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1. Broadband ultraviolet-visible frequency combs from cascaded high-harmonic generation in quasi-phase-matched waveguides

   https://doi.org/10.1364/JOSAB.427086  

   Rutledge, Jay; Catanese, Anthony; Hickstein, Daniel_D; Diddams, Scott_A; Allison, Thomas_K; Kowligy, Abijith_S (July 2021, Journal of the Optical Society of America B)

   High-harmonic generation (HHG) provides short-wavelength light that is useful for precision spectroscopy and probing ultrafast dynamics. We report efficient, phase-coherent harmonic generation up to the ninth order (333 nm) in chirped periodically poled lithium niobate waveguides driven by phase-stable $\le <\#comment/>12\mathrm{n}\mathrm{J}$, 100 fs pulses at 3 µm with 100 MHz repetition rate. A mid-infrared to ultraviolet-visible conversion efficiency as high as 10% is observed, among an overall 23% conversion of the fundamental to all harmonics. We verify the coherence of the harmonic frequency combs despite the complex highly nonlinear process. Accommodating the extreme spectral bandwidth, numerical simulations based on a single broadband envelope equation with only quadratic nonlinearity give estimates for the conversion efficiency within approximately 1 order of magnitude over a wide range of experimental parameters. From this comparison between theory and experiment, we identify a dimensionless parameter capturing the competition between three-wave mixing and group-velocity walk-off of the harmonics that governs the cascaded HHG physics. We also gain insights into spectral optimization via tuning the waveguide poling profile and pump pulse parameters. These results can inform cascaded HHG in a range of different platforms. 

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2. Chireix amplifier with enhanced bandwidth using active load

   https://doi.org/10.1109/IEEE-IWS.2018.8400841  

   Roblin, Patrick; Chang, Hsiu-Chen (May 2018, 2018 IEEE MTT-S International Wireless Symposium (IWS))

   The use of an active load has been recently proposed for the realization of power-efficient broadband balanced amplifiers. The application of an active load to a dual-input Chireix amplifier is investigated in this paper for the purpose of increasing their bandwidth. An embedding device model is used to established the optimal non-Foster loads required for both the transistors to remain operating in class F as the operating frequency deviates from the center frequency. Given the transistors must operate with a constant voltage swing between backoff and peak, it is found necessary for the two transistors to operate with different load impedances as the frequency varies. The required load impedance and outphasing angles for the Chireix operation are obtained using a generalized eigenvalue problem using the Y-matrix of the Chireix combiner loaded with the active load. It is verified that using an active load, it is possible to maintain a high efficiency not only at peak power but also under various backoff power levels over a bandwidth of 1 GHz. Within a 200 MHz bandwidth, the PA is predicted to be able to maintain an efficiency larger than 79% for 6 dB backoff. Further work is required to experimentally validate the proposed technique. 

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3. Ultrafast 1  MHz vacuum-ultraviolet source via highly cascaded harmonic generation in negative-curvature hollow-core fibers

   https://doi.org/10.1364/OPTICA.395688  

   Couch, David_E; Hickstein, Daniel_D; Winters, David_G; Backus, Sterling_J; Kirchner, Matthew_S; Domingue, Scott_R; Ramirez, Jessica_J; Durfee, Charles_G; Murnane, Margaret_M; Kapteyn, Henry_C (July 2020, Optica)

   Vacuum-ultraviolet (VUV) light is critical for the study of molecules and materials, but the generation of femtosecond pulses in the VUV region at high repetition rates has proven difficult. Here we demonstrate the efficient generation of VUV light at megahertz repetition rates using highly cascaded four-wave mixing processes in a negative-curvature hollow-core fiber. Both even- and odd-order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with high energy resolution of $\sim <\#comment/>40\mathrm{m}\mathrm{e}\mathrm{V}$. In contrast to direct high harmonic generation, this highly cascaded harmonic generation process requires lower peak intensity and therefore can operate at higher repetition rates, driven by a robust $\sim <\#comment/>10\mathrm{W}$fiber-laser system in a compact setup. Additionally, we present numerical simulations that explore the fundamental capabilities and spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source can enhance the capabilities of spectroscopies of molecular and quantum materials, such as photoionization mass spectrometry and time-, angle-, and spin-resolved photoemission. 

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4. Fault location in microgrids: a communication‐based high‐frequency impedance approach

   https://doi.org/10.1049/iet-gtd.2018.5166  

   Beheshtaein, Siavash; Cuzner, Robert; Savaghebi, Mehdi; Golestan, Saeed; Guerrero, Josep_M (February 2019, IET Generation, Transmission & Distribution)

   This paper proposes a novel method to locate faults in an AC‐meshed microgrid. To this end, a set of features is first extracted and selected from the measured signals and fed to a Support Vector Machine (SVM) to detect the occurrence of fault. Then, the Distributed Generator (DG) with the lowest amount of fundamental voltage, which is the closest one to the fault, injects an appropriate voltage/current harmonic. As the faulted section has the lowest impedance value from the Point of Common Coupling of the DG, the harmonic current of the corresponding line has the highest value. Based on this fact, the first candidate DG sends a notification signal to the second candidate DG, in which the fault occurs between them. Finally, the impedances in the injected frequency are measured from these two DGs and fed into a multi‐class SVM to locate the faulted line. The proposed method has the ability to locate faults for islanded and grid‐connected microgrids with variable configurations. Real‐time simulation results are taken by OPAL‐RT to show the effectiveness of the proposed method in the meshed microgrid. 

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5. Nonlinear Dynamics of a Two Members Angle-Shaped Energy Harvester

   https://doi.org/10.1115/SMASIS2022-90623  

   Danzi, Francesco; Tao, Hongcheng; Joodaky, Amin; Gibert, James M. (September 2022, ASME 2022 Conference on Smart Materials, Adaptive Structures and Intelligent Systems)

   In this article we propose a theoretical investigation of the nonlinear dynamical response of a class of planar resonators dubbed the V-Shaped resonator. The resonators are intended for energy harvesting purpose and are designed to exhibit two-to-one internal resonance. In particular, we navigate the design space for the generalized V-shaped resonator to investigate the influence of shape parameters on the performance of the Vibration Energy Harvester. Notably, we introduce two metrics that help elucidating the role of the shape parameter in dictating the behavior of the system in terms of peak voltage and operational bandwidth width. For simplicity, we consider that the system is subjected to harmonic excitations near its primary resonances. 

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