Application of digital holographic microscopy for inspection of micro-optical components

Micro-optical components. such as microlenses and microlens arrays are of growing interest in the fields of laser beam shaping, optical processing and similar applications. The characterization of these components require a fast and robust measurement technique, especially for simultaneous inspection of lenses manufactured in array structures e.g. on silicon wafers. Conventional interferometric techniques such as holographic or Mach-Zehnder interferometry provide high-precision information about wavefront aberrations and the shape of the component under test. However, these techniques require a high experimental and time effort for the characterization of micro-components. In this paper digital holographic microscopy as a measurement tool for the inspection of micro-components is presented, which has some advantageous properties with respect to other interferometric techniques. In conventional interferometry the resulting interferogram has to be magnified and imaged onto the CCD-target. The imaging and magnification optics can lead to additional wavefront aberrations and therefore to measurement errors. For the suppression of aberrations costly imaging systems have to be applied leading to a complex set-up. These aberrations do not appear in digital holography since no optical imaging and magnification system is used in the setup. Holograms are stored electronically without any imaging optics and the reconstruction and magnification is performed by numerical methods. Due to the reconstruction process a numerical representation of the recorded wavefront can be evaluated including amplitude and phase from one hologram only. In order to obtain a high lateral resolution approximations such as the Fresnel-approach are omitted in the numerical reconstruction procedure. Digital holographic microscopy has some advantageous properties with respect to conventional microscopic systems. Due to the numerical reconstruction the complex wavefront can be evaluated in different depths. Therefore, objects placed in different depths can be imaged in focus by reconstruction of one digital hologram. This property can be used to analyse the propagation of a wave influenced by an optical component. Parameters such as focal length and M-2 can be determined using this technique. The principles of digital holographic microscopy are presented including an estimation of the lateral resolution. This is compared to experimental investigations using the USAF-resolution chart. The resolution obtained enables microscopic application using digital holography. The technique is applied to the inspection of a microlens-array used for generation of Bessel beams. Applications of this technique are not limited to optical components. It can also be applied to shape and deformation measurements of opaque objects used in microsystem technology.