Characterization of aspherical micro-optics using digital holography: improvement of accuracy

The collimation of strongly diverging laser beams emitted by diode lasers is performed with aspherical micro-optical components. In order to obtain a good beam profile high-quality micro-lenses with a large numerical aperture compared to conventional lenses have to be applied. The characterization of these components using conventional interferometric techniques is not suitable, costly or inaccurate with respect to the required accuracy of the lens shape. Digital Holography as a measurement tool for the characterization of micro-optical components offers several advantageous properties with respect to other interferometric techniques, such as avoidance of aberrations introduced by imaging and magnification optics. The large numerical aperture of the microlenses under test leads to high fringe densities in the holograms which can not be resolved by CCD-detectors. In order to avoid this problem digital holography is combined with multiple wavelength and speckle techniques. A diffusing screen is placed directly behind the microlens in order to destroy the large divergence and at least two measurements with different wavelengths are performed for the recovery of the wavefront information. The speckle pattern in the numerical reconstruction of the wavefront reduces the accuracy of the resulting difference phase significantly. In this paper a technique for the reduction of speckle noise is proposed which is not based on classical filtering techniques such as median filters. Several holograms of the same object under test are recorded with different speckle patterns. A proper averaging taking into account the properties of the wrapped phases leads to a improvement of the accuracy up to 1/60 of the wavelength. Results of the characterization of aspherical microlenses using the new technique are presented.