ALLVAR Alloys enable the passive athermalization of optics. These  offer a high magnitude Negative Thermal Expansion (NTE) in the axial direction, and a positive expansion in the radial direction. This NTE can compensate for the natural changes in an optic’s focal length as temperature changes. This thermal compensation allows reduced length, weight, and complexity of optical assemblies, like the one shown to the left.

The Athermalization Issue

The series system schematic to the right displays a common way to use two different materials (𝐿1 and 𝐿2) to match the thermal expansion of the focal length (𝛿𝑓).

A particularly challenging issue arises in infrared optics when large changes in refractive index with temperature (𝑑𝑛/𝑑𝑇) create a negative coefficient of thermal defocus, 𝛿𝑓.

The current solution to the negative 𝛿𝑓 problem is the parallel system displayed to the left. The trade off is the increased diameter, length and weight with decreased mechanical stiffness and stability when compared to the simpler series system.

With ALLVAR NTE Alloys, you can:

  • Achieve athermalization of the design
  • Decrease thermal compensator length, diameter, and weight
  • Simplify design and manufacturing.
A Solution by the Numbers The plot below demonstrates how ALLVAR Alloys can simultaneously open the achievable series system design window and decrease the length of commonly used Aluminum and Delrin parallel systems. The term 𝐿2/𝑓 is used to quantify the increase in total thermal compensator length of the parallel system compared to the series system. It represents the increased length required per unit of focal length.

Coefficient of Thermal Defocus versus Optic Length – Shorter Athermalization with ALLVAR Comparing commonly used Aluminum and Delrin to ALLVAR Alloy 30 and Delrin, the savings are immediately apparent. Not only does the current theoretical parallel system window extend down to -30×10-6 K-1, but the ALLVAR-Delrin system can reduce compensator length up to 40%.

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