• Korean Journal of Optics and Photonics
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• v.31 no.6
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• pp.247-258
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• 2020

In this paper, based on a stochastic parallel gradient descent (SPGD) algorithm we study phase control of a coherent-beam-combining system under turbulent atmospheric conditions. Based on the statistical theory of atmospheric turbulence, we carry out the analysis of the phase and wavefront distortion of a laser beam propagating through a turbulent atmospheric medium. We also conduct numerical simulations of a coherent-beam-combining system with 7- and 19-channel laser beams distorted by atmospheric turbulence. Through numerical simulations, we characterize the phase-control characteristics and efficiency of the coherent-beam-combining system under various degrees of atmospheric turbulence. It is verified that the SPGD algorithm is capable of realizing 7-channel coherent beam combining with a beam-combining efficiency of more than 90%, even under the turbulent atmospheric conditions up to cn2 of 10-13 m-2/3. In the case of 19-channel coherent beam combining, it is shown that the same turbulent atmospheric conditions result in a drastic reduction of the beam-combining efficiency down to 60%, due to the elevated impact of the corresponding refractive-index inhomogeneity. In addition, by putting together the number of iterations of the SPGD algorithm required for phase locking under atmospheric turbulence and the time intervals of atmospheric phenomena, which typically are of the order of ㎲, it is estimated that hundreds of MHz to a few GHz of computing bandwidth of SPGD-based phase control may be required for a coherent-beam-combining system to confront such turbulent atmospheric conditions. We expect the results of this paper to be useful for quantitatively analyzing and predicting the effects of atmospheric turbulence on the SPGD-based phase-control performance of a coherent-beam-combining system.

• Journal of forensic and investigative science
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• v.2 no.2
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• pp.5-20
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• 2007

During the period from 2001 to 2005, 29,892 burglaries took place in Korea with the approximate average annual number - 5,978 cases. This study was conducted to analyze the reported burglaries and the result was summarized as follows. There were 8,605 residential invasion burglaries (28.8%) as the most frequent characteristic pattern. The exit was used as the most frequent invasion route for 4,031 invasion burglaries (64.3%), and an unlocked exit door or window was used as the most frequent invasion method for 2,462 invasion burglaries (28.6%). The hours just after midnight (between 00:00 and 04:00) were the most frequent time for invasion burglary to occur. Also, 5,652 burglaries occurred on Wednesday which was twice higher than on Sunday (2,988 burglaries). It was shown that the number of persons injured during burglaries were 260 deaths and 10,610 injuries. The places of the highest occurrence were the street with 10,183 burglaries (34%) and then residential place with 7,527 burglaries (approximately 25%). One-man burglary was the highest complicity: 15,012 offenders (56.1%). The knife was used as the most frequent instrument for 6,498 burglaries (24,3%) what is rare, while no criminal tool or instrument was used for 15,631 burglaries (58.4). During the period from 2001 to 2006, 1,506 burglaries occurred in Daegu and the average annual number was 251 burglaries. Among those,515 residential invasion burglaries (34.2%) took place and the average annual number was approximately 86 cases. The hours just after midnight (between 00:00 and 04:00) were the most frequent time for invasion burglary to occur (194 cases, 37.7%), the place of the highest invasion occurrence was the residential place (259 cases, 50.3%), and the exit was used as the most frequent invasion route (87 cases, 37.7%). An unlocked exit door or window was the most frequent invasion method (65 cases, 25.1%). In addition, pretending to be a delivery man, visitor or following the victim methods were used for 26 burglaries (10%). It is apparent that personal preventive measures against crimes, as well as governmental and social measures, play an important role in preventing burglaries. In particular, based on the analyzed result that an unlocked window or exit door was most frequently used for reported burglaries, it seems that there is a lack of understanding of crime prevention while little effort has been made to prevent crimes. Although everyone knows that locking a door is one of the basic measures to prevent crimes, many people tend to pay little attention to lock a door properly so burglary takes place. This study, therefore, is intended to encourage people to pay more careful attention to crime prevention, in order to help reduce the probability of burglary. With the recent improvement in social understanding of scientific crime investigation, a wide variety of police professions, including crime analysts, crime victim police counselors and coroners, have been prepared to develop the scientific crime investigation and crime analysis. In addition, it is hoped that further this study will contribute to encourage studies on crime prevention to be carried out in the future.