Harmonic Pulse Research Articles

Spectrum reconstruction of experimentally produced nano-colloids using Mie theory

Aim: Synthesis of plasmonic nanoparticles, characterization, size detection by modeling. Methods: Colloidal plasmonic gold and silver nanoparticles were prepared by laser ablation with a 1,064 nm pulsed Nd:YAG laser with equal fractional volume and then were illuminated with its 532 nm second harmonic pulse. After the illumination process, we observed 1 nm blue shift in peak position of gold colloid and 4 nm blue shift in silver colloid. We observed a variation in the dielectric function due to nanoparticle size reduction in both samples. Using micrograph, size distribution was plotted and with the help of Mie theory and size dependent dielectric function, we reconstructed absorption spectrum to best fit the experimental spectrum and we estimated 12- and 16-fold increase in the number of Au and Ag nanoparticles respectively, due to illumination. Results: We have estimated size distribution of produced nanoparticles. Conclusions: We produced silver and gold colloids with ablation of their foils in water without any surfactant, and then we fragmented the nanoparticles colloids with an intense nanosecond laser and studied the effect of illumination on peak position and size distribution of colloids.

41.6nm/44 fs Mamyshev Yb-doped fiber laser with bound-state solitons and multiple harmonics

A circular Mamyshev Yb-doped fiber laser was experimentally designed, and multiple pulse modes, including single-pulse, bound-state pulse, and harmonic mode-locked pulse, were observed by modulating the seed source and pump power. Single-pulse mode-locking with a single-pulse energy of 68.6 nJ was demonstrated at a repetition frequency of 12.1 MHz with a spectral half-height width of 41.6 nm and a compressed pulse duration of 44 fs. Results were also obtained for bound-state and harmonic mode-locked pulses, both with post-compression pulse durations of less than 100 fs. The experiments provide validation of the dynamics of a wide range of pulses in Mamyshev lasers. These findings have implications for the further exploration and understanding of pulse dynamics within the optical domain.

Control Performance Assessment of a Zero Harmonic Distortion Grid-Forming Converter for Medium Voltage Islanded Microgrids

The world is currently witnessing a rapid transformation in the production and utilization of electrical energy. The traditional centralized generation model for electric power is swiftly evolving into a more decentralized system, known as Distributed Generation (DG), which incorporates renewable energy sources situated closer to end-users. This shift towards DG has paved the way for the emergence of grid-forming converters, which play a pivotal role in enhancing voltage and frequency stability within microgrids (MGs) and isolated applications. This study focuses on assessing the performance of the Zero Harmonic Distortion (ZHD) in both stand-alone and parallel operation modes. The distinctive feature of this converter lies in its inherent ability to generate a sinusoidal voltage source without the need for capacitive filtering components, which can adversely affect cost, efficiency, and size while potentially contributing to resonance problems. This is achieved through a judicious combination of harmonic cancellation within a three-winding transformer and the utilization of Selective Harmonic Elimination Pulse Width Modulation (SHE PWM) dismissing a closed-loop control structure. Simulation and hardware-in-the-loop results presented in this work demonstrate the satisfactory performance of the ZHD grid-forming converter.

The influence of resonant light pulses on high harmonic generation in solids

Abstract We have theoretically studied the high harmonic generation (HHG) in solids driven simultaneously by a mid-infrared (MIR) laser and a high-order harmonic pulse with energy around the band gap between the valence band and conduction band. By adding this resonant harmonic light pulse with the relative intensity ratio of 4\%, the high-order harmonic emission from the crystal is enhanced by 1-2 orders of magnitude. The yield of HHG in solid increases monotonically with the relative strength of the resonant harmonic pulses. In addition, we also found that HHG dynamics from the $k$ channel around the boundary of the Brillouin zone can be selectively enhanced by adjusting the frequency of the resonant high-order harmonic pulse. The resonance-enhanced HHG and $k$ channel selection effect in solids is also investigated by using the three-band semi-conductor Bloch equation for HHG in ZnO. We also find that the harmonic in the plateau region driven by adding a resonant light field to the strong MIR driving field has less red-shifted compared with the case driven by the MIR driving field alone.

A Novel NR-DA-Based ANN for SHEPWM in Cascaded Multilevel Inverters for Renewable Energy Applications

Harmonics is the major power quality problem caused by nonlinear devices, leading to malfunction or operational halt of the system. Low-order harmonics increase vibration and heat generation in motors. Therefore, controlling harmonics in the output waveform is the prime target for industrial applications to avoid economic loss. Selective harmonic elimination pulse width modulation (SHEPWM) is one of the techniques used for eliminating or minimizing selected harmonics in the output voltage waveform. This paper utilizes the Newton-Raphson method and Dragonfly Algorithms to calculate optimum switching angles for a Cascaded H-bridge Multilevel inverter (CHBMLI). The algorithms use non-linear equations to calculate the Switching Angles of MLI. The Dragonfly algorithm requires several iterations to reach an optimum solution. For complex problems, this algorithm becomes computationally expensive and time-consuming. A lookup table addresses the limitation by offline training an Artificial Neural Network (ANN) to generate the optimum switching angle for a given modulation index. Neural Fitting Tool in MATLAB software is used to train the ANN model. The simulation is performed using MATLAB SIMULINK software for both 5-level and 7-level CHBMLI configuration. The Dragonfly algorithm-based ANN achieves THD 8.84% when the modulation index (M) equals 0.8 for a 7-level inverter and THD 14.91% for a 5-level inverter and effectively minimizes third and fifth-order harmonics.

Highly Circularly Polarized Monochromatic High Harmonic Pulses via a Reflective Phase Retarder at 27.9 eV

Highly Circularly Polarized Monochromatic High Harmonic Pulses via a Reflective Phase Retarder at 27.9 eV

Passively harmonic mode-locked GHz U-band femtosecond pulse generation based on soliton self-frequency shift.

We demonstrate a GHz U-band fiber laser harnessing soliton self-frequency shift (SSFS). The seed source is a passively harmonic mode-locked (HML) fiber laser based on carbon nanotubes (CNTs) polymer film. By adjusting the pump power and polarization controller (PC) appropriately, the repetition rate can be tuned up to 1.104 GHz with a side mode suppression ratio (SMSR) of 42.5 dB. The seed source with different repetition rates can be frequency shifted from C-band to U-band in a piece of dispersion-shifted fiber (DSF) after amplification. We selected three repetition rates (457.8 MHz, 700.1 MHz, and 1104.1 MHz) and systematically studied the impact of different repetition rates on SSFS. The broad tuning range of wavelengths covering the entire U-band can be obtained by adjusting the pump power of the main amplifier. To the best of our knowledge, the demonstrated laser is the first U-band femtosecond fiber laser based on passively HML with a repetition rate over 1 GHz.

Generation of sub-10-fs deep and extreme ultraviolet pulses for time-resolved photoemission spectroscopy.

We present a light source capable of generating sub-10-fs deep UV (DUV) and extreme UV (EUV) pulses for use in time-resolved photoemission spectroscopy. The fundamental output of a Ti:sapphire laser was compressed using the multi-plate method and mixed with the uncompressed second harmonic in a filamentation four-wave mixing process to generate sub-10-fs DUV pulses. Sub-10-fs EUV pulses were generated via high-order harmonic generation driven by the second harmonic pulses that were compressed using Ar gas and chirped mirrors. The minimum cross correlation time between 267 and 57 nm (corresponding to 21.7 eV) was measured to be 10.6 ± 0.4 fs.

Single-shot complete characterization of synthesized laser pulses and the nonlinear frequency-conversion process.

High-energy-synthesized laser pulses through a nonlinear frequency-conversion process with different characteristics, such as polarization, central wavelength, and pulse duration, play important roles in materials science, high-energy physics, and ultrafast optics. In this study, we present an improved transient-grating frequency-resolved optical gating based on a self-referenced and reflective structure, which enables the single-shot complete measurement of complex high-power synthesized laser pulses in the broadband range and analysis of the nonlinear frequency-conversion process of ultrashort pulses. The waveform/spectrum evolution of both the fundamental and second harmonic pulses in a nonlinear frequency-conversion process with different injected energies was studied for the first time using this method. Moreover, the method was numerically and experimentally verified to be able to completely characterize double pulses with spectral and temporal separation, including the relative phase between the two components. This method has considerable potential for studying the complex physical processes of high-power synthesized laser fields.

High signal-to-noise ratio harmonic mode-locking Mamyshev oscillator at 1550 nm

Harmonic mode-locking is the effective method to improve ultrafast pulse repetition rate for the current research. However, this way of increasing the repetition rate based on pulse splitting has inherent instability characteristics, manifested as low signal-to-noise ratio and loss of mode-locking. In this paper, Mamyshev regenerative oscillation mode-locking technique is used to effectively improve the signal-to-noise ratio of pulses by compressing the nonlinear chirp in the cavity under the condition of near-zero dispersion. In the experiment, we successfully achieved a harmonic mode-locked pulse of up to 16 orders, corresponding to a repetition rate of 78.56 MHz, and the signal-to-noise ratio of the highest harmonic order reached 70 dB. Our research not only solves the inherent defect of low repetition rate of Mamyshev oscillator, but also effectively improves the signal-to-noise ratio of harmonic mode-locked pulse. This mode-locked structure based on Mamyshev oscillator provides reference and operability for improving the stability of ultrafast fiber lasers.

Second-harmonic generation of ultrashort pulses in refractive index linearly modulating quantum dot structure

While other works use segments of nonlinear crystal to compensate for walk-off, this work utilizes a quantum dot (QD) nanostructure for such compensation. Such nanocrystal structures allow two benefits: the high nonlinear susceptibility in these nanocrystals and the high surface density of QDs in the structure. Good results are obtained for pre-chirping and modulation parameters. The results are computed at 10 and 100 kV/cm applied electric fields. Compared to ordinary nonlinear crystals, the pulse width, intensity, and efficiency of the second harmonic (SH) pulse are all higher in QD nanocrystals, even with a low electric field applied. Higher applied fields give higher results.

New control scheme for a dynamic voltage restorer based on selective harmonic injection technique with repetitive controller

Repetitive controller and selective harmonic injection technique (SHI) in medium and low voltage distribution networks improve dynamic voltage restorer (DVR) DC bus voltages as well as nullify power quality (PQ) problems. DVRs use sinusoidal pulse width modulation (SPWM) firing control, but DC bus use seems to be limited, affecting density, cost, and power packaging. By adding 1/6th of the 3rd harmonic waveform to the basic waveform, SPWM yields the developed model. According to the findings, 15% of DC bus usage improves and produces high voltage AC. Nevertheless, just control systems perturb PQ. The proposed controller uses feed forward and feedback to enhance transient response and justify stable zero error. 3rd third harmonic injection pulse width modulation (THIPWM) improves total harmonic distortion (THD) in the proposed scheme. Power system computer aided design (PSCAD) simulation produced high accuracy for THIPWM and repetitive controllers.

Post-compression of powerful femtosecond pulses after second harmonic generation

A proof-of-principle experiment of highly efficient (38%) second harmonic generation was implemented at a ∼1TW/cm2 intensity of a 45 fs pulse in a composite nonlinear sample–a 1 mm KDP crystal glued onto a 1 mm fused silica substrate. The attenuated replica of the second harmonic pulse (455 nm) was compressed down to 28.6 fs by means of dispersive mirrors, with a significant reduction in both the wings and the far temporal contrast. The peak power of the second harmonic was ∼74% of the power of the fundamental harmonic, which ensured a three-fold increase in the focal intensity.

A Novel Second Harmonic Voltage Suppression Control for PSFB Converter in Dual-Stage Single-Phase Rectifier

The inherent second harmonic power pulse in single-phase grid-connected rectifiers leads to a noticeable output voltage ripple and thus results in the degradation of the system. A novel second harmonic suppression control is introduced in this paper to address this issue. The key point of the proposed control lies in the real-time prediction of the phase-shifted full-bridge (PSFB) converter’s desired duty cycle through accurate PSFB converter modeling. By accurately predicting the ideal duty cycle, the proposed control facilitates a significant enhancement in the control loop’s gain specifically at the second harmonic frequency, thereby improving the overall system performance. To validate the theoretical analysis, a series of simulations and experiments were conducted. The results demonstrate the effectiveness of the proposed second harmonic suppression control in mitigating second harmonic output voltage ripple.

Harmonic domain-wall dark pulse generation using unpumped thin-core thulium fiber as interferometer

In this work, the harmonic domain-wall dark pulse is experimentally demonstrated in C-band fiber laser. The domain-wall dark pulse was self-started at pump power of 112 mW. The fundamental dark pulse repetition rate was measured as 0.96 MHz, pulse duration of 250 ns and highest average pulse energy of 3.24 nJ. As the power increased, dark pulses up to the 7th harmonic order were visualized and quantified. Pulse temporal width reduced as harmonic order increased, with the minimum pulse width of 60 ns recorded in the 7th order. The harmonic dark pulse fiber laser demonstrated adequate stability, with signal-to-noise ratios above 50 dB throughout all harmonic operations. The endeavor of harmonic dark pulse dark soliton could lay the groundwork for future sustainable industrial growth.

Generation of high-energy, sub-20 fs deep-UV pulses in a twin-crystal third harmonic generation scheme.

Ultrashort deep ultraviolet (DUV) pulses serve as indispensable tools for investigating molecular dynamics on the femtosecond scale. Nonlinear frequency upconversion of near-infrared (NIR) light sources in a sequence of nonlinear crystals is a common method for their generation. However, preserving the temporal duration of the starting source encounters challenges owing to phase-matching bandwidth limitations within the harmonic generation process. Here we propose an approach for circumventing this limitation and demonstrate it for the case of generation of the third harmonic of 800 nm pulses in a two-stage scheme (second harmonic generation succeeded by sum-frequency mixing of the fundamental and second harmonic pulses). Expanding the bandwidth of the DUV pulse involves the utilization for the last mixing process of two nonlinear crystals, detuned to convert opposite sides of the spectrum. The implementation of this approach yields 20 µJ, 263 nm DUV pulses as short as 19 fs after compression. The setup is very compact and extremely stable due to the common-path scheme, which makes it very interesting for a variety of advanced ultrafast spectroscopy applications.

Efficient prediction of attosecond two-colour pulses from an X-ray free-electron laser with machine learning

X-ray free-electron lasers are sources of coherent, high-intensity X-rays with numerous applications in ultra-fast measurements and dynamic structural imaging. Due to the stochastic nature of the self-amplified spontaneous emission process and the difficulty in controlling injection of electrons, output pulses exhibit significant noise and limited temporal coherence. Standard measurement techniques used for characterizing two-coloured X-ray pulses are challenging, as they are either invasive or diagnostically expensive. In this work, we employ machine learning methods such as neural networks and decision trees to predict the central photon energies of pairs of attosecond fundamental and second harmonic pulses using parameters that are easily recorded at the high-repetition rate of a single shot. Using real experimental data, we apply a detailed feature analysis on the input parameters while optimizing the training time of the machine learning methods. Our predictive models are able to make predictions of central photon energy for one of the pulses without measuring the other pulse, thereby leveraging the use of the spectrometer without having to extend its detection window. We anticipate applications in X-ray spectroscopy using XFELs, such as in time-resolved X-ray absorption and photoemission spectroscopy, where improved measurement of input spectra will lead to better experimental outcomes.

Harmonic Elimination Using Grid Integrated Hybrid Renewable Energy System with Optimized Control for Nano Scale Based 31-Level MLI

This research focuses integration of grid integrated hybrid renewable energy system (HRES) with optimized control techniques for harmonic elimination using Nano Scale Based 31-Level Multi-Level Inverter (MLI). The proposed system combines Renewable Energy Sources (RES), such as Solar Photovoltaic (PV) and Wind Energy Conversion System (WECS) assisted with Doubly Fed Induction Generator (DFIG) to generate power for grid integration. Interleaved KY converter with novel Rain Optimized Algorithm assisted Artificial Neural Network (ROA-ANN) based Maximum Power Point Tracking (MPPT) is established, which is a hybridized approach extracting maximum PV power and optimizes power output for utilization. Excess energy generated from hybrid power sources is utilized to charge battery. The DC supply from hybrid sources is transferred to silicon carbide switches of 31 MLI for elimination of harmonics using Selective Harmonic Elimination Pulse Width Modulation (SHE-PWM). The 31-Level MLI facilitates conversion of DC voltage to AC with assistance of Particle Swarm Optimized Proportional Integral (PSO-PI) controller. Finally, controlled AC supply is transferred to LC filter, resulting in effective grid synchronization with reduced harmonics. The proposed system’s performance is validated using MATLAB Simulink, and obtained results demonstrate effectiveness with reduced total harmonic distortion (THD) of 0.66% in contrast to state-of-art approaches

Wind Energy Conversion System using Cascading H-Bridge Multilevel Inverter in High Ripple Scenario

This paper presents wind energy conversion system using CHB MLI and phase interleaved boost converter to overcome high voltage and current ripple. Developments in power electronics technology have a direct impact on advances in wind energy conversion systems. WECS output voltage may fluctuate depending on wind speed. For WECS to maintain a constant output voltage, a power converter is required. This paper explains how to configure a phase-interleaved boost converter and voltage controller to maintain a stable intermediate circuit voltage in the system. The proposed cascading H-bridge multilevel inverter (CHB MLI) converts DC/AC using a novel topology. Separate DC sources are not utilizing the suggested topology. Multi-level inverters are operated by specific harmonic disposal pulse width modulation, or SHE-PWM are utilized. PMSG voltage, CHB-MLI voltage, boost converter voltage and rotor speed have seven different levels of simulated waveforms. Among the many advantages of three-phase alternating DC-DC boost converters, that are high efficiency, fast dynamics and very low current ripple. The output voltage is increased to 400 V using a three-phase interleaving converter, which also maintains a higher efficiency of about 98%. DC-DC power converters have proven to be essential components of many applications and topologies. The interleaving technique is necessary to address the main disadvantages of DC-DC power converters: increased voltage, reduced efficiency, and rippled current. Interleaved boost converters have several advantages, including lower switching losses, lower voltage and current ripple, increased efficiency, and more. To improve the converter's overall functionality, the "n" parallel converter is connected through an interleaved boost converter. This paper presents a performance analysis of a multiphase alternating boost converter to reduce high voltage and current ripple. MATLAB/Simulink is used for analysis and simulation of phase-interleaved boost converters.

Preparation of amorphous silicon-doped Y2O3 aerogel enabling nonlinear optical features for ultrafast photonics

Abstract Amorphous aerogels with the microscopic nanoscale three-dimensional meshes provide superb platforms for investigating unique physicochemical properties. In order to enhance the physical, thermal and mechanical performances, one efficient and common approach is integrating diverse functional materials. Herein, we report a simple strategy to fabricate the amorphous silicon doped Y2O3 aerogels with the post-gelation method under the N2/EtOH supercritical atmosphere. The impact of Si concentration on the nonlinear optical properties is investigated for the first time. The maximum modulation depth is 1.65 % with a saturation intensity of 0.78 MW cm−2 with the 1-ps laser excitation at 1590 nm. Finally, we incorporated the silicon-doped Y2O3 aerogel based saturable absorber (SA) into an erbium-doped fiber laser (EDFL) and achieved various mode-locking operations at different wavelengths in the super C band, in terms of the conventional soliton, harmonic soliton molecules pulses, and dual-wavelength soliton mode-locking. Overall, this work confirms that amorphous silicon-doped Y2O3 aerogels are good nonlinear optical materials and pave a way for the ultrafast photonic and nonlinear optical applications with amorphous materials in near future.