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<jats:p>Traditional compressed sensing algorithm is used to reconstruct images by iteratively optimizing a small number of measured values. The computation is complex and the reconstruction time is long. The deep learning-based compressed sensing algorithm can greatly shorten the reconstruction time, but the algorithm emphasis is placed on reconstructing the network part mostly. The random measurement matrix cannot measure the image features well, which leads the reconstructed image quality to be improved limitedly. Two kinds of networks are proposed for solving this problem. The first one is ReconNet's improved network IReconNet, which replaces the traditional linear random measurement matrix with an adaptive nonlinear measurement network. The reconstruction quality and anti-noise performance are greatly improved. Because the measured values extracted by the measurement network also retain the characteristics of image spatial information, the image is reconstructed by bilinear interpolation algorithm (Bilinear) and dilate convolution. Therefore a second network USDCNN is proposed. On the BSD500 dataset, the sampling rates are 0.25, 0.10, 0.04, and 0.01, the average peak signal-noise ratio (PSNR) of USDCNN is 1.62 dB, 1.31 dB, 1.47 dB, and 1.95 dB higher than that of MSRNet. Experiments show the average reconstruction time of USDCNN is 0.2705 s, 0.3671 s, 0.3602 s, and 0.3929 s faster than that of ReconNet. Moreover, there is also a great advantage in anti-noise performance.</jats:p>

<jats:p>We report the experimental results of hybrid four-wave mixing and fluorescence signals from nitrogen-vacancy (NV) centers in diamond. The fluorescence signals are slowed owing to dark state. The observed delay time of light slowing due to interconversion between NV<jats:sup>−</jats:sup> and NV<jats:sup>0</jats:sup> is about 6.4 μs. The relative intensities of read-out signals change with the wavelength and power of writing pulse. Based on light slowing, we present the model of all-optical time division multiplexing. The intensity ratio in different demultiplexed channels is modulated by the wavelength and power of control field. It has potential applications in quantum communication and all-optical network.</jats:p>

<jats:p>We demonstrate a 300-km+200-km cascaded coherent phase transfer via fiber link. The transfer is divided into a 300-km span and a 200-km span with independent phase locking loops, aiming to extend the phase control bandwidth of the whole link. The phase noise and transfer instability of the cascaded transmission are investigated and compared with those in the case of a single-span 500-km transfer. We achieve the transfer instabilities of 1.8 × 10<jats:sup>−14</jats:sup> at 1 s, 8.9 × 10<jats:sup>−20</jats:sup> at 10<jats:sup>4</jats:sup> s for the 300-km + 200-km cascaded transmission, and 2.7 × 10<jats:sup>−14</jats:sup> at 1 s for the 500-km single-span transfer.</jats:p>

<jats:p>An InP optical 90° hybrid based on a × 4 MMI coupler with a deep ridged waveguide is designed and fabricated. The working principle of the 90° hybrid is systematically introduced. Three-dimensional beam ropagation method (3D BPM) is used to optimize the structure parameters of the 90° hybrid. The designed compact structure is demonatrated to have a low excess loss less than –0.15 dB, a high common mode rejection ratio better than 40 dB, and a low relative phase deviation less than ± 2.5°. The designed hybrid is manufactured on a sandwitched structure deposited on an InP substrate. The measured results show that the common mode rejection ratios are larger than 20 dB in a range from 1520 nm to 1580 nm. The phase deviations are less than ± 5 ° in a range from 1545 nm to 1560 nm and less than ± 7° across the C band. The designed 90° optical hybrid is suitable well for realizing miniaturization, high-properties, and high bandwidth of coherent receiver.</jats:p>

<jats:p>We propose a fiber-solid hybrid system which consists of a semiconductor saturable absorber mirror (SESAM) mode-locked fiber seed with a pulse width of 10.2 ps and a repetition rate of 18.9 MHz, a two-level fiber pre-amplifier and a double-passing end-pumped Nd:YVO<jats:sub>4</jats:sub> amplifier. In the solid-state amplifier, to enhance the gain and the extraction efficiency, a specially designed structure in which the seed light passes through the gain medium four times and makes full use of population inversion is used as the double-passing amplifier. Besides, the beam filling factor (the ratio of the seed light diameter to the pump light diameter) and the thermal lens effect of the double-passing amplifier are considered and its optical-to-optical conversion efficiency is further improved. To preserve the beam quality of the double-passing amplifier, a new method of spherical-aberration self-compensation based on the principles of geometrical optics is used and discussed. Our system achieves a maximum average power of 9.5 W at the pump power of 28 W, corresponding to an optical-to-optical efficiency of 27%. And the beam quality factor <jats:italic>M</jats:italic> <jats:sup>2</jats:sup> reaches 1.3 at the maximum output power.</jats:p>

<jats:p>Fe<jats:sup>2+</jats:sup>:ZnSe thin films are prepared on sapphire substrate at room temperature by electron beam evaporation and then annealed in vacuum (about 1 × 10<jats:sup>–4</jats:sup> Pa) at different temperatures. The influences of thermal annealing on the structural and optical properties of these films such as grain size and optical transmittance are investigated. The x-ray diffraction patterns show that the Fe<jats:sup>2+</jats:sup>:ZnSe thin film is preferred to be oriented along the (111) plane at different annealing temperatures. After the film is annealed, the full-width-at-half-maximum ( FWHM ) of the x-ray diffraction peak profile (111) of the film decreases and its crystal quality is improved. Scanning electron microscope images show that the films are more dense after being annealed. Finally, the sample is used as a saturable absorber in ZBLAN fiber laser. The annealed Fe<jats:sup>2+</jats:sup>:ZnSe thin films can be used to realize stable <jats:italic>Q</jats:italic>-switching modulation on ZBLAN fiber laser. The results demonstrate that the Fe<jats:sup>2+</jats:sup>:ZnSe thin film is a promising material for generating the high-power pulses of mid-infrared <jats:italic>Q</jats:italic>-switched fiber lasers.</jats:p>

<jats:p>CO<jats:sub>2</jats:sub> laser rapid ablation mitigation (RAM) of fused silica has been used in high-power laser systems owing to its advantages of high efficiency, and ease of implementing batch and automated repairing. In order to study the effect of repaired morphology of RAM on laser modulation and to improve laser damage threshold of optics, an finite element method (FEM) mathematical model of 351 nm laser irradiating fused silica optics is developed based on Maxwell electromagnetic field equations, to explore the 3D near-field light intensity distribution inside optics with repaired site on its surface. The influences of the cone angle and the size of the repaired site on incident laser modulation are studied as well. The results have shown that for the repaired site with a cone angle of 73.3°, the light intensity distribution has obvious three-dimensional characteristics. The relative light intensity on <jats:italic>z</jats:italic>-section has a circularly distribution, and the radius of the annular intensification zone increases with the decrease of <jats:italic>z</jats:italic>. While the distribution of maximum relative light intensity on <jats:italic>y</jats:italic>-section is parabolical with the increase of <jats:italic>y</jats:italic>. As the cone angle of the repaired site decreases, the effect of the repaired surface on light modulation becomes stronger, leading to a weak resistance to laser damage. Moreover, the large size repaired site would also reduce the laser damage threshold. Therefore, a repaired site with a larger cone angle and smaller size is preferred in practical CO<jats:sub>2</jats:sub> laser repairing of surface damage. This work will provide theoretical guidance for the design of repaired surface topography, as well as the improvement of RAM process.</jats:p>

<jats:p>We theoretically propose a narrowband perfect absorber metasurface (PAMS) based on surface phonon polaritons in the terahertz range. The PAMS has unit cell consisting of a silver biarc on the top, a thin polar-dielectric in the middle and a silver layer at the bottom. The phonon polaritons are excited at the interface between the silver biarc and the polar dielectric, and enhance the absorption of the PAMS. The absorption peak is at 36.813 μm and the full width half maximum (FWHM) is nearly 36 nm, independent of the polarization and incidence angle. The electric fields are located at the split of the biarc silver layer and the quality factor <jats:italic>Q</jats:italic> is 1150. The FWHM decreases with the decreasing split width. When the thickness of the bottom layer is larger than 50 nm, the narrow band and high absorption are insensitive to the thickness of those layers. The designed absorber may have useful applications in terahertz spectra such as energy harvesting, thermal emitter, and sensing.</jats:p>

<jats:p>The propagation of a probe field through a four-level Y-type atomic system is described in the presence of two additional coherent radiation fields, namely, the control field and the coupling field. An expression for the probe response is derived analytically from the optical Bloch equations under steady state condition to study the absorptive properties of the system under probe field propagation through an ensemble of stationary atoms as well as in a Doppler broadened atomic vapor medium. The most striking result is the conversion of electromagnetically induced transparency (EIT) into electromagnetically induced absorption (EIA) as we start switching from weak probe regime to strong probe regime. The dependence of this conversion on residual Doppler averaging due to wavelength mismatch is also shown by choosing the coupling transition as a Rydberg transition.</jats:p>

<jats:p>A promising tool to detect micro-cracks in plate-like structures is used for generating higher harmonic Lamb waves. In this paper, a method combining nonlinear S<jats:sub>0</jats:sub> mode Lamb waves with time reversal to locate micro-cracks is presented and verified by numerical simulations. Two different models, the contact acoustic nonlinearity (CAN) model and the Preisach–Mayergoyz (PM) model, are used to simulate a localized damage in a thin plate. Pulse inversion method is employed to extract the second and fourth harmonics from the received signal. Time reversal is performed to compensate the dispersion of S<jats:sub>0</jats:sub> mode Lamb waves. Consequently, the higher harmonics generated from the damaged area can be refocused on their source. By investigating the spatial distribution of harmonic wave packets, the location of micro-cracks will be revealed. The numerical simulations indicate that this method gives accurate locations of the damaged area in a plate. Furthermore, the PM model is proved to be a suitable model to simulate the micro-cracks in plates for generation of higher harmonics.</jats:p>