Showing papers in "Applied Optics in 2014"
TL;DR: Recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.
Abstract: Fiber lasers have seen progressive developments in terms of spectral coverage and linewidth, output power, pulse energy, and ultrashort pulse width since the first demonstration of a glass fiber laser in 1964. Their applications have extended into a variety of fields accordingly. In this paper, the milestones of glass fiber laser development are briefly reviewed and recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.
318 citations
TL;DR: An overview of recent advances in digital holography is presented, ranging from holographic techniques designed to increase the resolution of microscopic images, holographic imaging using incoherent illumination, phase retrieval with coherent illumination, and the holographic recording of depth-extended objects using a frequency-comb laser.
Abstract: This article presents an overview of recent advances in the field of digital holography, ranging from holographic techniques designed to increase the resolution of microscopic images, holographic imaging using incoherent illumination, phase retrieval with incoherent illumination, imaging of occluded objects, and the holographic recording of depth-extended objects using a frequency-comb laser, to the design of an infrastructure for remote laboratories for digital-holographic microscopy and metrology. The paper refers to current trends in digital holography and explains them using new results that were recently achieved at the Institute for Applied Optics of the University Stuttgart.
191 citations
TL;DR: The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented.
Abstract: The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS-nf/1f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n>1), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H2O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%–5%) of expected values.
186 citations
TL;DR: This paper proposes a method that can accurately calibrate a structured-light system even when the projector is not in focus, making it possible for high-accuracy and high-speed measurement with the binary defocusing method.
Abstract: A structured-light system with a binary defocusing technique has the potential to have more extensive application due to its high speeds, gamma-calibration-free nature, and lack of rigid synchronization requirements between the camera and projector. However, the existing calibration methods fail to achieve high accuracy for a structured-light system with an out-of-focus projector. This paper proposes a method that can accurately calibrate a structured-light system even when the projector is not in focus, making it possible for high-accuracy and high-speed measurement with the binary defocusing method. Experiments demonstrate that our calibration approach performs consistently under different defocusing degrees, and a root-mean-square error of about 73 μm can be achieved with a calibration volume of 150(H) mm×250(W) mm×200(D)mm.
155 citations
TL;DR: A discussion and a cryptanalysis of the optical phase-truncated Fourier-transform-based cryptosystem are presented and it is shown that the computing efficiency of the algorithm is improved and the number of iterations is much less than that by the specific attack, which has two iteration loops.
Abstract: A discussion and a cryptanalysis of the optical phase-truncated Fourier-transform-based cryptosystem are presented in this paper. The concept of an optical asymmetric cryptosystem, which was introduced into the optical image encryption scheme based on phase-truncated Fourier transforms in 2010, is suggested to be retained in optical encryption. A new method of attack is also proposed to simultaneously obtain the main information of the original image, the two decryption keys from its cyphertext, and the public keys based on the modified amplitude-phase retrieval algorithm. The numerical results illustrate that the computing efficiency of the algorithm is improved and the number of iterations is much less than that by the specific attack, which has two iteration loops.
112 citations
TL;DR: Extensive femtosecond laser damage threshold measurements of optical materials in both bulk and thin-film form are reported on, involving classical materials used in the fabrication of optical thin film components and their combination with codeposition processes.
Abstract: We report on extensive femtosecond laser damage threshold measurements of optical materials in both bulk and thin-film form. This study, which is based on published and new data, involved simple oxide and fluoride films, composite films made from a mixture of two dielectric materials, metallic films, and the surfaces of various bulk materials: oxides, fluorides, semiconductors, and ionic crystals. The samples were tested in comparable conditions at 1030 nm, 375 to 600 fs, under single-pulse irradiation. A large number of different samples prepared by different deposition techniques have been tested, involving classical materials used in the fabrication of optical thin film components (Ag, AlF3, Al2O3, HfO2, MgF2, Nb2O5, Pt, Sc2O3, SiO2, Ta2O5, Y2O3, and ZrO2) and their combination with codeposition processes. Their behaviors are compared with the surfaces of bulk materials (Al2O3, BaF2, CaF2, Ge, KBr, LiF, MgF2, NaCl, Quartz, Si, ZnS, ZnSe, and different silica glasses). Tabulated values of results are presented and discussed.
109 citations
TL;DR: The cryptanalysis has been carried out, which proves the robustness of proposed scheme against known-plaintext, chosen-plain text, and special attacks, and a simple optical setup for decryption has been suggested.
Abstract: We propose a novel nonlinear image-encryption scheme based on a Gerchberg-Saxton (G-S) phase-retrieval algorithm in the Fresnel transform domain. The decryption process can be performed using conventional double random phase encoding (DRPE) architecture. The encryption is realized by applying G-S phase-retrieval algorithm twice, which generates two asymmetric keys from intermediate phases. The asymmetric keys are generated in such a way that decryption is possible optically with a conventional DRPE method. Due to the asymmetric nature of the keys, the proposed encryption process is nonlinear and offers enhanced security. The cryptanalysis has been carried out, which proves the robustness of proposed scheme against known-plaintext, chosen-plaintext, and special attacks. A simple optical setup for decryption has also been suggested. Results of computer simulation support the idea of the proposed cryptosystem.
104 citations
TL;DR: A thorough analysis of the physical parameters that control the appropriate utilization of MOs in DHM is presented and a set of the criteria that consider the microscope features and the recording parameters to achieve DHM operation at the diffraction limit is derived.
Abstract: The utilization of microscope objectives (MOs) in digital holographic microscopy (DHM) has associated effects that are not present in conventional optical microscopy. The remaining phase curvature, which can ruin the quantitative phase imaging, is the most evident and analyzed. As phase imaging is considered, this interest has made possible the development of different methods of overcoming its undesired consequences. Additionally to the effects in phase imaging, there exist a set of less obvious conditions that have to be accounted for as MOs are utilized in DHM to achieve diffraction-limit operation. These conditions have to be considered even in the case in which only amplitude or intensity imaging is of interest. In this paper, a thorough analysis of the physical parameters that control the appropriate utilization of MOs in DHM is presented. A regular DHM system is theoretically modeled on the basis of the imaging theory. The Fourier spectrum of the recorded hologram is analyzed to evaluate the performance of the DHM. A set of the criteria that consider the microscope features and the recording parameters to achieve DHM operation at the diffraction limit is derived. Numerical modeling and experimental results are shown to validate our findings.
104 citations
TL;DR: A new Mueller matrix (MM) microscope is described that generalizes and makes quantitative the polarized light microscopy technique and can be applied to any visible wavelength.
Abstract: In this paper we describe a new Mueller matrix (MM) microscope that generalizes and makes quantitative the polarized light microscopy technique. In this instrument all the elements of the MU are simultaneously determined from the analysis in the frequency domain of the time-dependent intensity of the light beam at every pixel of the camera. The variations in intensity are created by the two compensators continuously rotating at different angular frequencies. A typical measurement is completed in a little over one minute and it can be applied to any visible wavelength. Some examples are presented to demonstrate the capabilities of the instrument.
99 citations
TL;DR: A reflection-mode continuous-wave terahertz imaging system was developed for application to food quality inspection, which requires fast, compact, and low-cost detection.
Abstract: In contrast to conventional x-ray food inspection systems that have difficulty in detecting low-density materials, a terahertz imaging system can even identify insects and plastics embedded in a food matrix. A reflection-mode continuous-wave terahertz imaging system was therefore developed for application to food quality inspection, which requires fast, compact, and low-cost detection. High-speed operation of the terahertz imaging system was achieved through the use of a beam-steering tool. A reasonable compromise between the spatial resolution and the scan length of an aspheric f-theta scanning lens could be achieved by optimizing the lens parameters.
96 citations
TL;DR: A new multiple-image encryption and decryption technique that utilizes the compressive sensing concept along with a double-random phase encryption (DRPE) is proposed, which results in a nonlinear encryption system that is able to overcome the vulnerability of classical DRPE.
Abstract: In this paper, a new multiple-image encryption and decryption technique that utilizes the compressive sensing (CS) concept along with a double-random phase encryption (DRPE) has been proposed. The space multiplexing method is employed for integrating multiple-image data. The method, which results in a nonlinear encryption system, is able to overcome the vulnerability of classical DRPE. The CS technique and space multiplexing are able to provide additional key space in the proposed method. A numerical experiment of the proposed method is implemented and the results show that the proposed method has good accuracy and is more robust than classical DRPE. The proposed system is also employed against chosen-plaintext attacks and it is found that the inclusion of compressive sensing enhances robustness against the attacks.
TL;DR: The computed values of mean squared error between the retrieved and the original image show the efficacy of the proposed scheme, and the scheme's sensitivity to the encryption parameters, and robustness against occlusion and multiplicative Gaussian noise attacks are investigated.
Abstract: We propose a method for fully phase image encryption based on double random-structured phase mask encoding in the gyrator transform (GT) domain. The security of the system is strengthened by parameters used in the construction of a structured phase mask (SPM) based on a devil’s vortex Fresnel lens (DVFL). The input image is recovered using the correct parameters of the SPMs, transform orders of the GT, and conjugate of the random phase masks. The use of a DVFL-based SPM enhances security by increasing the key space for encryption, and also overcomes the problem of axis alignment associated with an optical setup. The proposed scheme can also be implemented optically. The computed values of mean squared error between the retrieved and the original image show the efficacy of the proposed scheme. We have also investigated the scheme’s sensitivity to the encryption parameters, and robustness against occlusion and multiplicative Gaussian noise attacks.
TL;DR: With the limited-dynamics detector used, a maximum fourfold increase in contrast was demonstrated under bright background illumination using polarimetric difference image, and the efficiency of using polarized light for source contrast increase with different signal representations was compared.
Abstract: We report an experimental implementation of long-range polarimetric imaging through fog over kilometric distance in real field atmospheric conditions. An incoherent polarized light source settled on a telecommunication tower is imaged at a distance of 1.3 km with a snapshot polarimetric camera including a birefringent Wollaston prism, allowing simultaneous acquisition of two images along orthogonal polarization directions. From a large number of acquisitions datasets and under various environmental conditions (clear sky/fog/haze, day/night), we compare the efficiency of using polarized light for source contrast increase with different signal representations (intensity, polarimetric difference, polarimetric contrast, etc.). With the limited-dynamics detector used, a maximum fourfold increase in contrast was demonstrated under bright background illumination using polarimetric difference image.
TL;DR: First flight results over calibration sites as well as Monterey Bay in California have demonstrated good agreement between in situ and remotely sensed data, confirming the potential value of the sensor to the coastal ocean science community.
Abstract: The design, characteristics, and first test flight results are described of the Portable Remote Imaging Spectrometer, an airborne sensor specifically designed to address the challenges of coastal ocean remote sensing. The sensor incorporates several technologies that are demonstrated for the first time, to the best of our knowledge, in a working system in order to achieve a high performance level in terms of uniformity, signal-to-noise ratio, low polarization sensitivity, low stray light, and high spatial resolution. The instrument covers the 350–1050 nm spectral range with a 2.83 nm sampling per pixel, and a 0.88 mrad instantaneous field of view, with 608 cross-track pixels in a pushbroom configuration. Two additional infrared channels (1240 and 1610 nm) are measured by a spot radiometer housed in the same head. The spectrometer design is based on an optically fast (F/1.8) Dyson design form coupled to a wide angle two-mirror telescope in a configuration that minimizes polarization sensitivity without the use of a depolarizer. A grating with minimum polarization sensitivity and broadband efficiency was fabricated as well as a slit assembly with black (etched) silicon surface to minimize backscatter. First flight results over calibration sites as well as Monterey Bay in California have demonstrated good agreement between in situ and remotely sensed data, confirming the potential value of the sensor to the coastal ocean science community.
TL;DR: Line-scan Raman microscopy can be an effective and practical tool for obtaining endogenous microscopic chemical and molecular information from cell population and enables the study of intact microalgal cells at the colonial level and the identification of intra- and extracellular chemical constituents and metabolites.
Abstract: We evaluate the performance of line-scan Raman microscopy (LSRM), a versatile label-free technique, for high-throughput chemical imaging of cell population. We provide detailed design and configuration of a home-built LSRM system developed in our laboratory. By exploiting parallel acquisition, the LSRM system achieves a significant throughput advantage over conventional point-scan Raman microscopy by projecting a laser line onto the sample and imaging the Raman scattered light from the entire line using a grating spectrograph and a charge-coupled device (CCD) camera. Two-dimensional chemical maps can be generated by scanning the projected line in the transverse direction. The resolution in the x and y direction has been characterized to be ~600-800 nm for 785 nm laser excitation. Our system enables rapid classification of microparticles with similar shape, size, and refractive index based on their chemical composition. An equivalent imaging throughput of 100 microparticles/s for 1 μm polystyrene beads has been achieved. We demonstrate the application of LSRM to imaging bacterial spores by identifying endogenous calcium dipicolinate. We also demonstrate that LSRM enables the study of intact microalgal cells at the colonial level and the identification of intra- and extracellular chemical constituents and metabolites, such as chlorophyll, carotenoids, lipids, and hydrocarbons. We conclude that LSRM can be an effective and practical tool for obtaining endogenous microscopic chemical and molecular information from cell population.
TL;DR: The AMTM is a novel infrared digital imaging system that measures selected emission lines in the mesospheric OH (3,1) band to create intensity and temperature maps of the mesosphere around 87 km, allowing detailed measurements of wave propagation and dissipation at the ∼87 km level, even in the presence of strong aurora or under full moon conditions.
Abstract: Over the past 60 years, ground-based remote sensing measurements of the Earth’s mesospheric temperature have been performed using the nighttime hydroxyl (OH) emission, which originates at an altitude of ∼87 km. Several types of instruments have been employed to date: spectrometers, Fabry–Perot or Michelson interferometers, scanning-radiometers, and more recently temperature mappers. Most of them measure the mesospheric temperature in a few sample directions and/or with a limited temporal resolution, restricting their research capabilities to the investigation of larger-scale perturbations such as inertial waves, tides, or planetary waves. The Advanced Mesospheric Temperature Mapper (AMTM) is a novel infrared digital imaging system that measures selected emission lines in the mesospheric OH (3,1) band (at ∼1.5 μm) to create intensity and temperature maps of the mesosphere around 87 km. The data are obtained with an unprecedented spatial (∼0.5 km) and temporal (typically 30″) resolution over a large 120° field of view, allowing detailed measurements of wave propagation and dissipation at the ∼87 km level, even in the presence of strong aurora or under full moon conditions. This paper describes the AMTM characteristics, compares measured temperatures with values obtained by a collocated Na lidar instrument, and presents several examples of temperature maps and nightly keogram representations to illustrate the excellent capabilities of this new instrument.
Journal Article•
TL;DR: This work demonstrates a scanning imaging system based on a frequency-comb calibrated FMCW LADAR and real-time digital signal processing that can obtain three-dimensional images of a diffusely scattering surface at stand-off distances up to 10.5 m and is compatible with future optically integrated, on-chip systems.
Abstract: Non-contact surface mapping at a distance is interesting in diverse applications including industrial metrology, manufacturing, forensics, and artifact documentation and preservation. Frequency modulated continuous wave (FMCW) laser detection and ranging (LADAR) is a promising approach since it offers shot-noise limited precision/accuracy, high resolution and high sensitivity. We demonstrate a scanning imaging system based on a frequency-comb calibrated FMCW LADAR and real-time digital signal processing. This system can obtain three-dimensional images of a diffusely scattering surface at stand-off distances up to 10.5 m with sub-micrometer accuracy and with a precision below 10 µm, limited by fundamental speckle noise. Because of its shot-noise limited sensitivity, this comb-calibrated FMCW LADAR has a large dynamic range, which enables precise mapping of scenes with vastly differing reflectivities such as metal, dirt or vegetation. The current system is implemented with fiber-optic components, but the basic system architecture is compatible with future optically integrated, on-chip systems.
TL;DR: A method of locating the axial positions of both opaque and transparent objects in the reconstructed 3D field in the wavelet domain is proposed and validated by both simulated and experimental holograms of transparent spherical water droplets and opaque nonspherical coal particles.
Abstract: Depth-of-field extension and accurate 3D position location are two important issues in digital holography for particle characterization and motion tracking. We propose a method of locating the axial positions of both opaque and transparent objects in the reconstructed 3D field in the wavelet domain. The spatial–frequency property of the reconstructed image is analyzed from the viewpoint of the point spread function of the digital inline holography. The reconstructed image is decomposed into high- and low-frequency subimages. By using the variance of the image gradient in the subimages as focus metrics, the depth-of-field of the synthesis image can be extended with all the particles focalized, and the focal plane of the object can be accurately determined. The method is validated by both simulated and experimental holograms of transparent spherical water droplets and opaque nonspherical coal particles. The extended-focus image is applied to the particle pairing in a digital holographic particle tracking velocimetry to obtain the 3D vector field.
TL;DR: The overall result shows that SDSM is essentially functioning without flaws in catching the on-orbit degradation of the SD, and the most significant and direct impact of this work would be on the quality of the ocean color products that depend sensitively on moderate RSB (RSB) (M1-M8, M10, and M11).
Abstract: The reflective solar bands (RSB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (SNPP) satellite is calibrated by a solar diffuser (SD) whose performance is itself monitored by a solar diffuser stability monitor (SDSM). In this study, we describe the calibration algorithm of the SDSM, analyze the current two and a half years of calibration data, and derive the performance result for the SD, commonly called SD degradation or H-factors. The application of the newly derived vignetting functions for both the SD screen and the SDSM sun-view screen effectively removes the seasonal oscillations in the derived SD degradation and significantly improves the quality of the H-factors. The full illumination region, the so-called “sweet spot,” for both SD view and SDSM sun view is carefully examined and selected to ensure a consistent and an optimal number of valid data samples to reduce the sample noise owing to inconsistent or lack of samples. The result shows that SD degrades much faster at short wavelength as expected, about 28.5% at 412 nm but only 1.2% at 935 nm up to date. The performance of the SD degrades exponentially with time until 7 November 2013 but has since become flat. This sudden flattening of the SD degradation is a new phenomenon never previously observed for the degradations of the SD on VIIRS or other satellite sensors. The overall result shows that SDSM is essentially functioning without flaws in catching the on-orbit degradation of the SD. The most significant and direct impact of this work would be on the quality of the ocean color products that depend sensitively on moderate RSB (RSB) (M1–M8, M10, and M11). Two very important and key questions on the performance of the SD are also raised. One pertains to the directional dependence of the SD degradation result, and it is shown that the SD does not degrade uniformly in all directions as has been assumed by all SD calibration analyses. This has a definitive impact on the RSB calibration. Another is on the degradation of the SD at the shortwave infrared (SWIR) wavelengths, and it is shown that the zero degradation input for the RSB calibration would be erroneous. Last, the impact of the stray light on the SD since “first light” is cleanly exhibited in the improved SD degradation result.
TL;DR: Low spatial frequency holographic recording is explored to increase the angular and wavelength range of an individual grating, and results on the efficiency of holograms recorded in an acrylamide-based photopolymer at low spatial frequencies are reported.
Abstract: A holographic device is under development that aims to improve light collection in solar cells. The aim is to explore the potential of using photopolymer holographic optical elements (HOEs) to collect light from a moving source, such as the sun, and redirect it for concentration by a holographic lens. A working range of 45° is targeted for such a device to be useful in solar applications without tracking. A photopolymer HOE is capable of efficiently redirecting light, but the angular selectivity of a single grating is usually of the order of one degree at the thicknesses required for high efficiency. The challenge here is to increase the angular and wavelength range of the gratings so that a reasonable number may be multiplexed and/or combined to create a device that can concentrate light incident from a large range of angles. In this paper, low spatial frequency holographic recording is explored to increase the angular and wavelength range of an individual grating. Ultimately, a combination of gratings will be used so that a broad range of angles of incidence are accepted. A design is proposed for the combination of such elements into a holographic solar collector. The first step in achieving this is optimization of recording at low spatial frequency. This requires a photopolymer material with unique properties, such as a fast monomer diffusion rate. This paper reports results on the efficiency of holograms recorded in an acrylamide-based photopolymer at low spatial frequencies (100, 200, and 300 l/mm). The diffraction efficiency and angular selectivity of recorded holograms have been studied for various photopolymer layer thicknesses and different intensities of the recording beams. A diffraction efficiency of over 80% was achieved at a spatial frequency of 200 l/mm. The optimum intensity of recording at this spatial frequency was found to be 1 mW/cm2. Individual gratings and focusing elements with high efficiency and FWHM angles of 3° are experimentally demonstrated.
TL;DR: A head-mounted display that achieved high levels of color uniformity and optical efficiency and theoretical analyses showed that the proposed waveguide resolved the problems of thickness and limited FOV.
Abstract: We developed a head-mounted display (HMD) that achieved high levels of color uniformity and optical efficiency. The full-color holographic volume grating (HVG) attached on the specially designed wedge-shaped waveguide HMD system provided a 17° horizontal field of view (FOV). Theoretical analyses showed that the proposed waveguide resolved the problems of thickness and limited FOV. In this system, the HVG was recorded using a special sequential recording process on single photopolymer unit with 633, 532, and 473 nm wavelengths. The results confirm that the designed and fabricated waveguide can be employed in future commercial HMS.
TL;DR: An off-axis three-mirror system based on the primary mirror and tertiary mirror integrated on a single substrate obtained an axial symmetrical imaging quality about the x axis, and the axial symmetry aberration performance also brings considerable convenience to alignment and testing for the OTS.
Abstract: An off-axis three-mirror system (OTS) was designed based on the primary mirror and tertiary mirror (TM) integrated on a single substrate in order to solve the OTS drawbacks, such as the alignment difficulty and the large opto-mechanical weight. Furthermore, an optical freeform surface that can increase the optimizing degrees of freedom (DOF) was applied on the TM in order to achieve a wide field of view (FOV). An example with a focal length of 1200 mm, F-number of 12, and FOV of 10°×4° was given, and the maximum wave front error (WFE) RMS was 0.0126λ, indicating a good imaging quality. The design result shows that the number of alignment DOF was reduced from 12 to 6, and the weight of the mirror support assembly can also be lighter. An XY polynomial, established as an even function of x, was employed as the TM surface, so we obtained an axial symmetrical imaging quality about the x axis, and the axial symmetry aberration performance also brings considerable convenience to alignment and testing for the OTS.
TL;DR: Analysis of the borosilicate-to-borosilicate weld strength compares well to those obtained using similar welding systems based on femtosecond lasers, but there is a strong requirement to prepare surfaces to a high (10-60 nm Ra) flatness to ensure a successful weld.
Abstract: We report picosecond laser welding of similar and dissimilar materials based on plasma formation induced by a tightly focused beam from a 1030 nm, 10 ps, 400 kHz laser system. Specifically, we demonstrate the welding of fused silica, borosilicate, and sapphire to a range of materials including borosilicate, fused silica, silicon, copper, aluminum, and stainless steel. Dissimilar material welding of glass to aluminum and stainless steel has not been previously reported. Analysis of the borosilicate-to-borosilicate weld strength compares well to those obtained using similar welding systems based on femtosecond lasers. There is, however, a strong requirement to prepare surfaces to a high (10–60 nm Ra) flatness to ensure a successful weld.
TL;DR: A thermal rectification device concept based on far-field radiative exchange between two selective emitters, constituted by two multilayer samples made of metallic and semiconductor thin films, which shows a rectification ratio increasing with temperature up to 19% for a temperature difference of ΔT=370 K.
Abstract: We present a thermal rectification device concept based on far-field radiative exchange between two selective emitters. Rectification is achieved due to a large contrast between the two selective emitters’ thermo-optical properties. A simple device constituted by two multilayer samples made of metallic (Au) and semiconductor (Si and HDSi) thin films is proposed. This device shows a rectification ratio increasing with temperature up to 19% for a temperature difference of ΔT=370 K. Further optimization would allow larger rectification values. The presented results might be useful for energy conversion devices, engineering of smart radiative coolers/insulators, and development of thermal logical circuits.
TL;DR: A three-dimensional, time-resolved, OH planar laser-induced fluorescence (3D OH PLIF) technique that was developed to measure the relative OH concentration in multiphase combustion flow fields to show a comprehensive view of jetting events in gelled droplet combustion that was not observed with high-speed imaging or 2D OHPLIF.
Abstract: Imaging dynamic multiphase combusting events is challenging. Conventional techniques can image only a single plane of an event, capturing limited details. Here, we report on a three-dimensional, time-resolved, OH planar laser-induced fluorescence (3D OH PLIF) technique that was developed to measure the relative OH concentration in multiphase combustion flow fields. To the best of our knowledge, this is the first time a 3D OH PLIF technique has been reported in the open literature. The technique involves rapidly scanning a laser sheet across a flow field of interest. The overall experimental system consists of a 5 kHz OH PLIF system, a high-speed detection system (image intensifier and CMOS camera), and a galvanometric scanning mirror. The scanning mirror was synchronized with a 500 Hz triangular sweep pattern generated using Labview. Images were acquired at 5 kHz corresponding to six images per mirror scan, and 1000 scans per second. The six images obtained in a scan were reconstructed into a volumetric representation. The resulting spatial resolution was 500×500×6 voxels mapped to a field of interest covering 30 mm×30 mm×8 mm. The novel 3D OH PLIF system was applied toward imaging droplet combustion of methanol gelled with hydroxypropyl cellulose (HPC) (3 wt. %, 6 wt. %), as well as solid propellant combustion, and impinging jet spray combustion. The resulting 3D dataset shows a comprehensive view of jetting events in gelled droplet combustion that was not observed with high-speed imaging or 2D OH PLIF. Although the scan is noninstantaneous, the temporal and spatial resolution was sufficient to view the dynamic events in the multiphase combustion flow fields of interest. The system is limited by the repetition rate of the pulsed laser and the step response time of the galvanometric mirror; however, the repetition rates are sufficient to resolve events in the order of 100 Hz. Future upgrade includes 40 kHz pulsed UV laser system, which can reduce the scan time to 125 μs, while keeping the high repetition rate of 1000 Hz.
TL;DR: Results indicate that 633 nm is the best sensing wavelength due to the increased levels of sensitivity achieved at this wavelength in a refractive index sensor made of plastic optical fiber with tapered structure.
Abstract: This work reports a refractive index sensor made of plastic optical fiber (POF) with tapered structure. Transmission loss is measured when the external environment's refractive index changes from 1.33 to 1.41. Three wavelengths (532, 633, and 780 nm) are used to evaluate the sensitivity of the sensor, and results indicate that 633 nm is the best sensing wavelength due to the increased levels of sensitivity achieved at this wavelength. A biconical sensing structure is designed to enhance the sensitivity of the sensor. A sensitivity of 950 μW/RIU at 633 nm is obtained for a biconical sensing structure when launched power is 1 mW. Due to its sensitivity to the refractive index and simple construction, POF with tapered structure has potential applications in the biosensing field.
TL;DR: The main conclusions of the CLEANSPACE project are presented and a ground-based laser solution which can remove, at low expense and in a nondestructive way, hazardous debris around selected space assets appears as a highly promising answer.
Abstract: Studies show that the number of debris in low Earth orbit is exponentially growing despite future debris release mitigation measures considered. Specifically, the already existing population of small and medium debris (between 1 cm and several dozens of cm) is today a concrete threat to operational satellites. A ground-based laser solution which can remove, at low expense and in a nondestructive way, hazardous debris around selected space assets appears as a highly promising answer. This solution is studied within the framework of the CLEANSPACE project which is part of the FP7 space program. The overall CLEANSPACE objective is: to propose an efficient and affordable global system architecture, to tackle safety regulation aspects, political implications and future collaborations, to develop affordable technological bricks, and to establish a roadmap for the development and the future implantation of a fully functional laser protection system. This paper will present the main conclusions of the CLEANSPACE project.
TL;DR: A general method of surface profiling with phase-shifting interferometry techniques uses iterative linear regression to fit the sequence of interferograms to a physical model of the cavity, which is robust and rapid.
Abstract: A general method of surface profiling with phase-shifting interferometry techniques uses iterative linear regression to fit the sequence of interferograms to a physical model of the cavity. The physical model incorporates all important cavity influences, including environmentally induced rigid-body motion, phase shifter miscalibrations, multiple interference, geometry-induced spatial phase-shift variations, and their cross-couplings. By incorporating an initial estimate of the surface profile and iteratively solving for space- and time-dependent variables separately, convergence is robust and rapid. The technique has no restriction on surface shape or departure.
TL;DR: The studies presented here demonstrate that LIBS-SVM is a useful technique for the identification and discrimination of steel materials, and would be very well-suited for process analysis in the steelmaking industry.
Abstract: The feasibility of steel materials classification by support vector machines (SVMs), in combination with laser-induced breakdown spectroscopy (LIBS) technology, was investigated Multi-classification methods based on SVM, the one-against-all and the one-against-one models, and a combination model, are applied to classify nine types of round steel Due to the inhomogeneity of steel composition, the data obtained using the one-against-all and one-against-one models were ambiguous and difficult to discriminate; whereas, the combination model, was able to successfully distinguish most of the ambiguous data and control the computation cost within an acceptable range The studies presented here demonstrate that LIBS-SVM is a useful technique for the identification and discrimination of steel materials, and would be very well-suited for process analysis in the steelmaking industry
TL;DR: Simulated and real spectra experiments manifest that this algorithm can satisfactorily recover the overlap peaks as well as suppress noise and are robust to the regularization parameter.
Abstract: Spectroscopic data often suffer from common problems of band overlap and noise This paper presents a maximum a posteriori (MAP)-based algorithm for the band overlap problem In the MAP framework, the likelihood probability density function (PDF) is constructed with Gaussian noise assumed, and the prior PDF is constructed with adaptive total variation (ATV) regularization The split Bregman iteration algorithm is employed to optimize the ATV spectral deconvolution model and accelerate the speed of the spectral deconvolution The main advantage of this algorithm is that it can obtain peak structure information as well as suppress noise simultaneity Simulated and real spectra experiments manifest that this algorithm can satisfactorily recover the overlap peaks as well as suppress noise and are robust to the regularization parameter