This guide walks through an example setup for modeling a Wasatch Photonics holographic transmission grating in Ansys Zemax OpticStudio. The example uses a 1200 lines/mm, 840 nm circular grating with a 25.4 mm diameter and 20 mm clear aperture.
Example Grating Specifications
We’ll use the example drawing and specification table below to identify the key inputs for the Zemax model:
- Diameter
- Clear aperture
- Spatial frequency
- Angle of incidence
- Total thickness
- Substrate material
- Start a New Sequential File
Start a new file in Ansys Zemax OpticStudio and keep the system in Sequential mode.
In System Explorer, set the aperture type to Entrance Pupil Diameter. For this example, use a 20 mm aperture.
- Add Surfaces in the Lens Data Editor
In the Lens Data Editor, click the stop surface and insert additional surfaces above Row 1. Add enough surfaces to represent the incoming space, coordinate breaks, grating glass, diffraction grating surface, and image plane.
Change the first surface after the object surface to a thickness of 50 mm. This spaces the collimated light source from the grating.
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Add the First Coordinate Break
Set the surface type
Change the next surface (Row 2) to Coordinate Break by typing it directly or selecting it from the surface type dropdown.
Set the angle of incidence
In Tilt About X, enter 30.3°.
Confirm ray path
The 3D Viewer should now show rays traveling from the object to the first coordinate break.
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Define the Grating Thickness and Material
Update the surfaces that represent the two halves of the grating.
For a 3 mm thick grating, split the thickness into two 1.5 mm sections. Enter N-BK7 as the material for both glass sections.
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Add the Diffraction Grating Surface
Set the surface type
Change the surface between the two glass sections to Diffraction Grating.
Set the grating density
Parameter 1 (Lines/µm) should be set to the grating’s lines/mm divided by 1000.
For a 1200 lines/mm grating, enter 1.2 lines/µm.
Set the diffraction order
Set Parameter 2 (Diffraction Order) to -1.
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Add the Second Coordinate Break
Set the surface type and tilt
Change the output-side surface (Row 7) to Coordinate Break and set Tilt About X to 30.3°.
Add spacing before the image plane
Add 50 mm thickness to this coordinate break to create spacing before the image plane.
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Define the Grating Apertures
Define the aperture and dimensions of grating surfaces 3 through 6.
Set Surface 3 aperture
Set the aperture type for Surface 3 to Circular Aperture with a maximum radius of 12.7 mm to represent a 25.4 mm diameter grating (Figure 7a).
For rectangular gratings, a rectangular aperture may be used instead.
Set Surface 5 and 6 aperture references
For Surface 5 and 6, change Pickup From from None to Surface 3 (Figure 7b).
Set Surface 4 clear aperture
For Surface 4, which represents the diffraction grating, set a circular aperture with a maximum radius of 10 mm to represent the 20 mm clear aperture (Figure 7c).
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Add Multiple Wavelength Configurations
Open the Multi-Configuration Editor to add wavelength configurations.
Change the active configuration from MOFF to WAVE.
Insert additional configurations and enter the desired wavelengths in µm. For example:
- 700 nm = 0.700
µm - 840 nm = 0.840 µm
- 960 nm = 0.960 µm
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View Wavelength-Dependent Diffraction
In the 3D Viewer settings, set Color Rays By to Config # and set Configuration to All.
This allows you to visualize how different wavelengths diffract through the grating.
Make sure both coordinate breaks include the appropriate angle of incidence so the output beam, image plane, or downstream optics are positioned correctly.
Additional Modeling Note
When modeling a circular, square, or rectangular grating, it can be helpful to add an extra surface before the diffraction grating surface. This allows the model to preserve the full mechanical size before reducing to the grating’s clear aperture.
