This PDF file contains the editorial “‘‘Ball! High and tight...off the inside corner"” for OE Vol. 37 Issue 08

This PDF file contains the editorial “Guest Editorial: Special Section on High-Speed Photography” for OE Vol. 37 Issue 08

Two ruby laser systems employing multiple Q-switching technology are developed to provide ruby laser light for high-speed photography. Potential applications include ballistics and flow visualization as well as nondestructive test evaluation using laser imaging diagnostics such as photography, holography, and various interferometric techniques. The laser systems produce more than 50 pulses at repetition rates up to 500 kHz with nearly constant pulse-to-pulse energies. One system, based on a commercial laser, provides multiple pulses of holographic quality light with individual pulse energies of the order of 10 mJ, pulse widths of 50 ns, and a pulse train length of more than 300 µs. A new ruby laser system is developed to provide higher pulse energies, of the order of 350 mJ per pulse, with 10 ns pulse widths and 140 µs pulse train length. The method for multiple Q-switching by modulating the Pockels cell's quarter-wave voltage and the formation of an individual Q-switched pulse is investigated. The energy within the individual pulses formed in the oscillator cavity is successfully increased by propagating through an amplification section, without degradation of the temporal or pulse-to-pulse amplitude stability. Etalons for longitudinal mode selection and an iris for spatial mode selection are incorporated into the lower energy system and an image of a reconstructed hologram is presented. Camera capabilities and the implications of higher pulse energies are also investigated.

Proximity focused microchannel plate image intensifies (MCPIIs) with a mesh underlay photocathode are analyzed for their iris- ing time, which is found to be of the order of 650 ps. This is much longer than previously reported but is finally explained by the mesh thickness. The reduction of this effect requires further investigation. A newly introduced impedance match has real broadband characteristics, and the irising is fully caused by other effects. The minimum gate time observable was clearly below 1 ns. The mesh underlay MCPII did not open to its full diameter at the shortest applied times. The long laser diode pulse and a too flat voltage slope of the driving generator prevented exact results for subnanosecond timing. Continued development of the system is underway.

Pulsed laser ablation of blood clots in a fluid-filled blood vessel is accompanied by an explosive evaporation process. The resulting vapor bubble rapidly expands and collapses to disrupt the thrombus (blood clot). The hydrodynamic pressures following the bubble expansion and collapse can also be used as a driving force to deliver clotdissolving agents into thrombus for enhancement of laser thrombolysis. Thus, the laser-induced bubble formation plays an important role in the thrombus removal process. We investigate the effects of boundary configurations and materials on bubble formation with time-resolved flash photography and high-speed photography. Potential applications in drug delivery using microsecond laser pulses are also discussed.

Computer techniques and software are developed to study the dynamics of electron packets on a subpicosecond time scale in streak image tubes and photoelectron diffractometers. It is shown that the Coulomb spread of an electron packet substantially increases for a sufficiently narrow energy distribution of photoelectrons, due to disintegrative space charge effects inside the packet.

Field-assisted semiconductor photocathodes (pCs) based on In0.53Ga0.47As/InP heterostructures with a Schottky barrier for the spectral range of 1.0 to 1.7 µm are investigated. A technique is presented for manufacturing PCs. The method for ultrahigh vacuum transfer of the PC into vacuum devices is discussed. It is shown that the sensitivity of a PC in a sealed-out device at ? = 1.55 µm is two to three orders of magnitude higher compared to traditional Ag-Cs-O PCs, and approaches 400 µA/W at ? = 1.6 µm for PCs with Schottky barriers of Au film. It is shown that PCs with Schottky Au barriers can be activated with Cs only and without O, and the stability is considerably better than with a Ag barrier. A strong decrease of the internal photoeffect is discovered in the case when the PC is illuminated from the metallic film side. The developed PCs can be used for time analyzing tubes to record picosecond pulses with milliwatt peak intensity.

A new technique is proposed for the characterization of ultrashort optical pulses. The technique includes a simple mathematical treatment of the time-resolved spectrum of the signal that is obtained using an image converter camera. The theoretical description of the technique and several numerical examples modeling the ultrashort pulse reconstruction are presented. The dependence of the reconstruction result on the noise level is evaluated. It is shown that the temporal resolution of the technique can be up to two orders of magnitude higher than that for the used electro-optic device.

A new streak tube with the photocathode of 4x40 mm size is designed for the detection of dynamic spectra with a time resolution down to 2 ps and spatial resolution up to 35 lp/mm. The four-electrode design is adopted for a tube electron-optical system. A fine mesh accelerating electrode is placed at a distance of ~1 mm from the photocathode. A pair of deflecting electrodes are equipped with an electron trap, providing beam blanking.

A photoelectron gun as a source of a photoinduced, monoenergetic (energy spread <0.5 eV), well-collimated (divergence <0.5 deg), sharp (diameter <0.7 mm at the 1/e level), and ultrashort (²500 fs) bunch of electrons to be used for time-resolved electron diffraction (TRED) experiments is computer designed, assembled, and tested. in single-shot mode, it generates up to 103 of 30 keV electrons, and the electron pulse can be either measured in streak mode or focused onto a solid state target chosen from a set of interchangeable targets. High temporal resolution enables measurement with femtosecond precision of the diffraction pattern perturbation after the exiting laser radiation drops onto a target. Demonstration experiments with a 300- A1 target in transmission-type mode result in diffraction images of reasonable quality under accumulation of up to 4 x 104 500-fs photoelectron pulses.

An optical computerized tomography (OCT) technique is applied to research the radiation of an electrical element. An algebra iterative method is used to reconstruct the complicated temperature field of the radiator model including an opaque object. The result is consistent with the measurement of the thermocouple.

We describe a technique of three-dimensional computed tomography (3-D CT) using monochromatic x rays generated by synchrotron radiation, which performs a direct reconstruction of a 3-D volume image of an object from its cone-beam projections. For the development, we propose a practical scanning orbit of the x-ray source to obtain complete 3-D information on an object, and its corresponding 3-D image reconstruction algorithm. The validity and usefulness of the proposed scanning orbit and reconstruction algorithm were confirmed by computer simulation studies. Based on these investigations, we have developed a prototype 3-D monochromatic x-ray CT using synchrotron radiation, which provides exact 3-D reconstruction and material-selective imaging by using the K-edge energy subtraction technique.

The principle and structure of a new self-scanning laser interferometer are introduced. The output of a self-scanning photodiode array is modulated to generate an ac measurement signal, thereby avoiding dc drift and low-frequency disturbances. Experiments show that the device can be used in the range of 100 mm and with a resolution of less than 0.02µm.

A simple method to simultaneously measure axial strain and temperature using a surface-mounted Bragg-grating sensor is presented. This method uses a single, uniform-pitch Bragg grating with preloading that is only partly glued on the structure. To prevent the cross talk between two Bragg wavelengths reflecting from the glued and free sections, a preloading is applied to the fiber before it is glued. After releasing, two Bragg wavelengths are separately reflected from an infiber Bragg-grating sensor. The Bragg wavelength reflected from the sensor section not glued to the structure is used to measure temperature variations; that reflected from the glued sensor section is affected by both strain and temperature variations. Nevertheless, under conditions of no cross-talk sensitivity between strain and temperature, the variation in the spectral separation between two Bragg wavelengths is directly caused by the thermomechanical strain in the structures. After the temperature variation is obtained, the mechanical strain can be calculated if the thermal expansion coefficient of the structure is known. This method, therefore, supplies a simple, simultaneous measurement of axial strain and temperature. Based on the same concept, two alternative setups to measure axial strain and temperature simultaneously using an in-fiber Bragg-grating sensor are also introduced. This approach to the application of smart structures appears to be limited to surface-mounted structures. Nevertheless, it might be redesigned for embedded structures by inserting the Bragg grating fiber into a glass tube and partly fusing the fiber with the tube.

We report a far-field micro-imaging technique that is used for the observation and discrimination of the mode patterns in a micronsized hollow optical fiber as well as for the synthetic measurement of the fiber. By using an M-20x microscope objective lens, we obtained images, magnified by a factor of about 460, from the mode patterns at an output end facet of the hollow fiber with relative measurement accuracy better than 3%. This method can be used for clear identification of the mode patterns in the hollow fiber and detailed study of the relationship between the excitation conditions and the excited modes in the hollow fiber. Moreover, it is useful for the measurement of the geometrical sizes of the hollow fiber and fortesting the coupling efficiencies of the core and cladding modes in their mixed mode pattern. In addition, this method can be also used in the generation of a dark hollow laser beam with 10-^m dark-spot size and the measurement of the focused-spot size of a Gaussian laser beam with about 1-µrn diameter.

The quadtree method is usually used in fractal block coding, but its efficiency is very low. We propose a new scheme, merged quadtree partitioning (MQP), for efficient image compression. It is an improved quadtree method, and can merge the quadtree nodes on the same level and on the different levels to share one transformation. As the merged nodes describe irregular regions with boundaries approximating the image edges, which need only one transformation, the total number of transformations required by MQP is much less than by the quadtree method. Thus, we obtain a greater compression ratio. In addition, we speed up MQP method by using the relationship between the search of the merged range and that of its parent range, and obtain a shorter encoding time than with other partitioning schemes.

Image noise filtering has been widely perceived as an estimation problem in the spatial domain. We deal with it as an estimation problem in an uncorrelated transform domain. This idea leads to a generalization of the adaptive linear minimum mean square error (LMMSE) estimator for filtering noisy images. In our proposed method, the transform-domain local statistics obtained from the noisy image are exploited. Due to the fact that the transform-domain local statistics carry more information about the image than the spatial-domain local statistics do, improvement in noise filtering is gained overall and is particularly significant in the vicinity of edges.

A method of image analysis is proposed for detection of local defects in materials with periodic regular texture. A general improvement and enlargement of vision system capabilities for versatility, full automatism, computational efficiency, and robustness in their application to the industrial inspection of periodic textured materials is pursued. In the proposed method, a multiscale and multiorientation Gabor filter scheme that imitates the early human vision process is applied to the sample under inspection. The designed algorithm automatically segments defects from the regular texture. A variety of examples of fabric inspection are presented. In all of them defects are successfully segmented from the texture background.

Almost invisible Latin and Greek inscriptions were discovered on the shroud of Turin around the image of the face. This result was obtained by using digital image processing techniques. An original method is developed to eliminate the texture of the cloth and also to combine data from different photographs or from the same plate digitized under various conditions. According to some paleographists, the revealed characters are thought to date back to before the Middle Ages. This conclusion could be a new argument in favor of the authenticity of the shroud.

The extended averaging technique and the data windowing technique used in phase-stepping are investigated using the characteristic polynomials. It is shown that the phase measurement algorithms derived using the extended averaging technique are not in general more efficient and may require more samples than those obtained directly from the rules of the characteristic polynomial theory.

It is shown that micromirrors can be made by ablating a dyed plastic using visible light. To test the optical quality of the micromirrors, methods such as the Ronchi and Foucault tests are applied. A commercial interferometer is also used to find their aberrations.

High-radix number systems enable higher information storage density, less complexity, fewer system components, and fewer cascaded gates and operations. A simple one-step fully parallel high-radix signed-digit arithmetic is proposed for parallel optical computing based on new joint spatial encodings. This reduces hardware requirements and improves throughput by reducing the space-bandwidth product needed. The high-radix signed-digit arithmetic operations are based on classifying the neighboring input digit pairs into various groups to reduce the computation rules. A new joint spatial encoding technique is developed to present both the operands and the computation rules. This technique increases the spatial bandwidth product (SBWP) of the spatial light modulators (SLMs) of the system. An optical implementation of the proposed high-radix signed-digit arithmetic operations is also presented. It is shown that our one-step trinary signed-digit (TSD) and quarternary signed-digit (QSD) arithmetic units are much simpler and better than all previously reported high-radix signed-digit techniques.

An exact analytical solution for the design of an optical system without spherical aberration that satisfies the sine condition is presented. The system consists of two parts. The first two mirrors form a strictly afocal system with a spherical primary and the second convex aspherical mirror. The next two mirrors form a focusing system that compensates the coma aberration of the afocal system. A complete description of the optical design, with system data, is given. The spherical primary mirror has a diameter of 10 m. The focal length of the entire system is 152 m, length is 13.5 m, angular field is 7 arcmin, and its theoretical resolution is 0.03 arcsec.

Derived from an earlier development [fabrication of ultraprecise surfaces using a tube; patent pending (FAUST)], a new fabrication technique to produce on- and off-axis optical surfaces of revolution is presented. Although based on a shape-copying method, it is possible to generate different types of surfaces with the same machining tool. Load- controlled point-contact machining is applied using a small tool that is guided along a predetermined tool path, not requiring an in-process tool- path control. This fabrication technique constitutes a self-correcting process and is characterized by an advantageous error propagation between tool path and workpiece shape. The characteristics of this fabrication technique are discussed together with its application for the generation of on- and off-axis surfaces with conic sections as generators (''conic surfaces"). We present the design of a first setup for production of conic surfaces that facilitates the generation of all kinds of conic surfaces on the same machine, featuring a pantograph enabling the production of different scales of the surfaces. A discussion of first experimental data is also presented.

Binary amplitude phase-only filters (BAPOFs) with high discrimination capability (DC) are presented. This enhancement is achieved by designing the region of support of the phase-only filters (POFs). We propose different approaches for maximizing the DC based on analytical expressions of the correlation function for real-valued input scenes. Computer simulation and experimental results are presented and discussed. The experimental results coincide with the numerical analysis and demonstrate that the optimized BAPOFs obtained with this new technique produce high discrimination capability among several objects. For comparison, the optimization of the DC of the POF is formulated as an annealing problem.

We present a palmprint approach to identifying individuals. Some significant features covering both geometrical and structural characteristics can be extracted from the palmprint to distinguish a person from others. The experiments show that this approach can be effectively used as a new biometric technology for automated personal identification.

Nonexpansive symmetric extension is preferred over merely periodic extension because it leads to far fewer losses in boundaries and edges after compression. The generalization of the results in 1-D cases to arbitrary nonseparable multidimensional cases remain open problem. By properly classifying the symmetry of finite-supported 2-D signals and filters, we study the properties of decimation, interpolation, and convolution in the two-band nonexpansive symmetric extension of 2-D nonseparable filters, where the quincunx sampler and diamond-shaped linear phase filters are considered and the conditions for retaining symmetry during various operations are obtained, which are prerequisites for perfect reconstruction. According to these properties, we sketch out a detailed procedure for implementation and test its correctness on the real image. The strategies and results can be generalized for applications in dimensions higher than two.

Conventional synthesis filters in subband systems lose their optimality when additive noise (due, for example, to signal quantization) disturbs the subband components. The multichannel representation of subband signals is combined with the statistical model of input signal to derive the multirate state-space model for the filter bank system with additive subband noises. Thus the signal reconstruction problem in subband systems can be formulated as the process of optimal state estimation in the equivalent multirate state-space model. Incorporated with the vector dynamical model, a 2-D multirate state-space model suitable for 2-D Kalman filtering is developed. The performance of the proposed 2-D multirate Kalman filter can be further improved through adaptive segmentation of the object plane. The object plane is partitioned into disjoint regions based on their spatial activity, and different vector dynamical models are used to characterize the nonstationary object-plane distributions. Finally, computer simulations with the proposed 2-D multirate Kalman filter give favorable results.

An optically addressed spatial light modulator using the twisted smectic-C* liquid crystal effect in the light-modulating layer and an intrinsic hydrogenated amorphous silicon in the photosensitive layer is fabricated. The device incorporates a layer of pixelated aluminum mirrors, sandwiched between photosensitive and modulating layers, to increase the reflectivity of the modulated output read beam. The device is capable of achieving an intrinsic gray-scale optical output. A spatial resolution of 27 lp/mm (i.e., 54 lines/mm) at 50% modulation depth driving at 2.0 kHz and a maximum frame rate of 2.5 kHz at 80% modulation depth for which a frame cycle includes erase, write, and read operations have been demonstrated. Other operating characteristics, i.e., the sensitivity and contrast ratio, are reported.

Recently, the application of a scale-space filter for speckle noise reduction in electronic speckle pattern interferometry correlation fringes was reported. The filter, derived from information theory and statistical mechanics considerations, is an iterative, adaptive, and clustering algorithm that performs edge-preserving smoothing. To apply the filter, the kernel size and the number of iterations must be previously set. We show that both parameters can be chosen depending on image features, e.g., presence or absence of sharp edges due to shadows, holes, or physical limits of the test object. When high-contrast edges are present, it is also shown that better noise reduction and edge preservation are obtained when the convergence of the solution of the filter is truncated. Both computer-simulated and experimental correlation fringes are used to test the scale-space filter.

Based on the dual-beam illumination principle of Leendertz, an electronic speckle pattern interferometer (ESPI) is developed to determine separately two orthogonal components of the in-plane displacement field. In the arrangement described, the lengths of the illumination beams were reduced to make the optical setup compact. The CCD camera for recording images is placed outside the area enclosed by the optical path, and adequate working space is provided between the camera and the test rig, so that adjustments can be made without interfering with the light beams. Using a customized telephoto lens, different areas in the range of 5.6x4.0 and 14x10 mm2 can be examined at a distance of 550 mm from the test piece. Combined with the blind-hole-drilling method, it is applied to residual stress determination, which is a challenging task due to stress concentration and severe decorrelation in the area neighboring the drilled hole.

A new method of electronic speckle pattern interferometry (ESPI) for in-plane deformation measurements is presented. Unlike usual ESPI methods used for in-plane deformation measurements with two symmetrical smooth illuminating waves, we introduce miniature speckling elements and thin laser beam illumination of them. The miniature elements serve for generating two symmetrical speckled waves. These miniature speckling elements are produced as reflection or transmission holograms. Nonholographic speckling elements were also tested. We present a family of flexible electronic speckle pattern interferometers for in-plane deformation analysis based on our method. Due to their simplicity, compactness, and low cost, the devices are ideally suited for industrial automated inspection. Experimental results obtained with the novel ESPI devices are given.

In order to make appropriate technology choices for the system implementation of optical interconnects, the issues of reliability and manufacturability must be included in any figure of merit (FOM) calculation.