Towed linear array array based on fiber laser hydrophone which had much research potentiality owing to its higher acoustic pressure sensitivity, smaller size and lower difficulty of multiplexing. Flow noise is one of the main sources of noise in the towed linear array system. It will compress the dynamic range of fiber laser hydrophone and reduce the detection ability to small signal. In this paper, the structure of a 4 channel fiber laser hydrophone towed linear array was presented, and the data of sea trial of flow noise was analyzed. The results shows that the influence of flow noise mainly concentrate on the low frequency band which under 500Hz, and the background noise level has increased nearly 70-80dB when the array is towed at the speed of 3 kn.
Optical fiber laser hydrophone which has more research potentiality and value owing to higher acoustic pressure sensitivity, smaller size and lower difficulty of multiplexing. However the detecting capacity to low frequency signal of optical fiber laser hydrophone will be limited because of the low frequency noise such as 1/f noise and thermal noise of optical fiber laser and pumped laser. In order to suppress these noises, the iterative discrete wavelet transformation algorithm was designed which used multi-scale trait of wavelet transform. The different spectral components of underwater acoustic signal were separated and the noises below 1kHz were eliminated on the basis of the target signal amplitude would not be weakened. The measured data acquisitive from anechoic tank showed that the algorithm reduced the noise below 1kHz nearly 50dB and the Signal to Noise Ratio(SNR) is improved from 55.23dB to 84.05dB.
Feature description and matching are at the base of many computer vision applications. However, traditional local descriptors cannot fully describe all information of features, and there are so many feature points and so long local descriptors that the matching steps are time-consuming. In order to solve these problems. This paper proposed a new efficient method for description and matching, called TSMwGLD (the two-step matching with global and local Descriptors). In TSMwGLD, first, it designed a simple global descriptor and then found N best-matching points by using global descriptors, and at the same time it could eliminate lots of points which didn’t match in global information. Next, the method continued the matching step to find the best-matching point by using the local descriptors of N candidate points. So the whole matching process could become faster because the distances between global descriptors with the size of 8 were computed more easily than local descriptors with the size of 64 in SURF. The experimental results show that TSMwGLD results in increased accuracy and faster matching than original method. Especially for blurred images with textures, the matching time is less than tenth of original and the whole description and matching process is about two times faster than SURF.
This paper proposes an improved method of edge detection based on the mean shift algorithm. A pixel of an image calculated by the mean shift algorithm eventually converges to a peak point of probability density of the image. The pixel which is farther from the peak point has a greater mean shift vector and higher probability to be an edge pixel. The gradient of the mean shift vector of an edge pixel is a local maximum. During the mean shift iterations, the mean shift vector decreases by steps. Therefore, the vector of the first step is representative, while it is unnecessary to calculate each pixel to its convergence. This reduces the amount of computation and promotes the efficiency of the algorithm in a large extent. First, the image is smoothed by the mean shift filter, and the gradient of the mean shift vector is computed. Then, the local maximum is found by using non-maxima suppression on the gradient, which thins the edges detected. Finally, dual-threshold is used to detect and link edges. The edges detected have more accuracy and continuity. Experimental results show that the proposed method outperforms the conventional methods while suppressing noise and preserving edges.
A scheme using CO2 laser beam to thermally fix selective layers or lumps of photorefractive holograms to integrate and miniaturize an optical 3D system into a single block of LiNbO3 crystal is suggested. Theoretical and experimental investigations on laser local thermal fixing are given, and a stage of the modified gamma network is experimentally demonstrated.
A scheme of using carbon-dioxide laser beams to thermally fix layers of photorefractive holograms to implement a solid-state optical system in a single block of LiNbO3 crystal is suggested. In this paper, a modified gamma network is adapted to demonstrate, and a stage of such a network is constructed in a block of crystal. To limit the heating region with the shape similar to the heating beam, two carbon-dioxide laser beams of narrow width are used to laterally illuminate a layer of crystal, and additional absorbers are attached on the crystal surfaces outside the heating region to guide the heating flows. Analytical discussion based on the heat- transfer equations and the proof-of-principle experimental result are given. It can be thus seen that it is possible to package an optical system into a miniaturized single-block of photorefractive crystal.
In this paper, an in real time and in situ fixing method by heating with a carbon-dioxide laser beam is suggested to thermally fix a small local hologram in a bulk of Fe:LiNbO3 photorefractive crystal. To form a heating-up to 100 - 200 degree Celsius volume with the shape similar to the laser beam, heat-guiding technique is developed. Based on the heat transfer equations, different heating modes with or without metal absorbers for heat-guiding by using a continuous or pulsed laser beam are analyzed. The optimal mode may be the case of pulsed heating with absorbers. On this basis, experiments are designed and demonstrated. It is seen that the fixing process by carbon-dioxide laser beam is very short compared with the process by oven and the fixing efficiency is quite high.
A new type of anisotropic scattering ring, accompanying strong isotropic conical scattering and isotropic scattering oscillation, is observed on a screen behind a Cu:KNSBN crystal pumped by a extraordinary beam. The latter plays a key role in the forming of the former.
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