Coatings - Thin Film Layered Coating

The Thin Film Layered Coating type is used to create a single or multilayer thin film coating model that can be attached to any surface. The characteristic matrix of the coating stack is used to determine the appropriate reflection and transmission properties as a function of wavelength and angle of incidence and the substrate material is determined automatically from the surface that the coating is assigned to. The Thin Film Layered Coating can also be exported to an Encrypted Thin Film model.

Navigation

The Thin Film Layered Coating can be accessed in the following ways:

•Open a new coating dialog and select "Thin Film Layered Coating" as the coating type.

Controls

Control	Inputs / Description	Defaults
Name	Name of the coating.	Coating n
Description	Description of the coating.
Type	Coating type. Select "Thin Film Layered Coating" from the drop-down menu.	Sampled Coating
Thin Film Layered Coating Options
Wavelength	Enter the coating design wavelength in microns. This value is used when calculating the optical thickness of the layers if the Thickness parameter is toggled to use Wavelengths. Otherwise, this parameter has no effect.	Default wavelength
Angle	Enter the coating design angle in degrees. This value is used when calculating the optical thickness of the layers if the Thickness parameter is toggled to use Wavelengths. Otherwise, this parameter has no effect.	0 (normal incidence)
Thickness	Enter the units for layer thicknesses. Choices are wavelengths (at the design wavelength), microns or Geometry units (system units).	Microns
Substrate	Enter substrate location (first layer or last layer). The substrate material is determined to be that with the highest refractive index of the two materials assigned to the surface. In this way, the ordering of the coating stack is unambiguous given the incident direction of a ray, as long as the materials are unique.	At Last Layer
Grp#-Layer#	Grp# refers to the coating group, Layer# refers to the location of the layer within the assigned Grp#. Multiple layers can be combined into a group (see application note below).	1-1
Thickness	Enter the layer thickness in the selected units. When units of wavelengths are entered the thickness value is the optical thickness, n*d, where n is the index of refraction at the designated wavelength and d is the physical thickness of the layer.	0
Material	Enter the coating material from the pull-down menu. Available materials are those defined in the Materials folder of the object tree.	Air
Repeat Count	Number of times the layer or group is repeated.	1

OK	Accept settings and close the dialog.
Cancel	Discard settings and close the dialog.
Help	Get coating help.

Application Notes

Importing thin film prescriptions

FRED supports the import of thin film coating prescriptions from The Essential MacLeod (*.csv) and Optilayer (*.frt). This feature can be accessed by right mouse clicking in the spreadsheet area of the coating definition and selecting "Import Prescription from File" from the list menu.

Encrypted Thin Film Export

The Thin Film coating model can be exported to FRED's Encrypted Thin Film file format by right mouse clicking in the Thin Film coating dialog and selecting, "Export to Encrypted File". Please refer to the Encrypted Thin Film help topic for more information. This capability is intended to allow coating vendors to distribute accurate coating models to their customers for raytracing analysis while also protecting the coating vendor's intellectual property.

A "Save As" dialog will be opened when a Thin Film is requested to be exported to an Encrypted Thin Film file. If the Thin Film coating model was generated by reading in a prescription from one of the supported file formats, the exported Encrypted Thin Film file will re-use the original file name with "_encrypted.fec" appended on. Otherwise, the name of the coating will be used (blank spaces are replaced with "_") and "_encrypted.fec" will be appended on. The name and description string of the Thin Film Layered coating are saved in the *.fec file and are assigned to the Encrypted Thin Film coating when the *.fec file is imported. The *.fec file extension stands for FRED Encrypted Coating.

After choosing a file name and location, the user will be prompted to configure wavelength sampling for the exported model. The refractive index values of the constituent thin film materials are stored as sampled data in the encrypted coating model. Consequently, the entered wavelength sampling should span the wavelength range over which the coating is intended to be used and should include a sufficient number of samples such that the refractive index values are accurately represented over the wavelength range. Using more sample points in the exported data will produce a better match between the Encrypted Thin Film coating model and the original Thin Film coating model.

Please note that the encrypted coatings files used in ZEMAX and CODEV are not compatible with FRED and vice versa; each software uses its own proprietary encryption tool.

Legal note: The encryption method used is a “medium grade” encryption algorithm intended to prevent non-professional hackers from gaining access to the encrypted information. The user assumes all responsibility for the use of this feature.

Coating Stack Orientation

The substrate material is determined to be that with the highest refractive index of the two materials assigned to the surface. In this way, the ordering of the coating stack is unambiguous given the incident direction of a ray as long, as the materials are unique. In the case where the materials assigned to the surface are identical (ex. Cubed Beamsplitter) and refractive index values cannot be used to determine the substrate, the convention is that the first layer of the coating is located at Z = 0 on the surface and the N'th layer is located the appropriate distance away along the +Z direction.

Using Birefringent Materials in Layers

When a layer in the thin film stack is defined using a birefringent material, the ordinary refractive index values of the material will be used in the thin film calculation. The birefringent nature of the material is not considered.

Group and layer #'s

Coating layers can be combined into groups by highlighting the desired layers (drag your mouse in the "Designation" column of the coating spreadsheet area), right mouse clicking and selecting "Make into group". Each group can then be assigned a repeat count, allowing thin film prescriptions to be entered quickly and efficiently.

Coatings and raytrace controls

Coatings are used in conjunction with the Raytrace Controls to determine ray propagation.

•To continue propagating a reflected ray, both the Raytrace Control and the Coating must allow the reflection.

•To continue propagating a transmitted ray, both the Raytrace Control and the Coating must allow the transmission.

•Scattered rays are subject to the same limitations. Currently, an absorbing surface cannot generate scattered rays.

Copying to a new FRED model

Copies between FRED models will always transfer new material and coating information.

Quarter Wave AR coating

This example shows a single layer quarter wave thickness magnesium fluoride (n = 1.38) coating. It is sometimes referred to as V-coat because this describes the shape of the reflectance curve as a function of wavelength. For this example, minimum reflectance is achieved at a wavelength of 0.55 microns at a 45-degree angle of incidence. Note that the Wavelength, Angle, and Thickness settings are all used by FRED to calculate the physical thickness of the layer. The incident media is air and the substrate material is glass.

2 layer AR coating

This example shows a coating with 2 thin film layers. Coating layers are appended, inserted, or deleted using the context menu that pops up with a right mouse click. The first layer is a quarter wave of Magnesium Fluoride.

The second layer is a high index coating (Ti02, n = 2.2) one half-wave thick.

The incident media for this coating is air and the substrate is glass. The order of intersection is Air | MgF2 | Ti02 | Glass. Note that the Group number of the second layer has been incremented by one. FRED allows grouping and repetition of multiple layers, as shown in the Dichroic Filter example.

Dichroic Filter

This example shows a 3 layer coating sequence that is repeated multiple times. To group a sequence of layers, select the first layer in the sequence with a left mouse click and, while holding down the mouse button, drag the pointer down to the last layer. When the desired layers are highlighted, right mouse click and select "Make Into Group" from the list menu, as shown below.

The Repeat Count is the number of times this coating sequence is repeated. Note that the group number for all three layers is the same. A group can be undone by again selecting all the layers in the group, right mouse clicking and selecting the option "Make into Individual Layers". The Repeat Count is automatically reset to 1 when a coating group is split into layers.

Narrowband Filter

A narrowband filter is constructed using long sequences of alternating high and low quarter wave layers separated by a half-wave high index layer, as shown below.