Aiming at the error problems in the computation of the fast Fourier transform (FFT) of the traditional Richards-Wolf focusing field in the xy-plane and xz-plane, this study proposes an improved FFT integration method, which aims to improve the computational accuracy through the integration boundary correction. The method is achieved by finely adjusting the integration boundary grid sampling point and integration area, specifically adjusting the integration boundary grid sampling point to the grid centre in Algorithm 1 and to the centre of the boundary polygon in Algorithm 2. In Algorithm 3, the boundary polygon is divided into triangles and the centres of the triangles are taken as the sampling points, and the product of the function value and the corresponding area at these points is calculated as the contribution to the FFT integration. Simulation results show that compared with the traditional FFT algorithm, the improved FFT algorithm significantly improves the computational accuracy while reducing the number of sampling points required, effectively reducing the computation time and improving the computational efficiency. This result fully confirms the effectiveness of the FFT algorithm based on the integral boundary correction in optimising the computational accuracy of the RichardsWolf focusing field FFT, which provides new powerful tools and methods for the research in the fields of holographic optical tweezers and the analysis of the optical field distribution near the focusing plane, and it is expected to push forward the technological advancement and the application development of these fields.
With the advent of the 5G era, there is an increasing demand for high-speed and high-capacity information transmission technology. Multi-wavelength Brillouin fiber lasers (MBFL) have several advantages such as large frequency shift spacing, a wide wavelength range, narrow spectrum width, high power output, and high stability. Due to these advantages, MBFL have a wide range of applications in optical communication, optical fiber sensing, lidar, and optical measurement. MBFL with multiple Brillouin frequency shift spacing have emerged the preferred light source for high-capacity dense wavelength division multiplexing (DWDM) systems. However, the narrow frequency spacing of Brillouin frequency shift makes demultiplexing difficult. To address this issue, this article reviews the latest progress in the research on MBFL. This includes MBFL with multiple Brillouin frequency shift spacing, as well as MBFL with switchable frequency shift spacing. The frequency shift spacing range covered in this article is 10GHz to 60GHz. Furthermore, the article provides a detailed introduction to the applications of MBFL in temperature sensors and microwave photons. Finally, the challenges of future development of MBFL are summarized, which how to achieve more practical lasers, such as wider frequency shift intervals, higher power, and higher signal-to-noise ratio.
Due to the natural dispersion effect of the material itself, chromatic aberration is produced when light is incident, so it is important to eliminate chromatic aberration, especially for metalens. We use the phase change material (PCM) Ge2Sb2Te5 (GST-225) in the wavelength range of 4 to 5 μm and design two-layer vortex metalenses, the first one at a single wavelength of 5 μm, to verify the feasibility of a two-layer metasurface for vortex metalens. The second is a two-layer vortex achromatic metalens, where the two layers are modulated by the propagation phase and geometric phase, respectively, and the design of the vortex achromatic metalens is achieved by a combination of compensation phase and adjustment of the crystallization rate of the PCM. The results show that the designed metasurface achieves achromatic effect in the designed wavelength band, which provides an idea for the design of the achromatic vortex metalens.
Metamaterials are widely used as an artificial composite material in optical systems, however, almost all imaging systems suffer from chromatic aberrations due to their inherent dispersion. In this paper, we demonstrate the achromatic bifocal metalens based on phase change materials by tuning the crystalline fraction of Ge2Sb2Te5 in a linear-phasegradient metasurface, which can realize off-axis and co-axial achromatic bifocal metalens in the continuous waveband from 9.5 to12μm. Simulation results show that average focusing efficiency of the off-axis metalens in the working band is about 28.96%, average focusing efficiency of the co-axial metalens is 50.2% with the full width at half-maximum is close to the diffraction limit. This design will play an important role in the field of fluorescence microscopy, which can achieve achromatic objective and eyepiece metalens with different focal lengths over a wide waveband.
Based on double-layer Sb2S3 material, a method is proposed to realize adjustable metalens at 1.31 μm using geometric phase to regulate the incident beam. Different functions are realized by changing different states of Sb2S3. In design 1, the lower layer of phase change material is set as a half-wave plate in the amorphous and crystalline states, and the upper layer can be switched between half-wave plate and full-wave plate when it is in the two states so that the focal length of the metalens can be switched between F1 and F2. The full width at half maximum (FWHM) is close to the diffraction limit, and the focus efficiency can reach 69%. In design 2, the upper layer is always in the amorphous state with the high transmittivity. An optical switch is realized when the lower layer is used as a half-wave plate with the transmittivity close to 0 in the crystalline state. At the same time, a bifocal metalens with the focusing efficiency of 62% can be realized when the lower layer is used as a half-wave plate with the high transmittivity in the amorphous state. By adjusting the material crystallization rate, bifocal metalens with various intensities are realized. Our design has great potential in optical imaging, scanners that combine beam switches and lenses, and dual-function devices.
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