Hans J. Tiziani

Optical particle trapping with computer-generated holograms written on a liquid-crystal display.

Computer-generated holograms written on a liquid-crystal display can be used to generate dynamic light fields of arbitrary shape. This method was used to simultaneously trap polystyrene particles laterally and to displace them independently of one another.

Multi-functional optical tweezers using computer-generated holograms

Optical tweezers are capable of trapping microscopic particles by photon momentum transfer. The use of dynamic computer-generated holograms for beam shaping allows a high flexibility in terms of trap characteristics and features. We use a liquid crystal display (LCD) to display the holograms. Efficiency losses caused by the periodic electrode structure of the LCD have been clearly reduced by use of an optically addressed spatial light modulator. We realized multiple traps, which can hold and move at least seven silica spheres independently in real time. We also demonstrate the controllability of trapped particles in three dimensions without the need for mechanical elements in the setup.

Short-coherence digital microscopy by use of a lensless holographic imaging system

An optical system based on short-coherence digital holography suitable for three-dimensional (3D) microscopic investigations is described. The light source is a short-coherence laser, and the holograms are recorded on a CCD sensor. The interference (hologram) occurs only when the path lengths of the reference and the object beam are matched within the coherence length of the laser. The image of the part of the sample that matches the reference beam is reconstructed by numerical evaluation of the hologram. The advantages of the method are high numerical aperture (this means high spatial resolution), detection of the 3D shape, and a lensless imaging system. Experimental results are presented.

Speckle interferometry with temporal phase evaluation for measuring large-object deformation.

We propose a new method for measuring large-object deformations byusing temporal evolution of the speckles in speckleinterferometry. The principle of the method is that by deformingthe object continuously, one obtains fluctuations in the intensity ofthe speckle. A large number of frames of the object motion arecollected to be analyzed later. The phase data for whole-objectdeformation are then retrieved by inverse Fourier transformation of afiltered spectrum obtained by Fourier transformation of thesignal. With this method one is capable of measuring deformationsof more than 100 mum, which is not possible using conventionalelectronic speckle pattern interferometry. We discuss theunderlying principle of the method and the results of theexperiments. Some nondestructive testing results are alsopresented.

Three-dimensional image sensing by chromatic confocal microscopy

In the image of a confocal microscope, only those parts of an object appear bright that are located in the focal plane of the objective. Because of an axial chromatic aberration deliberately introduced into the microscope objective, the location of the focal plane depends on the wavelength used. By using a white-light source and examining an object with a depth variation less than the axial range of the chromatic focus, we find that all parts of the object appear sharp and bright in the image, but according to its height they appear in different colors. A camera with black-and-white film sequentially combines, with three selected chromatic filters, intensity and tone of color of each object point. For each tone of color one can assign a height by using a calibration curve. This assignment could be made unequivocal by the selection of filters with adequate chromatic transmission.

Simultaneous three-dimensional dynamic deformation measurements with pulsed digital holography

The three deformation components x, y, z of a vibrating object are measured simultaneously by use of digital holography with a double-pulse ruby laser source. The object is illuminated from three different directions, each optically path matched with three reference beams such that three independent digital holograms are formed and added incoherently in one single CCD image. The optical phase difference between the two recordings taken for each hologram is quantitatively evaluated by the Fourier-transform method so that a set of three phase maps is obtained, representing the deformation along three sensitivity vectors. The total object deformation is obtained as a vector resultant from the data of the three phase maps. To give the full three-dimensional (3-D) description, the shape of the object is measured by the two-wavelength contouring method. Experiments are performed with a cylinder as the test object, transiently and harmonically excited. The 3-D deformation and shape measurement results are presented graphically.

Three-dimensional analysis by a microlens-array confocal arrangement

Scanning confocal microscopy is now well developed and applied. As an alternative to a laser spot to be scanned, parallel processing can be obtained when a two-dimensional structure is moved through the focal plane and a series of image sections is recorded. Surface topography is determined by analysis of the normalized intensity of the appropriate image points, i.e., a search of the intensity maximum leads to surface coordinates. With a high numerical aperture of the optical system, the half-width of I(z) is small, and the topography can be calculated with high accuracy. But with a high numerical aperture, only small object fields can be reproduced. As an alternative to the Nipkow disk for parallel processing, high-numerical-aperture microlenses are combined in an array. The reproducible object field is then limited by the size of the array and the number of lens and detector elements.

Digital double pulse-TV-holography

Abstract A digitial double-pulsed holographic system will be described. Two separate holograms of an object under test are recorded within a few microseconds using a CCD-camera and stored in a frame grabber. Different recording arrangements for Fresnel hologram, quasi-Fourierand image-plane hologram are discussed and compared. The holograms are reconstructed digitally using a computer. The phases of the two reconstructed wave fields are calculated from the complex amplitudes. The deformation information is obtained from the phase difference. In the case of the image-plane hologram the deformation can be obtained by using a sinusoid-fitting method without reconstruction of the wavefront. Experimental results are presented.

A study of the use of laser speckle to measure small tilts of optically rough surfaces accurately

A new method is proposed where small tilts can be measured with high accuracy even in the presence of a lateral shift. For this purpose the speckling of the object, an optically rough surface, is recorded in the Fourier plane before and after a tilt. By illuminating the developed plate with a laser, fringeswith spacings inversely proportional to the tilt angle can be observed.

Double pulse-electronic speckle interferometry

In this paper we describe a double-pulsed electronic speckle interferometry system. Two separated speckle patterns are recorded within few microseconds using a charge-coupled device camera, the two images are then stored in a frame grabber and the subtraction method is applied. A quantitative analysis of the fringes using the spatial-carrier phase-shift method is presented. The results show that this method is well suited to study transient vibrations.