What is thermal lensing?
Thermal lensing causes the focus position of a lens to change as the optics are heated or cooled.
How to get rid of thermal lensing?
By determining the thermal time constant of the optical system using the Haas BWA-MON or by using the Haas LTI TLC optics which passively mitigate thermal lensing.
Thermal lensing is a significant challenge in high-power laser applications, particularly for collimators and focusing lenses. This phenomenon arises due to the inherent thermal properties of optical materials, notably the coefficient of thermal expansion (α, measured in 10-6 °C) and the temperature coefficient of refractive index (dn/dT, also in 10-6 °C). As these materials absorb laser energy, temperature variations lead to changes in their refractive indices and physical dimensions, causing unintended alterations in the lens’ focusing behavior.
Traditionally, mitigating thermal lensing has involved the use of low-expansion glasses, such as fused silica. While fused silica offers a low coefficient of thermal expansion, it does not possess a correspondingly low dn/dT. Consequently, the dn/dT of the glass significantly impacts the thermal lensing effect. Thermal management of the lens mechanics has been one of the primary methods to address this issue. However, since optical glass generally exhibits poor thermal conductivity, substantial temperature gradients can develop across the lens, adversely affecting its performance. Even with thermal management, these gradients can lead to performance issues, necessitating several minutes for the optical system to stabilize thermally before the focus shift remains within the Rayleigh range.
In response to these challenges, Haas Laser Technologies has developed the TLC optics, a novel solution designed to passively compensate for thermal lensing in high-power laser systems. Protected under U.S. Patent No. 8,274,743, this innovative optic design employs a proprietary merit function that balances the dn/dT between two high-power laser optical materials, along with optimized thicknesses, air spacing, and curvatures. This approach minimizes thermal lensing across a broad temperature range, from 20°C to 200°C. As a result, applications utilizing the TLC optics remain in focus from the moment the laser is activated until it is turned off, eliminating the need for a warm-up period.
The effectiveness of this design is illustrated in Figure 2, which compares a simple fused silica best-form singlet at ambient temperature (depicted with blue rays) to the same lens subjected to a significant temperature gradient (shown with green rays). The shift in focus due to thermal effects is evident. In contrast, Figure 5 provides a close-up view of the Haas TLC doublet (as seen in Figure 1) with the same focal length and temperature gradient, demonstrating its superior thermal stability and consistent focusing performance under varying thermal conditions.
Figure 2: Best Form Focusing Lens under thermal load
Figure 3: Zoom of best form focusing lens at temperature extremes
By addressing the root causes of thermal lensing through advanced optical design and material selection, the Haas TLC optics represents a significant advancement in high-power laser optics, ensuring reliable and precise performance in demanding applications.
Figure 4: Close up of TLC objective focus at temperature extremes
Using the Haas LTI patented BWA-MON®, we measured a customer’s 10 kW, multimode fiber laser and their F125 mm, all-fused silica collimation lens for thermal lensing. Figure 5 is a screenshot of the measurement that shows a focus shift of a 1 mm over 1 minute.
Figure 5: A 1-minute thermal lensing measurement of a 10 kW, multimode fiber laser
Figure 6: The same fiber laser of Figure 5 but with a Haas LTI TLC collimator
Figure 6 shows the same laser from Figure 5 but with a Haas LTI TLC collimator and the beam waist location over another minute. The thermal shift was less than 100 microns over the same time frame. Equally important to observe between Figure 5 and Figure 6 is that the all-fused silica lens is experiencing tremendous thermal stress to the point that severe astigmatism manifests. At about 5 seconds, you can see the two axes cross, and the astigmatism gets worse as the thermal load increases. Whereas the relative position of the beam axes does not change with the TLC optics. This is due to the coefficient of refractive index of the glass and the thermal expansion are compensated by material choice and mechanical design.
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