This PDF file contains the editorial “Reflections and Resolutions” for OE Vol. 50 Issue 01

We examine the practicability of the visible-light communication (VLC) transceiver for bidirectional high-speed and short-range communications. A visible-light peripheral interface could be a novel approach, visualizing the veiled security feature of light by employing visible lights as communication media beyond its intended applications, such as illumination and display. The proposed VLC transceivers are implemented with edge-an emitting laser diode and a silicon photodiode, which is primarily designed to operate in a full duplex mode at 120 Mbit/s. The shielding method that is employed to reduce the light cross-coupling effect inside the VLC transceiver is proposed and experimentally investigated. The bit error rate performance of the proposed VLC transceiver is examined with respect to the transmission distance and the coverage.

Three kinds of spectrometers based on off-axis Schmidt and Schmidt-Cassegrain cameras are presented; they have been used for several instruments studies, mainly for European Space Agency and Agenzia Spaziale Italiana. Both dispersive prism and grating based configurations have interesting characteristics, such as: simplicity, low cost, high efficiency, small volume and weight, very low sensitivity to polarization and great flexibility also in multichannel (wavebands) configurations. The image quality is high, even with low relative apertures and great fields of view, allowing a very good correction of smile and keystone. The compensation of the slit curvature induced by a prism disperser is also demonstrated. This family of spectrometers was the topic of three patents, belonging to Selex-Galileo, while the intellectual property belongs to A. Romali et al.

Lightweight, active, silicon carbide mirrors can increase the capability of space-based optical systems. However, launch survival is a serious concern for such systems, with the vibrations and acoustics from launch threatening to damage the optics. Therefore, a dynamic, state-space launch model has been developed with which one can quickly analyze the survival probability of many designs and also directly analyze launch load alleviation techniques. This paper discusses the launch model from which launch stress and survival probability are obtained, as well as launch load alleviation techniques that may increase the probability of launch survival. Three launch load alleviation techniques are presented and analyzed: whole spacecraft isolation, passive shunt circuits using the existing embedded actuators, and active damping using the existing actuators. All of the techniques reduce the launch stress, but at the expense of mass and complexity. The launch model allows for early identification of lightweight, active mirror designs which will survive launch, and the alleviation techniques expand the feasible design space by decreasing the launch stress and increasing the probability of launch survival.

Concentrating photovoltaic (CPV) technology has recently gained interest based on its scalability and expected low levelized cost of electricity. The reliability of encapsulation materials used in CPV systems, however, is not well established. For example, the present qualification test for CPV modules includes only real-time ultraviolet (UV) exposure, i.e., methods for accelerated UV testing have not yet been developed. To better define the stress inherent to CPV systems, the UV and infrared spectra transmitted through representative optical systems were evaluated. Measurements of optical components are used to assess expected optical performance and quantify damaging optical exposure. Optical properties (transmittance, refractive index, reflectance, and absorptance) of candidate materials (including PMMA, soda-lime glass, borosilicate glass, and quartz refractors), components (including Ag- and Al-enabled reflectors), and encapsulants (including EVA, ionomer, PDMS, PPMS, polyolefin, and PVB) were identified. The activation spectrum was calculated for the representative optical systems using an assumed action spectrum to compare the expected damaging dose of UV radiation delivered to the cell encapsulation. The dose and flux analysis identifies the significance of IR relative to UV exposure for CPV systems. Because UV light is typically more highly attenuated, the UV dose within the encapsulation may not greatly exceed the unconcentrated global solar condition, but the thermal load scales nearly directly with the geometric concentration. Relative to a previous analysis for crystalline silicon cell technology, the analysis here is performed for III-V multijunction technology. Novel aspects here also include additional materials (such as TPU encapsulation) and additional components (transmission through silicone on glass lenses, antireflective coatings, and the front glass used with reflective systems, as well as reflection off of the cell).

A multisegment stitching method is proposed to get the profile of steep aspheric surfaces with even 90-deg edge slope. The profile is divided into several segments overlapping each other, and the segments are measured with a high-precision stylus profilometer one by one. Then an iterative algorithm is used to stitch all of the segments together, based on the minimization of inconsistency among the overlapping regions. A convex hyperbolic surface is measured with three-segment stitching, and the result is approved by full-profile testing. We also demonstrate the method with a steep elliptical surface, measured and stitched by five segments. Finally, discussions are given on the error introduced by sagittal displacement of the segments.

The dynamics of material response following initial localized energy deposition by the laser pulse on the material's surface is still largely unknown. We describe a time-resolved microscope system that enables the study of the sequence of events and the individual processes involved during the entire timeline from the initial energy deposition to the final state of the material, typically associated with the formation of a crater on the surface. To best capture individual aspects of the damage timeline, this system can be configured to multiple imaging arrangements, such as multiview image acquisition at a single time point, multi-image acquisition at different time points of the same event, and tailored sensitivity to various aspects of the process. As a case example, we present results obtained with this system during laser-induced damage on the exit surface of fused silica.

Metallic paint has been widely used to provide for special visual appearance to various products so that photorealistic rendering of this material has been an important issue in the development of new products. We introduce a new approach that predicts the reflectance of metallic paint while considering manufacturing parameters. Our main idea is to simulate the appearance of various metallic paints having different composition of constituent materials by combination of measured bidirectional reflectance distribution functions and thereby to find a paint that provides optimal appearance of a product with appropriate composition of constituent materials. We mainly focused on two paint parameters, average size and density of aluminum flakes, because they significantly affect the appearance of metallic paint. We also present a compact representation to approximate a large size of measured data using a few curve functions. We demonstrate the efficiency and usability of our reflectance estimation method using some examples.

3-D metrology systems are used from examination and qualification of micro manufacturing techniques to reverse engineering of microstructures. For this purpose, the generation of capable and reliable 3-D coordinates in large numbers is essential. Combining classical triangulation with a new technique of calibrating measurement labels results in high rates of reliable 3-D metrology data. The triangulation setup consists of a stereomicroscope mounted on a 5-axis-gantry to generate all necessary points of view. Measurement labels are projected through one port of the stereomicroscope while their positions are localized through the second port. Providing a calibration rule is defined, the measurement system assigns each localized position to an absolute coordinate. For registration purposes, measurement labels are projected on a semitransparent mirror. Localizing measurement labels simultaneously with an observation and a calibration camera in connection with the working distance defines the calibration rule. By modifying the working distance between calibration camera and stereomicroscope, the measurement volume is scanned. The grid of the calibration camera chip acts as a measurement standard for the measurement labels. This approach features the spatial registration of a huge amount of measurement labels, covering the measurement volume, without building up the optical model of the imaging in a short time.

We demonstrate the generation of a collimated hollow beam of variable diameter using a pair of convex and concave metal axicon mirrors. Using these homemade inexpensive convex and concave metal axicon mirrors of equal cone angle, we have generated a good quality hollow beam with high conversion efficiency (∼76%) from a Gaussian input beam. The focusing of the generated collimated hollow beam has been compared with that of a diverging hollow beam.

Triethylamine (C₆H₁₅N) is widely used to stabilize the discharges of wavelength-nonselective transversely excited atmospheric (TEA) CO₂ lasers. In this paper, the influences of triethylamine on the discharge stability and line characteristics of a high-energy wavelength-selective TEA CO₂ laser are experimentally studied. The laser has 49 lasing lines ranging from 9.23 μm to 10.74 μm, and the energy of the strongest lines is above 50 J. However, the energy of the laser pulses is very unstable. The discharge stability is improved considerably by adding small amounts of triethylamine in the working gas mixture. In addition, lines in the 9 μm band are weakened greatly, and many of them even can no longer oscillate. In contrast, there are no distinct influences on the 10 μm lines. The results obtained in this study demonstrate the applied range of triethylamine in stabilizing the discharges of wavelength-selective CO₂ lasers.

The atmospheric aerosol scattering is considered as a major distortion mechanism in laser detection. Its influence on the target-reflected signal in incoherent frequency-modulation continuous-wave (FMCW) short-distance laser detection, which has not yet been well addressed, has been theoretically and experimentally studied in this paper. Assuming intensity of one-particle backscattering as an interference signal, the expression of this interference signal is calculated based on the model of one-particle backscattering and the Mie theory. Only single scattering is considered when optical thickness of the particle group is low. Broadenings of the hybrid signal formed with the particle group backscattering signal and the target-reflected signal are emulated, combining the model of particle group backscattering with the particle distribution function. The broadening time caused by the particle group is far less than the minimum response time of the photodiode. Experimental results confirm that the output voltage of the hybrid signal is identical to the output voltage of the target-reflected signal, except for attenuation in amplitude. Therefore, the incoherent FMCW short-distance laser detection shows a good capability of antiaerosol interference, which is very important for signal processing and target identification.

A novel liquid-level sensor based on a fiber Bragg grating and carbon fiber composite diaphragm is proposed and demonstrated. The sensing principle and finite element analysis result are described. Because the carbon fiber composite diaphragm's thickness is 0.2 mm and thinner than that of other materials, the sensitivity of the liquid-level sensor is improved. The experimental results show that sensitivity can reach 0.185 nm/m of water height. Based on the high sensitivity and the simple structure of the sensor, this sensor can find applications in the area of liquid level sensing.

Many organisms have diverse hair cells to instantaneously perceive the change of surroundings so that they can keep away from threats. These organs can precisely detect the tiny variations of airflow, water flow, sound, or pressure, and also resolve their affecting directions. Through this brilliant inspiration by the insects' cilia, we decided to design and develop a hair sensor for detecting two-dimensional airflow and pressure waves by using photoelectronic principles. The hair sensor inherently consists of an artificial cilium supported by an elastic membrane. A light-emitting diode and a quadrant photodiode are used as the photoelectronic sensor. The airflow or pressure wave directly stimulates the cilium to sway, and this motion contributes to let the projected light beam shift over the quadrant photodiode, whose four photodiodes produce then corresponding output signals. Because of dynamic and high-sensitive properties of the photoelectronic sensor, the hair sensor we developed possesses a high measurement resolution to be able to detect very tiny stimulation and its affecting direction. According to its multifaceted characteristics and simple structure, the hair sensor can be applied in numerous potential application fields, such as intrusion alarm system, noise detection system, as well as a tactile sensor.

An optical fiber using palladium as sensitive layer is characterized in the range of 450 to 900 nm. The sensitive layer is deposited on the outside of a multimode fiber, after removing the optical cladding. The sensor is based on a measurement technique that uses the surface plasmon resonance effect. A continuous change in output intensity is observed as a function of the hydrogen concentration between 0.5% and 4% H₂ by volume in Argon. The response shows that the transmitted intensity can either decrease or increase, depending on the selected wavelength. This behavior is directly related to the change in reflectance upon hydrogenation between the polarization s and p. The loading time is 30 s and the unloading time is 90 s in a mix of argon and 10% of oxygen. The detectors show a good reproducibility.

A method to detect delamination in a composite material, using a surface mounted fiber Bragg grating (FBG) sensor, is proposed in this paper. In this method, the static strain is measured to accurately detect the delamination area. Experiments are carried out and results show the relation between the wavelength shift and the distance between the FBG center and the delaminated area. The sensitivity of this method is also discussed. Results show the feasibility of this technique for structural health monitoring applications.

Superprism-based deflection of an optical beam is observed in a photonic crystal composed of a triangular lattice of pillars infiltrated with a liquid crystal. The device is based on a Silicon-on-insulator substrate and operates in the telecommunications band. The experimental results show a wavelength shift of 0.76 μm/nm, in reasonable agreement with simulations. Temperature-based control of the liquid crystal properties is also shown to modulate the superprism characteristics.

We mainly concentrate on the problem of gain broadening and flattening of erbium-doped fiber amplifier (EDFA) using dual forward-pumping configuration in which we use pumping both at 980 and 1480 nm wavelengths. The gain cascading of two EDFAs pumped separately at 980 and 1480 nm combines the advantage of high gain characteristics of 980 nm pumping and broad bandwidth characteristics of 1480 nm pumping. The use of increased number of channels in the present dense wavelength division multiplexing (DWDM) optical network requires a flat gain spectrum across the whole usable bandwidth. The use of very high-speed integrated circuit hardware description language (VHDL-AMS) in lightwave system modeling has been explored. Its application in designing wideband EDFA has not been reported thus far. This work facilitates to design broadband EDFA operating in both the C-band and L-band regions with VHDL-AMS. The amplifier design is made of a dual stage, copumped at 980 and 1480 nm, which delivers an average output power of -16 dBm over the entire 3-dB bandwidth of 80 nm.

The plastic substrates, reflective layers, dyes, and adhesives of four archival-grade, recordable DVDs and one standard-grade recordable DVD were analyzed to determine their chemical compositions and/or physical dimensions. Chemical analyses by attenuated total internal reflection Fourier transform infrared spectroscopy, time-of-flight secondary ion mass spectrometry, x-ray photoelectron spectroscopy, energy-dispersive x-ray/scanning transmission electron microscopy, and Rutherford backscattering spectrometry show that all these DVDs use very similar polycarbonate plastic substrates and acrylate-based adhesives, but different reflective layers and dye write layers. In addition, physical measurements by atomic force microscopy show differences in the DVD groove depth, width, and other dimensions. These chemical and physical analyses may help explain variations in DVD lifetimes and facilitate development of the next-generation archival-grade DVDs.

The development of spatial light modulators enables the application of active holographic optical elements in electronic speckle pattern interferometry. In our work adaptive comparative measurement is done, where an optically reconstructed image of a recorded or simulated single or double exposure hologram is used for holographic illumination of another object. In this paper, we present experimental results of measuring the difference of two deformations using this technique. The displacement difference can also be obtained numerically, if the wavefront used as a coherent illuminating mask does not belong to an existing object. This type of interferometer can easily adapt to the change of measuring conditions.

Photopolymer materials exhibit good characteristics when used as holographic recording media. Extensive studies have been carried out on the behavior of the various chemical components in such materials, with photosensitizers in particular receiving much attention. In all previous analysis of photopolymer kinetics, the effects of photosensitiser diffusion have been neglected. For rapid sequential holographic recordings in photopolymers, for example, in an application such as holographic data storage, dye diffusion effects may become more pronounced. Therefore, we examine the dye diffusion effects of erythrosine B in an acrylamide/polyvinyl alcohol material. This is achieved using simple experimental techniques and a proposed theoretical model.

he standard method for modeling the atmospheric short-exposure modulation transfer function (MTF) is due to Fried. This method involves evaluating the impact of amplitude and phase modulations of the propagating wavefront after the removal of the mean wavefront tilt. This tilt is determined by a least-squares analysis. We retain a term involving the correlation between tilt-corrected phase and computed tilt vector that was formerly assumed to be zero. Inclusion of this term yields a new turbulence-strength-dependent effect that degrades performance at high integrated turbulence levels and also predicts super-resolution effects at lower integrated turbulence levels. An analytical approximation is derived from this analysis that describes the new MTF model as a function of three dimensionless parameters related to the angular frequency, system aperture diameter, and integrated turbulence strength.

Henyey-Greenstein (H-G) phase function is typically used as an approximation to Mie phase function and its shortcomings have been discussed in numerous papers. But the judicious criterion of when the H-G phase function would be valid is still ambiguous. In this paper, we use the direct sample phase function method in transmittance calculation. A comparison of the direct sample phase function method and the H-G phase function is presented. The percentage of the multiple scattering in Monte Carlo transfer computations is discussed. Numerical results showed that using H-G phase function led to underestimating the transmittance. The deflection of root means square error can be used as a criterion. Although the exact calculation of sample phase function requires slightly more computation time, the rigorous phase function simulation method has an important role in the Monte Carlo radiative transfer computation problems.

Traditional two-dimensional (2-D) barcodes, such as the QR code and PDF417, do not carry visual information. This work proposes a visually meaningful 2-D barcode (or 2-D image code) that carries not only hidden information, but also visual information. Watermarking is employed to hide information in a meaningful cover image to produce the 2-D image code. The 2-D image code can be printed or displayed and then captured using a camera-equipped mobile device. The hidden information is extracted for innovative applications. To resist distortions in the print-and-photo process, a perceptual shaping algorithm, based on Watson's DCT-based perceptual model, is proposed for use in the data hiding procedure. The detection performance that can be achieved using two classes of position-detection patterns is evaluated, and the best pattern is then used as a part of the 2-D image code. A web service is provided for generating the 2-D image codes and a Java-based decoder is developed for mobile phones. The results of practical experiments reveal the applicability, robustness, and high capacity of the proposed 2-D image code.

The H.264/AVC standard utilizes temporal prediction tools to obtain a good coding performance by reducing temporal redundancy between frames. However, because H.264/AVC does not efficiently encode video sequences that have a local illumination change, the coding performance of H.264/AVC is dropped when the local illumination changes occur in video. We propose an adaptive motion compensation method using neighboring pixels and motion vector refinement to efficiently encode the local illumination changes. By using the proposed method, we achieve a bit savings up to 6.86% with peak signal-to-noise-ratio gain compared to H.264/AVC.

We present an efficient deinterlacing method via mathematical modeling of the neighbor pixels in the local region. The local surface model is designed using the quadratic equation having two-dimensional coordinate variables. Unlike conventional deinterlacing methods, the proposed method avoids using directional difference measures, resulting in reduced limitation on the number of considering edge directions. By modeling the local surface, it is easier to derive the true characteristic of the local region in a natural image than utilizing the directional difference measure. In order to decide the optimal coefficients of the surface model, the neighbor pixels around the current pixel to be interpolated are utilized. Once the coefficients are determined, the surface model estimates the pixel intensity of the current pixel to be interpolated. Simulation results show that the proposed surface model-based deinterlacing method minimizes the interpolation error. Compared to the traditional deinterlacing methods and Wiener filter-based interpolation method, the proposed method improves the subjective quality of the interpolated edges while maintaining a higher peak signal-to-noise-ratio level.

Generation of chaos from acousto-optic modulators with an electronic feedback has been studied for several years. Such chaotic signals have an important application in providing secure encryption in free-space optical communication systems. Lyapunov exponent is an important parameter for analysis of chaos generated by a nonlinear system. The Lyapunov exponent of an acousto-optic system is determined and calculated in this paper to understand the dependence of the chaotic response on the system parameters such as bias, feedback gain, input intensity and initial condition exciting the cell. Analysis of chaos using Lyapunov exponent is consistent with bifurcation analysis and is useful in encrypting data signals.

Multimodal biometrics provides high recognition accuracy and population coverage by using various biometric features. A single finger contains finger veins, fingerprints, and finger geometry features; by using multimodal biometrics, information on these multiple features can be simultaneously obtained in a short time and their fusion can outperform the use of a single feature. This paper proposes a new finger recognition method based on the score-level fusion of finger veins, fingerprints, and finger geometry features. This research is novel in the following four ways. First, the performances of the finger-vein and fingerprint recognition are improved by using a method based on a local derivative pattern. Second, the accuracy of the finger geometry recognition is greatly increased by combining a Fourier descriptor with principal component analysis. Third, a fuzzy score normalization method is introduced; its performance is better than the conventional Z-score normalization method. Fourth, finger-vein, fingerprint, and finger geometry recognitions are combined by using three support vector machines and a weighted SUM rule. Experimental results showed that the equal error rate of the proposed method was 0.254%, which was lower than those of the other methods.

As increasing urbanization is taking place worldwide, earthquake hazards pose serious threats to lives and properties for urban areas. A practical earthquake prediction method appears to be far from realization. Generally, the traditional instruments for earthquake detection have the disadvantages of high cost and size. To solve these problems, this paper presents a new method which can detect earthquake intensity using video capture device. The main method is based on a new proposed motion vector algorithm with simple but effective methods to immediately calculate acceleration of a predefined target object. By estimating the motion vector variation, the movement distance of predefined target object can be computed, and therefore the earthquake amplitude can be defined. The effectiveness of the proposed scheme is demonstrated in a series of experimental simulations. It is shown that the scheme successfully detects the earthquake occurrence and identifies the earthquake amplitude from video streams.

Spatial-temporal interest points (STIPs) are popular in human action recognition. However, they suffer from difficulties in determining size of codebook and losing much information during forming histograms. In this paper, spatial-temporal interest regions (STIRs) are proposed, which are based on STIPs and are capable of marking the locations of the most "shining" human body parts. In order to represent human actions, the proposed approach takes great advantages of multiple features, including STIRs, pyramid histogram of oriented gradients and pyramid histogram of oriented optical flows. To achieve this, cumulative histogram is used to integrate dynamic information in sequences and to form feature vectors. Furthermore, the widely used nearest neighbor and AdaBoost methods are employed as classification algorithms. Experiments on public datasets KTH, Weizmann and UCF sports show that the proposed approach achieves effective and robust results.

An H.264-based fast 3-D video compression algorithm is presented by reducing coding complexity of P slices in a depth map sequence using a fast-mode decision scheme. On the basis of the relationship of coding modes of collocated macroblocks on texture and depth frames, an early SKIP-mode decision scheme and a selective prediction-mode decision scheme have been developed to rapidly encode depth map sequences. Experimental results show that the algorithm can save the entire encoding time by 79% on average compared to conventional a depth map sequence coding algorithm, while introducing only negligible loss in peak signal-to-noise ratio value.

On the basis of the work of Chen, Zhou, Luo, and Du [Opt. Lett. 33, 753-755 (2008)], we propose a new method using nonperiodic sampling to design a high-transmittance structured lens. The sizes of the holes are decided by the phase requirements, and the holes are arranged with a fixed interval in the vertical direction while a variable interval in the horizontal direction. With this design method, holes can be arranged more compactly and the fill factor of the holes can be increased effectively. Numerical simulation is performed through the finite-difference time-domain method, and the results indicate that the transmittance of the lens climbs to 73.6%, which reaches almost 3.5 times that achieved in the above-mentioned work. Meanwhile, the simulated focal length agrees well with the theoretical design result with the error of 1.1% and the full width at half maximum of the focal spot is obtained ∼1.16 times of the diffraction limit.

In this paper, we exploit the nonlinearity inherent in four-wave mixing in organic photorefractive materials and demonstrate edge enhancement, contrast conversion, and defect enhancement in a periodic structure. With the availability of these materials, which have large space-bandwidth products, edge enhancement, contrast conversion and defect enhancement are possible. Some simulation results also are provided.

We investigate new numerical speckle images using diffuser provided with some classes of amplitude apertures. The 1st class is based on circular aperture modulation, the 2nd class assumes modulated elliptical apertures, while the 3rd is devoted to a cardiac, and comet deformed apertures. We have reconstructed the images of all apertures and using the concept of Fourier transform. We computed the point spread functions (PSF) for the modulated circular, elliptical, and the cardiac apertures.

The soliton-like interactions of two low divergence fields propagating in unbiased self-defocusing photorefractive media are examined. The interactions appear to be a result of factors that are not dissimilar to those between two steady state screening solitons. Analysis of the conditions for the interactions show a cutoff for the degree of beam divergence required for the soliton-like interactions to occur. The analysis also indicates that the strength of the interaction increases as the beam divergence decreases.

The corrected approach for increasing the filed-of-view of the polarization imaging spectrometer based on Savart polariscopes is presented.