Hollow-Core Fiber (HCF) has attracted great interest from researchers because of its high damage threshold and small nonlinearity compared with solid-core fiber. However, how to reduce the loss of HCF has always been an urgent problem to be solved. Aiming to solve the problem, we propose a novel Hollow-Core Negative Curvature Fiber (HC-NCF) with an elliptical nested tube and a circular nested tube. The structure of this HC-NCF is relatively simple, which greatly reduces the difficulty of fabrication. Finite element modeling has been used to simulate and calculate the Confinement Loss (CL) and Bending Loss (BL) of the fiber with different nested tube structures. Results show that the CL of the LP01 mode is as low as 6.48×10-6 dB/km at the interesting wavelength of 1.06 μm. It exhibits a minimum CL of 5.28×10-6 dB/km at 1.01 μm with maintaining a loss of less than 0.003 dB/km over 1020 nm (0.77 μm to 1.79 μm) bandwidth. In addition, we proposed the HC-NCF has been confirmed to have better-bending resistance. Within a bending radius of 5–40 cm, the HC-NCF has a BL below 3.75×10-4 dB/km at a 10 cm bending radius; the BL is below 1.03×10-5 dB/km at a 40 cm bending radius.
KEYWORDS: 3D image processing, Underwater imaging, Digital micromirror devices, 3D modeling, 3D acquisition, Absorption, Signal attenuation, Sensors, Scattering
Obtaining a clear image of the underwater environment with classical imaging methods is still a challenging task due to the large amounts of noise caused by absorption and scattering nature of water and complex light interactions. By contrast, ghost imaging (GI) is a second-order correlation imaging technique, which has the disturbance-free ability in severe backscattering and high absorption conditions. In this paper, a physical model of three-dimensional ghost imaging (3DGI) through turbid water was set up based on laser detection and ranging (LADAR) and GI system, which considering the light-field transmission, the effects of absorption and scattering of water and the interaction between light field. Then the quality of 3DGI with different turbid water conditions and over different propagation distances was investigated by numerical calculation. The results show that the proposed 3DGI scheme is capable to reconstruct the object of long-distance in highly turbid underwater environments, and the reconstruction quality is closely related to the turbidity of the water and transmission distance. This work provides a reference to underwater 3DGI application, and maybe a better alternative strategy for underwater imaging.
Polarized skylight sensor can calculate the heading angle by detecting the polarization patterns of skylight and overcome many inherent defects of the conventional navigation methods. This paper develops a real-time bionic polarized skylight sensor. In order to eliminate the sensor’s hardware errors, an indoor calibration experiment is conducted. We also propose an image processing method to enhance the sensor’s robustness in the urban environment. The comparative experiment shows that both calibration experiment and image processing algorithm can achieve good effects.
With the development of machine vision technology, in the process of visual navigation with images, it is necessary to match the local geometric features or global features of the images; however, the matching of local geometric features is low in accuracy and difficult to be used in tracking. In contrast, template-based global feature matching can directly use the information of the entire image, and it has high robustness to illumination variations and occlusions, so it has attracted widespread attention. At present, the classical matching algorithms based on templates mainly include Sum of Absolute Differences (SAD), Sum of Squared Differences (SSD), Normalized Cross Correlation (NCC), and Mutual Information (MI). In order to make it more reasonable to evaluate and compare the performance of the algorithms, in this paper, we decided to compare Mean Absolute Differences (MAD), Mean Square Differences (MSD), Zero-mean Normalized Cross Correlation (ZNCC), and Normalized Mutual Information (NMI). During the experiment, the Gaussian noise, illumination variations and occlusion were applied to the current image to simulate complex navigation scenes, and then matched it with the template images. The matching values obtained by the above four matching algorithms in different scenes were collectively called as alignment metric values. The matching effects of the four algorithms were evaluated from the following aspects including the smoothness of the metric value, the number of local extremums and whether the best position was in the correct alignment position. The results showed that the accuracy of MSD was greatly affected by noise and was not suitable for scenes interfered by noise, the number of local extremums of ZNCC changed greatly under the conditions of noise, illumination changes, and occlusion, the alignment metric values became unsmooth. In comparison, the NMI showed good robustness and accuracy in different conditions.
There is a strong demand for beam steering aiming to reposition an optical beam in various fields of applications, such as optical communications, light detection and ranging, microscopies, displays. In this paper, we present a beam steering method actuated by a hydraulic polymer elastic membrane. A thick polydimethylsiloxane (PDMS) membrane is placed underneath a thin PDMS membrane. The beam steering angle can be varied adaptively through inputting different hydraulic pressure. The thin PDMS membrane deforms significantly and the thick PDMS membrane deforms slightly since there is enough thickness difference between thick and thin PDMS membrane when the liquid pressure is applied. The model of the adaptive beam steering method is built and some simulation experiments are carried out by COMSOL Multiphysics software. The effects of some model parameters are analyzed. The parameters include the thickness ratio, the distance between the thick PDMS membrane and the acrylic frame, the length of the thick PDMS membrane, and liquid pressure are studied. We can find that the linearity of the effective refractive surface increases with the increase of thickness ratio and decreases when the thick PDMS membrane is close to the acrylic frame. The beam steering angle increases with increasing of the liquid pressure. We also find that there is a tradeoff between the length of the thick PDMS membrane and the range of the beam steering angle. The results show that the appropriate thickness ratio of the thick and thin PDMS membrane is 7:1. The beam steering angle is 0°~33.46° when the liquid pressure is 0 kPa~8 kPa. The length of the thick PDMS membrane is 7.5 mm. This paper can be used to design an adaptive beam steering device actuated by the hydraulic pressure.
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