This article introduces one novel and useful feature in STAR of Ansys Zemax OpticStudio 25R1 (Enterprise), STAR stress birefringence. It enables many new capabilities and possibilities in OpticStudio.
Authored by Chenfeng Gu
Introduction
Stress birefringence is a consequence of stress acting on a transmissive optical material due to the photoelastic effect and will result in wavefront error and polarization changes in an optical system.
Stress birefringence in optical materials is a multifaceted issue, stemming from a variety of factors that can be attributed to. These include the mechanical stress induced by external pressures such as those from mounting hardware, the influence of primary manufacturing processes with their inherent material process variables, and the additional stress introduced by secondary manufacturing processes like grinding, cutting, heating, and bending. Furthermore, physical damage from handling, which can result in chips, cracks, or scratches, as well as material inhomogeneities such as bubbles or inclusions, contribute to birefringence. The thermal effects of manufacturing processes, which create temperature differentials, and the pressures exerted during plastic molding, also play a role in inducing stress within the material.
Typical technologies affected by stress birefringence are ones such as optical lithography, data storage, high-energy lasers, LCD projectors, free-space optical communication, consumer electronics and defense, AR/VR/XR systems and so on. For example, high stress in thin display glass may result in poor material yield due to fracturing during processing or handling. In some cases, high stress can degrade the optical performance of finished parts. In lithographic applications, this can lead to poor quality images. For polarization dependent and polymer-based AR/VR systems, analysing the impact of stress birefringence from manufacturing processes is becoming a crucial step in the development.
Internal stress in Injection molded-lens
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STAR stress birefringence in OpticStudio could allow users to calculate the stress birefringence effect in “Real Wavefronts” through the combination of FEA stress data fitting and non-uniform gradient index (GRIN) ray tracing. It considers the direction of the propagation of the wavefronts and calculates the polarization and wavefront error induced by the non-uniform stress field. Its effects are considered in all Analysis tools, including the Polarization tools such as Polarization Ray Trace, Polarization Pupil map, etc.
In this article, we’ll explain how STAR stress birefringence works, and how to evaluate the impact of stress birefringence using OpticStudio.
How STAR stress birefringence works
Required inputs for stress birefringence
Stress birefringence calculation requires two external inputs: stress FEA data files and stress-optical coefficients.
For stress FEA data files, nine columns in a tab-delimited format provide the stress tensor normal and shear components, with columns in the following order:
[X position, Y position, Z position, SX, SY, SZ, SXY, SYZ, SXZ].
The position units should be in mm while the stress tensor components units should be in MPa.
At the same time, the material catalog is being updated to include stress-optical coefficients. The AGF file can include stress birefringence material properties. Below the IT lines there will be a new BD line (BD for Birefringence Data). The line(s) in the AGF file look like this:
The coefficients data will be provided in the format as following, holding four values:
- Wavelength at which the Stress Optical Coefficient was measured (in micrometers)
- Stress Optical Coefficient K = K11-K12 (unit 10-6 mm²/N)
- -K11 photoelastic coefficient for light oscillating parallel to the direction of stress (unit 10-6 mm²/N)
- -K12 photoelastic coefficient for light oscillating perpendicular to the direction of stress (unit 10-6 mm²/N)
For stress birefringence simulation, both stress-optical coefficients K11 and K12 are required. If stress-optical coefficients are defined for a single wavelength, the model considers them to be constant across the spectral range. If the coefficients are defined for more than a wavelength, then an interpolation is made to consider the variation across the spectrum.
STAR stress birefringence workflow
When external inputs have been well prepared, you can use functions under the STAR tab to simulate and analyse the stress birefringence effects:
Multiphysics Data Viewer can be used to view the FEA stress datasets and verify the correct file before assigning it to an optical surface and performing a numeric fit. Through changing the settings, normal and shear stresses can be checked separately.
Next, through Multiphysics Data Loader, you can import stress datasets and assign them to optical surfaces. Once you have verified that the datasets are all aligned, click OK (Fit Multiphysics Data), then the fitting is done.
Fit Assessment provides controls to adjust the numerical fit settings for each stress dataset to its associated surface while Alignment Check allows for an overall view of the alignment of stress datasets in a single window.
Under the Data Summary, Volumetric Data Summary shows an overview of the surfaces that have Volumetric datasets assigned and provides quick controls to enable or disable the effects of these datasets, including stress datasets. Specially, there is one “Ray Tracing Model “choice for stress birefringence.
STAR can use two different ray tracing models for stress birefringence analysis. Ray Tracing model can be selected for each surface with loaded stress data in the Volumetric Data Summary:
- GRIN ray tracing (Default) – Utilizes Ansys Zemax OpticStudio’s GRIN tracer to propagate the ray through the system. This option might be more accurate but will require more computational resources. It is recommended that this method be used for imaging analyses (more detailed discussion can be found in the helpfile).
- Linear propagation – Assumes that the ray path variation within the material is negligible. As a result, the ray follows a linear path through the optic. The optical path difference is considered but the ray follows a linear path through the material. This option will likely be faster but may be slightly less accurate. It is recommended that this method be used for polarization analysis (more detailed discussion can be found in the helpfile).
Stress birefringence causes a phase difference, or retardance between both polarizations. Thanks to the System Viewer function, when Data Type is selected as Stress, you may select among Lens Retardance (OPD), Lens Retardance (Phase Shift), Point Retardance (OPD), Point Retardance (Phase Shift), Index or Change in Index choices following what you need. More detailed explanations can be found in the help file.
The Performance Analysis enables the ability to see how each stress dataset that is assigned to an optical surface is contributing to the overall change in the specified performance metric. Four types of metrics can be selected for stress birefringence analysis:
- Spot size - RMS Spot Size
- Wavefront error - RMS Wavefront Error
- RMS Retardance (OPD) – RMS Retardance in nanometers
- RMS Retardance (Phase Shift) – RMS Retardance in radians
In addition to the nice analysis functions mentioned above, you can use Analysis tools to evaluate too, for example, the Wavefront tools and Polarization tools.
With the Polarization Pupil Map and Wavefront Map, you can monitor the effects of stress as the rays pass through the system and compare the stressed and unstressed state to quantify the effect.
Conclusion
This article introduces the feature STAR stress birefringence in Ansys Zemax OpticStudio (Enterprise)25R1 release. It enables many new capabilities and possibilities in stress birefringence analysis. Combining with other multiphysics tools, an accurate simulation of the complete multiphysics workflow will be presented in next knowledge base article. Hope it will make your daily work more efficient and creative. Looking forward to your feedback on this feature through community posts or email!