Radio interferometry applied to the observation of cosmic-ray induced extensive air showers

We developed a radio interferometric technique for the observation of extensive air showers initiated by cosmic particles. In this proof-of-principle study we show that properties of extensive air showers can be derived with high accuracy in a straightforward manner. When time synchronisation below $$\sim $$ ∼ 1 ns between different receivers can be achieved, direction reconstruction resolution of $$< 0.2^\circ $$ < 0 . 2 ∘ and resolution on the depth of shower maximum of $$<10$$ < 10 g/cm$$^2$$ 2 are obtained over the full parameter range studied, with even higher accuracy for inclined incoming directions. In addition, by applying the developed method to dense arrays of radio antennas, the energy threshold for the radio detection of extensive air showers can be significantly lowered. The proposed method can be incorporated in operational and future cosmic particle observatories and with its high accuracy it has the potential to play a crucial role in unravelling the composition of the ultra-high-energy cosmic-particle flux.

Interferometric Technique for the Spectral Characterization of High Frequency Current-Modulated Mid-Infrared Semiconductor Lasers

We describe two methods, based on Michelson interferometery, that enable the determination of the absolute wavelength of current-modulated semiconductor lasers. By non-linear regression of the instantaneous frequency of the interference signal, the rate of change of the wavelength of the radiation can be inferred. Alternatively, the absolute wavelength can be directly calculated from the maxima and minima of the interference signal. In both cases a reference absorption line enables the determination of the absolute wavelength. Both methods offer respective advantages. The methods allow a highly resolved wavelength measurement under lower kilohertz range current-modulation with relatively little effort. As a result, we present the rates of wavelength change and absolute wavelengths exemplarily for a specific interband cascade laser. It is furthermore shown that the spectral dynamic range of the laser decreases with increasing modulation frequency.

Efficient Optical Sensing Based on Phase Shift of Waves Supported by a One-Dimensional Photonic Crystal

Interferometric methods of optical sensing based on the phase shift of the Bloch surface waves (BSWs) and guided waves (GWs) supported by a one-dimensional photonic crystal are presented. The photonic crystal, composed of six SiO2/TiO2 bilayers with a termination layer of TiO2, is employed in the Kretschmann configuration. Under resonance condition, an abrupt phase change is revealed, and the corresponding phase shift is measured by interferometric techniques applied in both the spectral and spatial domains. The spectral interferometric technique employing a birefringent quartz crystal is used to obtain interference of projections of p- and s-polarized light waves reflected from the photonic crystal. The phase shifts are retrieved by processing the spectral interferograms recorded for various values of relative humidity (RH) of air, giving the sensitivity to the RH as high as 0.029 rad/%RH and 0.012 rad/%RH for the BSW and GW, respectively. The spatial interferometric technique employs a Wollaston prism and an analyzer to generate an interference pattern, which is processed to retrieve the phase difference, and results are in good agreement with those obtained by sensing the phase shift in the spectral domain. In addition, from the derivative of the spectral phase shifts, the peak positions are obtained, and their changes with the RH give the sensitivities of 0.094 nm/%RH and 0.061 nm/%RH for the BSW and GW, respectively. These experimental results demonstrate an efficient optical sensing with a lot of applications in various research areas.

High Spatial-Resolution Digital Phase-Stepping Shearography

Digital phase-stepping shearography is a speckle interferometric technique that uses laser speckles to generate the phase map of the displacement derivatives of a stressed object, and hence can map the stresses of a deformed object directly. Conventional digital phase-stepping shearography relies on the use of video cameras of relatively lower resolution, in the order of 5 megapixels or lower, operating at a video rate. In the present work, we propose a novel method of performing high spatial resolution phase stepping shearography. This method uses a 24 megapixel still digital imaging device (DSLR camera) and a Michelson-type shearing arrangement with an edge-clamped, center-loaded plate. Different phase-stepping algorithms were used, and all successfully generated shearograms. The system enabled extremely high-resolution phase maps to be generated from relatively large deformations applied to the test plate. Quantitative comparison of the maximum achieved spatial resolution is made with the video-rate cameras used in conventional shearography. By switching from conventional (video) imaging methods to still imaging methods, significantly higher spatial resolution (by about 5 times) can be achieved in actual phase-stepping shearography, which is of great usefulness in industrial non-destructive testing (NDT).

Graphical Optimization of Spectral Shift Reconstructions for Optical Backscatter Reflectometry

Optical backscatter reflectometry (OBR) is an interferometric technique that can be used to measure local changes in temperature and mechanical strain based on spectral analyses of backscattered light from a singlemode optical fiber. The technique uses Fourier analyses to resolve spectra resulting from reflections occurring over a discrete region along the fiber. These spectra are cross-correlated with reference spectra to calculate the relative spectral shifts between measurements. The maximum of the cross-correlated spectra—termed quality—is a metric that quantifies the degree of correlation between the two measurements. Recently, this quality metric was incorporated into an adaptive algorithm to (1) selectively vary the reference measurement until the quality exceeds a predefined threshold and (2) calculate incremental spectral shifts that can be summed to determine the spectral shift relative to the initial reference. Using a graphical (network) framework, this effort demonstrated the optimal reconstruction of distributed OBR measurements for all sensing locations using a maximum spanning tree (MST). By allowing the reference to vary as a function of both time and sensing location, the MST and other adaptive algorithms could resolve spectral shifts at some locations, even if others can no longer be resolved.

Photoelastic Properties of Trigonal Crystals

All possible experimental geometries of the piezo-optic effect in crystals of trigonal symmetry are studied in detail through the interferometric technique, and the corresponding expressions for the calculation of piezo-optic coefficients (POCs) πim and some sums of πim based on experimental data obtained from the samples of direct and X/45°-cuts are given. The reliability of the values of POCs is proven by the convergence of πim obtained from different experimental geometries as well as by the convergence of some sums of POCs. Because both the signs and the absolute values of POCs π14 and π41 are defined by the choice of the right crystal-physics coordinate system, we here use the system whereby the condition S14 > 0 is fulfilled (S14 is an elastic compliance coefficient). The absolute value and the sign of S14 are determined by piezo-optic interferometric method from two experimental geometries. The errors of POCs are calculated as mean square values of the errors of the half-wave stresses and the elastic term. All components of the matrix of elasto-optic coefficients pin are calculated based on POCs and elastic stiffness coefficients. The technique is tested on LiTaO3 crystal. The obtained results are compared with the corresponding data for trigonal LiNbO3 and Ca3TaGa3Si2O14 crystals.

Munch and optical coherence tomography: unravelling historical and artist applied varnish layers in painting collections

Effective care of large-scale museum collections requires planning that includes the conservation treatment of specific groups of art works, such as appropriate cleaning strategies. Optical coherence tomography (OCT) has been successfully applied as a non-invasive method for the stratigraphic visualisation of the uppermost transparent and semi-transparent layers in paintings, such as varnishes. Several OCT case study examples have further demonstrated the capabilities of the non-contact interferometric technique to measure the thickness of the various varnish layers, to help monitor cleaning and associated optical changes, and to detect past restorations. OCT was applied for the detection of varnishes to 13 paintings by Edvard Munch (1863–1944) owned by the Norwegian National Museum of Art. The paintings have a controversial and complex varnish history and are displayed as a group according to their acquisition legacy. A prototype high-resolution portable SdOCT instrument was used in combination with complementary imaging techniques. Questions concerning thickness, stratigraphy and the identification/location of the artist’s original varnish layers and/or pigmented glazes were addressed. Findings confirmed the complexity of the historical layers present and provided new evidence for Munch’s use of transparent and semi-transparent layers as part of an occasional, localised varnishing and/or glazing technique.

Self-referenced hologram of a single photon beam

Quantitative characterization of the spatial structure of single photons is essential for free-space quantum communication and quantum imaging. We introduce an interferometric technique that enables the complete characterization of a two-dimensional probability amplitude of a single photon. Importantly, in contrast to methods that use a reference photon for the phase measurement, our technique relies on a single photon interfering with itself. Our setup comprises of a heralded single-photon source with an unknown spatial phase and a modified Mach-Zehnder interferometer with a spatial filter in one of its arms. The spatial filter removes the unknown spatial phase and the filtered beam interferes with the unaltered beam passing through the other arm of the interferometer. We experimentally confirm the feasibility of our technique by reconstructing the spatial phase of heralded single photons using the lowest order interference fringes. This technique can be applied to the characterization of arbitrary pure spatial states of single photons.

Analysis of Salt Lake Volume Dynamics Using Sentinel-1 Based SBAS Measurements: A Case Study of Lake Tuz, Turkey

As one of the largest hypersaline lakes, Lake Tuz, located in the middle of Turkey, is a key waterbird habitat and is classified as a Special Environmental Protection Area in the country. It is a dynamic lake, highly affected by evaporation due to its wide expanse and shallowness (water depth <40 cm), in addition to being externally exploited by salt companies. Monitoring the dynamics of its changes in volume, which cause ecological problems, is required to protect its saline lake functions. In this context, a spatially homogeneous distributed gauge could be critical for monitoring and rapid response; however, the number of gauge stations and their vicinity is insufficient for the entire lake. The present study focuses on assessing the feasibility of a time-series interferometric technique, namely the small baseline subset (SBAS), for monitoring volume dynamics, based on freely available Sentinel-1 data. A levelling observation was also performed to quantify the accuracy of the SBAS results. Regression analysis between water levels, which is one of the most important volume dynamics, derived by SBAS and levelling in February, April, July and October was 67%, 80%, 84%, and 95% respectively, for correlation in the range of 10–40 cm in water level, and was in line with levelling. Salt lake components such as water, vegetation, moist soil, dry soil, and salt, were also classified with Sentinel-2 multispectral images over time to understand the reliability of the SBAS measurements based on interferometric coherence over different surface types. The findings indicate that the SBAS method with Sentinel-1 is a good alternative for measuring lake volume dynamics, including the monitoring of water level and salt movement, especially for the dry season. Even though the number of coherent, measurable, samples (excluding water) decrease during the wet season, there are always sufficient coherent samples (>0.45) over the lake.