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.

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.

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.

A germline variation in the progesterone receptor gene increases transcriptional activity and may modify ovarian cancer risk.

Recently, we and others have detected a haplotype of the human progesterone receptor gene (PR). This haplotype consists of a 320-bp insertion in intron G together with point mutations in exons 4 and 5 and was named PROGINS. Whereas the exon 5 mutation is silent, the mutation in exon 4 results in a V660L substitution. Interestingly, this genetic polymorphism was seen to cosegregate with an increased risk of sporadic ovarian cancer in different ethnic groups. Our data provide evidence for the existence of an epidemiological link between a mutated progesterone receptor allele and ovarian cancer (odds ratio, 3.02; 95% confidence interval, 1.86-4.91). Functional characterization of the mutated receptor protein revealed a greater transcriptional activity compared with the wild-type receptor. By contrast, hormone binding and hormone dissociation rates were similar in both receptor proteins. We found that the increased transcriptional activity was due to increased stability resulting in higher expression of the mutant protein. Thus, the long-lasting hyperactivation of progesterone receptor-driven genes secondary to the increased transcriptional activity of the mutated progesterone receptor may participate in ovarian carcinogenesis. This is of special interest, because only a few genetic markers are available for the majority of women diagnosed with sporadic ovarian cancer.

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.

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.

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.