This PDF file contains the editorial “Editorial: Keeping Old Texts Alive” for OE Vol. 44 Issue 05

We propose a novel demosaicking method based on the linearity property of a local color distribution. With the proposed technique, the color filter array can be demosaicked with less "confetti" types of errors and fringe artifacts than many current demosaicking methods. Furthermore, edge details are well preserved.

A multichannel bidirectional dynamic data transmission system (DDTS) through a rotary interface with one fiber is designed based on a fabricated single pass fiber optic rotary joint. The feasibility of transmission system is tested at both 1310- and 1550-nm wavelength bands. The performance of this DDTS was measured using optical spectrum analyzer and lightwave multimeter. The insertion losses of DDTS were 1.55 and 1.20 dB at 1310- and 1550-nm wavelength bands, respectively. The total bandwidth of the DDTS is more than 170 nm.

We present an all-fiber filter with symmetric forward and feedback lattice structures. Using the proposed filter, Gaussian-like spectra can be equalized in the maximally flat sense while the output power is maximized. As an illustrative example, the proposed filter is designed to flatten the output spectrum of a superluminescence light-emitting Diode (SLED). Compared to existing filters, the proposed filter achieved better performance in experiments.

A monolithically integrated high-speed true-time-delay line with embedded electrically switchable Bragg gratings was developed and characterized. Such a monolithically integrated device structure leads to a significantly reduced device size and allows a more precise RF phase control. A 40 ps reprogrammable optical true-time delay increment was acheived with a very fast switching speed of <50 µs.

An optical micro-shadowgraph for the height measurement of a micro-solderball on a semiconductor wafer is proposed. The micro-shadow image resulting from an oblique illumination onto the protruded solderball/bump on the wafer is clearly captured. Experimental investigation shows that accurate solderball height measurement can be readily obtained.

This PDF file contains the editorial “Editorial: Special Section on Applications of Laser Ablation and Micromachining” for OE Vol. 44 Issue 05

We report experiments on the ablation of arsenic trisulphide and silicon using high-repetition-rate (megahertz) trains of picosecond pulses. In the case of arsenic trisulphide, the average single pulse fluence at ablation threshold is found to be >100 times lower when pulses are delivered as a 76-MHz train compared with the case of a solitary pulse. For silicon, however, the threshold for a 4.1-MHz train equals the value for a solitary pulse. A model of irradiation by high-repetition-rate pulse trains demonstrates that for arsenic trisulphide energy accumulates in the target surface from several hundred successive pulses, lowering the ablation threshold and causing a change from the laser-solid to laser-plasma mode as the surface temperature increases.

Direct-write micro- and nanostructuring laser technologies are very important for the fabrication of new materials and multifunctional devices. Using tightly focused femtosecond laser pulses one can produce submicrometer holes and periodic structures in metals, semiconductors, and dielectrics on arbitrarily shaped surfaces. The achievable structure size is not restricted by the diffraction limit. It is determined by material properties and the laser pulse stability. We report investigations of possibilities to use femtosecond laser pulses for nanostructuring of different materials.

Embedded waveguides and integrated optical devices are fabricated from a computer-aided design file by scanning the focus spot in amorphous silica, using femtosecond laser pulses (800 nm, <125 fs, at 1 kHz). The effect of laser processing conditions on the optical properties of direct-written waveguides and an unusual laser-induced birefringence in an optically isotropic glass are reported. Several integrated optical devices, including Y coupler, directional coupler, and Mach-Zehnder interferometer, are made to demonstrate the simplicity and flexibility of this technique in comparison to the conventional waveguide fabrication processes.

We present some applications of femtosecond laser ablation for microfluidics. A doubled Ti:sapphire femtosecond laser (λ=400 nm; pulse width, 90 fs; pulse energy up to 350 µJ; pulse repetition rate, 1 kHz) is used to microstructure passive microfluidic devices (channels, reservoirs, through-holes) in polymers [polymethyl methacrylate (PMMA), polyimide (PI, Kapton)] and glass (Pyrex). These materials are selected because of their extended use in the fabrication of microfluidic chips. In all cases, channels of some tenths of micrometers are obtained with a good-quality finishing for fluid transport. In the same sense, reservoirs and holes are produced. These latter elements are fabricated in larger dimensions to combine them with channels in some presented prototypes. The well-known feature of ultrashort pulses is that no edge effects are observed because the absence of thermal effects enables a good sealing. For PMMA, polymer bonding technologies are used. For Pyrex, the well-known silicon sealing by anodic bonding is chosen. In both cases, the fabricated prototypes work properly with a good flow behavior and no leakage is observed.

Following advances in ultrafast laser technology as a reliable tool for material probing and processing, we discuss various options for control and optimization. The possibility to tailor the temporal shape of ultrashort laser pulses enables extended opportunities for material processing. The concept of optimizing laser interactions is based on the possibility to regulate the energy delivery so that control of laser-induced phenomena can be achieved and quality structures can be realized. An experimental demonstration of the possibility to design excitation sequences tailored with respect to the material response is described, laying the groundwork for adaptive optimization in materials structuring. We show that under particular irradiation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. This is particularly important for processing brittle materials. Further examples are given to illuminate the possibility to optimize the kinetic properties of ions emitted from laser-irradiated semiconductors, using excitation sequences synchronized with the solid-to-liquid transformation time. Versatile sub-kilo-electron-volt ion beams are obtained, exploiting transitions to supercritical fluid states with minimal energetic expenses. Temporally selective irradiation can thus open up efficient thermodynamic paths, unfolding interesting perspectives for "intelligent," feedback-assisted processing of materials.

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta₂O₅ film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO₂, HfO₂, Al₂O₃, and SiO₂ films. A theoretical model is presented that explains these findings.

Deep-UV (248 nm, 193 nm) excimer laser ablation is used to pattern thin oxide films or layer stacks. The ablation of extended areas as well as submicrometer structuring is possible. Due to their optical, chemical, and thermal stability, these inorganic films are better suited for optical applications compared to organic films, especially if UV transparency is required. The ablation of SiO₂, Al₂O₃, HfO₂, and Ta₂O₅ layers and combinations is investigated. Patterned films can be used as masks, diffractive phase elements, or gratings.

We address the peculiarities of femtosecond laser ablation of both metallic and dielectric materials. The ablation process is investigated using two numerical models. For metals, a hydrodynamic model is used that describes the laser light absorption together with heat and pressure wave propagations and the material motion. This model is used to study laser ablation at different fluences for two metals with different strengths of the electron-ion coupling. In these calculations, the role of the temperature-dependent electron heat conduction is demonstrated. For dielectrics, material ionization and laser light absorption processes are modeled in both one and two dimensions. The saturation of the light absorption, and, hence, of the ablation depth, is shown to take place in dielectric materials at sufficiently large laser intensities. The role of this effect on the shape of the craters is examined. This saturation effect is demonstrated to be a consequence of the interplay between the ionization and the light absorption processes.

A novel autofocusing technique using a self-imaging approach is developed for a space camera having large aperture and long focus. Its optical system consists of an emitting system, receiving system, and two pentagonal prisms. The emitting system has three parts: a light source, beamsplitter, and aim slit. The receiving system includes a field lens, two separate lenses, and CCD. Using two little pentaprisms instead of a large planar mirror makes the autofocus system simple and compact. A defocused signal is acquired by measuring the change of the slit self-image position imaged on the CCD. The algorithm to calculate the movement of the slit self-image position is analyzed to achieve subpixel position estimation. The results of the experiments show a ±10-µm precision of autofocusing in a range of ±5-mm defocusing. The system demonstrates high resolution, easy adjustment, and high reliability. Also this method can meet the requirements of various accurate focus measurement systems.

Lightweight mirrors experience optical image degradation due to mechanical loadings such as self-weight, polishing pressure, and vibration. Optical surface deformation of a lightweight primary mirror is an important factor that affects optical performance. We use topology optimization to design a lightweight primary mirror under self-weight and polishing pressure. For the optimization, we used a 3-D model of the mirror and based our calculations on the rms surface error of the mirror as an objective function constrained by the maximum weight of the mirror. In the first example of topology optimization, we consider the mirror's self-weight loading. In the second example, we include the polishing pressure. We present the results of the optimized design topology for the mirror with various mass constraints. To examine the optimal design results, we manufacture a prototype of the mirror.

Differential phase detection (DPD) and push-pull methods are widely used to generate tracking error signals in reading and writing high-density digital disks. Radial and tangential tilts, which will deteriorate the performance of the servo system, occur while the disk is spinning. Such influences vary with different tracking error generating methods and physical parameters of the disk. An analytic description of the disk diffraction model is obtained by diffraction theory. Disk radial tilt effects on tracking error signal and cross talk of read only and rewriteable disks are simulated and analyzed. A tracking error signal offset caused by radial tilt is 0.06 µm/deg on a DVD-ROM disk using the DPD method, 0.15 µm/deg using the push-pull method, and 0.12 µm/deg (0.10 pitch/deg) using the DPD method on a DVD-RAM disk. The DPD method exhibits better performance than the push-pull method when the disk is tilted on a DVD-RAM disk. The relationship between the tracking error signal offset and the groove depth is obtained in advance. The intertrack cross talk is also analyzed when disk tilt occurs.

Based on a vertical scanning system with a line-structured light source, data registration of several 3-D sensors is performed by a calibration process using a linear partition method. Data integration is fulfilled by using a relatively simple but efficient technique called the average coordinates method. The principle and solution procedure of each method is deduced in detail. Two 3-D sensors, a round filament target and an object with certain edge length, are used to verify the theory, and experimental results show that the mean relative error is less than 1%. With no requirement for solving nonlinear equations and other camera parameters, the method proposed is easy to adopt, runs fast, and still is high in precision, which meets the demands of 3-D measurement of a relatively big object.

A controllable, refractive, electro-optical liquid crystal (LC) beam deflector is studied both theoretically and experimentally. The principle of operation of this device is based on a beam deflection from a large gradient of refractive index, generated in an LC layer by a transversal electric field, formed between two lateral electrodes, in a simple three-electrode LC cell. A continuous, controllable deflection of a Gaussian laser beam, focused into the narrow region at the electrode gap is demonstrated. Maximum deflection angles of ~18 to 25 deg, depending on the LC birefringence, as well as a high deflection efficiency of up to 98% are demonstrated experimentally, backed by a theoretical analysis.

A hybrid C- plus L-band fiber amplifier module with three amplifier stages, which provides an operation range of 1540 to 1600 nm, is proposed and experimentally demonstrated. The proposed amplifier consists of two erbium-doped fiber amplifier (EDFAs) and a semiconductor optical amplifier (SOA) in cascade. Compared with a traditional L-band EDFA having longer EDF length, this amplifier can reduce the total EDF length to 44 m. As a result, 41.5 dB gain and a 3.7-dB noise figure are achieved at 1562 nm over the bandwidth of 1540 to 1600 nm, while the input signal power is –30 dBm. In addition, the behavior and performance of the proposed gain-clamping amplifier module over the C- plus L-band is investigated experimentally under different operation conditions.

We describe a systematic procedure that uses experimental and numerical methods to analyze the continuous-wave power performance of diode end-pumped solid-state lasers. For the general case, saturation, excited-state absorption, and thermal lensing effects are considered and integral equations are derived to study the evolution of the pump and laser beams in the gain medium. As an application of the method, we consider two different diode end-pumped Nd³⁺:YVO₄ lasers operating at 1064 and 1342 nm. Experimental efficiency data were first analyzed to determine the stimulated emission cross sections and the resonator losses. The best-fit laser parameters were then used to calculate the optimum crystal length that maximizes the output power of the laser. The described method should prove useful in the design of a wide range of efficient diode-pumped solid-state lasers.

IR lasers are widely used in electro-optical applications, especially in detector characterization systems. These lasers can be extremely sensitive to fluctuations in the operational temperature of their cavity and other environmental factors. Due to these influences, the laser output signal normally fluctuates randomly. These variations make it difficult to characterize the laser waist position and exact focus, which in turn causes difficulty with detector measurement. We apply a multivariate statistical approach to characterize and filter these variations and to calculate the "best focus" of a carbon dioxide laser operating at 10.6 µm. Using this method, the "best focus" can be calculated with great accuracy and can be easily implemented during postsignal processing. Also, this technique can potentially be applied to other situations in which laser signal instability is significant.

A compact microchip laser pumped by a single fiber-coupled semiconductor diode laser is developed for a space-borne scanning laser radar instrument. A commercial off-the-shelf component is used for the pump laser and undergoes a rigorous qualification approach to meet the requirements for the space-borne application. The qualification and testing process for the commercial pump laser is derived based on a nonstandard piece part screening plan and is presented along with the test results. These tests include mechanical, vibration, thermal cycling, and radiation tests as well as a full destructive parts analysis. Accelerated lifetests are also performed on the packaged device to demonstrate the ability to meet an operational lifetime of 5000 h. The environmental testing approach would be applicable to space qualification of a variety of commercial photonic systems, particularly in cost-constrained missions.

Proposed and demonstrated is an angstrom level optical path length (OPL) detection sensitivity polarization multiplexed heterodyne acousto-optic interferometric sensor. Because both the phase-coded and the reference rf signals in the design are generated by the same optics, the heterodyne detected rf signals generated contain correlated phase noise characteristics that essentially cancel out during the mixing process in the rf domain. Hence, the proposed four-beam design leads to a super stability OPL sensor with angstrom level experimentally demonstrated measurement stability. A proof-of-concept experiment measures the voltage-dependent phase change due to the refractive index change of a liquid crystal sensor chip.

We present a chirped fiber Bragg grating sensor that should be suitable for esophageal motility studies. The device uses the time-dependent group delay response of a chirped fiber Bragg grating to measure the peristaltic pressure wave that propagates down the esophagus with the transport of a bolus to the stomach. In contrast to existing transducers that only measure at discrete points, the output of this device is a continuous function of length along the esophagus. This paper presents ex-vivo experimental results. There is a linear relation between the wavelength location of the maximum phase perturbation and the position along the sensor where the perturbation occurred. The maximum phase change itself is directly proportional to the magnitude of the applied load at a specific position.

A novel fiber optic pressure sensor system with self-compensation capability for harsh environment applications is reported. The system compensates for the fluctuation of source power and the variation of fiber losses by self-referencing the two channel outputs of a fiber optic extrinsic Fabry-Pérot interfrometric (EFPI) sensor probe. A novel sensor fabrication system based on the controlled thermal bonding method is also described. For the first time, high-performance fiber optic EFPI sensor probes can be fabricated in a controlled fashion with excellent mechanical strength and temperature stability to survive and operate in the high-pressure and high-temperature coexisting harsh environment. Using a single-mode fiber sensor probe and the prototype signal-processing unit, we demonstrate pressure measurement up to 8400 psi and achieved resolution of 0.005% (2σ=0.4 psi) at atmospheric pressure, repeatability of ±0.15% (±13 psi), and 25-h stability of 0.09% (7 psi). The system also shows excellent remote operation capability when tested by separating the sensor probe from its signal-processing unit at a distance of 6.4 km.

We demonstrate with examples a technique of improving the accuracy of the inverse Wentzel-Kramers-Brillouin (WKB) method for refractive-index profiling of single-mode graded-index optical planar waveguides.

In graded-index glass waveguides a key factor for the design of components and devices is usually the accurate determination of the refractive index profile as a function of depth. The problem of reconstructing the index profile using the measured effective refractive indices has been faced by several authors. Here we suggest a new expression, linear combination of two analytical functions, which is able to provide a good approximation of the solution of the differential equation that rules the thermal diffusion process. The use of this equation, in connection with the general approach of inverting the Wentzel-Kramer-Bruilloin (WKB) eigenvalue equation, leads to a simple, fast, and accurate computation of the refractive index profile obtained by ion exchange. The application of the method to various Ag-exchanged waveguides in different glass matrices, with better accuracy than most of the usual procedures, has been demonstrated.

A novel S- to C-band erbium-doped fiber amplifier (EDFA) module over a 94-nm operation wavelength from 1476 to 1570 nm is experimentally investigated and demonstrated. This proposed module composes an S-band EDFA and a C-band erbium-doped waveguide amplifier (EDWA) with a coupled-structure. The 31.4-dB peak gain with 5.6-dB noise figure, and 30.5-dB peak gain with 4.7-dB noise figure are observed at 1506 and 1534 nm in this configuration when the input signal power is –30 dBm, respectively.

Wavelength-domain tracking (WDT) is a new concept for application in multiple-beam optical storage systems. Tracking errors caused by use of multiple beams on curved tracks are avoided in WDT systems by optimizing separations of focused beam spots. The control of beam spot separations is accomplished by wavelength tuning of individual laser sources. Feasibility of WDT is demonstrated by implementation and testing of a single-beam WDT tracking servo. Also, a novel method of error signal extraction by low-frequency modulation is proposed.

A novel, compact optoelectronic hardware neural network architecture based on an optical broadcast scheme is proposed and demonstrated. The basic cell in the system is composed of electronic neurons that share the same time-distributed, optical broadcast input. The system combines the computational strengths of electronics and the communication strengths of optics, and employs a modular reconfigurable architecture that is potentially scalable to a very large number of neurons while maintaining compactness. These characteristics are realized in an architecture that combines the integration of the processing elements in complementary metal-oxide semiconductor (CMOS) technology with the construction of efficient optical interconnection elements with focusing properties based on multiplexed volume holograms.

A new 2-D quality-guided phase-unwrapping algorithm, based on the placement of the branch cuts, is presented. Its framework consists of branch cut placing guided by an original quality map and reliability ordering performed on a final quality map. To improve the noise immunity of the new algorithm, a new quality map, which is used as the original quality map to guide the placement of the branch cuts, is proposed. After a complete description of the algorithm and the quality map, several wrapped images are used to examine the effectiveness of the algorithm. Computer simulation and experimental results make it clear that the proposed algorithm works effectively even when a wrapped phase map contains error sources, such as phase discontinuities, noise, and undersampling.

First, the phase differences between s- and p-polarizations of a circularly polarized heterodyne light beam reflected from the emulsion layer, and that from its substrate, are measured, respectively. The measured data are substituted into specially derived equations, so the refractive indices of the emulsion layer and its substrate can be calculated. Second, the variations of phase differences between s- and p-polarizations due to the wavelength shifts and the extraction of the holographic plate in a modified Michelson interferometer are measured. Then, the thickness of the emulsion layer and its substrate can be estimated based on the measured values of refractive indices, the wavelength shifts, and the phase difference variations. This method has some advantages, such as high resolution and easy operation in only one optical configuration.

A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.

The effects of laser beam multimode content on the angle-of-arrival fluctuations are examined for free-space optics (FSO) access systems. Multimode excitation is represented by coherent addition of Hermite-Gaussian modes. Mean square angle-of-arrival fluctuations are formulated using our previously reported multimode phase structure function. Numerical evaluations are obtained for practical FSO links operating at 1.55- and 0.85-µm wavelengths with link spans of up to 5 km. Mode content is arranged by sequentially grouping all possible mode combinations starting from the single fundamental mode (TEM₀₀) up to a certain higher order (n,m). Angle-of-arrival fluctuations are found to be of the order of several tens of microradians, except for the cases when the mode group terminates with an odd mode or when the extremely higher order modes are present. In these instances, the fluctuations will rise to the radian level. From these results, it is concluded that the performance of a practical FSO receiver, having a field of view of several milliradians, will not be substantially affected by the angle-of-arrival fluctuations due to multimode excitation, provided that the mode content of source excitation is confined to mode indices below 20.

We present an unconventional approach to passively expand the field of view of optical receivers for free-space laser communications. The illumination area, given by the movement of a focused light spot due to different angles of reception, is reduced to a smaller area without expanding the propagation angle of the spot. The working principle of this approach is based on combining a photonic crystal's negative refractive behavior with properly bent crystal boundaries. The device's optical behavior is determined by means of rigorous electromagnetic computation, but the overall working principle of illumination area reduction can be well visualized by the ray trace method. In addition, we estimate the influence of light modulation for high-speed data transmission. Since the discrete structure of photonic crystals only approximates a bent boundary with an arbitrary inclination angle, a slight modification is introduced into the crystal's structure to enable a more flexible design. Although such a modification influences the negative refractive behavior, the function of field-of-view expansion is still verified and confirmed by means of electromagnetic computation.

Laser communication systems developed for mobile platforms, such as satellites, aircraft, and terrain vehicles, require fast wide-range beam-steering devices to establish and maintain a communication link. Conventionally, the low-bandwidth, high-steering-range part of the beam-positioning task is performed by gimbals that inherently constitutes the system bottleneck in terms of reliability, accuracy and dynamic performance. Omni-Wrist^TM, a novel robotic sensor mount capable of carrying a payload of 5 lb and providing a full 180-deg hemisphere of azimuth/declination motion is known to be free of most of the deficiencies of gimbals. Provided with appropriate controls, it has the potential to become a new generation of gimbals systems. The approach we demonstrate describes an adaptive controller enabling Omni-Wrist^TM to be utilized as a part of a laser beam positioning system. It is based on a Lyapunov function that ensures global asymptotic stability of the entire system while achieving high tracking accuracy. The proposed scheme is highly robust, does not require knowledge of complex system dynamics, and facilitates independent control of each channel by full decoupling of the Omni-Wrist^TM dynamics. We summarize the basic algorithm and demonstrate the results obtained in the simulation environment.

Imagers based on focal plane arrays (FPAs) risk introducing in-band and out-of-band spurious responses, or aliasing, due to undersampling. IR systems can use microscan (or dither) to reduce aliasing. We describe a generic microscan technique and the benefits of microscanning, including an analysis of and experiments on four-point microscan employed in IR imagers, in which the image is mechanically shifted by 1/2 pixel between fields, in each dimension. Our purpose is to describe the benefits of microscanning for IR systems employing sensitive detectors. Through analysis and experiments on production systems, we show that microscanning is an effective way to improve the resolution of imaging systems. In addition, we present experimental data that shows that this increased resolution results in lower minimum resolvable temperatures (MRTs) than an equivalent nonmicroscanned system; and that this improvement in MRT is accompanied by an increase in detection, recognition, and identification (DRI) range performance in a real-world system. The microscan hardware can also be used to null out residual gimbal jitter in a stabilized imaging system, resulting in a jitter reduction of 35 to 50%. We show that this technique, known as microscan stabilization (MSS), is complementary to microscan, and further increases the imaging system performance.

We propose a new method for image encryption and decryption in which the image is broken up into bit planes. Each bit plane undergoes a jigsaw transform. The transformed bit planes are combined together and then encrypted using random phase masks and fractional Fourier transforms. The different fractional parameters, the random phase codes, and the jigsaw transform index form the key to the encrypted data. This increases the robustness of the encryption system by several orders of magnitude. Different variations of the juxtaposition of the pieces of the image are also considered. These include the rotated version of the jigsaw pieces. The computational complexity of the bit-plane-based jigsaw algorithm is further improved using the third dimension (i.e., along different bit planes) for scrambling as well. The results of computer simulation are presented to verify the proposed idea and analyze the performance of the proposed techniques.

Many face recognition algorithms/systems have been developed in the last decade and excellent performances have also been reported when there is a sufficient number of representative training samples. In many real-life applications such as passport identification, only one well-controlled frontal sample image is available for training. Under this situation, the performance of existing algorithms will degrade dramatically or may not even be implemented. We propose a component-based linear discriminant analysis (LDA) method to solve the one training sample problem. The basic idea of the proposed method is to construct local facial feature component bunches by moving each local feature region in four directions. In this way, we not only generate more samples with lower dimension than the original image, but also consider the face detection localization error while training. After that, we propose a subspace LDA method, which is tailor-made for a small number of training samples, for the local feature projection to maximize the discrimination power. Theoretical analysis and experiment results show that our proposed subspace LDA is efficient and overcomes the limitations in existing LDA methods. Finally, we combine the contributions of each local component bunch with a weighted combination scheme to draw the recognition decision. A FERET database is used for evaluating the proposed method and results are encouraging.

We present the abilities of diffractive elements for imaging with extended depth of focus. The elements of interest belong to the class of diffractive structures focusing incident light into a segment of the optical axis. We describe the imaging properties of the two following elements of this kind: the annular axicon and the light sword optical element (LSOE). In particular, the point spread functions and the modulation transfer functions of axicons and LSOEs are analyzed experimentally and numerically in detail. The obtained results correspond to different defocusing parameters. The performed experiments confirm the usefulness of axicons and LSOEs for imaging with extended depth of focus.

A new approach for the measurement of a refractive index is presented, following the introduction of a novel feature based on the selection of a large number of angles of incidence along a direction of symmetry at the prism-ambient interface, without moving the incident beam. This gives rise to very high sensitivities over a refractive index range determined by the divergence of this beam.

The improved Newton algorithm is proposed to apply to the inversion of the light-scattering data for particle sizing. A theoretical analysis is performed in detail and the experimental results are also given. The experimental results show that, compared with the projection algorithm and the improved projection algorithm, the presented improved Newton algorithm is much more stable and speedy and has high precision.

No abstract available.

A multilayered infrared Ge/Si quantum-dot photodetector is fabricated by pulsed laser deposition. Forty successive Ge quantum dot layers, each covered with a thin Si layer, are deposited. Deposition is monitored by in situ reflection high-energy electron diffraction and the morphology is further studied by ex situ atomic force microscopy. Current-voltage measurements reveal typical diode characteristics, while responsivity measurements show an absorption peak around a 2-μm wavelength