Terahertz (THz) sources, ranging from 0.1 THz to 10 THz, between microwaves and infrared waves, have important applications in spectral detection, medical imaging, communication, etc. Difference frequency generation (DFG) is an effective method for generating terahertz with the characteristics of low cost, simple structure, widely tunable range, no threshold, and room-temperature operation. This paper reviews various optical terahertz sources of difference frequency generation based on nonlinear crystals, including DFG with inorganic crystals, DFG with organic crystals, DFG with quasi-phase-matching (QPM) crystals, DFG in waveguides, cavity-enhanced DFG, and cascaded DFG. Their recent advances, as well as their advantages and disadvantages, are fully present and discussed. This review is expected to provide a comprehensive reference for researchers in this field and a quick understanding of optical THz sources of difference frequency generation with nonlinear crystals.

We demonstrate a compact, high-power, quasi-continuous-wave (QCW) end-pumped 1319 nm Nd:YAG slab amplifier laser with good beam quality. The laser is based on a QCW pulse Nd:YAG master oscillator and Nd:YAG slab amplifier with multi-pass zigzag architecture. The amplifier operates at a pulse repetition frequency of 500 Hz and pulse width of $\sim 105\mathrm{\mu s}$, delivering a maximum output power of 51.5 W under absorbed pump power of 217.8 W and corresponding to an extraction efficiency of 14.2%. The beam quality factor is measured to be $M_x^2=1.61$ and $M_y^2=1.81$ in the orthogonal directions. To the best of our knowledge, this is the first compact, high-power, high-beam-quality QCW Nd:YAG amplifier at 1319 nm based on a multi-pass zigzag slab structure.

We propose a parity-baseline segmented planar imaging method with more frequency components by changing different baseline selection to achieve better imaging in the rotational process. A theoretical model of the parity-baseline segmented planar imaging system with rotational operation is built to analyze the imaging effect. The simulation results show that the imaging quality of the parity-baseline segmented planar imaging system has an approximately 20% increase by rotation compared with the conventional system. In addition, the CLEAN algorithm in radio astronomy imaging is also applied to the parity-baseline segmented planar imaging system, and the image quality is further improved by 55%. Such a new imaging method holds great potential toward astronomical observation and detection.

Orbital angular momentum (OAM) signal mode crosstalk and energy loss induced by atmospheric turbulence are challenging phenomena commonly occurring in high-capacity and high-speed OAM-based free-space optical communications. In this paper, we combine the optical pin beam (OPB) that resists atmospheric turbulence with the quadratic vortex phase (QVP) to realize a communication transmission beam based on weakening the degradation of the OAM signal mode. The probability densities of a single OAM mode of an optical pin vortex beam carrying a QVP (QVP-OPVB), after passing through an anisotropic non-Kolmogorov turbulent atmosphere, are theoretically derived. Through numerical simulation, it is found that QVP can be regarded as a superposition of multiple OAM modes, and the energy transfer between OAM modes during the propagation can compensate for the degradation of the OAM spectrum caused by atmospheric turbulence. Compared to the OAM spectra of the Gaussian vortex beam carrying QVP (QVP-GVB), the QVP-OPVB has better anti-crosstalk performance in a turbulent environment with a long propagation distance and a small receiving aperture. This beam has applications in free-space optical communication to enhance the interference resistance of the communication link.

A high-efficiency laser coherent combination method based on the virtue of the self-transverse acceleration effect of Airy beams is proposed and demonstrated by converting an array of Gaussian beams into Airy beams array to reduce laser beams separation distance. The coherent combination procedures of circular array for both Airy and Gaussian beams are theoretically and experimentally studied. The results show that the central intensity of the laser coherent combination of the circular array of Airy beams is 14.33 times higher than that of the Gaussian beams array, and the combined efficiency of the circular array of Airy beams is 1.64 times higher than that of the Gaussian beams circular array. The method may provide an efficient way to increase combination efficiency of laser coherent combination array and may have a broad application prospect in optical communication, particle capture, material processing.

We propose a high-stability compact interference image detection by an adjustable baseline with real-time calibration based on phase feedback compensation, which is formed by traditional lens pairing. A theoretical model of the interference detection system is established to analyze the effect of reconstruction. The jitter of interference signal caused by environmental disturbance is solved effectively by phase feedback compensation and the corresponding fluctuations is less than 1.06%. The interferometric detection system can switch between the detection mode and the calibration mode through the optical switch. Three targets are successfully reconstructed with different baseline interference data. The relative position error between the calculated coordinates and the measured coordinates of the point target is less than 4%. Such the high stability interference detection system has good prospect for fast aiming interconnection of optical communication and electro-optical reconnaissance.