Pump Wave Research Articles

Features of Narrowband Stimulated Electromagnetic Emission Depending on the Effective Radiated Power of the EISCAT/Heating Facility

Estimates of excitation thresholds and the analysis of spectral features of narrowband stimulated electromagnetic emission (NSEE) depending on the electric-field intensity of an extraordinary polarized HF pump wave have been carried out. They are based on results obtained during power stepping EISCAT/Heating experiments. The HF pump wave was radiated toward the magnetic zenith at frequency of 5.423 MHz. The effective radiated power was changed from 55 to 360 MW. NSEE was recorded in the vicinity of St. Petersburg at a distance of ~1200 km away of the EISCAT/Heating facility. Calculations of the electric field of a powerful HF radio wave near the reflection altitude taking into account the nondeviation absorption in the underlying layers were performed. The threshold (minimum) values of the electric field required for the NSEE excitation were determined.

1.76 μm on-chip gain characteristics of Tm3+ in a rib waveguide based on Tm3+-doped LiNbO3 thin-film on insulator

A theoretical study has been performed on 1.76 μm on-chip amplification characteristics of Tm3+ in a rib waveguide of LiNbO3 on insulator (Tm:LNOI) under 795 nm wavelength pumping. Steady-state rate equations and propagation equations of signal and pump waves are established on the basis of a three-level model of Tm3+ system with the cross relaxation process 3H4→3F4: 3H6→3F4 included. The validity of the three-level model is demonstrated by referencing previously reported experiment result of Er3+-doped LNOI waveguide amplifier and by examining the effect of Tm3+ blue upconversion emissions on the 1.76 μm amplification. Besides the relations of signal gain to propagation length, launched pump power and input signal power, the effects of waveguide cross-section geometry and Tm3+ concentration on the gain characteristics are also studied. The results show that the waveguide cross-section geometry affects definitely the gain performance via its effect on mode field. The Tm3+ concentration affects the signal gain also via its effect on the 3H4→3F4: 3H6→3F4 cross relaxation coefficient, besides the conventional effect that higher active ion concentration yields larger signal gain. A comparison with conventional Ti4+(Zn2+)-diffused Tm3+-doped LN [Ti(Zn):Tm:LN] waveguide shows that the Tm:LNOI waveguide has much better gain performance than the Ti(Zn):Tm:LN waveguide, including much stronger pump power and propagation distance dependences of signal gain, much higher signal gain and much lower threshold pump power. These features are associated with significantly increased Tm3+ population inversion extent due to ultra-compact mode field and larger overlapping factor of mode field and Tm3+ population profiles in the Tm:LNOI waveguide.

Low-noise phase-sensitive optical parametric amplifier with lossless local pump generation using a digital dither optical phase-locked loop.

The low noise figure of phase-sensitive amplifiers (PSAs) is attractive for optically pre-amplified measurement and communication systems. However, a major practical implementation difficulty pertains to the requirement of phase-locked signal, idler, and pump waves. Previously, injection locking to a co-propagating weak pump pilot or tapping portions of the received signal (lossy) for carrier re-generation have been used. Here we present a novel, lossless approach without any pump pilot, that generates a phase-locked receiver-local pump within the PSA using a digital dither-based optical phase-locked loop. We experimentally demonstrate a 2 dB noise figure with a low 0.3 dB penalty due to imperfect locking. By comparing the phase-locking performance in a PSA to that in a 50/50-coupler, we discuss and predict potential performance improvements connected to loop delay and laser phase characteristics.

Efficient coastal inundation early-warning system for low-lying atolls, dealing with lagoon and ocean side inundation in Tarawa, Kiribati

Tarawa is a low-lying atoll in the Gilbert Island group, capital of the Republic of Kiribati and home of nearly 70.000 inhabitants. With limited land area, rapid population growth and urbanization, strong interannual sea level variability induced by ENSO and sea level rise, Tarawa is highly vulnerable to coastal flooding. In this context, Early Warning Systems are a proven cost-effective climate adaptation measure to strengthen community resilience. In virtually enclosed atolls, the water level experienced at the shore is compounded by tides, sea level anomaly, storm surge and the contribution of waves. While wave setup and runup, are the primary components driving inundation along the ocean-facing shorelines, sea level anomaly, wind setup and wave pumping through the atoll rim contributes more inside lagoons. In this paper we present an efficient process-based approach to forecast flooding events along both, the ocean and lagoon coasts of atoll islands. With the intention of being highly scalable to other island countries, the system has been designed as a lightweight and accurate tool, that provides actionable and user-friendly water level predictions 7 days in advance. Publicly available global forecast products are ingested by a high-resolution wave model and tailor-made metamodels to translate ocean forcings to water levels at the shore. In absence of a comprehensive topography dataset, extreme value distributions of 27-year hourly water levels were evaluated every 500 m along the coast to define and communicate different levels of warnings according to the recurrence interval of the forecasted event. The long-term wave climate, nearshore wave transformation, and water levels produced under this work increase Tarawa's risk knowledge and support informed investments and future development strategies. While this system will significantly enhance ocean services in Kiribati, improving baseline data still remains a critical need for government and communities to support informed decision making to better cope with increased coastal hazards.

Numerical Analysis of Dual-Wavelength Tungsten-Tellurite Fiber Raman Lasers with Controllable Mode Switching

Fiber laser sources in the spectral range near 1.7–1.8 μm are in highly demand for a lot of applications. We propose and theoretically investigate a dual-wavelength switchable Raman tungsten-tellurite fiber laser in the 1.7–1.8 µm range which can produce two stable modes at frequencies separated by ~7 THz with a pump at 1.55 µm. The Raman waves shifted by 19.8 THz (mode 1) and 27.5 THz (mode 2) from the pump frequency can be generated near two different maxima of the Raman gain spectrum (gain is higher at 19.8 THz and twice lower at 27.5 THz). We numerically simulate two-mode Raman lasing with allowance for energy transfer from the pump wave to modes 1 and 2, and from mode 1 to mode 2 due to inelastic Raman scattering. Diagrams of generation regimes depending on system parameters are constructed. We demonstrate controlled switching between two modes by changing the pump power. For the same intracavity losses for both Raman modes at relatively low pump powers, only mode 1 is generated. At medium pump power, generation occurs simultaneously in both modes. At relatively high pump power, only mode 2 is generated near the weaker maximum. This effect seems surprising, but a rigorous explanation with allowance for the nonlinear interaction between mode 1 and mode 2 is found. When losses for one of the modes change, switching of the generated regimes is also predicted.

Impact of Optically Pumped Nonequilibrium Steady States on Luminescence Emission of Atomically Thin Semiconductor Excitons.

The interplay of the nonequivalent corners in the Brillouin zone of transition metal dichalcogenides (TMDCs) has been investigated extensively. While experimental and theoretical works contributed to a detailed understanding of the relaxation of selective optical excitations and the related relaxation rates, only limited microscopic descriptions of stationary experiments are available so far. Here we present microscopic calculations for the nonequilibrium steady state properties of excitons during continuous wave pumping exemplary for monolayer MoSe_{2}. We find sharp features in photoluminescence excitation spectra and degree of polarization which result from phonon assisted excitonic transitions dominating over exciton recombination and intervalley exchange coupling.

Six-wave interaction with double wavefront reversal in multimode waveguides with Kerr and thermal nonlinearities

Spatial selectivity of six-wave radiation converters, which perform double wavefront conjugation of a signal wave in long multimode waveguides with both Kerr and thermal nonlinearities, is studied. Waveguides with infinitely conductive surfaces, with a parabolic refractive index profile, were used. It is shown that the spatial structure of the first pump wave does not affect the quality of doubled wavefront conjugation in a waveguide with Kerr nonlinearity, but only slightly affects the quality of doubled wavefront conjugation in a waveguide with thermal nonlinearity. A decrease in the radius of the second Gaussian pump wave on the back face of the waveguide leads to an improvement in the quality of the doubled wavefront reversal both in the case of six-wave interaction in the Kerr and thermal nonlinearities. In a parabolic waveguide, when the zero mode of the waveguide is excited by pump waves at a constant frequency of the second pump wave, an increase in the frequency of the first pump wave worsens the quality of the double wavefront conjugation.

Stimulated Raman scattering by circularly polarized quadruple Gaussian laser beam and co-propagating electron beam in plasma

This study investigates the interplay between a co-propagating relativistic electron beam and a quadruple Gaussian laser beam in plasma, focusing on the suppression of stimulated Raman scattering (SRS) growth. The presence of the laser beam induces the excitation of a pair of plasma waves and side-scattered electromagnetic waves. As the side-scattered wave and pump wave couple together, they exert a ponderomotive force on the electron beam and plasma electrons, resulting in an enhancement of the plasma wave amplitude. Nonlinear coupling between the density perturbation in the plasma, induced by the plasma wave, and the pump wave leads to the excitation of a nonlinear current responsible for the growth of the side-scattered electromagnetic wave associated with SRS. Furthermore, the growth rate of SRS is shown to be highly sensitive to the phase matching between the relativistic electron beam and the plasma wave. In cases of phase mismatch, the growth rate experiences a significant reduction. Additionally, the effectiveness of the electron beam in driving the stimulated Raman process is greatly affected by the energy spread of the electron beam. A substantial reduction in effectiveness is observed due to this energy spread.

On the longitudinal wave pumping in fluid-filled compliant tubes

This study investigates the physics of the longitudinal stretching-based wave pumping mechanism, a novel extension of the traditional impedance pump. In its simplest form, an impedance pump consists of a fluid-filled elastic tube connected to rigid tubes with a wave generator. These valveless pumps operate based on the principles of wave propagation in a fluid-filled compliant tube. Cardiovascular magnetic resonance imaging of the human circulatory system has shown substantial stretching of the aorta (the largest compliant artery of the body carrying blood) during the heart contraction and recoil of the aorta during the relaxation. Inspired by this dynamic mechanism, a comprehensive analysis of a longitudinal impedance pump is conducted in this study where waves are generated by stretching of the elastic wall and its recoil. We developed a fully coupled fluid–structure interaction computational model consisting of a straight fluid-filled elastic tube with longitudinal stretch at one end and a fixed reflection site at the other end. The pump's behavior is quantified as a function of stretching frequency and tube wall characteristics. Our results indicate that stretch-related wave propagation and reflection can induce frequency-dependent pumping. Findings suggest a non-linear pattern for the mean flow–frequency relationship. Based on the analysis of the propagated waveforms, the underlying physical mechanism in the longitudinal impedance pump is discussed. It is shown that both the direction and magnitude of the net flow strongly depend on the wave characteristics. These findings provide a fundamental understanding of stretch-related wave pumping and can inform the future design of such pumps.

A simulation study of electron-scale turbulence generation by nonlinear wave–wave interaction in laser-produced plasmas

A laboratory simulation of a model based on the nonlinear interaction of plasma waves is presented to study the electron-scale magnetic turbulence in the magnetized plasma. In this perspective, the model equations of the Trivelpiece–Gould (TG) mode and extraordinary mode (pump wave) are developed, taking the relativistic change in electron mass and the ponderomotive force into account. Laboratory simulations utilizing the pseudo-spectral method along with the predictor–corrector scheme and finite difference method are performed to solve the formulated coupled model equations. The propagation angle (θ) of the TG mode from the magnetic field affects the dispersive properties of the dynamics (TG mode), which, in turn, impact the density perturbation, scale size of the filamentary structures, density harmonics, magnetic field enhancement, and the spectral index of the turbulence generation. The simulation results reveal that the observed turbulent spectra resemble the magnetic turbulence reported in various studies of the interaction of the intense laser with plasma at the laboratory astrophysics scale relevant to astrophysical events. A simplified model in the paraxial limit is also given to understand the effect of the propagation direction (angle of propagation θ) of the TG mode on the localized structures of the pump laser beam in the magnetized plasma.

Ballistic dynamics of emergent soliton from Airy pulse in a medium with linear optical potential

We numerically investigate the propagation dynamics of Airy pulse in dispersive and nonlinear medium with time-dependent external optical potential. These time-dependent optical potentials are created by the co-propagating strong pump wave. In the presence of linear optical potential, soliton shedding from signal Airy pulse is observed, which demonstrates ballistic motion. However, emergent soliton can either be advanced or delayed depending upon the sign of the potential and the extent of temporal advancement or delay is directly proportional to the magnitude of the potential strength. In addition, the temporal shift of the emergent soliton is found to be dependent on the input pulse power. Moreover, we show ballistic motion of the emergent soliton by incorporating an initial linear phase as well as linear chirp (quadratic phase) parameters in the input Airy pulse. Furthermore, the effect of truncation factor on the propagation dynamics is explored. These results provide new insights on the manipulation of the signal Airy pulse dynamics in dispersive and nonlinear media.

Parametric excitation of electron acoustic wave by Langmuir wave in the Earth’s inner magnetospheres

A kinetic model for the parametric decay of Langmuir wave (LW) into sideband LW and electron acoustic wave (EAW) in plasmas containing hot and cold electrons is developed. The parametric instability for LW decaying into EAW in the Earth’s inner magnetosphere is numerically analyzed. The maximum growth rate and the range of wavenumber for the existence of parametric instability increase with the increase of pump wave strength and hot electron concentration. The frequency of excited EAW corresponding to the maximum grow rate goes up as the enhancement of pump wave strength, but lowers as the reduction of hot electron concentration. The frequencies of excited EAW are comparable to the observed frequencies of whistler wave in the inner magnetosphere. The growth rate will be about a few tens per second when pump LWs is strong enough. The results obtained may be helpful for the explanation of the observed electrostatic field spectrum in the inner magnetosphere.

Vibro-acoustic modulation-based bolt looseness detection method using frequency sweep probe waves

ABSTRACT Traditional vibro-acoustic modulation methods for bolt looseness detection often use a probe wave with single frequency. However, using a single frequency probe wave not only reduces the detection accuracy but also increases the difficulty of selecting the frequency of the probe wave. To overcome these drawbacks, this paper proposes a bolt looseness detection method based the vibro-acoustic modulation using a frequency sweep probe wave. For the low-frequency pump wave, the excitation frequency is set as the resonant frequency of the bolted joint to enhance the modulation effect. For the high-frequency probe wave, a frequency sweep excitation is used to replace the single frequency excitation, which improves the detection accuracy and overcomes the difficulty of selecting the probe frequency. A signal processing method is proposed to extract modulation information from the complex response signal obtained under a frequency sweep excitation. A nonlinear modulation index is defined using the extracted modulation information to quantify the loosening level of bolts. An experimental setup is designed to implement the vibro-acoustic modulation detection method on a bolted joint. Experimental results demonstrate that the proposed method is effective for detecting bolt looseness and has high detection sensitivity and accuracy for the early bolt loosening.

Detection of barely visible impact damage in composite plates using non-linear pump-probe technique

In the last decades, composite materials have been increasingly used in aircraft structures as a mean to reduce the weight. For safety reasons, periodic checks of impact-induced damage need to be performed. In that framework, ultrasound non destructive testing has proven its ability to detect macro-defects such as delamination. However, at early stages, Barely Visible Impact Damage (BVID) may not be detected via linear ultrasound techniques. In recent years, non-linear ultrasound has gained traction for the inspection of such defects. However, detection of the weak non-linear defect signature buried in ambient noise remains challenging, and depends on an empirical choice of excitation frequencies. This study focuses on the evaluation of a non-linear pump probe ultrasound inspection technique applied to BVID in multilayer carbon fibre reinforced composites used in aeronautics. In particular, the influence of various experimental parameters on the measured non-linear response is studied. A low frequency pump wave (via a shaker) and a high frequency wave (via a piezoelectric transducer) are transmitted to the medium while a second transducer records the ultrasonic response. Both pump and probe waves are sent in the form of a few secondslong frequency sweep, in the audible and ultrasonic range respectively. Appropriate post processing is then applied to the recorded signals to retrieve the non-linear response of the defect. The procedure is repeated on several composite plates with and without impact damages. The nonlinear response appears in the form of modulation sidebands in the frequency spectrum of the post-processed signals. The relative amplitudes of the side bands obtained for the various samples with various defect sizes and excitation amplitude or frequency content are studied. Optimal experimental parameters were obtained and led to a good detection of defects. The limits of the method are also discussed.

Improving the Accuracy and Resolution of Filter- and Frequency-to-Time Mapping-Based Time and Frequency Acquisition Methods by Broadening the Filter Bandwidth

In this article, a method for improving the accuracy and resolution of the filter-and frequency-to-time mapping (FTTM)-based time and frequency acquisition system in the fast sweep scenario is proposed by broadening the filter bandwidth. It is found that the width of the generated pulse via FTTM is mainly determined by the impulse response of the filter when the sweep rate is very fast. Therefore, appropriately increasing the filter bandwidth can significantly reduce the pulsewidth, so as to improve the measurement accuracy and resolution. To verify the validity, short-time Fourier transform (STFT) and microwave frequency measurement (MFM) systems employing the stimulated Brillouin scattering (SBS)-based FTTM are further investigated by inserting a frequency-sweep pump wave for SBS gain bandwidth broadening. The improvement of accuracy and frequency resolution is well confirmed by comparing the measurement results with and without SBS gain spectrum broadening. The MFM accuracy is improved by more than ten times compared with other reported works under a similar fast sweep rate, and the frequency resolution of the STFT is also much improved compared with our former results.

On the Issue of Possibility of Reducing Anomalous Absorption of Extraordinary Pump Wave in a Wide Range of Plasma Densities

On the Issue of Possibility of Reducing Anomalous Absorption of Extraordinary Pump Wave in a Wide Range of Plasma Densities

Smart patterning for topological pumping of elastic surface waves.

Topological pumping allows waves to navigate a sample undisturbed by disorders and defects. We demonstrate this phenomenon with elastic surface waves by strategically patterning an elastic surface to create a synthetic dimension. The surface is decorated with arrays of resonating pillars that are connected by spatially slow-varying coupling bridges and support eigenmodes located below the sound cone. We establish a connection between the collective dynamics of the pillars and that of electrons in a magnetic field by developing a tight-binding model and a WKB (Wentzel-Kramers-Brillouin) analysis. This enables us to predict the topological pumping pattern, which we validate through numerical and experimental steering of waves from one edge to the other. Furthermore, we observe the immune nature of the topologically pumped surface waves to disorder and defects. The combination of surface patterning and WKB analysis provides a versatile platform for controlling surface waves and exploring topological matter in higher dimensions.

Stimulated Raman side and backscatter instabilities of crossed laser beams in plasma

Stimulated Raman scattering (SRS) instability due to two nonlinearly coupled laser beams propagating in homogeneous, thermal plasma is analyzed. The temporal evolution equations for pump, scattered and Langmuir waves excited due to the interaction are set up. Using numerical techniques temporal evolution of amplitudes of pump, scattered and resultant Langmuir waves for side and back SRS instabilities have been obtained in the non-relativistic regime. The temporal growth rate, saturation time and saturation amplitude of waves excited by two crossed (perpendicular) lasers are studied and compared with the single beam case. The model considers pump depletion to be the dominant saturation mechanism. The study points towards a new concept for generation of high amplitude Langmuir waves.

Implications of liquid impurities filled in breaking cracks on nonlinear acoustic modulation response: Mechanisms, phenomena and potential applications

Supervised strategies for ultrasonic-based cracks detection in existing structures has been one of particular concerns in the engineering community. However, current researches mostly focus on the clean microcracks, while ignoring the fact that such cracks will be contaminated with liquid impurities (lubricants, silicone oil, etc.) due to its exposure to the operational environment. Actually, the presence of liquid may lead to a non-negligible change in the acoustic performance and interfere with damage diagnosis. The central contribution of this work is to reveal the nonlinear acoustic mechanisms and phenomena induced by the gas–liquid–solid coupling interfaces in such liquid-filled cracks and investigate their potential applications for such crack detection. Firstly, the state equation for a liquid-filled cavity in beams is established to determine the nonlinear dependences of stress–strain relationship at crack wall on the viscous and capillary pressure in liquid. Secondly, a modified acoustic modulation method, in which swept signal is used as the pumping wave to sufficiently trigger and enhance the nonlinear behaviours at the gas–liquid–solid interfaces without requiring exact reference information, is utilized for detecting sub-millimeter cracks filled with different liquid. The results suggest that the liquid fillers may cause considerable increase of the damage-related acoustic nonlinearities involving hysteresis nonlinearity, nonlinear elastic behavior and dissipative nonlinearity. Additional analysis provides compelling evidence for the following valuable conclusions: 1) the capillary pressure is a key factor that promotes conducive to the nonlinear elastic behavior that is the primary source of the first-order sum-difference frequency; 2) the viscous pressure contributes to hysteresis as well as the nonlinear dissipation, and the latter promotes the second-order sum frequency. Further, such nonlinear performance especially dissipation offer promising prospects for nonlinear acoustic diagnostic of such cracks. All results obtained in this study will aid in understanding the nonlinear wave processes and further improving the reliability of crack detection in engineering.

Four-wave mixing on thermal and resonant nonlinearities with feedback for object and signal waves

Background. The need to create highly efficient four-wave radiation converters in order to use them in adaptive optics systems, for real-time processing of complex spatio-temporal fields, in interferometry, quantum cryptography, etc. requires along with the use of traditional methods to increase the efficiency of such converters by increasing the interaction length, radiation power density, effective value of the nonlinear susceptibility of the development of new methods. One of these methods is a method based on the imposition of feedback on one or more interacting waves. Aim. The influence of feedback on the amplitude reflection coefficient of the degenerate four-wave radiation converter on thermal and resonant nonlinearities has been considered. Methods. The amplitude reflection coefficient of a degenerate four-wave radiation converter on thermal and resonant nonlinearities in the presence of feedback for both object and signal waves has been analyzed by a numerical method based on the multiple passage of the signal and object waves of the nonlinear layer in the ring resonator. Results. It was shown, that the difference in the reflection coefficients in the presence and absence of feedback for both object and signal waves increase monotonically with a growth in the pumping waves intensity and does not depend on the parameter characterizing the relationship between thermal and resonant nonlinearities at the approximation of a small reflection coefficient. Conclusion. The range of values of the absorption coefficient in which the imposition of feedback for both object and signal waves leads to an increase in the efficiency of the four-wave radiation converter has been established.