Robust interferometer with external phase-shift control.

We describe a robust interferometer with external phase-shift control that does not require moving parts. The optical architecture resembles a common-path device in which the interfering waves propagate together in one collimated beam passing through the test sample. The collimated beam is incident on a calcite plate, which produces a polarization selective lateral translation and superposition of the reference and test waves. The characteristic features of the proposed interferometer, i.e. one-beam single-element scheme combined with external phase-shift control without moving parts, make a highly vibration insensitive device. Validation experiments are presented.

Polarization analyzer for all the states of polarization of light using a structured polarizer.

Polarization analyzers are an essential measuring tool to improve the characteristics of optical components and optimize them with respect to a useful application in optical networks. We describe an instrument of this kind, which consists of two crossed birefringent wedges and acts as a continuous structured polarizer for all the states of polarization of light. We analyze this device theoretically by using the Poincaré-sphere and the Jones-matrix method and verify our results in a number of experiments with quartz wedges and red filtered light. Different realizations of this instrument are discussed, and an application as a beam splitter for all the states of polarization is proposed.

Modeling the avalanche diode as a photon detector in quantum optical interferometers.

Avalanche diodes (ADs) are widely used to count photons in quantum interferometry. In reality they do not count photons, but click once when a bunch of photons arrives in a light pulse. We model this behavior in typical quantum optical interferometers like the Hong-Ou-Mandel beam splitter and the Mach-Zehnder interferometer, and compare it with the behavior of the photon-number-resolving (PNR) detector and the Hanbury-Brown-Twiss detector in these measuring devices. Our results show that quantum interferometric measurements with biphotons could be performed with single ADs, if the noise of the diodes could be reduced. Even a single PNR detector can be used in these interferometers, if the variance of the measurement is determined, since it reveals information about biphoton interference in contrast to the single detector counting rate.

Negative refraction and lensing at visible wavelength: experimental results using a waveguide array.

Experimental results showing "negative refraction" and some kind of "lensing" -in the microwave-infrared range- are often presented in the literature as undisputable evidence of the existence of composite left-handed materials. The purpose of this paper is to present experimental results on "negative refraction" and "lensing" at visible wavelengths involving a waveguide array formed by a tight-packed bundle of glass fibers. We will demonstrate that the observed phenomena are not necessarily evidence of the existence of left-handed materials and that they can be fully explained by classical optic concepts, e.g. light propagation in waveguides.

Edge enhancement and image equalization by unsharp masking using self-adaptive photochromic filters.

A new method for real-time edge enhancement and image equalization using photochromic filters is presented. The reversible self-adaptive capacity of photochromic materials is used for creating an unsharp mask of the original image. This unsharp mask produces a kind of self filtering of the original image. Unlike the usual Fourier (coherent) image processing, the technique we propose can also be used with incoherent illumination. Validation experiments with Bacteriorhodopsin and photochromic glass are presented.

Nonlinearity in the rotational dynamics of Haidinger's brushes.

Haidinger's brushes are an entoptic effect of the human visual system that enables us to detect polarized light. However, individual perceptions of Haidinger's brushes can vary significantly. We find that the birefringence of the cornea influences the rotational motion and the contrast of Haidinger's brushes and may offer an explanation for individual differences. We have devised an experimental setup to simulate various phase shifts of the cornea and found a switching effect in the rotational dynamics of Haidinger's brushes. In addition, age related macular degeneration reduces the polarization effect of the macula and thus also leads to changes in the brush pattern.

Tomographic multiaxis-differential optical absorption spectroscopy observations of Sun-illuminated targets: a technique providing well-defined absorption paths in the boundary layer.

A novel experimental procedure to measure the near-surface distribution of atmospheric trace gases by using passive multiaxis differential absorption optical spectroscopy (MAX-DOAS) is proposed. The procedure consists of pointing the receiving telescope of the spectrometer to nonreflecting surfaces or to bright targets placed at known distances from the measuring device, which are illuminated by sunlight. We show that the partial trace gas absorptions between the top of the atmosphere and the target can be easily removed from the measured total absorption. Thus it is possible to derive the average concentration of trace gases such as NO(2), HCHO, SO(2), H(2)O, Glyoxal, BrO, and others along the line of sight between the instrument and the target similar to the well-known long-path DOAS observations (but with much less expense). If tomographic arrangements are used, even two- or three-dimensional trace gas distributions can be retrieved. The basic assumptions of the proposed method are confirmed by test measurements taken across the city of Heidelberg.

Polymer-dispersed liquid-crystal voltage sensor.

We present a novel electric-field and voltage sensor based on the electro-optical properties of polymer-dispersed liquid-crystals (PDLCs). In principle, the transmittance of PDLCs is a nonlinear function of the applied electrical field. To measure an AC field we superposed to it a known DC field. This allowed us to achieve linearization of the PDLC response and to measure transmittance changes independently of the light-intensity level variations. Validation experiments are presented.

Spatial self-filtering with polarizer sheets.

A method for phase visualization and edge enhancement by spatial self-filtering by use of a polarizer sheet in the Fourier plane of an optical processor is described. Light absorbed by the polarizer sheet induces a thermal lens, which, in turn, produces selective action on certain spatial frequencies of the image to be processed. Some experiments that demonstrate the self-filtering action of the proposed system are presented.

Linear focusing by a plane grating with curved grooves.

We study the field diffracted by a plane grating with curved (parabolic) grooves. We will demonstrate that when a monochromatic plane wave is incident on a grating with parabolic grooves the diffracted field has a focal line whose position depends on the curvature radius of the parabolas and the incidence angle of the light onto the grating. The effect described has potential applications in grating-based devices for focusing light without requiring any additional optics.

Phase modulation by polarization recording in bacteriorhodopsin: application to phase-shifting interferometry.

A novel phase-control method with application to phase-shifting interferometry is presented. The linear polarization state of an external (green) light beam is recorded on a bacteriorhodopsin film, and this polarization state is read by a circular polarized (red) laser beam. By reading the bacteriorhodopsin film, the original (red) wave reverses its circularity and becomes phase shifted by an amount that is dependent on the polarization of the external (green) beam. This method of phase control can be applied in a two-beam interferometer in which the test and reference waves are orthogonally polarized, which allows one to obtain phase modulation without moving parts inside the interferometer.

Enhancing the phase profile and contrast of an interferogram by polarization recording in bacteriorhodopsin.

Interferometry is a technique for reconstructing the profiles of phase objects. We present a novel interferometric setup for generating interferograms with doubled phase profile and enhanced contrast compared with the standard interferogram. The proposed system consists of a two-beam interferometer in which the reference and test waves are circularly polarized orthogonally to each other. They are superposed upon a bacteriorhodopsin film, creating a polarization grating that is distorted by the phase of the test object. This polarization pattern is read by a polarized He-Ne beam. We show analytically and experimentally that, when the zero diffraction order is removed, an interferogram with doubled phase profile and enhanced contrast is obtained.

Generation of nondiffracting beams by spiral fields.

In this paper we demonstrate that spiral fields generate nondiffracting dark beams. A collimated laser beam incident on a compact disc, i.e., a commercial CD, was used as mask for the generation of spiral fields. We study theoretically and experimentally the intensity distribution near the axis of the optical system.

One-beam interferometer by beam folding.

A novel one-beam interferometer based on beam folding is described. The device resembles a Mach-Zehnder interferometer in which the two arms are located together in one collimated beam. Different halves of the same beam interfere with the help of a mirror--with its reflecting surface along the axis of the optical system--placed near the focal plane of the imaging lens. Phase-delay control is achieved by application of an electrical potential to a Pockels cell, which permits the use of techniques of phase-stepping interferometry.