Digital holography as a tool for testing high-aperture micro-optics

The collimation of strongly diverging laser beams emitted by high-power diode lasers is performed with aspherical micro-optical components which have large numerical apertures up to NA=0.7. Conventional interferometric techniques do neither deliver the resolution nor the test frequency and are too costly for an industrial application.Digital holography is presented as a measurement tool for the characterization of micro-optical components in this paper providing a robust and fast technique with a simple and compact setup.The setup presented in this contribution does not apply any imaging or magnifying optics thus avoiding any additional aberrations in the wavefront which would falsify the measurement. The reconstruction process is performed in the computer giving direct access to intensity and phase of the measured wavefront. Additional experimental efforts like, e.g. phase shifting techniques are not needed.The large numerical aperture of the microlenses leads to high fringe densities in the holograms which cannot be resolved by CCD-detectors directly. In order to avoid this problem digital holography is combined with a multiple wavelength technique and a diffusing screen to capture the highly diverging wavefront. An averaging method to reduce the introduced speckle noise is applied and results of the characterization of refractive cylindrical microlenses are presented.