Thermal control for ion beam figuring of thin-layer unimorph deformable mirror

To improve the effective deformation of the unimorph deformable mirror (DM), ion beam figuring (IBF) is applied for manufacturing the initial surface figure. However, heat produced during IBF can cause comparable internal stress and thermal deformation on the adhesive layer, which causes the mirror's figure to be unsatisfactory. Consequently, the whole manufacturing process, especially thermal distribution on the adhesive, should be monitored and controlled. A 19-unit thin-layer unimorph DM is adopted as the object to simulate and measure the distribution law of temperature field on the adhesive during the IBF process. IBF parameters are optimized according to the characteristics of the adhesive and the manufacturing simulation results. With a Phi 3-mm diaphragm integrated on IBF, the processing temperature is reduced below the adhesive's glass transition temperature T-g. Actual figuring results report over 50% reduction on both peak-to-valley (PV) and root-mean-square (RMS) of DM with the value from 310 to 141 nm and from 38 to 17 nm, respectively. This precision has satisfied the self-corrected requirement of general unimorph DMs, which is usually <20 nm RMS. The voltage fitting coefficient of self-correcting for most electrodes is reduced to <0.05, which means the significant improvement in the effective deformation and correction ability. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)