The flat-top beam is required in lots of practical applications. However, the semiconductor laser which is widely used as light source has a Gaussian or Gaussian-like energy distribution. In this work, a beam shaping system consisting of aspherical circular lens, Powell lens and cylindrical lens is proposed to transform the semiconductor laser beam into a collimated flat-top beam. The parameters of the Powell lens are designed based on the energy conservation between the incident and output beams, and the cylindrical lens is used for beam collimation. Simulation and experimental analysis are conducted to investigate the energy distribution of the shaped beam at various distances. The results demonstrate a strong agreement between the theoretical expectations and experimental observations, confirming the feasibility and scientific validity of the shaping system. This approach provides an effective method for shaping Gaussian beams into collimated flat-top beams.
The in-situ investigation of interstellar dust has becoming one of the focuses in deep space exploration. It is of great importance since it provides us with key information about the origin and evolution of planets. To measure the size of a single slow dust particle, a laser-based optical measurement system was designed and calibrated accordingly. In this system, a detection area of 50mm*50mm laser curtain was generated using a diffractive optical element (DOE) and a cylindrical lens, which transform the laser beam in Gaussian profile to a laser sheet with a rectangular uniform profile. When a dust particle passes through the laser curtain, scattered light will be generated and collected by a compound parabolic concentrator (CPC) and then be converted into an electrical signal by a PIN photodiode. The amplitude of electrical signals, which are directly related to the scattered light flux, are used to extract the particle size. Standard spherical SiO2 sample particles of different sizes were used in the calibration activities. Satisfied agreements have been achieved between the theoretical result calculated using the Lorenz-Mie theory and the experimental result.
Using the Finite-Difference Time-Domain method (FDTD), the properties of the three-dimensional photonic jet generated by the spheroidal particles illuminated by a Bessel beam are studied. The internal and near-field spatial distributions with the change of the ellipticity of the micro-spheroid are investigated for the cases of Bessel beams with different polarizations. The simulation results show that by varying the ellipticity and the polarization of Bessel beam, it is possible to obtain localized photon fluxes having different power characteristics and spatial dimensions. This can be of interest for several applications, such as ultramicroscopy, nanolithography, low-loss coupled resonance waveguides.
An arbitrary order Bessel beam with arbitrary incidence is generated numerically in finite-difference time-domain (FDTD) method using a total-field/scattered-field (TF/SF) approach. This is implemented by decomposition of Bessel beam into a series of plane waves, which are projected into the FDTD simulation domain. The off-axis incidence case is realized by tuning the arrival time when the elementary plane wave gets to the center area of simulation domain, and the oblique incidence case is implemented by rotating the plane waves through Euler angles. Numerical examples concerning backscattering radar cross-sections (RCS) are presented to demonstrate the validity, accuracy, and capability of the proposed method. The results in this paper provide an efficient way to investigate the interactions of Bessel beams and particles with complex shape and composition using FDTD.
In this paper, an analysis of scattering properties of aggregated particles illuminated by an arbitrary shaped beam is implemented using GLMT. A theoretical treatment for an aggregate of particles illuminated by an arbitrarily incident beam is briefly presented, with special attention paid to the calculation of beam shape coefficients of a shaped beam. The theoretical treatment and the home-made codes are verified by making a comparison between our numerical results and those obtained using a public available T-Matrix code MSTM. Good agreements are achieved which partially indicate the correctness of both codes. Furthermore, some new numerical results concerning the scattered fields of aggregated particles illuminated by a focused Gaussian beam are presented.
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