Additive manufacturing is increasingly used for optical applications, especially for the production of the optical elements. However, larger elements usually require further post-processing steps of the optical surfaces and are not printable as monolithic multi-element systems. Nevertheless, optical systems can still benefit significantly by utilising the design freedom for the mounting structures. We designed a fully monolithic and additively manufactured mounting structure that is robust against mechanical and thermal influences from the environment and passively compensates their effects. We present our design approach and prove its feasibility by stressing an imaging lens and evaluating its optical performance.
Passive athermalization techniques for optical systems combine materials in the mounting structure with different coefficients of thermal expansion (CTE) to minimize thermally driven shifts. However, such an approach requires complex structures to be manufactured with multiple high-precision opto-mechanical components. Our concept utilises a monolithic and additively manufactured mounting structure and a housing made of a second material to generate mechanical stresses caused by temperature fluctuations. The difference in coefficients of thermal expansion induces these mechanical stresses during temperature changes, resulting in elastic deformations of the inner monolithic structure. The magnitude of the local deformation of the monolithic structure is adjusted via the stiffness between the optical elements. This allows to control the displacement for each optical element such that their positions remain unaffected by a thermal load and thus passively athermalizes the optical system.
Additive Manufacturing (AM) processes improved to such an extent, that they are now applied for production of optical elements like lenses, mirrors and light guides. However, since the quality of these manufacturing processes still does not meet typical surface roughness specifications for optical surfaces, except on the micrometer scale, additional effort for post processing is required. It is therefore more convenient to use the potential of AM to build the mounting structures of optical systems and use more suitable technologies for the optical surface production. For that reason, we develop additively manufactured and monolithic mounting structures using the design freedom and flexibility of AM and combine them with macroscopic lenses of excellent optical quality. To evaluate our concept, we measured the modulation transfer function (MTF) of a commercial photo lens. We replaced its mounting structure with our additively manufactured and monolithic one and measured the MTF again. It could be demonstrated that the replaced mounting structure has a nearly identical optical performance and even an improvement in the off-axis fields, due to less tolerance chains.
Additive Manufacturing (AM) is often not considered as a manufacturing technique for mounting structures, because the assembly of optical systems usually requires small manufacturing tolerances, that are currently hard to achieve. But in comparison to conventional manufacturing techniques, AM has the advantage of being able to easily individualize each manufactured structure. Hence, mounting structures can be designed and adapted such, that the measured inner decentrations of conventionally manufactured optical elements to be mounted can be compensated. With this, the error budget contributions from optical elements can be minimized, either to relax their or their mounting structure tolerances or to reduce the overall system tolerance. To prove this concept, we designed an optical system to quantify small displacements of optical elements within a simple to replace additive manufactured mounting structure. From analysing the recorded intensity distribution, one can accurately quantify the lateral decentration of one of the optical elements in respect to the other one. The first experimental results show, that it is feasible to control and adjust the decentration of an optical element mounted in an additively manufactured structure.
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