Optical Review最新文献

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.

A dual-frequency ultra-wideband line-to-circular polarization converter based on metasurfaces is proposed in this paper. The metasurface is composed of a symmetrical semicircular structure connected to a rectangular metal strip with a deflection direction at a 45° angle to the y-axis, and its dielectric substrate layer is fabricated on the metal layer. When y-polarized waves are incident, the polarization converter can convert linearly polarized incident waves into circularly polarized reflected waves. The frequency range with an axial ratio less than 3 dB is 14.0–20.1 THz and 23.5–37.4 THz, respectively. The obtained bandwidth is 6.1 THz and 13.9 THz, which generates left-handed circularly polarized waves (LCP) and right-handed circularly polarized waves (RCP). The linear circular polarization converter unit is compact and easy integration. When the oblique incidence angle is 40°, the minimum elliptical polarization conversion rate within the working axis ratio 3 dB bandwidth is above 80%. When the polarization angle is 20°, the elliptical polarization in the operating frequency band is also above 78%. As the reflective polarization converter has such excellent characteristics, which make it having high research value for electromagnetic devices in the terahertz frequency band.

We demonstrated an efficient MoS₂-based eye-safe passively Q-switched Nd:GGG laser under 880 nm LD pumping. The maximum average output power was about 1.05 W with a slope efficiency of about 7.0%. The maximum pulse repetition rate and the shortest pulse width were 365.5 kHz and 146 ns respectively. To the best of our knowledge, this work represented the MoS₂ firstly used as a saturable absorber (SA) in Q-switched Nd:GGG laser operating at 1.4 μm wavelength, which deepened the understanding of the Q-switched performance of MoS₂ in this spectral region.

Interferometric techniques are widely used to obtain the real-time phenomena in a majority of applications considering its advantages of non-intrusively, mapping changes in path length as small as the wavelength of the light source used, etc. Interferometers often recorded the changes in the path length in the form of fringe orientation. Depending upon the complexity of the phenomena under observation, interferometric fringes are formed from simple fringe ordinations to complex fringe structures. Extracting phase map from these fringe patterns are highly crucial to evaluate the phase change and the corresponding property change. In order to use the interferometers in its full potential, it is necessary to develop robust and accurate algorithms for phase extraction. In this paper, we discuss 2-D Hilbert transforms (HT) and 2-D continuous wavelet transforms (CWT) techniques to analyze fringe patterns of different orientation and inter-fringe spacing. Simulated interferograms were used to generate implemented fringe patterns with different orientation and spacing. MATLAB software is used to generate these simulated image. In order to check the efficiency of the mentioned algorithm, a white Gaussian filter with different noise levels (0%, 5%, 10%) has been added to the simulated interferograms. Interferometric fringes with noise is a common phenomenon in many transient and turbulent processes. Moreover, the in-build noise in optical configuration and external noises in the testing environment also adds to the noise. Hence, it is necessary to test the accuracy of algorithm for enhanced noises. The results show that the HT algorithm was difficult to implement on complex patterns and was also difficult to implement for a high degree of noise interference. The two-dimensional CWT, on the other hand, has good performance on complex striped patterns and can handle noise disturbances with a good degree of accuracy. The continuous wavelet transform is insensitive to noise interference patterns and gives accurate results. The processing time is very short and it can handle both simple and complex fringe patterns within a very short period of time. It can also be concluded that the 2-D CWT algorithm is very effective and robust.

A relative humidity (RH) sensor based on in-fiber Michelson interferometer (MI) is proposed, which consists of a short piece of graded-index multimode fiber (GIMMF) followed by a 2-core fiber (2CF), whose end face is terminated by a thick aluminum film. The GIMMF excites cladding modes into the pigtail 2CF via the mismatched-core splicing interface. The core-cladding modes are reflected back by the aluminum film and recoupled to the core of lead-in SMF through the GIMMF. A well-defined interference pattern is obtained as the result of core-cladding mode interference. The experimental results show that a configuration with a 10 mm pigtail 2CF at a wavelength of 1552.78 nm has a good linear response to relative humidity with the sensitivity of – 0.044 dB/%RH in the range of 35–95%RH. Meanwhile, the selected monitoring peak provides a better temperature sensitivity of 65 pm/℃ in the range of 35 –85 ℃. In addition, the aluminum film is manufactured by physical vapor deposition (PVD), which greatly enhances the compactness of the film and improves the contrast of the interference fringes; the manufacturing method has high repeatability.