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1. Analysis of Nonlinear Fiber Kerr Effects for Arbitrary Modulation Formats

   https://doi.org/10.1109/JLT.2022.3213182  

   Rabbani, Hami ; Hosseinianfar, Hamid ; Rabbani, Hamed ; Brandt-Pearce, Maite ( January 2023 , Journal of Lightwave Technology)

   Coherent optical transmission systems can be modeled as a four-dimensional (4D) signal space resulting from the two polarization states, each with two quadratures. Recently, nonlinear analytical models have been proposed capable of capturing the impact of Kerr nonlinearity on 4D constellations. None of these addresses the inter-channel nonlinear interference (NLI) imposed by arbitrary modulation formats in multi-channel wavelength division multiplexed (WDM) systems. In this paper, we introduce a general nonlinear model for multi-channel WDM systems that is valid for arbitrary modulation formats, even asymmetric ones. The proposed model converges to the previous models, including the EGN model, in the special case of polarization multiplexed systems. The model focuses on the cross-phase modulation (XPM) nonlinear term that lies at the heart of the NLI in multi-channel WDM systems operating on standard high dispersion single-mode fiber. We show that strategic mappings of the modulation format's coordinates to the polarization states can reduce the NLI undergone by these formats. 

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2. Machine learning-assisted nonlinear-impairment-aware proactive defragmentation for C + L band elastic optical networks

   https://doi.org/10.1364/JOCN.440214  

   Jana, Rana Kumar ; Chatterjee, Bijoy Chand ; Singh, Abhishek Pratap ; Srivastava, Anand ; Mukherjee, Biswanath ; Lord, Andrew ; Mitra, Abhijit ( December 2021 , Journal of Optical Communications and Networking)

   Efficient resource allocation and management can enhance the capacity of an optical backbone network. In this context, spectrum retuning via hitless defragmentation has been presented for elastic optical networks to enhance efficient spectrum accommodation while reducing the unused fragmented spaces in the spectrum. However, the quality of service committed in a service level agreement may be affected due to spectrum retuning. In particular, for transmission beyond the conventional C band, the presence of inter-channel stimulated Raman scattering can severely degrade the quality of the signal during defragmentation. To conquer this problem, this paper proposes, for the first time to our knowledge, a signal-quality-aware proactive defragmentation scheme for the$\mathrm{C}+\mathrm{L}$band system. The proposed scheme prioritizes the minimization of the fragmentation index and quality of transmission (QoT) maintenance for two different defragmentation algorithms, namely, nonlinear-impairment (NLI)-aware defragmentation (NAD) and NLI-unaware defragmentation (NUD). We leverage machine learning techniques for QoT estimation of ongoing lightpaths during spectrum retuning. The optical signal-to-noise ratio of a lightpath is predicted for each choice of spectrum retuning, which helps to monitor the effect of defragmentation on the quality of ongoing lightpaths (in terms of assigned modulation format). Numerical results show that, compared to a baseline algorithm (NUD), the proposed NAD algorithm provides up to 15% capacity increment for smaller networks such as BT-UK, while for larger networks such as the 24-node USA network, a capacity benefit of 23% is achieved in terms of the number of served demands at 1% blocking.

    

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3. 4D printed shape memory metamaterial for vibration bandgap switching and active elastic-wave guiding

   https://doi.org/10.1039/D0TC04999A  

   Li, Bing ; Zhang, Chao ; Peng, Fang ; Wang, Wenzhi ; Vogt, Bryan D. ; Tan, K. T. ( February 2021 , Journal of Materials Chemistry C)

   null (Ed.)

   Acoustic/elastic metamaterials that rely on engineered microstructures instead of chemical composition enable a rich variety of extraordinary effective properties that are suited for various applications including vibration/noise isolation, high-resolution medical imaging, and energy harvesting and mitigation. However, the static nature of these elastic wave guides limits their potential for active elastic-wave guiding, as microstructure transformation remains a challenge to effectively apply in traditional elastic metamaterials due to the interplay of polarization and structural sensitivity. Here, a tunable, locally resonant structural waveguide is proposed and demonstrated for active vibration bandgap switching and elastic-wave manipulation between 1000–4000 Hz based on 3D printed building blocks of zinc-neutralized poly(ethylene- co -methacrylic acid) ionomer (Surlyn 9910). The ionomer exhibits shape memory behavior to enable rearrangement into new shape patterns through application of thermal stimuli that tunes mechanical performance in both space and time dimensions (4D metamaterial). The thermally induced shape-reorganization is programed to flip a series of frequency bands from passbands to bandgaps and vice versa . The continuously switched bandwidth can exceed 500 Hz. Consequently, altering the bandgap from “on” to “off” produces programmable elastic-wave propagation paths to achieve active wave guiding phenomena. An anisotropic cantilever-in-mass model is demonstrated to predict the self-adaptive dynamic responses of the printed structures with good agreement between the analytical work and experimental results. The tunable metamaterial-based waveguides illustrate the potential of 4D printed shape memory polymers in the designing and manufacturing of intelligent devices for elastic-wave control and vibration isolation. 

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4. Domain-dependent strain and stacking in two-dimensional van der Waals ferroelectrics

   https://doi.org/10.1038/s41467-023-42947-3  

   Shi, Chuqiao ; Mao, Nannan ; Zhang, Kena ; Zhang, Tianyi ; Chiu, Ming-Hui ; Ashen, Kenna ; Wang, Bo ; Tang, Xiuyu ; Guo, Galio ; Lei, Shiming ; et al ( December 2023 , Nature Communications)

   Van der Waals (vdW) ferroelectrics have attracted significant attention for their potential in next-generation nano-electronics. Two-dimensional (2D) group-IV monochalcogenides have emerged as a promising candidate due to their strong room temperature in-plane polarization down to a monolayer limit. However, their polarization is strongly coupled with the lattice strain and stacking orders, which impact their electronic properties. Here, we utilize four-dimensional scanning transmission electron microscopy (4D-STEM) to simultaneously probe the in-plane strain and out-of-plane stacking in vdW SnSe. Specifically, we observe large lattice strain up to 4% with a gradient across ~50 nm to compensate lattice mismatch at domain walls, mitigating defects initiation. Additionally, we discover the unusual ferroelectric-to-antiferroelectric domain walls stabilized by vdW force and may lead to anisotropic nonlinear optical responses. Our findings provide a comprehensive understanding of in-plane and out-of-plane structures affecting domain properties in vdW SnSe, laying the foundation for domain wall engineering in vdW ferroelectrics.

    

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5. Two-Dimensional Structure of Flow Channels and Associated Upward Field-Aligned Currents: Model and Observations

   https://doi.org/10.3389/fspas.2021.737946  

   Lyons, Larry. R. ; Nishimura, Yukitoshi ; Wang, Chih-Ping ; Liu, Jiang ; Bristow, William. A. ( August 2021 , Frontiers in Astronomy and Space Sciences)

   Flow bursts are a major component of transport within the plasma sheet and auroral oval (where they are referred to as flow channels), and lead to a variety of geomagnetic disturbances as they approach the inner plasma sheet (equatorward portion of the auroral oval). However, their two-dimensional structure as they approach the inner plasma sheet has received only limited attention. We have examined this structure using both the Rice Convection Model (RCM) and ground-based radar and all sky imager observations. As a result of the energy dependent magnetic drift, the low entropy plasma of a flow burst spreads azimuthally within the inner plasma sheet yielding specific predictions of subauroral polarization stream (SAPS) and dawnside auroral polarization stream (DAPS) enhancements that are related to the field-aligned currents associated with the flow channel. Flow channels approximately centered between the dawn and dusk large-scale convection cells are predicted to give significant enhancements of both SAPS and DAPS, whereas flow channel further toward the dusk (dawn) convection cell show a far more significant enhancement of SAPS (DAPS) than for DAPS (SAPS). We present observations for cases having good coverage of flow channels as they approach the equatorward portion of the auroral oval and find very good qualitative agreement with the above RCM predictions, including the predicted differences with respect to flow burst location. Despite there being an infinite variety of flow channels’ plasma parameters and of background plasma sheet and auroral oval conditions, the observations show the general trends predicted by the RCM simulations with the idealized parameters. This supports that RCM predictions of the azimuthal spread of a low-entropy plasma sheet plasma and its associated FAC and flow responses give a realistic physical description of the structure of plasma sheet flow bursts (auroral oval flow channels) as they reach the inner plasma sheet (near the equatorward edge of the auroral oval). 

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