Abstract. Small scale gold mining activity in some mountainous areas of China has caused a series of Hg pollution problems. Preliminary study indicated that impacts of rig pollution on the surrounding environment, air, water and soil are dramatic, and human contamination requires attention.
It has been demonstrated that an estimate of an empirical Green's function (EGF) can be extracted from the ocean ambient noise cross-correlation functions, which can provide an alternative method for ocean acoustic tomography. However, the requirement for a long recording time to obtain EGFs with a high signal-to-noise ratio limits the application. This article focuses on using array signal processing to accelerate the convergence rate of EGFs between two horizontally separated arrays. With the extracted EGFs and data assimilation, ocean sound speed profiles (SSPs) can be inverted every 2 h in shallow water. The experimental results indicate that the variation in ocean SSPs can be reconstructed with reasonable agreement using an average variance of 1.14 m/s over three months.
Based on periodically poled lithium niobate, a scheme for optical signal temporal cloak and encryption in the optical physical layer is proposed and numerically studied, in which the amplitude and phase information of the symbols to be sent are detected and transmitted while that of all other symbols will be hidden from the receiver. Meanwhile, the phase and amplitude of the symbols that are not hidden can be encrypted according to arbitrary encryption rules in the optical physical layer. The simulation results show that there is almost no ripple in the cloak time window, which makes the performance of temporal cloak quite well. The output signals with high Q factor can be obtained when the linewidths of the tunable lasers are <300 kHz. Two simple encryption rules are designed, 43.75% of binary codes will be misinterpreted by eavesdropper if the signal is encrypted with such two rules in the demonstration.
We proposed and numerically studied a traceless encryption approach for physical layer security in coherent optical communications system, the most attractive advantage of which is that it is hard for eavesdroppers to be aware that the transmission signal has been encrypted because the modulation formats of encrypted signal are still the regular ones, i.e. traceless encryption. In the proposed approach, the phase only or the combination of phase and amplitude dimensions can be used for encryption and decryption. Three simple encryption rules are designed and used to investigate the encryption security performance of the scheme, in which the QPSK signal can be encrypted to be as 8PSK, QPSK and 8QAM. The results show that three simple encryption rules can cause 37.5%, 25%, 62.5% of user signal binary codes to be misinterpreted by the eavesdroppers, respectively. When the modulation formats of encrypted signal and user signal are identical, the scheme can not only cover up the real information, but also have a potential application at misleading eavesdroppers. The impacts of the control light peak power at the receiver on the decryption performance are also analysed, the results indicate that the decryption performance of the scheme has a good tolerance to the peak power fluctuation of control light at the receiver.
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