Photonics

Based on the extended Huygens–Fresnel principle and mode expansion theory, we derive the expression for the Coherence-Orbital Angular Momentum (COAM) matrix of twisted Gaussian Schell-model (TGSM) beams propagating through non-Kolmogorov turbulence. Using numerical simulations, we compare the evolution characteristics of the COAM matrix in free space and under non-Kolmogorov turbulence conditions. The study analyzes the variation patterns in the absolute values, real parts, and imaginary parts of the COAM matrix elements under different topological charges, and provides a detailed investigation of the influence of various beam parameters and turbulence parameters on these elements. The results show that by selecting appropriate parameters, the negative impact of turbulence on the correlation between orbital angular momentum (OAM) modes can be effectively mitigated. This work provides theoretical support for parameter selection and optimization in atmospheric optical communication systems.

The fiber mode field overlap integral method is employed to analyze the influencing factors of coupling efficiency, as well as the effects of axial and radial alignment errors on coupling efficiency under different relative apertures of coupling lenses. The results indicate that there exists an optimal relative aperture of the coupling lens that maximizes coupling efficiency; however, at this optimal point, coupling efficiency is more susceptible to radial errors. Regarding axial and radial errors, when the relative aperture of the coupling lens is at its optimal value, the tolerance for alignment errors is minimal. Conversely, when the relative aperture exceeds the optimal value, both coupling efficiency and tolerance for alignment decrease. When the relative aperture is less than the optimal value, the requirement for installation accuracy decreases while the tolerance becomes larger. The trend of the simulation results aligns with the experimental data. This study provides instructive significance regarding the trade-off between coupling efficiency requirements and alignment accuracy in the design of actual polarization-maintaining fiber coupling systems.

Miniaturized long-wavelength infrared (LWIR) imaging systems are highly desirable for applications such as portable thermal sensing, unmanned surveillance, and medical diagnostics. Conventional refractive optics in the LWIR regime often require multiple lens configurations to extend depth of field (DoF), leading to increased size, weight, and cost. Although existing LWIR metalenses demonstrate competent capabilities, comprehensive approaches to DoF engineering have yet to be explored. Here, we demonstrate a miniature large-DoF camera using a metalens. The designed metalens features a 14 mm diameter aperture and weighs only 0.8 g while maintaining sharp focus over a working distance ranging from 1 m to 22 m. By leveraging subwavelength phase engineering, the metalens achieves high-resolution imaging with low aberration. The integrated camera exhibits an ultra-compact form factor, i.e., 2.3 cm × 2.3 cm × 1.2 cm (length × width × height) and weighs just 25 g. Experimental results confirm the superior DoF performance, enabling clear imaging across varying distances without mechanical refocusing. The advance provides a promising pathway toward ultra-compact, large-DoF LWIR imaging systems for applications ranging from autonomous vehicles to portable medical diagnostics and miniature surveillance devices.