High-power laser plays an important role in many fields, such as directed energy weapon, optoelectronic contermeasures, inertial confinement fusion, industrial processing and scientific research. The uniform nearfield and wavefront are the important part of the beam quality for high power lasers, which is conducive to maintaining the high spatial beam quality in propagation. We demonstrate experimentally that the spatial intensity and wavefront distribution at the output is well compensated in the complex high-power solid-state laser system by using the small-aperture spatial light modulator (SLM) and deformable mirror (DM) in the front stage. The experimental setup is a hundred-Joule-level Nd:glass laser system operating at three wavelengths at 1053 nm (1ω), 527 nm (2ω) and 351 nm (3ω) with 3 ns pulse duration with the final output beam aperture of 60 mm. While the clear arperture of the electrically addressable SLM is less than 20 mm and the effective diameter of the 52-actuators DM is about 15 mm. In the beam shaping system, the key point is that the two front-stage beam shaping devices needs to precompensate the gain nonuniform and wavefront distortion of the laser system. The details of the iterative algorithm for improving the beam quality are presented. Experimental results show that output nearfield and wavefont are both nearly flat-topped with the nearfield modulation of 1.26:1 and wavefront peak-to-valley value of 0.29 λ at 1053nm after beam shaping.
KEYWORDS: Pulsed laser operation, Pulse shaping, High power lasers, Laser systems engineering, Modulation, Solid state lasers, Distortion, Signal to noise ratio, Laser applications, Control systems
In high-power solid-state laser, initiative pulse shaping can help improve the output laser’s performance. The evaluation
for output laser pulse is also incomplete. In this paper, we propose a method of initiative pulse shaping by using arbitrary
waveform generator (AWG), and establish a relatively complete evaluation system for the output pulses shape
simultaneously. It achieves the super-Gaussian pulse output with high SNR (signal-to-noise ratio). As a consequence, a
square laser pulse with pulse adjustable width ~5ns, rising time 197ps is obtained. The power imbalance of the output
square pulse is 3.72%. The similarity between the eight-order super-Gaussian pulse and the one we get from experiment
reached 99%.
Anomaly detection becomes increasingly important in hyperspectral data exploitation due to the use of high spectral resolution which can uncover many unknown substances that cannot be visualized or known a priori. Unfortunately, in real world applications with no availability of ground truth its effectiveness is generally performed by visual inspection which is the only means of evaluating its performance qualitatively in which case background information provides an important piece of information to help image analysts to interpret results of anomaly detection. Interestingly, this issue has never been explored in anomaly detection. This paper investigates the effect of background on anomaly detection via various degrees of background suppression. It decomposes anomaly detection into a two-stage process where the first stage is background suppression so as to enhance anomaly contrast against background and is then followed by a matched filter to increase anomaly detectability by intensity. In order to see background suppression progressively changing with data samples causal anomaly detection is further developed to see how an anomaly detector performs background suppression sample by sample with sample varying spectral correlation. Finally, a 3D ROC analysis used to evaluate effect of background suppression on anomaly detection.
KEYWORDS: Thallium, Sensors, Data processing, Data communications, Image processing, Data compression, Nickel, Hyperspectral imaging, Satellite communications, Algorithm development
Band selection (BS) has advantages over data dimensionality in satellite communication and data transmission in the sense that spectral bands can be selected by users at their discretion for data analysis, while preserving data fidelity. However, to materialize BS in such practical applications several issues need to be addressed. One is how many bands required for BS. Another is how to select appropriate bands. A third one is how to take advantage of previously selected bands without re-implementing BS. Finally and most important one is how to process BS as number of bands varies. This paper presents a specific application to progressive band processing of anomaly detection, which does not require BS and can be carried out in a progressive fashion with data updated recursively band by band in the same way that data is processed by a Kalman filter.
KEYWORDS: Target detection, Data processing, Image processing, Spatial resolution, Detection and tracking algorithms, Hyperspectral imaging, Electrical engineering, Real time processing algorithms, Communication engineering, Sensors
Constrained energy minimization (CEM) has been widely used for subpixel detection. It makes use of the sample correlation matrix R by suppressing the background thus enhancing detection of targets of interest. In many real world problems, implementing target detection on a timely basis is crucial, specifically moving targets. However, since the calculation of the sample correlation matrix R needs the complete data set prior to its use in detection, CEM is prevented from being implemented as a real time processing algorithm. In order to resolve this dilemma, the sample correlation matrix R must be replaced with a causal sample correlation matrix formed by only those data samples that have been visited and the currently being processed data sample. This causality is a pre-requisite to real time processing. By virtue of such causality, designing and developing a real time processing version of CEM becomes feasible. This paper presents a progressive CEM (PCEM) where the causal sample correlation matrix can be updated sample by sample. Accordingly, PCEM allows the CEM to be implemented as a causal CEM (C-CEM) as well as real time (RT) CEM via a recursive update equation in real time.
Anomaly detection generally requires real time processing to find targets on a timely basis. However, for an algorithm to be a real time processing it can only use data samples up to the sample currently being visited and no future data samples can be used for data processing. Such a property is generally called “causality”, which has unfortunately received little interest in the past. Recently, a causal anomaly detector derived from a well-known anomaly detector, called RX detector, referred to as causal RXD (C-RXD) was developed for this purpose where the sample covariance matrix, K used in RXD was replaced by the sample correlation matrix, R(n) which can be updated up to the currently being visited data sample, rn. However, such proposed C-RXD is not a real processing algorithm since the inverse of the matrix R(n), R-1(n) is recalculated by entire data samples up to rn. In order to implement C-RXD the matrix R(n) must be
carried out in such a fashion that the matrix R-1(n) can be updated only through previously calculated R-1(n-1) as well as the currently being processed data sample rn. This paper develops a real time processing of CRXD, called real time causal anomaly detector (RT-C-RXD) which is derived from the concept of Kalman filtering via a causal update equation using only innovations information provided by the pixel currently being processed without re-processing previous pixels.
Characteristics of transmitted temporal profile of active stimulated Brillouin scattering (SBS) optical limiting are studied. Methods of impairing transmitted residual peak power and shortening the delay time of SBS are proposed. Based on coupled wave equations of SBS process, physical models of optical limiting based on active SBS are constructed. Using an implicit finite differencing in time and a downward differencing scheme in space, theoretical calculation models of active SBS limiting for numerical simulations are built up. Three parameters, i.e. residual peak power, delay time and limited power are defined renewably to characterize active SBS optical limiting. The relationship of transmitted temporal profile to seed peak power is obtained by numerical calculations. The influence of the injected seed power on transmitted residual peak power is analyzed in detail. It is shown that the residual peak power is weakened with strengthening of the seed power. By selecting suitable power of seed, residual peak is removed and consequently the ideal flat pulse output is achieved. On the basis of theoretical analysis, active SBS optical limiting temporal characteristics are examined experimentally with a 20 ns Nd:YLF laser and one nonlinear medium (CCl4). The experimental results are compared with analytical and numerical calculations.
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