Sep 22, 2022
STOP Analysis of High-Power Laser Systems - Part 5
High-power lasers are widely used in a variety of applications, such as laser cutting, welding, drilling etc. The effect caused by the absorption of laser light in the optical system is noticeable. The performance of such optical systems will be degraded by heating from the high-power laser, either due to bulk absorption of the lens materials or surface absorption via coatings. Modelling of such effects is necessary to ensure the focal length stability and the laser beam size and quality. In this series of 5 blogs, we simulate laser heating effects, including the change of refractive index due to the increased temperature in the lens materials and the structural deformation caused by mechanical stress and thermal elastic effect.
Analyzing STOP effects with the STAR module
Once a structural and thermal has been performed in your FEA package the data can be exported to a series of simple text files for easy import into OpticStudio with the STAR Module. In this post, we show how the full suite of OpticStudio analyses is available to help you quantify and understand the impact on the system’s optical performance.
Loading and Fitting FEA data in OpticStudio
Note: these steps require you to have a STAR Module license and OpticStudio.
Firstly, we open our original sequential optical system from part 1 of this series. We'll use the STAR module to apply structural and thermal data, from our FEA tool, and assess the impact on the nominal optical performance.
To Load FEA data we click STAR…FEA Data…Load FEA Data, browse to the location of our data, select all relevant files, and click Open. There are several folders with data from different time points in the analysis. Initially, we'll use data from the folder 'FEA_Data_800W_0010s'.
The dropdown menus are used to assign Structural and Thermal datasets to correct surfaces. The layout enables us to check whether the datasets are aligned well before selecting OK to fit the data.
The fit error can be checked through the Fit Assessment tool for each dataset. By default, the Rigid Body Motions (RBMs) within each data set are removed before fitting the surface deformation. This typically improves fit accuracy, but the user has full control of this setting.
The FEA dataset can be enabled or disabled for individual surfaces in the Structural Data Summary and Thermal Data Summary table.
With the STAR data enabled, we can check the analysis windows to see the impact of the FEA data on system performance, such as Wavefront Map, Spot Diagram, and Sag Map analysis.
ZOS-API import of FEA data into STAR and analysis of optical performance
We’ve shown how simple it is to manually import FEA data into STAR. An alternative is to use the power of the STAR API to automate the process. This can be particularly useful where, for example, there are multiple FEA data sets to analyze.
In this example, temperature and deformation files were acquired from FEA software for different surfaces at different time steps. In this case 10, 60, 600, 1800 and 3600 seconds.
In this section, we will demonstrate how to load the FEA data for multiple time steps to perform a transient analysis of the optical performance. We created a ZOS-API Matlab script to automate this process.
Functions included in the code
Six functions are created within the code. Brief descriptions are provided below.
ListFiles()takes the data folder as the input string argument. It reads the filenames inside the data folder and recognizes the surface number and whether it is a deformation or temperature file based on the naming rule. The output items are structure Data, and an integer file number, which is the count of files in the folder.
RemoveAllFEA()is a function to remove all the imported FEA data from the current system. It checks whether the temperature or deformation dataset is imported for each surface, and then unloads those already imported data.
FEALoad()is used to import and load the deformation and temperature datasets from a data folder. The coordinates of the FEA dataset can be transformed as global or local for each surface. Optionally the fit settings can be configured before the fitting, such as Remove RBMs before fit for structural data, and set GRIN Step for thermal data.
SpotDiagram()uses a code snippet from ZOS-API syntax example 22_seq_spot_diagram to plot a spot diagram at the image surface.
WavefrontMap()is used to get wavefront map data from the system and plot the results.
SagMap()is used to get sag map data for the current system and plot the results.
Get an analysis of optical performance by using the created functions
In the main function of the code, the existing FEA dataset will be removed from the system first, and then the temperature and deformation files from a time step folder are imported into the system. The Spot Diagram and Wavefront Map analyses are performed and results are plotted in Matlab.
Save as GIFs
The final function in the code saves the frame analysis figures from each timestep folder into a graphic file, in gif format.
Using the ZOS-API code
We use this interactive code as follows
Open the sequential lens file.
Click on Interactive Extension under Matlab to generate the interactive connection boilerplate code.
Click Programming…ZOS-API.NET Applications…Interactive Extension
Open the interactive code TransientAnalysis.m in MATLAB and adjust the following sections accordingly to customize your code:
- When the code is run the following gif files will be generated.
Below is an example of the outSpot.gif to show the Spot diagram evolvement for the nominal, 10s, the 60s, 600s, 1800s, and 3600s time intervals.
Here is an example of the outWavefront. This Gif to shows the evolution of the wavefront map from the nominal system through the five-time steps.
Results & Analysis
After loading both the Structural and Thermal FEA dataset into the system, we can then examine the effect of FEA data on system performance. For example, let’s look at the system with 10-sec laser exposure. Our nominal system is diffraction-limited as indicated by the Through focus RMS spot radius curve below.
We can add to the same plot the RMS spot radius through focus for the 10s laser exposure with only the thermal gradient effect in place, and also the 10s laser exposure with both structural deformation and the thermal gradient in place. As you can see, exposure to a high-power laser beam for 10 sec has destroyed the system performance. The RMS spot size grows from a few microns to almost 300 um.
Additionally, when analyzing the on-axis chief ray position on the image surface in the 10s exposed system, we notice that the chief ray has shifted significantly. REAR operand returns the radial position of the on-axis chief ray and it moved 109 um away from its nominal position. This results in the boresight error (BSER operand) which describes the angular deviation of the formed image.
If we re-focus the system using the Quick Focus tool, the image plane shifts by nearly 4mm and performance improve. We now see a smaller spot size and lower wavefront error but the performance is still far from being diffraction-limited. However, with a large amount of defocus removed, we can now observe some of the finer details in the wavefront map due to the thermal and structural effects.
Next, we’ll compare the wavefront error in the nominal system vs wavefront error in the system at 5-time steps, 10, 60, 600, 1800, and 3600 sec, respectively.
Based on the results, going from nominal to 10-sec laser exposure, we observed a significant increase in RMS spot radius and RMS wavefront error. This performance degradation is caused by the thermal elastic and thermal optic effect of laser heating. As time increases, the heat may become more evenly distributed through the system resulting in more uniform/smoother changes in the optics’ shape and index gradient. This might have contributed to the smaller spot size and lower RMS wavefront error for a longer exposure time.
STAR module also provides a tool, System Viewer for you to view the index gradient distribution in the lens volume. Below we show the Index gradient distribution at 5-time intervals, 10, 60, 600, 1800, and 3600 seconds, respectively.
In this system, the thermally induced effects result in a significantly larger focused spot and reduced system efficiency. These effects could be mitigated by changing to higher transmission optical coating or modifying the system housing to improve the cooling. The workflow-enabled by OpticStudio, OpticsBuilder and STAR make it straightforward to evaluate such design enhancements and iterate quickly to the optimum solution.
In this blog, we demonstrate an end-to-end workflow of Structural, Thermal, and Optical Performance analyses using the OpticStudio STAR module. We start with an optimized Sequential system, export to OpticsBuilder for optomechanical design, import back to OpticStudio Non-Sequential Mode, perform ray trace and use Detector Volume to capture the absorbed flux on each element due to laser heating. The results are then exported from OpticStudio Non-Sequential mode and shared with the FEA engineer for FEA analysis. After the FEA analysis is done, we use the STAR module to bring the structural and thermal FEA results back into OpticStudio Sequential mode to analyze performance degradation caused by laser heating.
The STAR module for OpticStudio provides a new capability to directly integrate FEA results into OpticStudio. This allows a more comprehensive study of the impact due to both the thermal and structural deformation caused by laser heating.
In combination the Zemax suite of tools, OpticStudio, OpticsBuilder, and STAR Module, enables design teams, to seamlessly integrate FEA data into their optical and optomechanical design workflow. In addition, the STAR API helps engineers streamline and automate the whole process.
Find the full article and downloadable sample files on our Knowledgebase here.
Experience the power of the Ansys Zemax optical design software for yourself, request a free trial today!
Julia Zhang, Senior Application Engineer, Ansys
Hui Chen, Senior Application Engineer, Ansys
Steven La Cava, Senior Application Engineer, Ansys
Chris Normanshire, Lead Application Engineer, Ansys