Numerical correction of optical vortex using a wrapped phase map analysis algorithm

We demonstrate experimentally a technique for the numerical correction of an optical vortex with a unitary topological charge. A developed algorithm based on the axial behavior of a reconstructed wavefront is used in the detection of the optical vortex. Optimizations of the number of axial phase maps and the window size used in the algorithm yield the precise coordinates of the vortex eye. The obtained coordinates and vortex handedness are used in designing a proper filter, facilitating numerical correction of the vortex phase map. The developed algorithm can be applied to absolute phase and phase difference maps obtained through any reconstruction method.     

Aperture synthesis in phase retrieval using a volume-speckle field

The resolution of the reconstructed wave by a phase-retrieval method using a volume-speckle field depends on the aperture defined by the size of the CCD array. The use of a larger aperture is introduced by measuring the speckle field at two different positions in the transverse plane and stitching the measurements together. Improvements in the quality of reconstructions are demonstrated experimentally and by computer simulations. Undesirable effects of camera tilt on the quality of reconstructions from synthetic aperture intensity measurements are experimentally observed and corrected.     

Fringe localization using focus measure

A method for automated evaluation of fringe localization using focus measure is demonstrated experimentally. Dual-illumination digital holographic interferometry is used to generate phase difference maps (PDMs) of a rough test object. Focus measure values of the PDMs along the axial direction (increment: 10λ=6.33 μm) yield an inverted bell-shaped curve which, in turn, facilitates the characterizations of the plane and region of fringe localization. The method can be applied in the distance measurement of rough objects and in the optimization of fringe visibility.     

Fringe analysis and enhanced characterization of sub-surface defects using fringe-shifted shearograms

A technique for fringe analysis using variance of directly fringe-shifted shearograms is demonstrated experimentally. Statistical variations in each of the fringe-shifted shearograms are calculated to obtain a contrast map which is then used to characterize sub-surface defects and mechanical loading points. The technique offers reduced computational load compared to Fourier transform-based fringe processing algorithms. The technique is implemented using both phase difference maps and interferograms. The latter allows the extension of the technique to physical (i.e., non-reconstructed) interferograms for the analysis of dynamic systems.     

Interferometric evaluation of angular displacements using phase retrieval

Phase retrieval is carried out using sequential intensity measurements of a volume speckle field and a wave propagation equation. Retrieved phases and phase subtraction facilitate the analysis of wavefronts before and after undergoing a small rotation. Angular displacement between incident planar wavefronts is determined from the unwrapped phase difference, phase diffuser aperture diameter, and the light source wavelength. Numerical simulations confirm the experimental results.     

Study on particle removal efficiency of an impinging jet by an image-processing method

 An image-processing method is proposed to obtain the distribution of the removal efficiency of particles on a plate by an air jet. This method can be used to measure particle removal from a flat surface by processing the image of the reflected light from the surface. Factors affecting the particle removal efficiency such as air pressure, distance between the nozzle and the impinging surface and the impinging angle are discussed. Optimal conditions are determined to obtain the most effective particle removal by the air jet. Received: 10 April 2001 / Accepted: 2 August 2001     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Stable isotopic studies of earthworm feeding ecology in tropical ecosystems of Puerto Rico

Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.