Ultralightweight deformable mirrors

Abstract

This paper presents a concept for ultralightweight deformable mirrors, based on a thin substrate of optical surface quality, coated with continuous active layers that provide separate modes of actuation at different length scales. This concept eliminates any kind of stiff backing structure for the mirror surface and exploits microfabrication technologies to provide tight integration of the active materials into the mirror structure, to avoid actuator print-through effects. Proof-of-concept, 10 cm diameter mirrors with an areal density of 0.6 kg∕m 2 have been designed, built, and tested to measure their shape-correction performance and verify the finite-element models used for design. The low-cost manufacturing scheme involves low-temperature processing steps (below 140°C) to minimize residual stresses, does not require precision photolithography, and is therefore scalable to larger diameters depending on application requirements.

Thin Films with Ultra‐low Thermal Expansion

“…Fabricated fi lms were experimentally and analytically studied for the thermal and optical properties for application as thermally stable mirrors. [ 2 ] The structure of the thin fi lms consists of a two-dimensional (2D) periodic bi-material lattice (see Figure 1 ) composed of hexagonal plates of a higher CTE material (aluminum, 23.1 × 10 −6 /°C) combined with a frame of a lower CTE material (titanium, 8.6 × 10 −6 /°C). [ 9 ] When heated, the thermal expansion of the hexagonal plate is accommodated by stretching and bending of the frame into the open spaces, leaving the frame's connection nodes stationary, and resulting in the low effective CTE, as illustrated in Figure 1 a.…”

“…[ 10 ] We designed the bi-material unit cells for our mirrors using 3D fi nite element simulations [ 10 ] (see Experimental Section), and targeted a CTE value for the whole fi lm of 1.1 × 10 −6 /°C, as shown in Figure 1 b, with consideration of space optics application. [ 2 ] The effective CTE and local thermal strain release within the 3D plate were parametrically studied considering the effects of geometry, constituent materials properties, outof-plane deformations, substrate effects, and effective boundary conditions. The unit cells consist of two metals, to achieve high Ultra-low coeffi cient of thermal expansion (CTE) is an elusive property; most conventional materials with low CTE can operate only in narrow temperature ranges and have poor mechanical properties.…”

“…The unit cells consist of two metals, to achieve high Ultra-low coeffi cient of thermal expansion (CTE) is an elusive property; most conventional materials with low CTE can operate only in narrow temperature ranges and have poor mechanical properties. Low CTE materials, particularly in thin fi lm form, are in demand for optical, [ 1,2 ] energy, [ 3,4 ] and microelectronic [ 5,6 ] applications, but their fabrication is challenging due to the poor machinability of conventional low-CTE materials. [ 7,8 ] Here, we engineer structured free-standing thin fi lms with ultra-low effective CTE operating in a wide temperature range, to be integrated as thermally stable space telescope mirrors.…”

“…[ 7,8 ] Here, we engineer structured free-standing thin fi lms with ultra-low effective CTE operating in a wide temperature range, to be integrated as thermally stable space telescope mirrors. [ 2 ] They consist of a periodic array of bi-metallic unit cells, which locally release thermal strains in a purely mechanical way. [ 9,10 ] The thermal and optical properties of thin fi lms were characterized experimentally.…”

“…The geometry of the array can be tailored to tune the effective CTE [ 9,11 ] on demand. These engineered thin fi lms with tunable CTE can provide solutions to thermal fatigue and failure of light-weight devices operating in extreme thermal environments, such as space optical systems, [ 1,2 ] MEMS/semiconductor devices including fl exible electronics, [ 5,6 ] biomedical sensors, [ 12,13 ] and solar energy applications. [ 3,4 ] Conventional materials expand when heated, because the interatomic length increases with the potential energy.…”