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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Laser induced thermal effects

Traditionally, OpticStudio models thermal effects using the Make Thermal tool. This tool sets up multiple configurations of the design at different temperatures to allow thermal variation of performance to be analyzed. However, the Make Thermal tool has certain limitations. First, the temperature specified is assumed to be uniform across the entire element. Second, the thermal expansion and contraction is estimated in a linear fashion based only on the CTE of the material. This is a much-simplified approach that does not account for the performance degradation caused by the temperature gradient distribution in the optic. It also lacks the ability to simulate the exact deformed shape of the optic. Instead, the structural deformation is simply approximated as a uniform flattening of the radius of curvature due to heating.

The STAR module for OpticStudio can fully address these limitations. It provides a new capability to directly integrate FEA results into OpticStudio with unparalleled ease and accuracy. This allows a more comprehensive study of the impact due to both the thermal and the structural deformation caused by laser heating.

In combination the Ansys Zemax suite of optical tools enables design teams, for the first time, to understand by seamlessly integrating FEA data into their optical and optomechanical design workflow:

• Design and optimize optics for high power laser system
• Easily share optical designs and analyze optomechanical packaging within your CAD platform
• Integrate with FEA packages to perform detailed impact assessment of analysis of structural and thermal effects on optical performance
• Analysis of absorbed power in optics and mechanics

Setting up the optical system

The optical system must deliver the laser beam to the workpiece in a highly controlled manner, and OpticStudio has all the tools needed to design the system for optimal performance. In this example, the focusing lens is an F-Theta lens which ensures a consistently tight focus, therefore high laser power, at different positions during the scan. It comprises a set of three focusing lenses and a protective window in front of the image plane.

A system shown below is commonly used in the industry. It consists of two Scan Mirrors, a set of three focusing lenses and a protective window in front of the image plane. Two mirrors rotate in different directions to make sure the laser scans on different positions on the working plane.

In this example, we’ll use a similar system but only contains one fold mirror. The focusing lens is an F-Theta lens. It ensures a tight focus therefore high laser power at different positions on the image plane during the scan. The F-Theta lens is designed to have small f-θ distortion so the system produces linear displacement on a relatively large image surface when the laser is scanned at a constant rate.

The system is first designed with 1064 nm wavelength. The effective focal length is 100 mm. The scan angle is 2.5 degrees. In the Merit Function Editor we add appropriate boundary constraints for both the glass and air thicknesses and set the Pupil Integration as Gaussian Quadrature with 4 rings and 6 arms. You can find this initial file, “starting point.zar”, in the article attachments.

We then perform global optimization to reach a system that has a reasonable spot size. From the RMS vs Field analysis, we see the performance of the system is now diffraction limited. This is just one of the many possible design variants found by OpticStudio global search algorithm. 

Prepare for further analysis

The final optimized system is now slightly adjusted to prepare for the following design and analysis. We will only use this system in the on-axis case, so all fields, except the 0 degree point, are removed. A fold mirror, with X-tilt of -90 degrees, is added 40 mm in front of the lenses, and set to be system Stop. The entrance pupil diameter is reduced to 18 mm and A fixed semi-diameter of 12.7 mm (diameter 25.4 mm) is assigned to all optical elements.

Open Surface Properties…Draw tab of mirror surface 4 and set the “Thickness” to 2.65mm. This setting will not only affect the drawing in the Layout window in Sequential Mode, but it will also define the thickness of the mirror when converting the system into Non-Sequential mode.

Without considering the temperature change in lenses, focusing of the system is stable within a certain depth of focus. This can be seen from the analysis Image Quality…RMS…RMS vs focus.

The sequential design is now complete. You can find this file, “Lens-3P_D25.4_2022.zar” in the article attachments for your reference.

Conclusion

We have set up our optical system and optimized for optical performance. The next step is to prepare our system for export to a CAD package and optomechanical design of the housing.