The diffraction characteristics of phase-type soft aperture with super-Gaussian transmittance were studied. It was proved that the phase-type soft aperture with super-Gaussian transmittance can improve the near-field intensity distribution of the beam and suppress the diffraction of the beam. Genetic algorithm was used to design the phase-type soft aperture, and was compared with the error diffusion algorithm. The design results demonstrates that the phase-type soft aperture designed by the genetic algorithm has a smaller RMS error and can suppress the diffraction modulation of the intensity in a large spatial range.
Holographic exposure mosaic technology is a feasible solution to fabricate large-area pulse compression gratings, where the mosaic grating method of developing region by region is one of the mosaic approaches. In this method, an exposed area of the substrate is firstly developed, and then the developed photoresist grating mask is put back into the previous exposure system. The next area grating mask is fabricated by aligning the interference fringes formed by the exposure beam and the developed real grating. However, since the unequal exposure and inconsistent development, the groove shapes of grating masks in two areas, including groove depth and duty cycle, will be different. When detecting the mosaic grating error, the differences of the groove shapes will cause the dislocation in the -1-order reflected diffraction wavefront at gap of the mosaic grating. It will be superimposed on the phase change caused by the lateral displacement error, so that the judgement of lateral displacement error will be seriously interfered. To solve this problem, the measurement method of the 0-order diffraction wavefront under multiple incident angles is proposed to precisely judge the lateral displacement error in the mosaic grating. In this paper, the grating diffraction analysis program based on the rigorous coupled-wave analysis is firstly written, and then the initial phase of 0-order reflected diffraction wavefront of mosaic grating mask is calculated. Subsequently, the sample is tested by atomic force microscopy and interferometer. By importing the measurement data into the searching program, the groove parameters of grating masks are obtained by the library matching method. Then, the lateral displacement error of the mosaic grating is further deduced. Finally, the feasibility of the proposed judgment method is confirmed by the experiment.
KEYWORDS: Holograms, Digital holography, Fresnel lenses, Computer generated holography, Spatial light modulators, Reconstruction algorithms, 3D modeling, 3D image processing, 3D displays, 3D image reconstruction
In this paper, a voxel-based algorithm for calculating phase-only three-dimensional (3D) computer generated holograms (CGHs) with occlusion effect is proposed. The 3D object can be firstly decomposed into a number of self-luminous voxels, which is the minimum imaging unit of the object. According to occlusion relationship, the 3D position and propagation direction of each voxel can be determined, where the axial position, lateral position and the propagation direction are precisely controlled by digital Fresnel lens, digital grating and hologram segmentation, respectively. Then, in order to reconstruct the 3D object composed of multiple voxels, the pre-calculated holograms for all of the voxels are synthesized with the interweaving arrangement method. Moreover, the look-up-table (LUT) method is used in the hologram generation process to improve the calculating speed. An experimental verification system for the proposed algorithm is constructed using a single SLM. The optical reconstruction results demonstrate that the CGHs generated by the proposed algorithm can successfully provide 3D sensation with occlusion effect.
Hyperspectral imaging spectrometer is an important measurement and analysis instrument, which combines imaging with spectrum technology. Nowadays, it has been widely used in research fields of atmosphere, ocean, geology, ecology, astronomy and so on. Convex blazed grating is a key component in the hyperspectral imaging spectrometer. At present, holographic-ion beam etching is an important method to fabricate convex blazed gratings. The common way of holographic-ion beam etching is that to etch the photoresist grating mask directly. However it is difficult to control the groove of photoresist mask accurately. This paper proposes a method of fabricating convex blazed grating by native substrate grating mask, ion beam etching and reactive ion beam etching are used to fabricate a native substrate grating mask based on photoresist grating. The diffraction efficiency of the convex blazed grating is investigated by FDTD (Finite Difference Time Domain) theory. The first-order diffraction efficiency can be over 45% within visible to near-infrared waveband through controlling the blaze angle from 6.4° to 7.2°. Furthermore, a convex blazed grating has been fabricated with the period of 2.45um, the blaze angle of 6.8° and the anti-blaze angle of 60°, theoretical analysis shows that the first-order diffraction efficiency is more than 50% within visible to near-infrared waveband.
Tunable Fabry-Perot filter is an important device in optics. It is widely used in high resolution spectroscopy, laser linewidth measurement, laser frequency stabilization and so on. The filtering characteristics of the Fabry-Perot filter are directly related to the cavity length, and the performance is greatly influenced by the parallelism of the cavity mirrors. For a tunable Fabry-Perot filter with parallel cavity, it is difficult to directly measure the cavity length and determine the parallelism of the cavity in the tuning process. In this paper, a tunable Fabry-Perot filter with an automatic cavity-length stabilization and monitoring system is designed. Four metallic electrodes are deposited on each cavity mirror of the FP filter, thus to form 4 parallel-plate type capacitive sensors. With the help of a capacitance measurement circuit and a micro-controller circuit, the capacitances of the sensors can be acquired. After calibration, the mean value of the capacitances is used to determine the cavity length of the FP filter, while the differential capacitance values in two orthogonal directions are used to monitor the tilt of the cavity mirrors. A feedback control system composed by PZTs, a MCU and the capacitive sensors is therefore constructed to automatically stabilize the F-P cavity length and to adjust the parallelism between the cavity mirrors. The automatic cavity-length stabilizing and monitoring system has the advantages of high accuracy, strong disturbance resistance and high measuring speed. Experimental results prove that it can effectively measure the cavity length of the tunable Fabry-Perot filter, and can stabilize the FP filter as well.
Multiview holographic 3D display based on the nano-grating patterned directional diffractive device can provide 3D images with high resolution and wide viewing angle, which has attracted considerable attention. However, the current directional diffractive device fabricated on the photoresist is vulnerable to damage, which will lead to the short service life of the device. In this paper, we propose a directional diffractive device on glass substrate to increase its service life. In the design process, the period and the orientation of the nano-grating at each pixel are carefully calculated accordingly by the predefined position of the viewing zone, and the groove parameters are designed by analyzing the diffraction efficiency of the nano-grating pixel on glass substrate. In the experiment, a 4-view photoresist directional diffractive device with a full coverage of pixelated nano-grating arrays is efficiently fabricated by using an ultraviolet continuously variable spatial frequency lithography system, and then the nano-grating patterns on the photoresist are transferred to the glass substrate by combining the ion beam etching and the reactive ion beam etching for controlling the groove parameters precisely. The properties of the etched glass device are measured under the illumination of a collimated laser beam with a wavelength of 532nm. The experimental results demonstrate that the light utilization efficiency is improved and optimized in comparison with the photoresist device. Furthermore, the fabricated device on glass substrate is easier to be replicated and of better durability and practicability, which shows great potential in the commercial applications of 3D display terminal.
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