Optical Review最新文献

A graded-index (GI)-type photonic crystal fiber (PCF) that has an α-power effective refractive index profile operates as a multimode optical fiber and can be applied to a collimator for single-mode PCFs. The α-power effective index distribution is realized by an α-power air hole diameter distribution with uniform air hole pitch. However, a pressure controlling in fiber drawing is difficult due to the difference of air hole diameters in fiber cross section. In this paper, we propose a GI-PCF with uniform air hole diameter while the air hole pitch varies in radial direction. Light propagation characteristics in the Gaussian beam excited GI-PCF with uniform air hole diameter were theoretically studied. The period and amplitude of mode field diameter variation along the propagation direction for the innermost air hole pitch, air hole diameter, and radial distribution profile of the pitch were clarified. The collimator application of GI-PCF with uniform air hole diameter for a conventional single-mode PCF was investigated. The suitable GI-PCF structure to obtain high optical coupling efficiency and good tolerance for GI-PCF length error was clarified. Wavelength characteristics of the GI-PCF collimator were also studied. It was clarified that the appropriate GI-PCF length was longer for shorter operating wavelength in collimator application.

We derive an equation that enables to get the floating distance of floating images of an arc 3D display by the radius of the arc, the angle of the arc 3D substrate, the light source illumination angle, and the observer’s angle. Conventional theoretical expression for the positions of the light source and observer relative to the center of the arc have been used to calculate the floating distance. However, when the arc3D substrate is inclined, it becomes more difficult to determine the floating distance from the actual positions of the light source and observer. In this paper, we derive an equation to approximate the floating distance from the positions of the light source and the observer while considering the tilt of the arc3D substrate and check the accuracy of the derived equation through experiments.

Aerial displays for providing road information require long-distance image formation and a compact installation space. This paper proposes a compact optical system for forming long-distance floating images by introducing a Fresnel lens in an aerial imaging by retro-reflection (AIRR) optical system. In the conventional AIRR optics, since the aerial image position is the plane-symmetrical position of the light source with respect to the beam splitter, the installation space for forming a long-distance aerial image becomes huge. Our proposed method uses the virtual image formed by a Fresnel lens as the light source in an AIRR optical system. This leads to a much longer distance from the beam splitter to the aerial image than the distance from the beam splitter to the light source. We developed a prototype long-distance floating aerial display system using a large-scale Fresnel lens. As a result, the distance from the LED panel to the beam splitter was halved. Furthermore, we used two beam splitters to form two aerial images by using a single LED panel. Long-distance floating images could be formed 3.4 m and 4.6 m away from the beam splitters and could be seen with the naked eye.

Soybean can be easily contaminated by Aspergillus flavus which can generate toxigenic and endanger human life and health. Due to the difficulty in detecting moldy phenomena at early stage by the naked eye and traditional machine vision technique, this paper proposes a classification method based on deep learning and optical coherence (OCT) techniques to detect moldy phenomenon of soybeans at early stage. The proposed method mainly includes three stages: the first stage is mildew information extraction, we use convolutional neural network (CNN) to extract image features. The input of traditional CNN is usually the whole image, and the output can not to reflect the fine-grained information. On this basis, we use the features extracted from the patch for the perception of fine-grained information (such as tiny mildew pixels). In the second stage, the features of the two channels are fused using the self-attention mechanism. In the third stage, the fused feature vectors containing the region information of moldy spots are used for classification. The results show that the proposed method is superior to the traditional CNN model in early mildew identification, with an average accuracy of 99.5% and have 15 points increasing to traditional CNN model, which proves the effectiveness of the method.

This study proposes the method of measuring 3D object shapes in an immersive space using a motion capture system. We report on the visualizing the distortion of acrylic panels mounted on a large aerial display and measuring the aberration of the aerial image using a motion capture system. Large aerial displays are made of large acrylic panels, which are subject to distortion due to their own weight. We succeeded in visualizing the shape of the acrylic plate by motion capture and 3D plotting of the positional information. Using a motion capture system, it was found that the aerial image formed by the distorted acrylic plate exhibits astigmatism, which is the difference between the vertical and horizontal focusing position. Furthermore, by drawing the shape of the side surface of the acrylic plate using poster papers, the coordinates were extracted from the imitation paper image, the radius of curvature of the acrylic plate was calculated, and the aberration was calculated. It was found that it is possible to measure the shape in an immersive space using the motion capture.

With the advantages of a large field of view, portability, and cost-effectiveness, lensless imaging has been applied widely nowadays. However, as a powerful tool for complete polarimetric characterization of microstructural and optical properties of a medium, Mueller matrix imaging has not yet been integrated in lensless imaging scheme. Here we propose a lensless inline polarization holographic system for high-speed and high-resolution Mueller matrix imaging. Liquid crystal variable retarders are introduced to realize high-speed response and avoid vibrations and positioning errors. We apply the blind deconvolution for depolarized imaging reconstruction and the back-propagation approach for polarization hologram reconstruction, respectively. The polarimetric imaging ability and resolution performance of the proposed technique are demonstrated. Furthermore, Mueller matrix images and certain quantitative polarimetric parameters of biological samples are calculated. The proposed method can be easily implemented and integrated in various lensless imaging techniques for on-chip polarimetric imaging.

In this study, we have proposed a digital holographic technique in which the frequency-modulated continuous-wave technique is introduced as a novel implementation of wavelength multiplexing in the time–frequency domain. In the proposed technique, the holograms are recorded with two wavelengths and the information of each hologram can be separated in the time–frequency domain by modulating the frequencies of two laser diodes at different modulation widths. Therefore, a temporal Fourier analysis is performed on each pixel of the time-series holograms whose intensity is modulated with two beat frequencies. And then, the holograms corresponding to the two wavelengths are extracted independently. Initially, a holographic system with two close wavelengths of 782.43 nm and 782.50 nm was designed to measure the surface profile of metallic gauge blocks with a known step-height of 1.16 mm in both experimental and numerical calculations. In addition, the measurement accuracy of the proposed system was investigated using both the experimental and the numerical results. Furthermore, the numerical calculation was conducted to investigate the origin of the periodic noise superimposed on the experimental results. Finally, the reduction method of the periodic noise was proposed, and the effect of the method was demonstrated using numerical calculations.

The technology of coherent tracking based on signal beam nutation is presented in the paper. The working principle and advantage of the technology are analyzed. We provided the optical axis error detection algorithm and the selection method of nutation parameter. A simulation model of coherent tracking system was built, and the simulation results show that when half of the nutation angle is between 0.463 and 2 µrad, the degradation of the receiving sensitivity is less than 1 dB and the noise equivalent angle (NEA) is less than 0.02 μrad. When the nutation frequency is better than 10 kHz, the coherent tracking system can compensate vibration influence of some common satellite platform. The technology provides a new thought for free-space optical communication system design.