STOP Analysis of high-power laser systems - part 2

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

Preparation for optomechanical design

Once the optical design is complete, the next stage is to create a mechanical housing for the optics. In addition to providing protection and arrangement of the optical system, the mounting design for the lenses and mirror will introduce a source of mechanical load. Furthermore, the mountings can serve as a heat sink to dissipate heat from the optics. We’ll explore these two issues later in the process, but for now we’ll focus on the design of the optomechanics. The interaction between OpticStudio and OpticsBuilder significantly streamlines this process. The Prepare for OpticsBuilder tool exports the optical system in a format that an optomechanical engineer can open directly in their CAD tool with all the information needed to create the housings.

Once the housing is complete the entire design can easily be exported back to OpticStudio Non-Sequential Mode. OpticStudio Non-Sequential Mode has the capability to treat every object as a detector to compute the absorbed flux on every optical and mechanical surface in the system. Additional detectors can record the absorbed flux inside the volume of lenses. Laser beams are propagated through the system as ray bundles and their every interaction with the components recorded. For more information on the different types and uses of detectors see here.

By harnessing the power of ZOS-API this stage can be automated with a script to retrieve the flux data stored on the detectors and output configured to meet the input requirements of your FEA package. The system geometry is also exported to the FEA tool as CAD parts.

This process consists of 4 stages:

  1. Convert the sequential system into Non-Sequential Mode and prepare for optomechanical design.
  2. Export the non-sequential system to OpticsBuilder for Creo or other CAD platforms to add in lens mounts, housing, and other mechanical components.
  3. Export the complete system back to OpticStudio Non-Sequential Mode. Add Detector Volume objects to record absorbed flux in the system. Perform ray trace and report absorbed flux in each element.
  4. Prepare and export data for FEA analysis.

Convert to NSC Group

Open the attached file ‘Lens-3P_D25.4_2022.zar’. We want to get the absorbed flux information from the system and use the data for FEA analysis. To proceed, we first convert the system into Non-Sequential Mode by using the File…Convert…Convert to NSC Group tool.

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During conversion, a source and detectors will be added automatically by the tool. The mirror surface is converted to an Off-axis Mirror (object 4).

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Modify Reference Object

The automatically generated non-sequential file locates objects relative to one another (as in the sequential system) rather than defining their locations directly in global coordinates. This can be seen from the Ref Object parameter in the non-sequential component editor (NSCE). Before we add the Detector Volume objects, it is convenient to modify the reference objects for all elements to refer to global coordinates.

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The Null objects that were previously used as coordinate references are no longer needed and can be safely deleted after the reference coordinates modification.

Change object type for the Source

In the example we use Source Gaussian, that better represents our laser beam profile, to replace the automatically generated Source Ellipse object. There are two specific parameters associated with the Source Gaussian object, the beam size and the position. To generate a collimated beam of rays, leave the position as zero. The beam size parameter defines the beam radius at the 1 over e^2 point in irradiance. In this example, we set Power(Watts) to 800 W, the beam size is 5 mm, and specify 20 Layout and 1e6 Analysis rays for the Source Gaussian.

Detector settings involved for the system

Drawing resolution and detector property setting for lenses

As well as adding Detector Volume objects, we will turn on the Object is A Detector option for all optical and optomechanical elements. This will enable us to record the absorbed irradiance on the faces of these objects. Most objects of arbitrary shape may be used as a detector that records incoherent irradiance data. This includes objects with flat faces, such as the polygon, STL, and rectangular volume objects. It is an option that can be enabled under Object Properties…Type...Detector section. When the option is ticked, each individual triangle used to draw the object becomes a single pixel, and the number of pixels is related to the drawing resolution of that object. The detected irradiance can either be displayed visually in the Shaded Model or shown as text listing in the Text tab of the Detector Viewer.

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Highlight rows 2-6 in the NSCE, open Object Properties…Type and tick the option 'Object Is A Detector'. Under the Draw tab adjust the Drawing Resolution to High, which will increase the number of pixels/mesh density used to render this object.

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Defining coatings for optical surfaces

The measured absorbed flux  in Non-Sequential Mode considers both the surface absorption due to  coatings and the bulk absorption of the lens materials. The transmissive elements have the anti-reflection coatings, and the mirror has high reflective coating. We use the simple IDEAL coating with the format IDEAL <name> T R TIR in the example. The three intensity coefficients in the syntax represent transmission, reflection, and total internal reflection. The absorption coefficient is computed automatically via A = 1.0 - R - T, to conserve energy. If the TIR value is omitted, 1.0 is assumed. We add the following two IDEAL coatings in the coating file for subsequent use. The edit can be done by clicking Libraries…Coating Tools…Edit Coating File. Save the edited coating file as "COATING_LASER.DAT".

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To apply coatings, choose Coat/Scatter tab under Object Properties. The coating is applied to individual faces of the object. For the front and side faces of the mirror, apply HR_LASER coating, and for front and back faces of transmissive elements (lenses and window), apply AR_LASER coating. Later the listed AL_LASER coating will be applied to the surfaces of the housing when the anodized aluminum mechanical parts have been added.

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Modify transmission data of lens materials

In the example we use Fused Silica as the lens material. It has low absorption and can provide high thermal stability. OpticStudio uses Beer’s law to calculate the absorption based on the Internal Transmission data available for that materials in the catalog. The F_SILICA material in the default INFRARED catalog has an ideal transmission value of 1 for 0.3-2.3 um wavelength range. To accurately model the bulk absorption of F_SILICA, we need to enter realistic transmission data. However, we cannot modify data in the default catalogs provided by OpticStudio. The SAVE button is greyed out.

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To use the real bulk absorption data of F_SILICA, as referenced from OHARA website, we must first save the above catalog as a new customized catalog, for example,  MYCATALOGAGF. Then we can edit the Transmission data of the F_SILICA in this customized glass catalog as shown below.

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Next, we need to load this new customized glass catalog MYCATALOGAGF in System Explorer…Material Catalogs section.

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The system at this point ‘Lens-3P_D25.4_NONSEQ_2022.ZAR’ can be downloaded from the article attachments.

With these modifications made our system is now ready to export to OpticsBuilder, and we’ll do this in the next article.

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

Next article: Coming soon!
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