The security issues of passive optical networks (PONs) have always been a concern due to broadcast transmission. Physical-layer security enhancement for the coherent PON should be as significant as ...improving transmission performance. In this paper, we propose the advanced encryption standard (AES) algorithm and geometric constellation shaping four-level pulse amplitude modulation (GCS-PAM4) pilot-based key distribution for secure coherent PON. The first bit of the GCS-PAM4 pilot is used for the hardware-efficient carrier phase recovery (CPR), while the second bit is utilized for key distribution without occupying the additional overhead. The key bits are encoded by the polar code to ensure error-free distribution. Frequent key updates are permitted for every codeword to improve the security of coherent PON. The experimental results of the 200-Gbps secure coherent PON using digital subcarrier multiplexing with 16-ary quadrature amplitude modulation show that the GCS-PAM4 pilot-based key distribution could be error-free at upstream transmission without occupying the additional overhead and the eavesdropping would be prevented by AES algorithm at downstream transmission. Moreover, there is almost no performance penalty on the CPR using the GCS-PAM4 pilot compared to the binary phase shift keying pilot.
T1ρ quantification has the potential to assess myocardial fibrosis without contrast agent. However, its preparation spin‐lock pulse is sensitive to B1 and B0 inhomogeneities, resulting in severe ...banding artifacts in the heart region, especially at high magnetic field such as 3 T. We aimed to design a robust spin‐lock (SL) preparation module that can be used in myocardial T1ρ quantification at 3 T. We used the tan/tanh pulse to tip up and tip down the magnetization in the spin‐lock preparation module (tan/tanh‐SL). Bloch simulation was used to optimize pulse shape parameters of the tan/tanh with a pulse duration (Tp) of 8, 4, and 2 ms, respectively. The designed tan/tanh‐SL modules were implemented on a 3‐T MR scanner. They were evaluated in phantom studies under three different cases of B0 and B1 inhomogeneities, and tested in cardiac T1ρ quantification of healthy volunteers. The performance of the tan/tanh‐SL was compared with the composite SL preparation pulses and the commonly used hyperbolic secant pulse for spin‐lock (HS‐SL). Feasible pulse shape parameters were obtained using Bloch simulation. Compared with HS‐SL, the quantification error of tan/tanh‐SL was reduced by 27.7% for Tp = 8 ms (mean ∆Q = 126.15 vs. 174.42) and 75.6% for Tp = 4 ms (mean ∆Q = 136.65 vs. 559.53). In the phantom study, tan/tanh‐SL was less sensitive to B1 and B0 inhomogeneity compared with composite SL pulses and HS‐SL. In cardiac T1ρ quantification, the T1ρ maps using tan/tanh‐SL showed fewer banding artifacts than using composite SL pulses and HS‐SL. The proposed tan/tanh‐SL preparation module greatly improves the robustness to B0 and B1 field inhomogeneities and can be used in cardiac T1ρ quantification at 3 T.
We used the tan/tanh pulse to tip up and tip down the magnetization in the spin‐lock preparation module (tan/tanh‐SL). The designed tan/tanh‐SL modules were implemented on a 3‐T MR scanner. The proposed tan/tanh‐SL preparation module greatly improves the robustness to B0 and B1 field inhomogeneities and can be used in cardiac T1ρ quantification at 3 T.
Turbulent flows over porous lattices consisting of rectangular cuboid pores are investigated using scale-resolving direct numerical simulations. Beyond a certain threshold which is primarily ...determined by the wall-normal Darcy permeability, ${{\mathsf{K}}_y}$, near-wall turbulence transitions from its canonical regime, marked by the presence of streak-like structures, to another marked by the presence of Kelvin–Helmholtz-like (K–H-like) spanwise-coherent structures. The threshold agrees well with that previously established in studies where permeable-wall boundary conditions had been used as surrogates for a porous substrate (Gómez-de Segura & García-Mayoral, J. Fluid Mech., vol. 875, 2019, pp. 124–172). In the smooth-wall-like regime, none of the investigated substrates demonstrate any reduction in drag relative to a smooth-wall flow. At the permeable surface, a notable component of the flow is that which adheres to the pore geometry and undergoes modulation by the turbulent scales of motions due to the interaction mechanism described by Abderrahaman-Elena et al. (J. Fluid Mech., vol. 865, 2019, pp. 1042–1071). Its resulting effect can be quantified in terms of an amplitude modulation (AM) using the approach of Mathis et al. (J. Fluid Mech., vol. 628, 2009, pp. 311–337). This pore-coherent flow component persists throughout the porous substrate, highlighting the importance of a given substrate's microstructure in the presence of an overlying turbulent flow. This geometry-related aspect of the flow is not accounted for when continuum-based models for a porous medium or effective representations of them, such as wall boundary conditions, are used. The intensity of the AM effect is enhanced in the K–H-like regime and becomes strengthened with larger permeability. As a result, structured porous materials may be designed to exploit or mitigate these flow features depending upon the intended application.
•Clarification of interaction of functional copolymer with sodium alginate.•Demonstration of usefulness of parameter Q to characterize hydrogels.•Detailed analysis of linear and nonlinear ...rheology.•Tailorable modulus, nonlinear behavior, and functional polymer dependence.•Extension of large amplitude oscillatory shear-techniques to hydrogels.
Here, we report on the effect of functional copolymer poly(N-isopropylacrylamide-co-4-vinyl-phenylboronic acid) (NIBA) on the rheology and network structure formed by sodium alginate (SA) through linear and nonlinear viscoelasticity measurements. The hydrogel moduli at pH 3 increased with increasing NIBA addition, while the yield point decreased. Furthermore, these hydrogels showed strain-softening behavior, weak G″-overshoot marking the onset of nonlinearity, and good self-healing properties after large deformation. The zero-strain nonlinearity parameter (Q0) was found to be more sensitive to NIBA-addition than the linear viscoelastic properties. The blends showed a clear peak in the startup test except for SA alone and the peak intensity increased with increasing NIBA-concentration. Finally, based on all data, gelation mechanism and interaction of SA and NIBA will be clarified.
We explore the two-dimensional flow past a cylinder undergoing forced transversal oscillations with the aim of elucidating the evolution of the topology of the wake under variation of the forcing ...amplitude at a Reynolds number of 100. In particular, we study the change from two single vortices (2S mode) to a pair and a single vortex ($\text {P}+ \text {S}$ mode) being shed per period. Matharu et al. (J. Fluid Mech., vol. 918, 2021, p. A21) showed that a dynamical symmetry-breaking pitchfork bifurcation plays a key role in this transition. We show that in addition to this bifurcation, a number of topological bifurcations in the vorticity field occur, both on the symmetric 2S branch and on the asymmetric $\text {P}+ \text {S}$ branch. The topological bifurcations are cusp bifurcations where an extremum of vorticity is created or destroyed. To describe the effect of the topological bifurcations we introduce an extended symbolic classification of the wake modes to account for the spatial variations of the vortex patterns that occur in the transition process. We identify four amplitude values that define critical stages in the transition, and provide a complete qualitative picture of the transition from 2S to $\text {P}+ \text {S}$ mode. We confirm the robustness of the observed transition process by simulations at Reynolds number 80.
Offset quadrature amplitude modulation OFDM (OQAM-OFDM) systems have low sidelobes but require complex receivers for broadband channels. To deal with this problem, cyclic prefix (CP)-based OQAM-OFDM ...systems have been proposed in the literature. In this paper, we study OQAM-OFDM and two existing CP based OQAM-OFDM systems and propose a new CP based system with good spectrum performance. We investigate their properties and analyze their power spectral densities (PSDs). We show that the continuous-time transmitted CP-OQAM-OFDM signal after the CP insertion is equivalent to a truncated signal obtained by inserting CP and cyclic suffix (CS) to the discrete-time input data symbols in the OQAM-OFDM system, which simplifies the calculation of the PSD of the transmitted CP-OQAM-OFDM signal and also implies that the PSD of the CP-OQAM-OFDM may be affected by the truncation. We show that the perfect reconstruction (PR) condition for the CP-OQAM-OFDM system is actually the same as that for the OQAM-OFDM system when the length of the prototype filter is not more than the length of the transmitted CP-OQAM-OFDM signal of each block excluding the CP. We also compare three CP-based OQAM-OFDM systems with windowed CP-OFDM systems. Numerical results show that the theoretical and simulated PSDs of the transmitted signals match well, CP-based OQAM-OFDM systems perform better than windowed CP-OFDM systems considering both the PSD and bit error rate (BER) performances, and the newly proposed CP-based OQAM-OFDM system has the best PSD performance in the three CP based OQAM-OFDM systems with an increase of the data rate overhead.
Coherent imaging of an attosecond electron wave packet Villeneuve, D. M.; Hockett, Paul; Vrakking, M. J. J. ...
Science (American Association for the Advancement of Science),
06/2017, Volume:
356, Issue:
6343
Journal Article
Peer reviewed
Open access
Electrons detached from atoms or molecules by photoionization carry information about the quantum state from which they originate, as well as the continuum states into which they are released. ...Generally, the photoelectron momentum distribution is composed of a coherent sum of angular momentum components, each with an amplitude and phase. Here we show, by using photoionization of neon, that a train of attosecond pulses synchronized with an infrared laser field can be used to disentangle these angular momentum components. Two-color, two-photon ionization via a Stark-shifted intermediate state creates an almost pure f-wave with a magnetic quantum number of zero. Interference of the f-wave with a spherically symmetric s-wave provides a holographic reference that enables phase-resolved imaging of the f-wave.
Ultrasonic imaging is crucial in the fields of biomedical engineering for its deep penetration capabilities and non-ionizing nature. However, traditional techniques heavily rely on impedance ...differences within objects, resulting in poor contrast when imaging acoustically transparent targets. Here, we propose a compact spatial differentiator for underwater isotropic edge-enhanced imaging, which enhances the imaging contrast without the need for contrast agents or external physical fields. This design incorporates an amplitude meta-grating for linear transmission along the radial direction, combined with a phase meta-grating that utilizes focus and spiral phases with a first-order topological charge. Through theoretical analysis, numerical simulations, and experimental validation, we substantiate the effectiveness of our technique in distinguishing amplitude objects with isotropic edge enhancements. Importantly, this method also enables the accurate detection of both phase objects and artificial biological models. This breakthrough creates new opportunities for applications in medical diagnosis and nondestructive testing.
In this paper, we implement an optical fiber communication system as an end-to-end deep neural network, including the complete chain of transmitter, channel model, and receiver. This approach enables ...the optimization of the transceiver in a single end-to-end process. We illustrate the benefits of this method by applying it to intensity modulation/direct detection (IM/DD) systems and show that we can achieve bit error rates below the 6.7% hard-decision forward error correction (HD-FEC) threshold. We model all componentry of the transmitter and receiver, as well as the fiber channel, and apply deep learning to find transmitter and receiver configurations minimizing the symbol error rate. We propose and verify in simulations a training method that yields robust and flexible transceivers that allow-without reconfiguration-reliable transmission over a large range of link dispersions. The results from end-to-end deep learning are successfully verified for the first time in an experiment. In particular, we achieve information rates of 42 Gb/s below the HD-FEC threshold at distances beyond 40 km. We find that our results outperform conventional IM/DD solutions based on two- and four-level pulse amplitude modulation with feedforward equalization at the receiver. Our study is the first step toward end-to-end deep learning based optimization of optical fiber communication systems.
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