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Multi-focus image fusion consists in the integration of the focus regions of multiple source images into a single image. At present, there are still some common problems in image fusion methods, such as block artifacts, artificial edges, halo effects, and contrast reduction. To address these problems, a novel, to the best of our knowledge, multi-focus image fusion method using energy of Laplacian and a deep neural network (DNN) is proposed in this paper. The DNN is composed of multiple denoising autoencoders and a classifier. The Laplacian energy operator can effectively extract the focus information of source images, and the trained DNN model can establish a valid mapping relationship between source images and a focus map according to the extracted focus information. First, the Laplacian energy operator is used to perform focus measurement for two source images to obtain the corresponding focus information maps. Then, the sliding window technology is used to sequentially obtain the windows from the corresponding focus information map, and all of the windows are fed back to the trained DNN model to obtain a focus map. After binary segmentation and small region filtering, a final decision map with good consistency is obtained. Finally, according to the weights provided by the final decision map, multiple source images are fused to obtain a final fusion image. Experimental results demonstrate that the proposed fusion method is superior to other existing ones in terms of subjective visual effects and objective quantitative evaluation.Generation of subwavelength beam sizes is a fascinating challenge with several implications. The observation of a 120 nm laser spot in the visible part of the spectrum is reported here. It has a size variation of less than 10% in a distance of $ 50\;\unicodex00B5\rm m $50µm along the axis of propagation. This so-called Arago spot results from the diffraction of the light from a laser diode by the edges of an absorbing disk. Applications are discussed and hollow beams carrying orbital angular momentum with a 400 nm diameter dark spot in the center are evidenced. This paves the way toward atom lithography via atom guiding or new spectroscopy on forbidden transitions.Three-dimensional and three-component (3D3C) velocity measurements have long been desired to resolve the 3D spatial structures of turbulent flows. Recent advancements have demonstrated tomographic particle image velocimetry (tomo-PIV) as a powerful technique to enable such measurements. The existing tomo-PIV technique obtains 3D3C velocity field by cross-correlating two frames of 3D tomographic reconstructions of the seeding particles. A most important issue in 3D3C velocity measurement involves uncertainty, as the derivatives of the measurements are usually of ultimate interest and uncertainties are amplified when calculating derivatives. To reduce the uncertainties of 3D3C velocity measurements, this work developed a regularized tomo-PIV method. The new method was demonstrated to enhance accuracy significantly by incorporating the conservation of mass into the tomo-PIV process. The new method was demonstrated and validated both experimentally and numerically. The results illustrated that the new method was able to enhance the accuracy of 3D3C velocity measurements by 40%-50% in terms of velocity magnitude and by 0.6°-1.1° in terms of velocity orientation, compared to the existing tomo-PIV technique. These improvements brought about by the new method are expected to expand the application of tomo-PIV techniques when accuracy and quantitative 3D flow properties are required.An unobscured optical system provides a good solution for a high-performance diffractive telescope. However, the unobscured diffractive telescope, such as an off-axis three-mirror diffractive telescope (OTDT), suffers from the complex calculation of initial parameters, strict tolerance, and fabrication of aspheric mirror diffractive optical elements (DOE). In this paper, a simplified unobscured design method, which is effective in achieving compact and loose tolerance, is proposed. Combining the deflection of an optical path and Schupmann achromatic theory, a novel unobscured diffractive telescope is rapidly converted from a coaxial transmissive system. Not only is the loose tolerance obtained, but the fabrication of an aspheric mirror DOE is avoided. The designed system, with a focal length of 400 mm, $f$f-number of 5, and working waveband of 612.8-652.8 nm, is analyzed. The results show that the imaging quality approached the diffraction limit within a field of view of $0.04^\circ \times 0.02^\circ $0.04∘×0.02∘. Compared with the OTDT, the designed system has great advantages in design step, DOE fabricating, and system alignment. It also provides a reference for unobscured diffractive telescope development.We demonstrate a uniaxial 3D profilometry system illuminating the sample with a linear polarization pattern and measuring a polarization camera. The linear polarization pattern is generated by a spatial light modulator and a quarter-wave plate in the optical system. The system can measure four different fringe patterns with a phase difference of 90 deg simultaneously in the polarization camera. Therefore, we can measure three-dimensional shapes in a single shot. We present the measurement principles of the system and show the results of a real-time 3D profilometry experiment.We report on the design, fabrication, and characterization of single longitudinal mode InAs/GaAs quantum dot lasers emitting at the 1.3 µm communication band. The influence of simply etched surface high-order gratings in the ridge of the Fabry-Perot lasers has been studied. A 35th-order surface grating is fabricated by standard photolithography to introduce the refractive index perturbation, which leads to the reduced mirror loss at the desired wavelength and thus realizing single longitudinal mode lasing. Stable single-mode operations are maintained at the injection current range of 45-100 mA with a side-mode suppression ratio up to 33 dB.Developments toward the implementation of a terahertz pulse imaging system within a guided reflectometry configuration are reported. Two photoconductive antennas patterned on the same LT-GaAs active layer in association with a silica pipe hollow-core waveguide allowed us to obtain a guided optics-free imager. Besides working in a pulsed regime, the setup does not require additional optics to focus and couple the terahertz pulses into the waveguide core, simplifying the global implementation in comparison with other reported guided terahertz reflectometry systems. The system is qualified for imaging purposes by means of a 1951 USAF resolution test chart. An image resolution, after a 53 mm propagation length, by about 0.707 LP/mm over the 400-550 GHz integrated frequency band, was obtained, thus providing a promising basis to pursue efforts toward compact guided pulse imagers for sample inspection within the terahertz range.A method of extending the imaging range through scattering layers around a reference point (RP) is realized. Objects within the entire correlation range of the RP can be totally recovered. By scanning the light source, objects within the memory effect (ME) range of the RP are completely recovered with high quality. By combining the shift of a camera to move the object to the center of observation view, objects far away from the RP are retrieved with an improved signal-to-noise ratio. The extended imaging range is about 3.5 times the ME range and more than 16 times the imaging range with normal static illumination. The RP can be imprecisely placed at a distance from the objects instead of precisely replacing them owing to the extended imaging range. This simple-system method forcefully breaks the limitation of the ME range and is very easy to implement in practical applications, which is meaningful for the research in scattering imaging.This paper proposes and studies the characteristics of a laser-driven optothermal microactuator (OTMA) directly operated in water. A theoretical model of optothermal temperature rise and expansion is established, and simulations on a 1000 µm long OTMA are conducted, revealing that its arm is able to expand and contract in response to the laser pulses in a water environment. Microactuating experiments are further carried out using a microfabricated OTMA. check details The results demonstrate that the OTMA can be practically actuated in water by a 650 nm laser beam and that the OTMA's deflection amplitude increases linearly with laser power. When irradiated by laser pulses with 9.9 mW power and 0.9-25.6 Hz frequencies, the OTMA achieves deflection amplitude ranging from 3.9 to 3.2 µm, respectively. The experimental results match well with theoretical model when taking the damping effect of water into account. This research may be conducive to developing particular micro-electromechanical systems or micro-optoelectromechanical devices such as underwater optothermal micromotors, micro-pumps, micro-robots, and other underwater microactuators.Geometric phase retarders-such as q-plates and S-waveplates-have found wide applications due to simplicity of operational principles and flexibility for the generation of azimuthally symmetric polarization states and optical vortices. Ellipticity of the polarization vector and phase of the generated beam strongly depend on the retardation of the plate. Real devices usually have retardation value slightly different than the nominated one. Previously unattended perturbation of the retardation leads to asymmetry in intensity distribution and variation of ellipticity of the local polarization vector of the generated beam. We elucidate that controlled and intentionally driven azimuthally variable, oscillating perturbation of the retardation reveals the possibility to avoid distortions in the generated beam and leads to the recovery of the symmetrically distributed intensity and polarization (with zero ellipticity) of the beam. Described recovery of the desired polarization state could find application for generation of the high purity beam with azimuthally symmetric polarization, in which the local polarization ellipse has zero ellipticity.In this paper, we report the performance of a bismuth-doped fiber amplifier at 1687 nm. This wavelength region is particularly interesting for laser-based spectroscopy and trace gas detection. The active bismuth-doped fiber is pumped at 1550 nm. With less than 10 mW of the seed power, more than 100 mW is obtained at the amplifier's output. link2 We also investigate the signal at the output when a wavelength-modulated seed source is used, and present wavelength modulation spectroscopy of methane transition near 1687 nm. A significant baseline is observed in the spectra recorded when the fiber amplifier is used. The origin of this unwanted background signal is discussed and methods for its suppression are demonstrated.In the method of surface reconstruction from polarization, the reconstructed area is generally non-rectangular and contains a large number of sampling points. There is a difficulty that the coefficient matrix in front of the height vector changes with the shape of the measured data when using the zonal estimation. The traditional iterative approaches consume more time for the reconstruction of this type of data. This paper presents a non-iterative zonal estimation to reduce the computing time and to accurately reconstruct the surface. The index vector is created according to the positions of both the valid and invalid elements in the difference and gradient matrices. It is used to obtain the coefficient matrix corresponding to the general data. The heights in the non-rectangular area are calculated non-iteratively by the least squares method. At the same time, the sparse matrix is applied for handling the large-scale data quickly. link3 The simulation and the experiment are designed to verify the feasibility of the proposed method.