Since the central part of laser beam cannot be transmitted through the traditional Cassegrain antenna, thus a large energy loss is caused. To improve the transmission efficiency of traditional Cassegrain antenna, a pair of lenses is designed and set between the laser source and traditional Cassegrain antenna in this paper. The parameters of the pair of lenses and Cassegrain antenna system is determined and the energy distribution after the laser beam passing through the pair of lenses is discussed in detail. Based on three dimensional vector refraction and reflection theory, the ray tracing of the entire communication system and the chart of the transmission efficiency are simulated with MATLAB. From the chart, it can be concluded that proper laser beam shaping can improve the transmission efficiency of the entire communication system up to 100% at the wavelength of 1550 nm under ideal condition. After taking several practical factors such as the reflectivity of the mirror composing the Cassegrain antenna, the transmissivity and chamfering of the pair of lenses and the dispersion of the material into account, the transmission efficiency of the entire system can still be increased to 95.54% at the wavelength of 1550 nm. Compared with designing complex antenna systems, the method presented in this paper is more practical and convenient for optical communication.
A new type of laser radar system with off-axis parabolic rotating surfaces and a hyperbolic plane-convex lens configuration is designed in this paper. Three dimensional vector theory of reflection and refraction are utilized to design and analyze the structural parameters of the system. Ray tracing simulation are performed and results show that the new system can greatly decrease energy loss which is caused by central reflection from the secondary reflector in cassegrain-type antenna. In ideal conditions, the divergence angle of the transmitting rays can be compressed to 0.04 mrad. The incident lights will converge to the fiber core if the incident angle is less than 0.65 μmad. This design provides a practical way to improve performance of laser radar system.
We propose a hollow-core photonic bandgap fiber (HC-PBGF) with background composed of two materials to support orbital angular momentum (OAM) modes. Numerical models are set up to figure out the effective indexes and confinement losses over 1.3-2.0 μm. Simulation results show that this fiber can support more than 48 OAM modes, of which the effective indexes satisfy the condition for effective index separation (<10-4) and the confinement loss keeps under 10-7 dB/m over 1.3-2.0 μm. According to the comparison between fibers with same structure but comprising one or two background materials, adopting two materials to compose background is an effective method to significantly improve the performance of OAM-supporting HC-PBGF. The HC-PBGF proposed here is competitive in dealing for OAM multiplexing for optical communication systems.
One-dimensional photonic crystal exhibits unbelievable performance in designing large-mode-area fiber and fiber Bragg gating for high power fiber laser. However, the property of one-dimensional photonic crystal is sensitive to its structure and refractive index distribution, which may change due to the non-negligible thermal effect resulting from the hyperthermal working condition. In this paper, the thermal effect on one-dimensional photonic crystal is analyzed on the basis of heat transfer theory, Bragg reflection theory and finite element method(FEM). Firstly, with the help of heat transfer theory and finite element method, the temperature fields of the one-dimensional photonic crystal subjected to different heating sources are calculated. By making use of the calculation results, the deformation of the photonic crystal bringing from the temperature field is estimated. Then, the thermal effect on the transmission spectrum of the one-dimensional photonic crystal is analyzed. These studies not only provide important information for the manufacture of high power fiber laser but also may help the designers of fiber laser to find methods of counteracting the thermal effect.
A model is proposed to realize light collimation by surface modes that propagate on a corrugated surface around the output of a photonic crystal (PhC). The dispersion relation of the surface corrugation is given for a number of surface terminations. Self-collimation effect simulation can be obtained compared to the structure without surface corrugation. Precision collimation is shown at the chosen frequency, which matches with the self-collimation frequency in the PhCs.
By introducing a line defect and a V-shaped defect into two-dimensional triangular lattice photonic crystals, a 60 deg beam bend, a Y-shaped beam slitter, and a one-to-three beam splitter are designed based on the self-collimation effect of photonic crystals (PCs). By analyzing the equi-frequency contours calculated with the plane wave expansion method, the frequency for self-collimation in the PC is determined. By using the finite-difference time-domain method, the beam bending efficiency and splitting ratios of the device as a function of the defect radius are calculated. These results are confirmed by computational simulations that provide field distributions of light propagating in the structures. The designed beam bend and splitters offer more choices for beam control in the design of photonic integrated circuits.
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