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The efficiency of a hollow beam received by the Cassegrain antenna coupling into a single-mode fiber is low, and converting the hollow beam into a solid beam can remarkably improve the coupling efficiency. In this paper, shaping diffractive optical elements (DOEs) are designed through a modified Gerchberg–Saxton algorithm (MGS) with Fresnel diffraction. Further, the MGS algorithm can be applicable in the issue of circular symmetric beam shaping. The properties of the system with/without shaping DOEs are analyzed and compared. According to the simulation results, in consideration of the energy loss of the antenna, DOEs, and coupling lens, the total transmission efficiency of the receiving antenna system at 1550 nm wavelength can reach 77.81%. In addition, the system with shaping DOEs can better adapt for coupling lenses with different focal lengths, and the variation of the maximum coupling efficiency of the DOEs shaping system at different focal lengths studied in this paper is within 2.00%, which is 6.73% lower than that of the lens shaping system. The research results provide an idea of reverse design for improving a coupling system, which can also provide inspiration for other optical system designs.

The transmission characteristics of the Bessel-Gaussian (BG) beam propagation in free space and turbulence atmosphere after passing through the Cassegrain antenna system are researched in this paper. The research results show that the half-cone angles of the beam and parameters of the optical system can be used to control the propagation trajectory and the cross-talk among different orbital angular momentum (OAM) modes of the BG beam. The BG beam has a focusing property when propagating in free space after passing through the collimating optical system. The trend of cross-talk among different OAM modes is different for focused BG beam and non-focused beam, and the focusing beam can potentially reduce cross-talk among different OAM modes at the receiver plane.

A ring focus reflector is proposed for transmitting a perfect vortex (PV) beam, and the transmission characteristics of the PV beam with different topological charges in free space after passing through the reflector are studied. The reflector parameters can be determined by fitting the structural formula, and PV beams of different orders transmit with small spot sizes at the same time. The transmission trajectory calculated by the diffraction formula is consistent with the ray tracing results. The research results show that the reflector can achieve a high level of transmission efficiency of beams with different topological charges, which is conducive to the multiplexing of PV beams.

The ray transfer matrix method is widely used for analyzing beam-transmission properties and designing multi-pass systems. In this paper, a modified ray transfer matrix method is proposed for tracing an accurate non-sequential ray in three-dimensional space based on a vector reflection theory. The modified ray transfer matrix method can be utilized to analyze a ray transmitting between arbitrary surfaces, which is not confined to rotational symmetrical structures. There is no need to project rays onto two perpendicular planes, nor to introduce a paraxial approximation in our calculation. Since the length and angle of every ray can be calculated accurately, almost no deviation is accumulated in multi-pass system. The modified ray transfer matrix method can be applied in optical design, especially in the design of multi-pass cavities and multi-pass cells.

In this paper, the propagation properties of the Laguerre-Gaussian (LG) beam passing through Cassegrain antenna system in a turbulent atmosphere have been researched. The accurate analytical function for the diffraction field of LG beam passing through Cassegrain antenna, the average intensity, and the cross-talk among different orbital angular momentum (OAM) modes of LG beam passing through Cassegrain antenna in Kolmogorov turbulent have been derived. The simulation results show that LG beam with ring-like distribution is selected to enhance the emission efficiency of Cassegrain antenna. The cross-talk among different OAM modes can remarkably reduce through using Cassegrain antenna. The analysis process can also apply to accurately analyzing the propagation properties of other kinds of beams through different optical systems in turbulent atmosphere.

The low efficiency of coupling a hollow Gaussian beam into single-mode fiber terribly decreases the transmission efficiency of an optical communication system. In this paper, a coupling system containing a shaping lens and a coupling lens is designed based on the vector theory of refraction. The simulation results show that our coupling system can improve coupling efficiency to 81.45%, and the efficiency can still reach 74.73% even though chamfering and other factors are considered. This paper may not only provide engineers with a more efficient coupling system but also provide designers with ideas of hollow–solid (or solid–hollow) beam shaping systems.

Analyzing optical antenna systems through geometrical optics is quite popular for its convenience, which, however, may cause remarkable errors. This paper uses both geometrical optics and diffraction theory to analyze the performance of a traditional optical antenna system, and the results calculated through the two methods are carefully compared. Based on the comparison, it shows the situations in which geometric optics will cause significant errors. In addition, a parameter γ is defined to quickly determine whether geometrical optics is suitable for analyzing optical antenna systems under some situations. This paper can help engineers quickly choose the proper method for designing and analyzing optical antenna systems.

To solve the serious loss problem for central energy of the Cassegrain antenna, this paper designs a beam-shaping system, named E-A system, to convert the elliptical divergent beam with astigmatism from a laser diode into an annular collimation beam. This E-A system, comprised only of lenses with quadratic surfaces, can simultaneously realize four functions, i.e., aberration correction, beam circularization, beam collimation, and dark hollow beam generation. By adjusting the parameters, the E-A system can be used to shape elliptical astigmatic beams with different parameters. Further, the E-A system can maintain good performance at wavelengths from 500 to 1647 nm. After considering various practical factors, the transmission efficiency of the entire transmitting system can be increased to 93.91% at a wavelength of 1550 nm.

We propose a novel annular focus antenna with an off-axial parabolic primary mirror and slanting hyperbolic secondary mirror. Ray tracing based on three-dimensional vector reflection theory is utilized to design and analyze the structural parameters of the antenna. In ideal conditions, there is no geometric loss in theory and the transmitting efficiency increases by 24.54% compared with the conventional Cassegrain optical antenna. The transmitting efficiency is also calculated in the conditions of off-axial and on-axial defocus. The simulated results demonstrate that this design can increase transmission efficiency of the optical antenna effectively.

Designing a cavity with a high quality factor for omnidirectionally emitting laser (OEL) can extend its potential applications in optical communication and biomedical detection. We demonstrate a method including five steps to design a high-Q cavity for OEL using a one-dimensional topological photonic crystal heterostructure. A Si/SiO2 fiber cavity for OEL with solid gain medium Er-doped SiO2 is designed following our design steps. The designed fiber can axially transmit the pump energy at low confine loss and act as a cavity for the radial emission of the exited beam, simultaneously. The quality factor of this fiber cavity is on the order of magnitude of 108. Moreover, a method of further improving the Q-factor is proposed. The results in this paper are not restricted to the solid gain medium, and they also can be applied to designing a cavity for optofluidic OEL or quantum dot OEL. Our study may provide not only the reference for OEL manufacture, but also a route for improving the performance of OEL.

In this paper, an asymmetric large-mode-area photonic crystal fiber (LMA-PCF) with low bending loss at a smaller bending radius is designed. The finite-element method with a perfectly matched layer boundary is used to analyze the performance of the PCF. To achieve LMA-PCF with low bending loss, the air holes with double lattice constants and different sizes at the core are designed. Numerical results show that this structure can achieve low bending loss and LMA with a smaller bending radius at the wavelength of 1.55 μm. The effective mode area of the fundamental mode is larger than 1000 μm2 when the bending radius is ≥10 cm. The bending loss of the fundamental mode is just 0.0113 dB/m, and the difference between the fundamental and high-order modes of the bending loss is larger than 103 when the bending radius is 10 cm. Simulation results show this novel PCF can achieve LMA and have effective single-mode operation when the bending orientation angle ranges in ±110°. This novel photonic crystal has potential application in high-power fiber lasers.

In this work, a novel design of a high birefringence photonic crystal fiber (HB-PCF) with multimode and low confinement loss is proposed. To achieve high birefringence, the core is designed as an elliptical region, which is enclosed by twelve small holes. Based on this design, replacing the two circular holes at the top and bottom of the core region with two elliptical holes can further improve the birefringence. At the wavelength of 1.55 μm, the birefringence of the fundamental mode (LP01) and the second-order mode (LP11) are 1.70×10−2 and 1.85×10−2, respectively. Meanwhile, the confinement losses maintain on orders of 1×10−5 dB/km (LP01) and 1×10−1 dB/km (LP11). After the effective refractive indices of two types of the proposed HB-PCF are calculated by the finite element method, the birefringence, confinement loss, bending loss, dispersion, and nonlinear coefficient are studied. These results reveal that the HB-PCF might be applied for polarization-maintaining and nonlinear optics.