We explore causes and possible techniques to minimize shape deformation of high-precision monocrystalline calcium fluoride optical surface formed during thermal annealing. Due to the anisotropic mechanical properties of calcium fluoride crystals, machining of lens shapes introduces defects into the crystal lattice. The thermal annealing thus leads to activation of the processes of recovery, resulting in the formation of characteristic surface structures causing both shape error and increased microroughness. The surface deformation can be gradually minimized by thermal treatment of the optical element and subsequent polishing steps to the order of units of nanometers, so they do not represent a fundamental problem for optical performance.
Plane-parallel optical elements, such as optical resonators, laser rods or wave plates, are widely used in many optical setups. Both planar surfaces of the element have to be manufactured with high precision in order to assure proper quality of the element and thus these surfaces have to be accurately measured. However, the interferometric measurement process, usually used for the surface form topography characterization, suffers from multiple interference patterns. This increases the total measurement error. In this paper the frequency-tuned interferometric method, that overcomes this problem, is demonstrated and compared to commonly used techniques using immersion liquid. Using a tunable laser source together with a frequency separation of the phase information from both surfaces, fringes from the first and second surface can be distinguished. Also, the interference between both surfaces can be used to obtain the wedge and thickness values of the specimen.
Ion Beam Figuring (IBF) has been used for nearly 20 years by several laboratories and companies as a highly deterministic method of final processing of ultra-precision optical elements. Nowadays, requirements for high precision optics demand to have full control over the ion beam, which includes both the ion beam profile and intensity. Electrostatic focusing using an Einzel lens setup provides a simple option to control the ion beam shape by changing voltage. This experimental study investigates the early stage development of an Einzel lens used to control an RF40 ion source. First results demonstrate the possibility to use an Einzel lens to control the ion beam profile and indicate possible future challenges this technology has to overcome when used in IBF machines.
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