Klaus Körner

Recent advances in digital holography [Invited]

This article presents an overview of recent advances in the field of digital holography, ranging from holographic techniques designed to increase the resolution of microscopic images, holographic imaging using incoherent illumination, phase retrieval with incoherent illumination, imaging of occluded objects, and the holographic recording of depth-extended objects using a frequency-comb laser, to the design of an infrastructure for remote laboratories for digital-holographic microscopy and metrology. The paper refers to current trends in digital holography and explains them using new results that were recently achieved at the Institute for Applied Optics of the University Stuttgart.

Testing micro devices with fringe projection and white-light interferometry

Optical sensors are very suitable for the analysis of microscopic structures and micro devices. We compare two very promising methods: the white-light interferometry and the fringe projection technique for the application to this task. The fringe projection is very useful for fast measurement of objects with vertical dimensions of some μm. White-light interferometry is especially useful for highly resolved 3-D measurements. Furthermore, we present a new technique, the scanning fringe projection (SFP), which enables absolute 3-D measurements with one single grating period.

Chromatic confocal spectral interferometry

Chromatic confocal spectral interferometry (CCSI) is a novel scheme for topography measurements that combines the techniques of spectral interferometry and chromatic confocal microscopy. This hybrid method allows for white-light interferometric detection with a high NA in a single-shot manner. To the best of our knowledge, CCSI is the first interferometric method that utilizes a confocally filtered and chromatically dispersed focus for detection and simultaneously allows for retrieval of the depth position of reflecting or scattering objects utilizing the phase (modulation frequency) of the interferometric signals acquired. With the chromatically dispersed focus, the depth range of the sensor is decoupled from the NA of the microscope objective.

Microscopic three-dimensional topometry with ferroelectric liquid-crystal-on-silicon displays

When three dimensional measurements are conducted with fringe projection, the quality of the grating used for the generation of the fringes is important. It has a direct influence on the achievable depth resolution in a given measurement setup. In the past, Ronchi grating or gratings written in nematic liquid-crystal displays or in digital micromirror devices have been used. We report on the application of a reflective ferroelectric liquid-crystal-on-silicon display as the fringe-generating element in a setup based on a stereo microscope. With this device the depth resolution of measurements by use of phase-shifting algorithms can be significantly improved compared with the application of a Ronchi grating or a nematic liquid-crystal display.

Short temporal coherence digital holography with a femtosecond frequency comb laser for multi-level optical sectioning

In this paper, we demonstrate how short temporal coherence digital holography with a femtosecond frequency comb laser source may be used for multi-level optical sectioning. The object shape is obtained by digitally reconstructing and processing a sequence of holograms recorded during stepwise shifting of a mirror in the reference arm. Experimental results are presented.

Chromatically dispersed interferometry with wavelet analysis

A new white-light interferometry point sensor utilizing a chromatically dispersed depth detection field is addressed. Monitoring the interference in the optical frequency domain allows for microscopic height detection without the necessity of a mechanical axial scan. The problem of limited dynamic range in previously reported spectral interferometric schemes is solved by forming a high-contrast interference window due to the chromatically dispersed focusing of the detection field. In a proof-of-principle experiment, the position of a reflecting object could be retrieved with a focus of 0.8 NA over an axial range of 30 microm by analyzing the phase of the emerging interference wavelets.

Correlated speckle noise in white-light interferometry: theoretical analysis of measurement uncertainty

The partial coherent illumination of the specimen, which is required for white-light interferometric measurements of optically rough surfaces, directly leads to speckle. The electric field of such speckle patterns strongly fluctuates in amplitude and phase. This spatially correlated noise influences the accuracy of the measuring device. Although a variety of noise sources in white-light interferometry has been studied in recent years, they do not account for spatial correlation and, hence, they cannot be applied to speckle noise. Thus, we derive a new model enabling quantitative predictions for measurement uncertainty caused by speckle. The model reveals that the accuracy can be attributed mainly to the degree of spatial correlation, i.e., the average size of a speckle, and to the coherence length of the light source. The same parameters define the signal-to-noise ratio in the spectral domain. The model helps to design filter functions that are perfectly adapted to the noise characteristics of the respective device, thus improving the accuracy of postprocessing algorithms for envelope detection. The derived expressions are also compared to numerical simulations and experimental data of two different types of interferometers. These results are a first validation of the theoretical considerations of this article.

Absolute macroscopic 3-D measurements with the innovative depth-scanning fringe projection technique (DSFP)

Summary We are presenting a new depth-scanning fringe projection technique (DSFP) on the basis of triangulation for the absolute 3-D measurement of a macroscopic scene. The absolute phase can be determined for any desired depth range of the measurement volume. Furthermore, the known limitations of the depth of focus occurring by light projection techniques, will be overcome. During the measurement procedure, an illuminated Ronchi grating in front of a projection lens is moved both in z- and x-directions by two perpendicularly arranged linear translation stages. This results in an oblique shift of the projected grating, resulting in a oblique scan of the projected fringe pattern in the measurement volume i.e. the object space. For a well-focussed image acquisition, a CCD-camera placed on the same z-stage in front of an observation lens takes a series of images of the scene in different z-positions. This focusing allows the use of large apertures and therefore a better exploitation of light. In our assembled sensor, the triangulation basis of 75 mm approximately corresponds to the distance of a persons pupils.

One-shot line-profiling white light interferometer with spatial phase shift for measuring rough surfaces

White light interferometry is a promising tool for industrial quality inspection. Since modern cameras offer a frame rate far above video-rate, the speed of these systems could be increased in order to fulfill the strong temporal constraints of inline inspection, i.e. the monitoring of every single part during the production process in just a few seconds. Its accuracy up to the sub-μm range enables even the detection of smallest defects like holes with a diameter of only a few microns and thus ensures a fast, contactless and high precision quality inspection. Due to the replacement of the mechanical phase shifting by a spatial phase shift, the commonly known white light interferometers could be extended to a one-shot line-profiling sensor. The main benefit of such a line-profiling technique is that also critical surfaces are accessible that deviate strongly from a plane shape, like rotary welds on cylindrical parts. It can be shown that the accuracy of the proposed system is comparable to the accuracy of conventional white light interferometers even on rough surfaces. Other parameters like lateral resolution and measuring range strongly depend on the optical setup and will be discussed in the following sections.

Improved micro topography measurement by LCoS-based fringe projection and z-stitching

Fringe projection is a commonly used method for 3D surface metrology. Numerous applications have demonstrated a measurement field from a few millimeters to several meters. To enable the measurement of micro systems with this method, a zoom stereo microscope from Leica was used as the basis for the implementation of a fringe projection microscope. A state of the art twisted nematic WUXGA LCD was used for flexible fringe generation. The high fill factor of this reflective LCoS in combination with a 500 Lumen LED and a 12 bit CCD camera delivers fringe patterns with high contrast. This allows us to measure objects with both a strong reflectivity variation and a low reflectivity. The second main objective was to increase the measurement field and the depth of field. Using the zoom system and exchangeable microscope objectives, the measurement fields could be changed quickly from 4 cm2 to less than 1 mm2. Depending on the measurement field, the depth of field was between 5.22 mm and 0.018 mm. However, this was often not sufficient to measure the complete depth of a 3D-object. The microscope system also features an integrated high precision motor stage, which is already used for system calibration. Based on this, we implemented a new z-stitching method where n measurements at different well determined z-positions of the motor stage were performed. The n resulting topography maps can be stitched together to get the complete depth map of the entire object. Thus the depth measurement range is only limited by the mechanics of the z-stage.