Measurement and Alignment Turning of Optical Assemblies
Alignment turning is the only method by which all relevant parameters of a mounted lens can be aligned, in particular the gap between two apex points and the contact surface.
In addition, a large number of different cell sizes can be processed. And finally, high precision turning machines achieve excellent production accuracies of up to 0.5 μm. The alignment turning method is described in the figures below (Fig. 3). In alignment turning, first the lens cell is fixed in an adjustable alignment chuck with the lens in place. Then the position of the optical axis of the lens to the spindle axis is measured with the OptiCentric® system. Using this alignment chuck, the lens is then aligned so that its two centers of curvature are located as closely as possible to the axis of rotation of the spindle.
Then the spindle is rotated and the contact surfaces of the cell are machined with a sharp (diamond) turning tool, resulting in a precisely machined surface of the cell, aligned parallel to the spindle axis. In addition to the outer surface of the cell, it is also possible to machine the front and rear contact surface during the turning process. To do this the turning tool is moved perpendicular to the spindle axis instead of along the optical axis.
The lens is already fixed in the cell during the alignment turning process. This means that low-stress adhesives with very long curing times can be used for highly precise optics. Similarly, the lenses can be crimped or held by screw rings when the available adhesives are not suitable for the intended application. Since the lens has been aligned at the beginning of the process, the optical axis of the lens and the mechanical axis of the cell are precisely aligned when the cell is machined on a turning machine corresponding to the optical axis of the lens. In order to achieve high accuracy TRIOPTICS has integrated additional measurement technology into its alignment turning stations, alongside the high-resolution autocollimators. These include tactile and optical distance sensors that ensure a highly accurate measurement of the relevant mechanical parameters. This means the highest precision is achieved by a gradual machining process, in which the cell accuracy is checked after each machining step. The cells used in alignment turning do not need to meet exceptionally tight tolerances before machining. The cell offset only needs to be large enough to meet the required tolerance after machining.
The distance from the lens vertex to the upper contact surface can be manufactured with an accuracy of up to ±0.5 μm. Similarly, the diameter can be manufactured with an absolute accuracy of ±2 μm. The remaining centration error can be reduced to less than 0.5 μm by using a suitable alignment chuck. Moreover, a low coherent interferometer can be used to measure the center thickness on the machine, so that the contact surfaces can be manufactured with high precision with respect to each lens vertex. Multiple mounted lenses manufactured by alignment turning are then assembled to an objective lens. The method of machining in the micron range ensures that all lenses are aligned to each other. This allows mounted lenses to be assembled in a tube without further adjustment steps.