prescription, right click on the coating folder and choose to "Create a new Coating...". In the window that opens, give the coating a suitable name and choose the type to be "Encrypted Thin Film", then press the "select file" button and navigate to the .fec file we just created. Make sure to change the substrate to "At last layer" then click OK:

In the next dialog that opens accept the sampling defaults, and the BBAR Encrypted coating will be added to the coating folder. To verify performance right click on the original coating and choose to plot the coating (we've plotted transmission as a function of S and P), then do the same for the encrypted version on the coating.

Identical performance of the original and encrypted coating

And we can see that, as expected, the encrypted coating has the same performance as the original. We recommend performing this test before sharing the encrypted coating with other FRED users.