Μm In Air Research Articles

Low-cost, high-volume, manufacturable 0.88 NA multi-wavelength diffractive lens array for optical document security

We design, manufacture, and characterize a high-numerical-aperture (NA=0.88, f/0.27), multi-wavelength (480 nm, 550 nm, and 650 nm) multilevel diffractive microlens array (MLA). This MLA achieves multi-wavelength focusing with a depth of focus (DoF) twice the diffraction-limited value. Each microlens in the array is closely packed with a diameter of 70 µm and a focal length of 19 µm in air. The MLA is patterned on one surface of a polymer film via UV casting, positioning the focal plane at the distal end of the polymer film. Each microlens focuses light at three design wavelengths into a focal spot with an estimated FWHM of ∼310nm. By placing this MLA directly on a standard high-resolution banknote print (minimum feature width of 10–15 µm), we demonstrate color-integral imaging for anti-counterfeiting. In contrast, refractive MLAs cannot achieve high-NA, multi-wavelength focusing or extended DoF. The extended DoF of our MLA ensures reliable performance despite variations in the polymer film’s thickness. Our MLA, produced via UV casting, enables extremely low-cost, high-volume production, making it ideal for flat optics in banknotes and document security.

Coupled effect of microstructure heterogeneity and hydrogen on local embrittlement of CGHAZ and IC-CGHAZ in X65 pipeline steel

The local embrittlement of the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated CGHAZ (IC-CGHAZ) in X65 pipeline steel was investigated using an in situ crack-tip opening displacement test in air and H2S, combined with microstructure-based simulation. The IC-CGHAZ exhibited significantly lower cracking resistance, with the fracture toughness reduced by 50% compared to CGHAZ in both environments (0.160 mm in air and 0.017 mm in H₂S for IC-CGHAZ, compared to 0.307 mm in air and 0.034 mm in H₂S for CGHAZ). The martensite/austenite (M/A) constituents showed exceptionally higher intrinsic hardness than the ferrite matrix. The continuous distribution of M/As along the prior austenite grain boundaries resulted in local hardening and the formation of heterogeneous hard zones in IC-CGHAZ. Such microstructure heterogeneity resulted in the unique partitioning of plastic strain localization and stress triaxiality that promotes premature fracture, particularly with the coupled effect of hydrogen. This study provides a novel perspective on the fracture mechanisms and the typical fracture morphology of IC-CGHAZs in pipeline steel under the coupled effect of microstructure heterogeneity and hydrogen.

Integrated line-field optical coherence tomography and scheimpflug imaging for corneal imaging

Previously we demonstrated a combined scanning-point optical coherence tomography (OCT) and Scheimpflug Imaging (SI) system, to provide cross-section images of the corneal layers and the whole anterior segment of the eye simultaneously. However, to fully realise the benefits of this dual imaging modality concept towards clinical practice, it is required that the OCT part of the system can achieve ultra-high axial resolution (<3 µm in air) and faster imaging speeds, without using prohibitively expensive components. To achieve this, a new imaging device integrating line-field OCT and SI, using a supercontinuum light source, is demonstrated. This line-field configuration enables both OCT and SI B-scan to be captured using the same light illumination in a single shot, which is the most significant improvement over single point scanning. This achieves an OCT axial resolution down to 2.1 µm in air (1.6 µm in corneal tissue) and an imaging speed up to 213 kA-Scans/s. Since the OCT and SI capture the identical corneal position and serve as cross-validation, this technique offers a great method for accurately determining the individual corneal thickness and refractive index, thereby minimizing individual variations, which can help determine the extent of cutting or correction needed in vision correction. Moreover, the precise measurement of corneal thickness provided by this technique allows for a better understanding of the biomechanical properties of the cornea. We demonstrate the efficacy of the proposed system by evaluating porcine and bovine eyes ex-vivo for determining the corneal refractive index and thickness.

Development of hybrid photonic integrated wavelength-tunable laser at 2 µm and its application to FMCW LiDAR.

This paper reports on the experimental demonstration of a fully integrated frequency-modulated continuous-wave (FMCW) LiDAR sensing system, operating at 2.0 µm. It makes use of a widely tunable hybrid external cavity laser based on the combination of GaSb gain chip and silicon waveguide circuits. The single-frequency laser operation over the full spectral bandwidth of the gain chip is secured using a frequency-selective filter, consisting of two sequential microring resonators in a Vernier configuration. To increase the mode-hop free wavelength tuning range while preserving the linewidth of the laser, the heater of the phase section placed along the bus waveguide is synchronously controlled with two independent heaters placed on each microring resonator. This laser is then implemented for the development of an FMCW LiDAR, consisting of all-optical fiber-based two independent unbalanced Mach-Zehnder interferometers: k-space interferometer for the linearization of continuously swept laser frequency and main interferometer for the measurement of the distributed back-reflection over the distance. The optical frequency of the laser is continuously swept over a ∼100 GHz range (or Δλ=1.47 nm at the operating wavelength) at a modulation speed of 100 Hz. Using this wavelength tunable laser, a light detection and ranging system (LiDAR) is experimentally demonstrated, showing a very high axial resolution of 1.36 mm in air with an extremely high precision of ∼9 µm at a 100 Hz measurement rate.

Two-site microendoscopic imaging probe for simultaneous three-dimensional imaging at two anatomic locations in tissues.

Systems that can image in three dimensions at cellular resolution and across different locations within an organism may enable insights into complex biological processes, such as immune responses, for which a single location measurement may be insufficient. In this Letter, we describe an in vivo two-site imaging probe (TIP) that can simultaneously image two anatomic sites with a maximum separation of a few centimeters. The TIP consists of two identical bendable graded index (GRIN) lenses and is demonstrated by a two-photon two-color fluorescence imaging system. Each GRIN lens has a field of view of 162 × 162 × 170 µm3, a nominal numerical aperture of 0.5, a magnification of 0.7, and working distances of 0.2 mm in air for both ends. A blind linear unmixing algorithm is applied to suppress bleedthrough between channels. We use this system to successfully demonstrate two-site two-photon two-color imaging of two biomedically relevant samples, i.e., (1) a mixture of two autofluorescent anti-cancer drugs and (2) a live hybrid tumor consisting of two spectrally distinct fluorescent cell lines.

Curvilinear catheter implantation in HDR prostate brachytherapy: feasibility study.

High-dose-rate (HDR) brachytherapy (BT) has been acknowledged as a widely utilized treatment for patients with intermediate- and high-risk prostate cancer, despite its side effects such as edema, incontinence, and impotence. Nevertheless, the treatment is consistently limited by the potential danger of excessive irradiation to organs-at-risk (OARs) like the urethra, bladder, and rectum. This study aims to introduce curvilinear catheter implantation in the prostate gland for HDR treatment. The objective is to improve the radiation dose distribution by offering access channels conformal to the prostate anatomy. This approach seeks to minimize toxicity to nearby OARs while utilizing a reduced number of needles, potentially leading to improved clinical outcomes. Curvilinear catheters were first pre-planned for an anonymized patient using Oncentra treatment planning system (TPS) and hybrid inverse planning optimization (HIPO) algorithm. The trajectories of the catheters were then analyzed using MATLAB to extract their radius of curvature. Tendon-driven active needles were then used to implant curvilinear catheters inside an anthropomorphic phantom. Proposed curvilinear catheter implantation resulted in significant improvement in terms of dosimetric constraints to the OARs and coverage to the prostate. Tendon-driven active needles were shown to be capable of realizing the required pre-planned curvatures inside prostate. It was shown that the active needle can realize a desired radius of curvature and a desired trajectory with an average accuracy of 9.1±8.6 and 1.27±0.50mm in air and inside a tissue-mimicking phantom, respectively. This work demonstrates the feasibility of using tendon-driven active curvilinear catheter implantation in prostate to improve the outcomes of HDR-BT via improved radiation dose distribution to the prostate and reduced toxicity to the OARs.

Particle-resolved hyperspectral pyrometry of metal particles

We present temperature histories of individual combusting metal particles using hyperspectral pyrometry. This method gives an increase in accuracy over traditionally used two- or three-color pyrometry, while maintaining temporal and spatial resolution. Temperatures can be determined between 1800 to >3000 K with a precision of typically <1%. It is shown that the maximum temperature of the burning iron particles increases from 2760 K to 2840 K with an increasing mean particle size from 32 to 54 μm in air with 21 % oxygen. The relatively high temperatures and its dependence on particle size are possibly related to flow field properties of the current experimental setup. Opportunities for this method, as well as future work, are discussed.Novelty and Significance Statement: In this article, a method to use a CCD camera and spectrograph as a hyperspectral detector, gaining a wavelength dimension while maintaining two spatial dimensions, is demonstrated and validated. This method is applied to measure the temperature of iron particles, a carbon free and circular energy carrier. This method is then used to prove that there is a particle size dependence on the maximum temperature, an open question which is often disputed in literature. This work will also add a dataset that can be used for the validation of numerical models. There are only two such datasets available for iron at this moment. Our analysis suggests that the heat release of iron is dependent on the slip velocity, possibly due to a circulating flow inside of the particle.

Theoretical and experimental study of heat and mass transfer, evaporation and ignition drops of monoatomic ethanol/butanol binary mixture in cold air

The study evaporation process results of lower alcohols' single drops (ethanol, butanol and their binary mixtures) with an initial droplet size of 1.5-2.5 mm in air at room temperature and atmospheric pressure are presented. To describe the heat and mass transfer of the fuel mixture's drops with different initial composition, prognostic evaluation of dynamics' changes fuels drops size, composition and temperature during their evaporation are proposed. Taking into account the temperature dependence of the heat of vaporization, an analytical dependence for evaluating the drop surface temperature of one-component evaporating fuels is proposed. Comparative analysis of the calculated model's data allowed to suggest a possible evaporation mechanism of multi-component fuel mixtures droplets and to explain some peculiarities in the behavior of the main characteristics this process. It is shown that the thermocouple wires qualitatively affects the behavior with time of temperature and droplet size. Formula for the drop's temperature during evaporation into account the influence of thermocouple are presented.

The Impact of PM10 and Other Airborne Particulate Matter on the Cardiopulmonary and Respiratory Systems of Sports Personnel under Atmospheric Exposure

Respirable particulate matter (PM10) is atmospheric particulate matter with a kinetic diameter of less than or equal to 10 μm in air. According to the definition of the World Health Organization, it is called thoracic-enterable particulate matter because it can enter the body through the respiratory tract and be deposited into the lungs or absorbed into the blood and lymphatic systems. The toxic substances in it can enter the bloodstream directly and cause serious harm to human health. In addition, PM10 has unique physiological and biological effects, making it an important area of atmospheric chemistry research. In this study, two urban neighborhoods and sports companies were selected for the purpose of investigating the effects of PM10 concentrations in the air of neighborhoods and workplaces on people living and working in these environments for a long period of time, as well as synergistic effects between PM10 concentrations and changes in temperature and the incidence of related diseases. By assessing the extent of PM10’s impact on the respiratory system, this study provides basic data for assessing the health hazards of particulate matter in community environments. This study also analyzed the synergistic effects between air pollutant concentrations, temperature changes, and the incidence of related diseases in two cities to investigate the spatial and temporal distribution characteristics of air pollution and the meteorological causes of pollution in China. On this basis, we established a prediction model for related sensitivity diseases to provide theoretical and technical support for the prediction of related sensitivity diseases on a nationwide scale. Meanwhile, our study also provides support to relevant government departments to formulate a scientific basis and preventive and control measures for dealing with air pollution and its effects on human health.

Tunable quasi-supercontinuum generation in a 1.7 µm spectral band for spectral domain optical coherence tomography

We present a system for generating a 1.7 µm quasi-supercontinuum (SC) as what we believe to be a new broadband light source for spectral domain (SD) optical coherence tomography (OCT). The quasi-SC source, whose spectral range and spectral shape can be arbitrarily changed, is based on wavelength-tunable soliton pulse generation and high-speed intensity modulation. A Gaussian-shaped quasi-SC was introduced into an SD-OCT system. A maximum axial resolution of 14 µm in air (corresponding to 10 µm in tissue) and an imaging sensitivity of 94 dB was obtained. We also demonstrated high-resolution imaging for tape stack, human nail and pig thyroid gland samples using a quasi-SC. From these results, we confirmed the usefulness of a quasi-SC for SD-OCT imaging.

Thermally tuned VCSEL at 850 nm as a low-cost alternative source for full-eye SS-OCT.

Swept-source optical coherence tomography (SS-OCT) demonstrates superior performance in comparison to spectral domain OCT with regard to depth ranging. The main driver of cost for SS-OCT systems is, however, the price of the source. Here we show a low-cost alternative swept source that uses a thermally tuned vertical-cavity surface-emitting laser (VCSEL) at 850 nm. Its center wavelength can be tuned by adjusting the operating temperature through modulation of the injection current. At 2 kHz sweep rate, the depth range of the system was 5 cm, with a sensitivity roll-off of under -3 dB across this range. The system achieved a sensitivity of 97 dB with a sample beam power of 0.3 mW and an axial resolution of 50 µm in air. To demonstrate the system performance in vivo, an eye of a healthy volunteer was measured, and full-eye scans were acquired at 25 and 50 kHz from the cornea to the retina. Based on our results, we believe that this technology can be used as a cost-effective alternative OCT for point-of-care diagnostics.

Low-Temperature Three-Channel Gas Analyzers in Optical Monitoring of Fire Hazard Liquefied Natural Gas Outflows

A network of three-channel low-temperature infrared-optical gas analyzers with a response time of 1 s is developed for detecting explosive and fire hazardous concentrations of gasified liquefied natural gas (LNG) at temperatures up to 150 K. Their characteristics are described in an analysis of its large-scale emissions into the atmosphere. It is shown that during pulsed LNG emissions in the form of submerged jets and spills on standard concrete and water, large-scale areas of mixtures of air, methane, and light alkane vapors form with an explosive concentration that fluctuates in time and volume. The evaporation of cryogenic methane droplets with diameters of more than 0.1 mm in air and methane vapor from temperatures T = 150–290 K has been studied theoretically and experimentally.

Accurate In Vivo Bowman's Thickness Measurement Using Mirau Ultrahigh Axial Resolution Line Field Optical Coherence Tomography.

PurposeThe purpose of this study was to assess the accuracy, repeatability, and performance limits of in vivo Mirau ultrahigh axial resolution (UHR) line field spectral domain (LF-SD) optical coherence tomography (OCT) for the measurement of Bowman's and epithelial thickness, and to provide a reference range of these values for healthy corneas.MethodsVolunteers with no history and evidence of corneal disease were included in this study. An in vivo graph search image segmentation of the central cornea was obtained at the normal interface vector orientation. The Mirau-UHR-LF-SD-OCT system used has an axial resolution down to 2.4 µm in air (1.7 µm in tissue), with an A-scan speed of 204.8 kHz and a signal to noise ratio (sensitivity) of 69 (83) dB.ResultsNine volunteers were included, one of whom wore contact lenses. The repeatability of mean Bowman's and epithelial thicknesses were 0.3 and 1.0 µm, respectively. The measured 95% population range for healthy in vivo thickness was 13.7 to 19.6 µm for the Bowman's layer, and 41.9 to 61.8 µm for the epithelial layer.ConclusionsThe measured thicknesses of Bowman's layer and the corneal epithelium using the Mirau-UHR-LF-SD-OCT were both accurate, with the range for healthy in vivo thicknesses matching prior confocal and OCT systems of varying axial resolutions, and repeatable, equaling the best value prior reported.Translational RelevanceT1. Development of a commercially viable clinical UHR OCT technology, enabling accurate measurement and interpretation of Bowman's and epithelial layer thickness in clinical practice.

Ultra-wideband signal generation and fusion algorithm for high-resolution terahertz FMCW radar imaging.

The terahertz frequency modulated continuous wave (THz FMCW) imaging has proved to be a novel nondestructive testing (NDT) technology for non-metal materials, and the large bandwidth is usually required to meet high range resolution demands in many applications such as multilayer sample under test (SUT). However, broadband THz hardware is difficult to design. In this paper, an ultra-wideband THz FMCW generation method is proposed, which provides frequency modulation bandwidths of up to 386 GHz by time-division multiplexing. Furthermore, an ultra-wideband signal fusion algorithm (USFA) is also proposed and significantly improves the range resolution to 0.46 mm in air. Results from the artificially constructed multilayer structure demonstrate the superiority and effectiveness of our method quantitatively.

Ultrahigh resolution spectral-domain optical coherence tomography using the 1000-1600 nm spectral band.

Ultrahigh resolution optical coherence tomography (UHR-OCT) can image microscopic features that are not visible with the standard OCT resolution of 5-15 µm. In previous studies, high-speed UHR-OCT has been accomplished within the visible (VIS) and near-infrared (NIR-I) spectral ranges, specifically within 550-950 nm. Here, we present a spectral domain UHR-OCT system operating in a short-wavelength infrared (SWIR) range from 1000 to 1600 nm using a supercontinuum light source and an InGaAs-based spectrometer. We obtained an axial resolution of 2.6 µm in air, the highest ever recorded in the SWIR window to our knowledge, with deeper penetration into tissues than VIS or NIR-I light. We demonstrate imaging of conduction fibers of the left bundle branch in freshly excised porcine hearts. These results suggest a potential for deep-penetration, ultrahigh resolution OCT in intraoperative applications.

A 403 MHz Wireless Power Transfer System With Tuned Split-Ring Loops for Implantable Medical Devices

A near-field wireless power transfer (WPT) system for implantable medical devices, such as pacemakers, is proposed. Operating at 403 MHz within the medical implants communication service (MICS) band, the WPT link constitutes a primary loop to be based outside the body as a transmitter and an implantable loop with a single-turn as a receiver. Simulation and experimental results show that the proposed link offers good power transfer efficiency (PTE) performance. The maximum measured PTE of the proposed link is 57.9&#x0025; at a transfer distance of 6 mm through 1 mm of air and 5 mm of body tissue. The maximum input power that can be supplied to stay within specific absorption rate safety guidelines is 159 mW. A rectifying circuit is designed to convert 403 MHz RF signals to dc for a 1.5 <inline-formula> <tex-math notation="LaTeX">$\text{k}\Omega $ </tex-math></inline-formula> load with a measured conversion efficiency of 73.2&#x0025;. The measured end-to-end PTE of the proposed WPT system is 42.4&#x0025;.

Broadband Wavelength Swept Laser With Bidirectional Photonic Resonance for High Resolution Optical Coherence Tomography

A light source with extended spectral bandwidth applied in optical coherence tomography (OCT) systems produces highly accurate micro-scale imaging structures owing to the attainable high-axial resolution. This study presents a wide tuning range of a wavelength-swept laser to improve the axial resolution. Linear cavity configuration was performed for bidirectional photonic resonance using two semiconductor optical amplifiers placed parallelly. A polygon scanner-based tunable filter produced a broad scanning range of 155 nm from 917 nm to 1072 nm and an axial resolution of 3.4 μm in air. Bench-top imaging of a phantom made of food wraps was performed with 22.6 μm transverse resolution to evaluate the performance of the swept laser for high-resolution OCT.

Simple Characterization Scheme for Optical Coherence Tomography Systems With Application to a Commercial and a Near-Isometric Resolution Fibre-Based System

Optical Coherence Tomography (OCT) is a rapidly growing imaging modality in biomedical optics. OCT can perform high-resolution, cross-sectional imaging of the microstructure of biological tissues by measuring the coherent spectrum from the backscattered light. OCT systems with broad spectral bandwidths are often constructed using free-space optics to avoid dispersion by fibre optic components. This paper presents a fibre-based OCT system at a centre wavelength of 1300 nm with an axial resolution of 3.8 µm in air, surpassing any previously reported values to the best of our knowledge. Despite the challenges in transporting a broadband spectrum using fibre-optics, the system investigation was motivated by the ever-increasing demand for commercialization of high-resolution OCT systems and simplification of construction. We also evaluate and demonstrate the direct measurement method for axial resolution using an air wedge. Imaging of biomedical and other samples is demonstrated using a high numerical aperture sample lens and compared with images from a commercial OCT system. We discuss the effect of the improved structural visibility by achieving image voxels closer to an isometric shape with a high NA sample lens.

Pencil-beam scanning catheter for intracoronary optical coherence tomography

Current gradient-index (GRIN) lens based proximal-driven intracoronary optical coherence tomography (ICOCT) probes consist of a spacer and a GRIN lens with large gradient constant. This design provides great flexibility to control beam profiles, but the spacer length should be well controlled to obtain desired beam profiles and thus it sets an obstacle in mass catheter fabrication. Besides, although GRIN lens with large gradient constant can provide tight focus spot, it has short depth of focus and fast-expanded beam which leads to poor lateral resolution for deep tissue. In this paper, a type of spacer-removed probe is demonstrated with a small gradient constant GRIN lens. This design simplifies the fabrication process and is suitable for mass production. The output beam of the catheter is a narrow nearly collimated light beam, referred to as pencil beam here. The full width at half maximum beam size varies from 35.1 µm to 75.3 µm in air over 3-mm range. Probe design principles are elaborated with probe/catheter fabrication and performance test. The <em>in vivo</em> imaging of the catheter was verified by a clinical ICOCT system. Those results prove that this novel pencil-beam scanning catheter is potentially a good choice for ICOCT systems.

The Effect of Displacement Amplitude on Fretting Wear Behavior and Damage Mechanism of Alloy 690 in Different Gaseous Atmospheres.

The effect of displacement amplitude on fretting wear behavior and damage mechanisms of alloy 690 in air and nitrogen atmospheres was investigated in detail. The results showed that in air, the friction coefficient gradually increased with the increase in displacement amplitude which conformed to the universal law. In nitrogen, however, it had the highest point at the displacement amplitude of 60 μm due to very strong adhesion. Whether in air or nitrogen, the wear volume gradually increased with the increase in displacement amplitude. The wear volume in air was larger than that in nitrogen except at 30 μm. At 30 μm, the wear volume in air was slightly smaller. With an increase in displacement amplitude, a transformation of fretting running status between partial slip, mixed stick-slip, and final gross slip occurred along with the change of Ft-D curves from linear, to elliptic, to, finally, parallelogrammical. Correspondingly, the fretting regime changed from a partial slip regime to a mixed regime to a gross slip regime. With the increase in displacement amplitude, the transition from partial slip to gross slip in nitrogen was delayed as compared with in air due to the strong adhesion actuated by low oxygen content in a reducing environment. Whether in air or nitrogen, the competitive relation between fretting-induced fatigue and fretting-induced wear was prominent. The cracking velocity was more rapid than the wear. Fretting-induced fatigue dominated at 30 μm in air but at 30–60 μm in nitrogen. Fretting-induced wear won the competition at 45–90 μm in air but at 75–90 μm in nitrogen.