Ultra-low threshold lasing through phase front engineering via a metallic circular aperture

Semiconductor lasers with extremely low threshold power require a combination of small volume active region with high-quality-factor cavities. For ridge lasers with highly reflective coatings, an ultra-low threshold demands significantly suppressing the diffraction loss at the facets of the laser. Here, we demonstrate that introducing a subwavelength aperture in the metallic highly reflective coating of a laser can correct the phase front, thereby counter-intuitively enhancing both its modal reflectivity and transmissivity at the same time. Theoretical and experimental results manifest a decreasing in the mirror loss by over 40% and an increasing in the transmissivity by 104. Implementing this method on a small-cavity quantum cascade laser, room-temperature continuous-wave lasing operation at 4.5 μm wavelength with an electrical consumption power of only 143 mW is achieved. Our work suggests possibilities for future portable applications and can be implemented in a broad range of optoelectronic systems.

Numerical modeling of the formation of D-T mixture spherical layer in micro-targets of LTS

The article presents a two-dimensional economical computational model of the formation of D-T mixture cryogenic layer in a spherical shell. The model is based on the description of the motion of the gas phase in the Boussinesq approximation. The thermal problem is a Stefan problem with a gas-solid phase transition. The technique is based on the finite volume method, the use of a structured mobile grid, whose movement is associated with the separation of the phase front, implicit approximations and the method of splitting two-dimensional equations in directions into one-dimensional equations. It is numerically shown that, due to natural radioactivity, the target is symmetrized. A calculated estimation of the symmetrization time for one geometry of the target with different filling coefficients is carried out.

Spatial phase retrieval of vortex beam using convolutional neural network

Vortex beam (VB) possessing spatially helical phase–front has attracted widespread attention in free-space optical communication, etc. However, the spiral phase of VB is susceptible to atmospheric turbulence, and effective retrieval of the distorted conjugate phase is crucial for its practical applications. Herein, a convolutional neural network (CNN) approach to retrieve the phase distribution of VB is experimentally demonstrated. We adopt a spherical wave to interfere with VB for converting its phase information into intensity changes, and construct a CNN model with excellent image processing capabilities to directly extract phase–front features from the interferogram. Since the interference intensity is correlated with the phase–front, the CNN model can effectively reconstruct the wavefront of conjugate VB carrying different initial phases from a single interferogram. The results show that the CNN-based phase retrieval method has a loss of 0.1418 in the simulation and a loss of 0.2344 for the experimental data, and remains robust even in turbulence environments. This approach can improve the information acquisition capability for recovering the distorted wavefront and reducing the reliance on traditional inverse retrieval algorithms, which may provide a promising tool to retrieve the spatial phase distributions of VBs.

INCREASE IN ACCURACY OF MEASURING AIR OBJECTS ANGULAR COORDINATES DURING MULTICHANNEL RECEPTION OF RADIO-LOCATION SIGNAL

Radars with a phased array antenna (PAA) which performs multi-channel radar signal reception are effective means of obtaining radar information about air objects in difficult conditions of air and jamming environment. Radar surveillance for radars with PAA is accompanied by a significant negative effect of tropospheric inhomogeneity, which causes a decrease in the accuracy of measurements of azimuth angles and air object‟s position due to fluctuations of the phase front of the received wave reflected from an air object. According to the results of research, the hypothesis of a normal distribution law of these fluctuations is accepted. The paper presents the results of estimating the root mean square errors of measuring the angular coordinates of the air object, which occur if the fluctuations of the phase front of the received signal‟s wave are not taken into account and analyzes the possibility of reducing such errors when the fluctuations are considered. The possibility of optimizing the angular measurements of air objects in digital radars with PAA is shown, which consists in taking into account the fluctuations of the phase front of the received signal in the algorithm of discrete (fast) Fourier transform, which is widely used to provide spatial measurements in modern digital radar stations. The results of previous studies were generalized, which makes it possible to evaluate the possibility of increasing the accuracy of angular measurements of air objects during multichannel reception of a radar signal in difficult conditions of radar operation.