Increasing your options on the athermal glass map.
As a ray tracer, what potential athermal designs could you unlock if optical your glass choices were unconstrained? By using ALLVAR Alloy 30 as a housing element, you can expand your design space from the limited yellow region to the expanded blue region in the figure below. With Alloy 30, more glasses are available for athermal designs.
In collaboration with Edmund Optics and experts from the University of Rochester, we recently explored how ALLVAR Alloys can be used to athermalize commercially available optical doublets. Athermalization is crucial for optical systems exposed to varying temperatures. Without it, optical components can suffer from defocus and distortion, which degrades performance. Through this project, we leveraged ALLVAR’s unique negative coefficient of thermal expansion (CTE) to address these challenges.
ALLVAR Alloys are unlike any other material available today. These negative thermal expansion alloys not only enable athermalization of off-the-shelf lenses, but expand the range of materials and design options for optical engineers by unlocking enhanced thermal stability and performance in optics. Here’s an in-depth look at our approach and results with ALLVAR alloys in athermalizing an off-the-shelf doublet lens.
Expanding the Athermal Design Space with ALLVAR Alloys
Athermalization in optics involves carefully balancing lens and housing materials with different thermal properties to maintain a stable focus across a range of temperatures. A popular tool for this process is the “athermal glass map,” which plots thermal power versus chromatic power for various lens materials. By connecting any two lens materials (L1 & L2) with a line and extending the line to the y-axis, designers can identify the housing thermal expansion (H) required to athermalize a fixed focus system.
However, using traditional materials like stainless steel, brass, or aluminum for lens housings significantly limits the selection of athermal glass combinations. These materials typically fall within narrow CTE ranges between +12 PPM/°C and +24 PPM/°C, which restricts design flexibility. This is where ALLVAR Alloys change the game. With a CTE of -30 PPM/°C at room temperature, ALLVAR Alloys can balance thermal effects in optical systems, opening up the athermal lens design space and athermalizing even systems that were not designed for changing thermal environments.
In this project, we used an ALLVAR Alloy 30 alongside Aluminum to stabilize the focus of a commercially off-the-shelf (COTS) doublet lens from Edmund Optics. The goal was to ensure that the lens maintained a stable focal length from 25°C to 50°C. By strategically combining materials with different thermal properties, we were able to create a housing that minimized thermal focus shift.
Case Study: Athermalizing a COTS Doublet Lens
Our team selected a doublet lens with a 25 mm outer diameter and 50 mm focal length for this experiment. We aimed to calculate the thermal defocus of the lens when exposed to temperature changes. To do this, we started by determining the thermo-optic coefficient, beta, which represents the rate at which the refractive index changes with temperature for each lens in the doublet. Beta is crucial to calculate the total thermal defocus of the doublet, as it takes into account both the thermal expansion of the material and the refractive index shift with temperature.
Using data from Schott’s catalog of glass materials, we calculated beta for each lens element. One surprising discovery was the effect of dn/dT (the rate of change in refractive index with temperature) on each element. For Lens Element 2, dn/dT was extraordinarily small, four orders of magnitude smaller than that of Lens Element 1. However, the thermo-optic coefficient of Lens Element 2 was larger than that of Lens Element 1. This was because thermal expansion, rather than dn/dT, dominated the thermo-optic coefficient for Lens Element 2 while the thermal expansion and dn/dT mostly canceled each other for Lens Element 1. This insight was an eye-opener: minimizing dn/dT alone does not necessarily athermalize a lens, as thermal expansion can play an equally or even more significant role.
After calculating beta for the entire doublet, we found it to be -11.2 PPM/°C, which equates to a 12-micron shift in focal length when the temperature increases by 25°C. This analysis provided the baseline data needed to design a housing to athermalize the COTS doublet.
Designing an Athermal Housing with ALLVAR and Aluminum
To counteract the thermal defocus in the COTS doublet, we next focused on the lens housing. An all-aluminum housing would typically expand by about 25 microns across the same 25°C temperature increase. With the focal length shrinking and the housing expanding, the two movements combined to create a net defocus of around 38 microns.
| Table Header | Table Header | Table Header | Table Header |
|---|---|---|---|
|
Tube |
Material |
α [°C-1] |
Calculated Length |
|
1 |
ALLVAR Alloy 30 |
-30×10-6 |
28.304 |
|
2 |
Aluminum 6061 |
23.6×10-6 |
15.257 |
Our solution involved designing a part Aluminum and part ALLVAR Alloy 30 housing. By strategically combining these materials, we could dial in the housing’s thermal expansion by using the ALLVAR Alloy 30’s negative thermal expansion to offset the Aluminum’s positive thermal expansion. We calculated that using a 28.3 mm length of ALLVAR Alloy 30 within the housing should theoretically maintain the focal length stability across the 25°C temperature range.
Experimental Validation of an Athermal Doublet with Shear Plate Interferometry
With the doublet and housing assembled, we set up an experiment to test our design using shear plate interferometry. Shear plate interferometry is an optical testing technique that allow acurate measurement of small focal length changes. We placed the COTS doublet lens inside the housing with a mirror at the focal length and placed the assembly inside a thermal chamber. Using a to create interference patterns. The interference patterns were created by shearing collimated helium-neon laser light through a shear plate, projected on a screen and captured on a CMOS camera. Interference patterns were collected at different temperatures and analyzed to determine the thermal focus shift.
This test method offered two main advantages. First, by taking multiple measurements, we achieved sub-micron accuracy in measuring the focal length. Second, shear plate interferometry is forgiving of minor misalignments, allowing us to focus on thermal effects without requiring precise realignment at each temperature.
Results and Observations
The results were promising. For the all-aluminum housing, we measured a positive +29µm defocus shift at 50°C, which was 9.1µm less than our calculated value of positive +38µm. The ALLVAR athermalized housing, on the other hand, displayed a -9.4µm focus shift at the same temperature. This result showed that the ALLVAR Alloy athermalized housing overshot the intended zero-defocus point by roughly 9µm; the same discrepancy between the calculated shift and the actual shift of the all-Aluminum housing. With higher order calculations or direct measurement of the all-Aluminum housing, ALLVAR could produce an athermal response. Despite this minor overshoot, the ALLVAR Alloy housing provided significant thermal stability, highlighting the potential of ALLVAR Alloys to athermalize a COTS lens.
Conclusion
This proof-of-concept study highlights the transformative potential of ALLVAR Alloys in optical design. By providing access to a wider range of materials for athermalization, ALLVAR alloys give optic designers access to more flexible and reliable optical system architectures. Our experiment demonstrated that integrating ALLVAR Alloys could significantly reduce focus shifts of COTS lenses due to temperature changes.
With their unique negative thermal expansion properties, ALLVAR Alloys pave the way for stable, high-performance optics in thermally dynamic environments. This advancement opens the door to applications in a variety of fields where temperature-induced focus shifts are a concern. As we continue to refine our understanding and calculations, ALLVAR Alloys hold promise for expanding the capabilities of optical systems and providing new solutions to complex thermal challenges.
Want to learn more? Please find the SPIE proceedings titled “Athermalizing mounted doublets with ALLVAR alloys“. Ready to design your own optics with ALLVAR Alloys? Reach out to us today.
