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The most common approach to optically generate and manipulate bubbles in liquids involves temperature gradients induced by CW lasers. In this work, we present a method to accomplish both the generation of microbubbles and their 3D manipulation in ethanol through optothermal forces. These forces are triggered by light absorption from a nanosecond pulsed laser (λ = 532 nm) at silver nanoparticles photodeposited at the distal end of a multimode optical fiber. Light absorbed from each laser pulse quickly heats up the silver-ethanol interface beyond the ethanol critical-point (∼ 243 °C) before the heat diffuses through the liquid. Therefore, the liquid achieves a metastable state and owing to spontaneous nucleation converted to a vapor bubble attached to the optical fiber. The bubble grows with semi-spherical shape producing a counterjet in the final stage of the collapse. This jet reaches the hot nanoparticles vaporizing almost immediately and ejecting a microbubble. This microbubble-generation mechanism takes place with every laser pulse (10 kHz repetition rate) leading to the generation of a microbubbles stream. The microbubbles' velocities decrease as they move away from the optical fiber and eventually coalesce forming a larger bubble. The larger bubble is attracted to the optical fiber by the Marangoni force once it reaches a critical size while being continuously fed with each bubble of the microbubbles stream. The balance of the optothermal forces owing to the laser-pulse drives the 3D manipulation of the main bubble. A complete characterization of the trapping conditions is provided in this paper.

Wavefront coding technique has been used to extend the depth of focus in an optical imaging system. An optical element called a phase mask allows coded images to be obtained since the point spread function remains almost invariant in an axial range. Subsequently, a computational technique is required to decode the acquired images. An optical-computational technique is proposed to use a phase mask for the coding stage and a convolutional neural network for the final restoration. Comparative results are made between cubic and trefoil profile phase masks and decoded using the traditional Wiener filter and a convolutional neural network. Image quality evaluation is done using the peak signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

Purpose: Investigations into the correction of presbyopia have considered lens design, clinical implications and the development of objective metrics such as the visual Strehl ratio. This study investigated the Jacobi–Fourier phase mask as an ophthalmic element in the correction of presbyopia. The goal was to develop a contact or intraocular lens whose performance was largely insensitive to changes in pupil diameter. Methods: Numerical simulations based on Fourier optics were performed to evaluate three different Jacobi–Fourier polynomials, with the aim of providing a range of clear vision (1 Dioptre (D)). Performance was evaluated for three pupil sizes (6, 4 and 2 mm), while polychromatic images were simulated using three different wavelengths (656.3, 587.6 and 486.1 nm). The Neural Transfer function was included in the simulation. To validate the method and results, we used the Visual Strehl combined objective metric (VSCombined) currently used in visual optics. This metric gives more weight to the phase transfer function and is more suitable for non‐symmetrical phase functions. Results: Numerical validation showed the suitability of the Jacobi–Fourier phase masks for extending the range of clear vision of presbyopic eyes, providing a visual acuity of at least 0.10 logMAR (6/7.5 Snellen) at all distances between 1 and 6 m. The results show a range of clear vision of 1D was not affected by changes in pupil size, an increase in retinal image contrast accompanied by image artefact reduction by increasing the radial order of the Jacobi–Fourier phase mask and a reduction of wavelength dependence of the retinal images. These results are supported by simulated images and the objective criterion VSCombined. Conclusions: The use of Jacobi–Fourier phase masks as ophthalmic elements for presbyopic correction show promising results, with a good range of clear vision and reduced dependence on pupil size and chromatic aberration. [ABSTRACT FROM AUTHOR]

The generation and manipulation of microbubbles by means of temperature gradients induced by low power laser radiation is presented. A laser beam (λ = 1064 nm) is divided into two equal parts and coupled to two multimode optical fibers. The opposite ends of each fiber are aligned and separated a distance D within an ethanol solution. Previously, silver nanoparticles were photo deposited on the optical fibers ends. Light absorption at the nanoparticles produces a thermal gradient capable of generating a microbubble at the optical fibers end in non-absorbent liquids. The theoretical and experimental studies carried out showed that by switching the thermal gradients, it is possible to generate forces in opposite directions, causing the migration of microbubbles from one fiber optic tip to another. Marangoni force induced by surface tension gradients in the bubble wall is the driving force behind the manipulation of microbubbles. We estimated a maximum Marangoni force of 400nN for a microbubble with a radius of 110 μm.

Introduction: Atmospheric turbulence is a kind of random vortex motion. A series of turbulent effects, such as fluctuation of light intensity, occur when laser is transmitted in atmospheric turbulence. Methods: In order to verify the possibility of detecting atmospheric turbulence by the Mie-scattering lidar, firstly, based on the power spectrum method, the Zernike polynomial method is used to simulate generation of the modified Von Karman turbulent phase screen by low-frequency compensation. By comparing the obtained phase structure function with the theoretical value, the accuracy of the method is verified. Moreover, the transmission process of the Gaussian beam from Mie-scattering lidar through the phase screen is simulated, and the transmission characteristics of the beam under modified Von Karman turbulence are obtained by analyzing the fluctuation of light intensity. Secondly, based on the guidance for simulation analysis, a Miescattering lidar system for detecting the intensity of atmospheric turbulence was developed in Yinchuan area, and the atmospheric turbulence profile was inverted by detected scintillation index. Results: The results show it is feasible to use the Zernike polynomial method perform the low-frequency compensation, and the compensation effect of low order is better than that of high order compensation. The scintillation index of simulation is consistent with the actual detection result, and has the very high accuracy, indicating that the atmospheric turbulence detection using Mie-scattering lidar is effective. Conclusion: These simulations and experiments play a significant guiding role for the similar lidar to detect atmospheric turbulence. [ABSTRACT FROM AUTHOR]

An experimental and theoretical study on the 3D trapping and manipulation of microbubbles by means low power laser-induced temperature gradients induced in ethanol by bulk light absorption (λ = 1550 nm) is presented. Two optical fibers were used: One for bubble generation (OFG) and the other for both trapping and manipulation (OFT). Light from a Q-switched pulsed laser (λ = 532 nm and pulse width τp = 5 ns) propagates in fiber OFG and gets absorbed at silver nanoparticles (AgNPs), previously photodeposited, at the distal end of a fiber optic core, generating the microbubbles. In the fiber OFT, light of low power CW laser was used to trap and manipulate the bubbles by thermocapillary induced by light bulk absorption in ethanol. The microbubble generated on OFG migrates toward the fiber OFT. The equilibrium between the buoyancy force FB, drag force FD and the Marangoni force (also known as thermocapillary force) FM gives rise to a 3D stably trapping and manipulation of the microbubble for the best time to our best knowledge.

The Bessel beams scattering of the fog particles were calculated by using the plane beams angle spectrum expansion method, and the effects of the topological charge and the half-conic angle of the Bessel beam on the differential scattering cross-section were analyzed by numerical calculation. Based on the scattering results of a single fog particle by a Bessel beam, by Monte Carlo method, the propagation characteristics of the Bessel beam in fogs with different visibility are simulated, and the effects of the wavelength, topological charge and semi-conic angle of the Bessel beam on transmissivity and reflectivity are analyzed. The studies show the self-healing ability of the Bessel beams, and the propagation distance of the Bessel beam is longer than that of the plane beams in fogs. [ABSTRACT FROM AUTHOR]

Infrared Thermography (IRT) is a non-contact, non-intrusive, and non-ionizing radiation tool used for detecting breast lesions. This paper analyzes the surface temperature distribution (STD) on an optimal Region of Interest (RoI) for extraction of suitable internal heat source parameters. The physiological parameters are estimated through the inverse solution of the bio-heat equation and the STD of suspicious areas related to the hottest spots of the RoI. To reach these values, the STD is analyzed by means: the Depth-Intensity-Radius (D-I-R) measurement model and the fitting method of Lorentz curve. A highly discriminative pattern vector composed of the extracted physiological parameters is proposed to classify normal and abnormal breast thermograms. A well-defined RoI is delimited at a radial distance, determined by the Support Vector Machines (SVM). Nevertheless, this distance is less than or equal to 1.8 cm due to the maximum temperature location close to the boundary image. The methodology is applied to 87 breast thermograms that belong to the Database for Mastology Research with Infrared Image (DMR-IR). This methodology does not apply any image enhancements or normalization of input data. At an optimal position, the three-dimensional scattergrams show a correct separation between normal and abnormal thermograms. In other cases, the feature vectors are highly correlated. According to our experimental results, the proposed pattern vector extracted at optimal position a=1.6 cm reaches the highest sensitivity, specificity, and accuracy. Even more, the proposed technique utilizes a reduced number of physiological parameters to obtain a Correct Rate Classification (CRC) of 100%. The precision assessment confirms the performance superiority of the proposed method compared with other techniques for the breast thermogram classification of the DMR-IR.

A Non-Binary Snow Index for Multi-Component Surfaces (NBSI-MS) is proposed to map snow/ice cover. The NBSI-MS is based on the spectral characteristics of different Land Cover Types (LCTs), such as snow, water, vegetation, bare land, impervious, and shadow surfaces. This index can increase the separability between NBSI-MS values corresponding to snow from other LCTs and accurately delineate the snow/ice cover in non-binary maps. To test the robustness of the NBSI-MS, regions in Greenland and France–Italy where snow interacts with highly diversified geographical ecosystems were examined. Data recorded by Landsat 5 TM, Landsat 8 OLI, and Sentinel-2A MSI satellites were used. The NBSI-MS performance was also compared against the well-known Normalized Difference Snow Index (NDSI), NDSII-1, S3, and Snow Water Index (SWI) methods and evaluated based on Ground Reference Test Pixels (GRTPs) over non-binarized results. The results show that the NBSI-MS achieved an overall accuracy (OA) ranging from 0.99 to 1 with kappa coefficient values in the same range as the OA. The precision assessment confirmed the performance superiority of the proposed NBSI-MS method for removing water and shadow surfaces over the compared relevant indices.

This paper reports the implementation of an optical fiber sensor to measure the refractive index in aqueous media based on localized surface plasmon resonance (LSPR). We have used a novel technique known as photodeposition to immobilize silver nanoparticles on the optical fiber end. This technique has a simple instrumentation, involves laser light via an optical fiber and silver nanoparticles suspended in an aqueous medium. The optical sensor was assembled using a tungsten lamp as white light, a spectrometer, and an optical fiber with silver nanoparticles. The response of this sensor is such that the LSPR peak wavelength is linearly shifted to longer wavelengths as the refractive index is increased, showing a sensitivity of 67.6 nm/RIU. Experimental results are presented.