Showing papers in "Optics and Laser Technology in 2014"

Novel image compression–encryption hybrid algorithm based on key-controlled measurement matrix in compressive sensing

Abstract: The existing ways to encrypt images based on compressive sensing usually treat the whole measurement matrix as the key, which renders the key too large to distribute and memorize or store. To solve this problem, a new image compression–encryption hybrid algorithm is proposed to realize compression and encryption simultaneously, where the key is easily distributed, stored or memorized. The input image is divided into 4 blocks to compress and encrypt, then the pixels of the two adjacent blocks are exchanged randomly by random matrices. The measurement matrices in compressive sensing are constructed by utilizing the circulant matrices and controlling the original row vectors of the circulant matrices with logistic map. And the random matrices used in random pixel exchanging are bound with the measurement matrices. Simulation results verify the effectiveness, security of the proposed algorithm and the acceptable compression performance.

A review of optical image encryption techniques

Abstract: In this paper we review a number of optical image encryption techniques proposed in the literature inspired by the architecture of the classic optical Double Random Phase Encoding (DRPE) system. The optical DRPE method and its numerical simulation algorithm are first investigated in relation to the sampling considerations at various stages of the system according to the spreading of the input signal in both the space and spatial frequency domains. Then the several well-known optically inspired encryption techniques are examined and categorized into all optical techniques and image scrambling techniques. Each method is numerically implemented and compared with the optical DRPE scheme, in which random phase diffusers (masks) are applied after different transformations. The optical system used for each method is first illustrated and the corresponding unitary numerical algorithm implementation is then investigated in order to retain the properties of the optical counterpart. The simulation results for the sensitivities of the various encryption keys are presented and the robustness of each method is examined. This overview allows the numerical simulations of the corresponding optical encryption systems, and the extra degree of freedom (keys) provided by different techniques that enhance the optical encryption security, to be generally appreciated and briefly compared and contrasted.

Microstructure and tensile properties of iron parts fabricated by selective laser melting

Abstract: Iron, as the basic industry material was extensively studied in the past, but it could still offer extended possibilities with the use of new processing techniques such as selective laser melting (SLM). In this work, the manufacturing of iron parts using SLM technology was investigated. The effect of processing parameters on density of the iron parts was studied. Fully dense iron parts have been fabricated at the laser power of 100 W using different laser scanning speeds. By means of metallographic observation and TEM characterization, it can be found that the grains size decreased with increasing scanning speed and high dislocation density was observed. Tensile specimens were fabricated using optimal parameters and mechanical tests allowed observing an ultimate tensile strength of 412 MPa and the yield strength of 305 MPa. Multiple self-strengthening mechanisms during SLM process are proposed to explain this high mechanical strength. The grain refinement seems to be the most significant strengthening mechanism, followed by work hardening arising from the high cooling rate.

An experimental–numerical investigation of heat distribution and stress field in single- and multi-track laser cladding by a high-power direct diode laser

Abstract: High-power direct diode laser (HPDDL) cladding offers several advantages in the laser surface modification and repair of high-value parts. The wider beam and uniform energy distribution in the direct diode laser provide a smooth heating and cooling cycle during the cladding process. Subsequently, lower dimensional distortion and thermally-induced stress occur during the process. In this paper, temperature evolution and molten pool dimensions as well as stress-and-strain fields were studied by utilizing experimental and numerical methods. A three-dimensional (3D) transient uncoupled thermo-elastic–plastic model was developed to simulate a thermal process during the single- and multi-track laser cladding and the thermally-induced residual stress in the laser cladding. The effect of latent heat and phase transformations are considered in the thermal analysis. The numerical results were validated by experimentally-measured values, and the maximum prediction error was 3.5%. The experimental results were collected by in-situ monitoring techniques (e.g., thermocouples and a high-speed CCD camera). The level of residual stresses at the cladded surfaces were measured by an X-ray diffractometer. In addition, the effect of scanning speed on the thermal and stress evolution was quantitatively discussed.

Selective laser melting additive manufactured Inconel 718 superalloy parts: High-temperature oxidation property and its mechanisms

Abstract: This work presented a comprehensive study of high-temperature oxidation behaviors and mechanisms of Selective laser melting (SLM) processed Inconel 718 superalloy parts using different methods including isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The experimental results revealed that the oxidation process of the tested parts processed at a lower volumetric laser energy density experienced the severe spallation. On reasonably increasing the applied volumetric laser energy density, the oxidation kinetics of the as-produced parts obeyed a parabolic law, exhibiting the significantly improved oxidation resistance performance. The constitutional phases within the oxidation film were identified and the corresponding formation mechanisms were elucidated in detail according to the thermodynamic principles. The cross-sectional morphologies of oxidized Inconel 718 parts indicated that the oxidation microstructure mainly consisted of an external oxidation layer and an internal oxidation zone. The oxidation process was controlled by the outward diffusion of oxide forming elements and inward penetration of oxygen, by which the interaction mechanisms between the microstructures and internal oxidation zones were clarified. On the basis of the experimental results and theoretical analyses, the physical oxidation mechanisms were accordingly established to illustrate the oxidation behaviors of SLM-processed Inconel 718 parts at elevated operative temperatures.

Cryptanalyzing a RGB image encryption algorithm based on DNA encoding and chaos map

Abstract: Recently, a RGB image encryption algorithm based on DNA encoding and chaos map has been proposed. It was reported that the encryption algorithm can be broken with four pairs of chosen plain-images and the corresponding cipher-images. This paper re-evaluates the security of the encryption algorithm, and finds that the encryption algorithm can be broken efficiently with only one known plain-image. The effectiveness of the proposed known-plaintext attack is supported by both rigorous theoretical analysis and experimental results. In addition, two other security defects are also reported.

Experimental and numerical investigation of temperature distribution and melt pool geometry during pulsed laser welding of Ti6Al4V alloy

Abstract: This paper reports on a numerical and experimental investigation of laser welding of titanium alloy (Ti6Al4V) for modeling the temperature distribution to predict the heat affected zone (HAZ), depth and width of the molten pool. This is a transient three-dimensional problem in which, because of simplicity, the weld pool surface is considered flat. The complex physical phenomenon causing the formation of keyhole has not been considered. The temperature histories of welding process were studied. It was observed that the finite volume thermal model was in good agreement with the experimental data. Also, we predicted the temperature as a function of distance at different laser welding speeds and saw that at each welding speed, the temperature profile was decreased sharply in points close to the laser beam center, and then decreased slightly in the far region from the laser beam center. The model prediction error was found to be in the 2–17% range with most numerical values falling within 7% of the experimental values.

The effects of short pulse laser surface cleaning on porosity formation and reduction in laser welding of aluminium alloy for automotive component manufacture

Abstract: Laser welding of aluminium alloys typically results in porosity in the fusion zones, leading to poor mechanical and corrosion performances. Mechanical and chemical cleaning of surfaces has been used previously to remove contaminants for weld joint preparations. However, these methods are slow, ineffective (e.g. due to hydrogen trapping) or lead to environmental hazards. This paper reports the effects of short pulsed laser surface cleaning on porosity formation and reduction in laser welding of AC-170PX (AA6014) aluminium sheets (coated with Ti/Zr and lubricated using a dry lubricant AlO70) with two types of joints: fillet edge and flange couch, using an AA4043 filler wire for automotive component assembly. The effect of laser cleaning on porosity reduction during laser welding using a filler wire has not been reported before. In this work, porosity and weld fusion zone geometry were examined prior to and after laser cleaning. The nanosecond pulsed Nd:YAG laser cleaning was found to reduce porosity significantly in the weld fusion zones. For the fillet edge welds, porosity was reduced to less than 0.5% compared with 10–80% without laser cleaning. For flange couch welds, porosity was reduced to 0.23–0.8% with laser cleaning from 0.7% to 4.3% without laser cleaning. This has been found to be due to the elimination of contaminations and oxide layers that contribute to the porosity formation. The laser cleaning is based on thermal ablation.

High performance SOI microring resonator for biochemical sensing

Abstract: In this work we have investigated different silicon-on-insulator (SOI) microcavities based on a planar geometry having a footprint on chip as small as 100 μm 2 with a ring, disk and hybrid configurations with the aim of being poorly intrusive for both in-body and out-of-body biosensing purposes. Accurate numerical results have been achieved by using the 3D finite element method and compared to 3D finite discrete time domain ones with a good agreement for both methods. The most promising resonator among the devices we have analyzed shows a Q -factor of the order of 10 5 , that allows a limit of detection for the sensor equal to 10 −6 RIU and a sensor sensitivity of 120 nm/RIU. The resonator has been designed for glucose biosensing, considering both the homogeneous sensing and the surface one, that enhances the sensor selectivity by the device functionalization with a glucose-oxidase (GOD) layer. The glucose concentration has been evaluated both with the microcavity surrounded by a water solution and with water only in the inner part of the cavity. The achieved performance is really attractive not only for the reduced size of the cavity, but also for the planar coupling configuration of the annulus and the waveguides composing the cavity since it appears to be a very promising configuration for the practical packaging of micro systems containing whispering gallery mode resonators. In this paper the concept of an on-chip platform for a high throughput and multichannel detection relying on an array of resonant cavities interacting with a single nanofluidic channel, is also discussed.

Particle swarm optimized multi-objective histogram equalization for image enhancement

Abstract: Histogram Equalization (HE) is a simple and effective technique for enhancing the contrast of the input image However, it fails to preserve the brightness while enhancing the contrast due to the abrupt mean shift during the process of equalization Many HE based methods have been developed to overcome the problem of mean shift But, they suffered from over-enhancement In this paper, a multi-objective HE model has been proposed in order to enhance the contrast as well as to preserve the brightness The central idea of this technique is to first segment the histogram of the input image into two using Otsu's threshold A set of optimized weighing constraints are formulated and applied on both the sub-images Then, the sub-images are equalized independently and their union produces the contrast enhanced, brightness preserved output image Here, Particle Swarm Optimization (PSO) is employed to find the optimal constraints This technique is proved to have an edge over the other contemporary methods in terms of entropy and contrast improvement index

Effect of laser pulse energy and wavelength on the structure, morphology and optical properties of ZnO nanoparticles

Abstract: In this work, the effects of the laser pulse energy and laser wavelength on the production of ZnO nanoparticles prepared by pulsed laser ablation of Zn metal plate in deionized water are investigated. The beam of a Q-switched Nd:YAG laser of 1064 and 532 nm wavelengths at 6 ns pulse width and different fluences is employed to irradiate the solid target in water. The ZnO nanoparticles were found to be hexagonal. The size distribution of generated ZnO nanoparticles is decreased by increasing the laser pulse energy. The rate of ZnO nanoparticles production is increased with increasing the laser pulse energy and is decreased with increasing the laser photon energy. ZnO nanoparticles were formed with different shapes depending on the laser pulse energy and laser wavelength. The bandgap energy for ZnO nanoparticles generated with 1064 nm laser pulse wavelength is calculated to be 3.59–3.89 eV.

Highly sensitive curvature sensor based on single-mode fiber using core-offset splicing

Abstract: A highly sensitive curvature sensor based on core-offset splicing is proposed and demonstrated. The whole fabrication process is quite simple and the sensor head is cost effective. Measurement results show that it has a high sensitivity of −22.947 nm/m −1 in the range from 0.35312 m −1 to 2.8127 m −1 . Temperature sensitivity of 77.6 pm/°C within the range of 20–80 °C has also been achieved, which implies the possibility for measurement of temperature. High sensitivity and low-cost make it a preferable candidate for small curvature sensing with high resolution in practical applications.

Real-time monitoring of laser hot-wire cladding of Inconel 625

Abstract: Laser hot-wire cladding (LHWC), characterized by resistance heating of the wire, largely increases the productivity and saves the laser energy. However, the main issue of applying this method is the occurrence of arcing which causes spatters and affects the stability of the process. In this study, an optical spectrometer was used for real-time monitoring of the LHWC process. The corresponding plasma intensity was analyzed under various operating conditions. The electron temperature of the plasma was calculated for elements of nickel and chromium that mainly comprised the plasma plume. There was a correlation between the electron temperature and the stability of the process. The characteristics of the resulted clad were also investigated by measuring the dilution, hardness and microstructure.

Retinal vessel enhancement based on multi-scale top-hat transformation and histogram fitting stretching

Abstract: Retinal vessels play an important role in the diagnostic procedure of retinopathy. A new retinal vessel enhancement method is proposed in this paper. Firstly, the optimal bright and dim image features of an original retinal image are extracted by a multi-scale top-hat transformation. Then, the retinal image is enhanced preliminarily by adding the optimal bright image features and removing the optimal dim image features. Finally, the preliminarily enhanced image is further processed by linear stretching with histogram Gaussian curve fitting. The experiments results on the DRIVE and STARE databases show that the proposed method improves the contrast and enhances the details of the retinal vessels effectively.

Diode lasers: From laboratory to industry

Abstract: The invention of first laser in 1960 triggered the discovery of several new families of lasers. A rich interplay of different lasing materials resulted in a far better understanding of the phenomena particularly linked with atomic and molecular spectroscopy. Diode lasers have gone through tremendous developments on the forefront of applied physics that have shown novel ways to the researchers. Some interesting attributes of the diode lasers like cost effectiveness, miniature size, high reliability and relative simplicity of use make them good candidates for utilization in various practical applications. Diode lasers are being used by a variety of professionals and in several spectroscopic techniques covering many areas of pure and applied sciences. Diode lasers have revolutionized many fields like optical communication industry, medical science, trace gas monitoring, studies related to biology, analytical chemistry including elemental analysis, war fare studies etc. In this paper the diode laser based technologies and measurement techniques ranging from laboratory research to automated field and industry have been reviewed. The application specific developments of diode lasers and various methods of their utilization particularly during the last decade are discussed comprehensively. A detailed snapshot of the current state of the art diode laser applications is given along with a detailed discussion on the upcoming challenges.

Propagation of a stochastic electromagnetic vortex beam in the oceanic turbulence

Abstract: By using the extended Huygens–Fresnel principle, we investigate the stochastic electromagnetic vortex beam propagating through oceanic turbulence. General formulas for the elements of the 2×2 cross-spectral density matrix of a stochastic electromagnetic vortex beam propagating through the oceanic turbulence are obtained. We study the changes in the spectral density, the spectral degree of coherence and the spectral degree of polarization of such a vortex beam with the help of the general formulas. It is shown by numerical calculations that, the beam profile will approach a Gaussian distribution in far field under the influence of oceanic turbulence. It is also interesting to find that the spectral degree of polarization of a stochastic electromagnetic vortex beam composed by isotropic sources on propagation in far zone will return to its value in the source plane.

Experimental and numerical investigations of hybrid laser arc welding of aluminum alloys in the thick T-joint configuration

Abstract: In the present investigation, a numerical finite element model was developed to simulate the hybrid laser arc welding of different aluminum alloys, namely 5××× to 6××× series. The numerical simulation has been considered two double-ellipsoidal heat sources for the gas metal arc welding and laser welding. The offset distance of the metal arc welding and laser showed a significant effect on the molten pool geometry, the heat distribution and penetration depth during the welding process. It was confirmed that when the offset distance is within the critical distance the laser and arc share the molten pool and specific amount of penetration and dilution can be achieved. The models and experiments show that the off-distance between the two heat sources and shoulder width have considerable influence on the penetration depth and appearance of the weld beads. The experiments also indicate that the laser power, arc voltage and type of the filler metal can effectively determine the final properties of the bonds, specifically the bead appearance and microhardness of the joints. The experiments verified the numerical simulation as the thermocouples assist to comprehend the amount of heat distribution on the T-joint coupons. The role of the welding parameters on the mechanism of the hybrid laser welding of the aluminum alloys was also discussed.

Surface modification of air plasma spraying WC-12%Co cermet coating by laser melting technique

Abstract: Tungsten carbide cermet powder with 12%Co was deposited on stainless steel substrate by air plasma spraying method. Two types of coatings were produced i.e. thick (430 µm) and thin (260 µm) with varying porosity and splat morphology. The coated samples were treated with CO2 laser under the shroud of inert atmosphere. A series of experimentation was done in this regard, to optimize the laser parameters. The plasma sprayed coated surfaces were then laser treated on the same parameters. After laser melting the treated surfaces were characterized and compared with as-sprayed surfaces. It was observed that the thickness of the sprayed coatings affected the melt depth and the achieved microstructures. It was noted that phases like Co3W3C, Co3W9C4 and W were formed during the laser melting in both samples. The increase in hardness was attributed to the formation of these phases.

Investigation of third-order nonlinear and optical power limiting properties of terphenyl derivatives

Abstract: A series of new chalcones containing terphenyl as a core and with different functional groups has been successfully synthesized by Claisen–Schmidt condensation method in search of new nonlinear optical (NLO) materials. Molecular structural characterization for the compounds was achieved by FTIR and single crystal X-ray diffraction. The third-order NLO absorption and refraction coefficients were simultaneously determined by Z-scan technique. The measurements were performed at 532 nm with 7 ns laser pulses using a Nd:YAG laser in solution form. The Z-scan experiments reveal that the compounds exhibit strong nonlinear refraction coefficient of the order 10 –11 esu and the molecular two photon absorption cross section is 10 –46 cm 4 s/photon. The results also show that the structures of the compounds have great impact on NLO properties. The compounds show optical power limiting behavior due to two-photon absorption (TPA).

Optical monitoring of high power direct diode laser cladding

Abstract: Laser cladding is one of the most advanced surface modification techniques which can be used to build and repair high-value components. High power direct diode laser (HPDDL) offers unique quality and cost advantages over other lasers (CO2, Nd:YAG). Especially its rectangular laser beam with top-hat intensity distribution makes HPDDL an ideal tool for large area cladding. In order to utilize this technique successfully, the development of on-line monitoring and process control is necessary. In this study, an optical monitoring system consisting of a high-speed CCD camera, a pyrometer, and an infrared camera was used to analyze the mass- and heat-transfer in the cladding process. The particle transport in flight was viewed by a high-speed CCD camera; the interaction between powder flow and laser beam was observed by an infrared camera; and the thermal behavior of the molten pool was recorded by the pyrometer and the infrared camera. The effects of the processing parameters on the laser attenuation, particle heating and clad properties were investigated based on the obtained signals. The optical monitoring method improved the understanding about mutual interrelated phenomena in the cladding process.

Cluster approach based multi-camera digital image correlation: Methodology and its application in large area high temperature measurement

Abstract: A cluster approach based multi-camera Digital Image Correlation (DIC) system has been developed to quantify dynamic material response at temperature up to 1200 °C The Monochromatic Light Illuminated Stereo DIC technique was embedded to eliminate surface radiance at high temperature. The employed measurement system not only takes advantage of a conventional 3D DIC system, but also provides a feasible way to enlarge the measurement field without losing effective resolution in the area of interest. Two pairs of pre-calibrated CCD cameras are used to measure a piece of sheet nickel alloy. The view of each pair of cameras covers about half of the specimen. To guarantee the continuity of the evaluation result, an overlapped area that is covered by all four cameras is used in the setup. Unlike the conventional data stitching technique which stitches data from different pairs of cameras, our system with the cluster approach technique, maps all data points into a universal world coordinate system before evaluating the contour, displacement, and strain. To evaluate our system, a specimen was loaded with infrared heaters, and the dynamic contour, displacement, and strain field was evaluated. The methodology of the employed system is introduced in this paper. The system has the potential to be expanded with more cameras to measure a very large surface with one shot.

Multiple-image encryption based on optical wavelet transform and multichannel fractional Fourier transform

Abstract: A multiple-image encryption scheme based on the optical wavelet transform (OWT) and the multichannel fractional Fourier transform (MFrFT) is proposed. The scheme can make full use of multi-resolution decomposition of wavelet transform (WT) and multichannel processing of MFrFT. The mentioned properties can achieve the encryption of multi-image and the encryption of single image. When encryption finished, each image gets its own fractional order and independent keys. Analysis of encrypted effects has been completed. Furthermore, the influence of WT type and order are analyzed, and the application and analysis of MFrFT are accomplished as well. Numerical simulation verifies the feasibility of the scheme and shows that the problem of insufficient capacity is better solved, and the flexibility of scheme increases. A simple opto-electronic mixed device to realize the scheme is proposed.

Effect of conjugation length on nonlinear optical parameters of anthraquinone dyes investigated using He–Ne laser operating in CW mode

Abstract: We report the studies on third-order optical nonlinearity and optical limiting of anthraquinone dyes. Z-scan technique was employed to evaluate the nonlinear parameters such as nonlinear absorption coefficient βeff and nonlinear index of refraction n2. Continuous wave He–Ne laser was used as the source of excitation. The estimated values of βeff, n2 and χ(3) are of the order of 10−3 cm/W, 10−5 esu and 10−7 esu respectively. The presence of donor and acceptor groups in the structure results in increase in conjugation length. This resulted in the enhancement of nonlinear optical parameters values of the dye. Multiple diffraction rings were observed when the samples were exposed to laser beam due to thermal lensing. Dyes exhibited good optical limiting behavior under the experimental conditions. The results indicate that the dyes investigated here are materialise as candidates for photonics device applications such as optical power limiters.

Detection of trace amount of arsenic in groundwater by laser-induced breakdown spectroscopy and adsorption

Abstract: LIBS technique coupled with adsorption has been applied for the efficient detection of arsenic in liquid. Several adsorbents like tea leaves, bamboo slice, charcoal and zinc oxide have been used to enable sensitive detection of arsenic presence in water using LIBS. Among these, zinc oxide and charcoal show the better results. The detection limits for arsenic in water were 1 ppm and 8 ppm, respectively, when ZnO and charcoal were used as adsorbents of arsenic. To date, the determination of 1 ppm of As in water is the lowest concentration of detected arsenic in water by the LIBS technique. The detection limit of As was lowered to even less than 100 ppb by a combination of LIBS technique, adsorption by ZnO and concentration enhancement technique. Using the combination of these three techniques the ultimate concentration of arsenic was found to be 0.083 ppm (83 ppb) for arsenic polluted water collected from a tube-well of Farajikandi union (longitude 90.64°, latitude 23.338° north) of Matlab Upozila of Chandpur district in Bangladesh. This result compares fairly well with the finding of arsenic concentration of 0.078 ppm in the sample by the AAS technique at the Bangladesh Council of Scientific and Industrial Research (BCSIR) lab. Such a low detection limit (1 ppm) of trace elements in liquid matrix has significantly enhanced the scope of LIBS as an analytical tool.

Optical characterization of sol–gel derived Nb2O5 thin films

Abstract: The optical properties of thin Nb2O5 films, obtained by spin coating of Nb sol on glass and silicon substrates are investigated. The Nb sol was prepared by a sonocatalytic method using niobium chloride dissolved in ethanol. A calculation procedure for reliable and unambiguous determination of optical constants (refractive index and extinction coefficient) and thickness of films is developed that uses transmittance and reflectance measurements and non-linear curve fitting. By means of the developed procedure, the impacts of the sol ageing and the post deposition annealing on the optical properties and thickness of the films are investigated. It is demonstrated that films with controlled thicknesses and refractive index values in the range from 1.82 to 2.20 (at wavelength of 600 nm) could be obtained using different concentrations of the Nb sol and post deposition annealing in the range 60–650 °C. The possibility of fabrication of single layer anti-reflective coating for silicon is discussed.

Development of Multi-Service (MS) for SAC-OCDMA systems

Abstract: A Multi-Service Optical Code Division Multiple Access (MS-OCDMA) code based on Spectral Amplitude Coding (SAC) is proposed in this paper. The advantage of proposed code on setting a variable number of users in a basic code matrix with a fixed code weight makes it more flexible in generating codewords. The appropriate quality of service required for various network applications can be provided by choosing a different number of users for the basic code matrix of MS code. The properties of the proposed code is compared with other OCDMA codes in terms of code length and maximum cross-correlation. The performance of the MS code mathematically analysed and probability of error for users is plotted as a function of the number of active users and optical received power. Shot, phase induced intensity and thermal noises are considered in mathematical analysis. Results show that by choosing code weight of 4 and optimizing number of users per sequence, the MS code supports up to 82 users, each operating at a bit-rate of 622 Mbps with reference to the Bit Error Rate (BER) of 10−9.

Optimization design of hybrid Fresnel-based concentrator for generating uniformity irradiance with the broad solar spectrum

Abstract: This paper presents a novel hybrid Fresnel-based concentrator with improved uniformity irradiance distribution on the solar cell without using secondary optical element (SOE) in the concentrator photovoltaic (CPV) system to overcome the Fresnel loss and to increase the solar cell conversion efficiency. The designed hybrid Fresnel-based concentrator is composed of two parts, the inner part and the outer part. The inner part is the conventional Fresnel lens, while the outer part is double total internal reflection (DTIR) lens. According to the simple geometrical relation, the profile of the proposed hybrid Fresnel-based concentrator is calculated as an initial design profile. To obtain good irradiance uniformity on the solar cell, optimal prism displacements are optimized by using a simplex algorithm for collimated incident sunlight based on different prism focus on different position principles. In addition, a Monte-Carlo ray-tracing simulation approach is utilized to verify the optical performance for the hybrid Fresnel-based concentrator. Results indicate that the hybrid Fresnel-based concentrator designed using this method can achieve spatial non-uniformity less than 16.2%, f -number less than 0.59 (focal length to entry aperture diameter ratio), geometrical concentrator ratio 1759.8×, and acceptance angle ±0.23°. Compared to the conventional Fresnel-based lens and the traditional hybrid Fresnel-based lens, the optimized concentrator yields a significant improvement in irradiance uniformity on the solar cell with a wide solar spectrum range. It also has good tolerance to the incident sunlight.

Investigation of optical and electrical properties of l-Cystein doped zinc thiourea chloride (ZTC) crystal for nonlinear optical (NLO) applications

Abstract: The single crystal of l -Cystein doped zinc thiourea chloride (ZTC) has been grown by slow evaporation technique. The optical study revealed that the doped ZTC crystal has high transmission with lowest cut off wavelength of 306 nm. The optical band gap was found to be 4.2 eV. The transition band gaps were studied using the photoluminescence spectrum. The incorporation of l -Cystein in ZTC was estimated qualitatively by FT-IR analysis. The presence of dopant was confirmed by energy diffraction X-ray analysis (EDAX) analysis. The lower dielectric characteristics of doped ZTC crystal were scrutinized by dielectric measurements. The high thermal stability of grown crystal was ascertained by TG/DTA analysis. The Second harmonic generation (SHG) efficiency measured using Nd-YAG laser is 1.96 times that of pure ZTC.

A novel laser vision sensor for weld line detection on wall-climbing robot

Abstract: In this paper, we present a cross-structure light (CSL) sensor, that consists a structured light projector and a camera, for weld line detection. The structured light projector projects cross laser beams on the weldment to form cross stripes, which are captured in images by a CCD camera for measurement. We use feature points, a planar target and a homograph matrix to calibrate the sensor. We also propose an effective approach to extract laser stripes in images for weld line detection. Experiments show that the CSL sensor can capture 3D information of the weldment with very low measurement error, and the weld line detection approach is effective in wall-climbing robotic platform navigation.

BER estimation for multi-hop RoFSO QAM or PSK OFDM communication systems over gamma gamma or exponentially modeled turbulence channels

Abstract: The optical wireless and in particular the radio-on-free-space-optical (RoFSO) communication systems are gaining popularity due to their high date rates, license free spectrum and adequate reliability at installation and operational costs which are much lower than comparable technologies. One significant disadvantage of these systems concerns the randomly time varying characteristics of the propagation path mainly caused by the atmospheric turbulence. In this work, we study the BER performance of a multi-hop RoFSO system which is using an orthogonal frequency division multiplexing (OFDM) scheme, with either quadrature amplitude modulation (QAM) or phase shift keying format (PSK), over atmospheric turbulence channels modeled with the gamma gamma or the negative exponential distribution. The individual RoFSO parts of the whole optical link are connected to each other by using regenerators relay nodes. The dominant impairments which are the most significant and have been taken into account are the atmospheric turbulence, the path losses, the nonlinear responsivity of the laser diode and the inter-modulation distortion effect. For this setup, we derive closed form mathematical expressions for the estimation of the BER performance for each individual OFDM RoFSO link and for the whole relayed optical communication system, as well. Finally, the corresponding numerical results, for common link׳s parameters, are presented.