High-End-Finishing procedures

For highest precision and to minimize form deviations, optics can be subjected to a final correction step, the so-called finishing. This serves for even more precise surface treatment by polishing and enables zonal corrections of even the finest deviations. asphericon offers two different high-end finishing processes for refining the surface structure of optical components:

• Magnetorheological polishing (Magnetorheological Finishing-Technology®) and
• Ångström Polishing.

These procedures will be explained and compared on the following. Initially, the focus will be on magnetorheological finishing (MRF). This correction method is a mechanical polishing process for optical surfaces. Targeted, local material removal corrects the optical surface and enables lowest surface roughness values (Rq < 0.3 nm). MRF technology enables the flexible design of high-quality precision optics by using the magnetorheological properties of the material components.

Magnetorheological Finishing

The MRF correction procedure works with a magnetorheological fluid consisting of magnetic particles, polishing agents and water. The MR fluid is continuously applied to a rotating wheel via a nozzle (see Fig. 1). Below the wheel surface is a magnet for generating a magnetic field that changes the viscosity of the “intelligent” fluid. The iron particles align and form a stiff structure adhering to the wheel; water and abrasive particles concentrate as a solidified, thin polishing layer on the surface (see Fig. 2). The workpiece is clamped in a movable holder and dipped into the polishing layer on the surface to be machined. Polishing of the workpiece surface begins (see Fig. 3). The position of the workpiece can be positioned fully automatically to save time. Targeted material removal enables the local correction of shape errors. This results in the lowest surface roughness values for optics of highest quality. MRF technology is characterized by maximum precision, determinism and repeatability in the manufacture of precision optics.

Figure: Process steps of MRF procedure

Advantages of MRF Technology

The MRF process impresses with its polishing and correction possibilities in the short-wave and long-wave range. The high determinism is reflected in very good error correction planning, which leads to best results in the polish. In addition, the process convinces through its simple applicability. The high flexibility of the MRF process allows easy adaptation to a wide variety of optical surfaces. Therefore, many optical components can be manufactured for a wide range of applications.

The following advantages can be noted for the correction procedure:

Fields of Application

The MRF refinement process has a wide range of applications because of the variety of optics that can be manufactured. Different geometries for the short- and long-wave range are easily attainable. Therefore, polishing within the framework of MRF technology can be carried out on classical optical elements such as spheres as well as on more complex surfaces such as aspheres (or free form optics). For example, lenses for microscopy are polished using MRF technology in order to keep scattered light to a minimum and generate razor-sharp images. The demands placed on optics installed in LIDAR systems are also very high, as they are exposed to particularly high environmental influences such as the sun and thus to particular heat. In order to guarantee optimum functionality, the optical components require a perfect optical surface, which can be provided by MRF technology.

The asphericon Ångström-Polishing Process

When every photon counts, scattered light is to be excluded or applications with high laser power are to be carried out, the asphericon Ångström-Polish allows roughness values of 5 Å (Rq measured on 1000x1000 µm measuring field, Rq according to ISO 10110) on optical surfaces. The process is linked to CNC machining. The optimized CNC process enables for the correction of short-frequency errors. Highest accuracy can be achieved through specially developed tools and technology. This allows to produce surfaces in an unprecedented form. The surface precision remains untouched.

The high-precision Ångström-Polishing, based on CNC production, is made possible by a patented control software specially developed by asphericon. The polishing process is controlled fully automatically and thus enables unique roughness values of 5 Å. The Ångström- Polishing can be illustrated using any chosen optical surface. The surface of no optics is ideal, since long-frequency errors lead to deviations from the ideal surface shape (see Fig. 1). On closer inspection, a long-frequency error contains many short-frequency errors - so-called spatial frequencies (see Fig. 2). The Ångström-Polishing completely corrects the short frequency errors by combining the optimized CNC process with a unique software control (see Fig. 3). What remains are the long-frequency errors, which can be corrected to a minimum using asphericon technology. The Ångström-Polishing can be used for the processing of various elements in the optical range, e.g. for mounted satellite lenses or for optics of an ultra-short pulse laser.

Fig. 1: Ideal surface deviates from real surface, long-frequency errors present

Fig. 2 & 3 - Left: Zoom before Ångström-Polishing (short frequency errors, local frequencies) Right: Zoom after Ångström-Polishing (short frequency errors removed, local frequencies corrected

Advantages of Ångström-Polishing

The low and precise material removal of the patented Ångström-Polishing makes the process stand out from other high-end finishing processes with an unprecedented level of accuracy in the short wavelength range. In the measured area of 1000 x 1000 µm, roughness values under 5 Å can be achieved by reducing the roughness values.

The advantages of Ångström-Polishing at a glance:

Application Examples

One advantage of the Ångström methodology is the wide range of applications for the most diverse optical elements that can be processed. The polishing of optical glasses (spheres and aspheres), crystals or e.g. IR materials is possible without problems and ensures an optimal utilization of the photons. The consistently flawless imaging properties of the processed elements make the High-End-Finishing process the ideal addition to the manufacturing process. High-precision surface tolerances and minimum surface roughness values are particularly important in the aerospace industry, especially in optics for satellites and satellite cameras. Also, in various measuring devices, such as atomic interferometry, high-precision optical components are required in order to achieve the best measurement results.

Summary - Comparison of the correction procedures

A comparison of the High-End-Finishing processes discussed above, shows each polishing process can be attributed special advantages. Depending on the production requirements, materials or required accuracy, each of the High-End-Finishing processes represented can convince with outstanding results.

The MRF technology is characterized by a broad correction spectrum in the short and long wavelength range and is therefore considered to be a kind of “allrounder of polishing processes”. The Ångström-Polish convinces with particularly low residual scatter and minimal roughness values.

The decision which High-End-Finishing method is chosen depends on various factors. Specific customer requirements are particularly important, and asphericon will find the most efficient solution to achieve these requirements. Depending on the material and the required accuracy, the various High-End-Finishing processes generate optimized solutions for the best surface qualities in a variety of optics.