STOP Analysis of high-power laser systems - part 3

High power lasers are widely used in a variety of applications, such as laser cutting, wielding, and drilling etc. The effect caused by absorption 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. Modeling of such effects is necessary to ensure the focal length stability and the laser beam size and quality. In this series of 5 articles we are going to simulate laser heating effects, including the change of refractive index due to the increased temperature in the lens materials, as well as the structural deformation caused by mechanical stress and thermal elastic effect.

Authored by: Julia Zhang, Hui Chen, Steven La Cava & Chris Normanshire

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Optomechanical design and analysis

Now the optical components have been designed we need to create the mechanical structures for mounting and housing the optics. There are a number of options for both preparing the mechanical components and importing back to OpticStudio for further analysis.

Preparing the mechanical parts

  • Using OpticsBuilder for Creo will enable you not only to create the required mechanical components, but also analyze their impact the rays passing through the system. You can interrogate specific ray paths of interest, add sources and detectors, all without leaving the CAD environment. The whole system can then be easily transferred back to OpticStudio.
  • Alternatively, you can import the optical components (as STEP files, for example) into another CAD package and design the mechanics around them, but without the easy data transfer and design insight provided by OpticsBuilder.

Importing to OpticStudio

  • Dynamic CAD Link. Using any of the CAD packages supported by OpticStudio Premium will enable you to import the mechanics as native CAD components with the underlying sketch parameters editable.
  • Using standard CAD formats, such as STEP or IGES. If your CAD platform is not supported by OpticStudio, then STEP and IGES files are a good alternative. These files can be generated from and imported to most CAD platforms. However, these files are not parametric and the underlying sketch elements are not editable.

The combination of OpticsBuilder and Dynamic CAD link is by far the most powerful and is the option we will demonstrate here. Please note that the STOP workflow (described in the other articles of this series) is possible with any of the other options outlined above , but at a cost of speed and efficiency.

Transferring the system to OpticsBuilder

Open ‘Lens-3P_D25.4_NONSEQ.ZAR’ in OpticStudio and click File…Prepare for OpticsBuilder.

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Next select the options desired in the pop-up window. To make the optical prescription editable, untick ‘Read only?’. Set your criteria for metrics such as spot size, image contamination and beam clipping, then click Prepare. This will generate a .ZBD file, which is the vehicle for passing data back and forth between OpticStudio and OpticsBuilder.Picture0.png

Open the CAD platform that has OpticsBuilder installed, in this case Creo.

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From the OpticsBuiler tab, select Import ZBD file, choose the file to import, then generate reference geometry. This allows the CAD parts to be mated with the optical components.Picture2.png

While in the OpticsBuilder table of the assembly window, right click on the optical components to reveal the prescription of each element. This is where the mechanical engineer can find the specifications (curvature, semi-diameter, etc.) of each optical element. This information is needed to build the housing elements that will encase the system. Note: Without OpticsBuilder this will need to be shared as a separate file or the mechanical engineer will need to pull the limited information available from a STEP file.

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Creating the lens housing

Next create sketches for the CAD components as would be done for any other project, define parameters within the sketch to create scalable relationships between elements.

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Use those sketches to create solids with tools such as revolve, extrude, etc. The example below is a sketch that was revolved about a center axis.

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Repeat this process for each of the parts created, then reopen the Assembly file that was created from the ZBD file. Next insert the parts created and mate them to the optics and each other.

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You can find the completed assembly in the OpticsBuilder samples file folder '\Documents\Zemax\Samples\OpticsBuilderCreo\Laser Induced Thermal Lensing Effect'

OpticsBuilder analyses

With OpticsBuilder the mechanical engineer will has the ability to run a ray trace within the CAD environment that will include interaction with both the optics and the CAD components. Rays can also be filtered by colored by different criteria to identify issues. In the example below, blue rays are passing through the system and red rays are being clipped by either a CAD part or an optical element.

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Another major step in validating the behavior of the optomechanical system is to check the performance using key optical metrics. These can be viewed in a concise results panel that will display after a simulation is run and it will show either a green check or a red bar dependent on whether the criteria specified in the Prepare for OpticsBuilder tool are satisfied or violated. This gives the mechanical engineer a quick reference if there are any major issues with the system. Other additional benefits available to a user of OpticsBuilder include the ability to view detectors in the system and place new sources and detectors in the system without returning to OpticStudio. These tools help the mechanical engineer understand the impact their mechanical design has on the optical system. This reduces the number of iterations sent to the optical engineer.

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In the image above , it is evident from the Detector Viewer panel that that the spot size looks the same with (right-hand image) and without (left) the CAD components. Other data reported shows RMS spot size, total hits of the number of rays reaching the detector, peak irradiance and total power at the detector plane. In this case we launched 10,000 rays, but only 9,998 are reaching the detector plane.

After interrogating the rays more it appears the rays are clipping the edge of an optical element and not a CAD component, as shown in the image below. This could be addressed in several ways, either the mechanical engineer can move the optical element (if “read only” was turned off at export and the element is supported for editing), or they can send the file back to the optical engineer to look at the issue in greater detail inside of OpticStudio. The final option is a hybrid, where the mechanical engineer makes the change, continues with the design and then send the file back to the optical engineer for review.

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The number of rays traced can be increased from the Settings tab of the OpticsBuilder ribbon to give a better understanding of the system.

Increasing the number of analysis rays to 10 million and adjusting the number of display rays to 50, it appears that the mirror is undersized for the input beam. This can then be compared, by the optical engineer, to the file in OpticStudio.

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A quick look at the original file in sequential mode shows that some rays miss the mirror. Note that in Sequential mode only intended path is traced. When a ray fails along the intended path that ray is truncated and does not continue to interact with other elements in the system, as illustrated below. In Non-Sequential Mode the ray would continue, which is why in OpticsBuilder the ray interacts with the housing after being clipped by the edge of the mirror, OpticsBuilder uses the non-sequential ray tracing engine.

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This was addressed in the final revision of the model where the beam and mirror are more adequately matched. This illustrates how design improvements and potential pitfalls can be discovered and corrected in a timely manner with the right workflow in place. It is worth noting that many initial designs are completed in Sequential Mode which, while a powerful design and analysis tool, models only the intended ray path. Non-Sequential Mode models a more robust scenario that includes both intended and unintended paths. Since OpticsBuilder files undergo a conversion from Sequential to Non-Sequential Mode and ray trace with Non-Sequential Mode, issues that may otherwise go unnoticed can be detected at this stage of the process. If a deeper analysis is deemed necessary, the files can be sent back to Non-Sequential Mode of OpticStudio for further scrutiny for things such as stray light or ghost analysis. This is made possible by the ZBD file format, which serves as a two-way street between OpticStudio and OpticsBuilder.

Accessing and modify optical properties

As well as geometric parameters (radius of curvature, semi-diameter) of the optical component, other optical properties, such as materials and thin-film coatings are automatically transferred from OpticStudio to OpticsBuilder. In addition, these optical properties can also be applied to the newly created mechanical components within the CAD environment.

To apply a surface property, either first select the CAD part from the OpticsBuilder part tab, then click Apply Surface Properties, on the OpticsBuilder.

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In the Component Summary there is a series of drop-down menus, one for each face of the CAD part. Select the desired surface property for each face. If a CAD part is created with OpticsBuilder installed and no property is designated, black anodized will be the default for all parts. Also, surface properties can be added to this list by either copying from the OpticStudio system folder the items used by your company's optical engineer or downloading vendor specific files.

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Other Options include adding coatings or scatter profiles to optical elements. This can be easily done inside of OpticsBuilder within the CAD environment or by sending it back to the optical engineer to perform the task in OpticStudio.

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Once the housing design is completed in OpticsBuilder, we save the finished file as a ZBD file that can be  opened in OpticStudio Non-Sequential Mode for further analysis. When opening the ZBD file in OpticStudio Non-Sequential Mode, any coatings and scattering profiles applied in will be automatically applied in OpticStudio. OpticsBuilder makes it easier to pass the system between a CAD environment and OpticStudio for ray tracing and maintains data integrity at each stage. This helps streamline the process of integrating mechanical components into an optical system.

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Exporting from OpticsBuilder has two options, one is to save as a STEP/IGES file the other is to save as a native CAD part, in this case a Creo part. If you are using OpticStudio Premium, the Dynamic CAD Link can be used in conjunction with OpticsBuilder. This allows the optical engineer to control the underlying sketch parameters of the CAD components inside of OpticStudio. Alternatively, the STEP/IGES file option will export all mechanical components as STEP/IGES parts whose geometries will not be editable in OpticStudio. It is important to note than in both cases all optical components are transferred back as, fully editable, native OpticStudio parts, and coating, material and scatter data is retained for all objects.

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The system at this point ‘system_NSC_2022.zar’ can be downloaded from the article attachments.

Conclusion

With initial design of the optical and optomechanical systems completed we can move the next stage in  our workflow. In the next article we’ll move back to OpticStudio and setup the non-sequential system to record the absorbed laser power on all optical and mechanical components.

This is the third article of the STOP Analysis of high-power laser systems series.

Next article: STOP Analysis of high-power laser systems - part 4
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