Pump Wave Research Articles

Growth of stimulated Raman instability by a density-modulated electron beam in laser-plasma interactions

A way to enhance the growth of stimulated Raman instability in laser-plasma interactions was investigated. This relies on the application of density modulation of a co-propagating electron beam in plasmas. In the stimulated Raman scattering process, an electromagnetic pump wave decays into a low-frequency wave and a scattered electromagnetic sideband wave. In this process, the pump wave produces an oscillatory velocity associated with the plasma electrons and the beam electrons. These oscillatory velocities combine with the existing low-frequency mode, producing ponderomotive force that drives high-frequency sideband waves. The sidebands couple to the pump wave, driving the beam-mode. A modulation of the electron beam density enhances the growth rate of the instability. The theoretical calculations show about 40% enhancements in growth of Raman instability at resonance (where the electron beam density modulation parameter approaches to unity) for the plasma density of the order of 1018 cm−3.

Impact of High-Power Cosh-Gaussian Beam on Second Harmonic Generation in Collisionless Magnetoplasma

The impact of high power Cosh-Gaussian (ChG) beam on Second harmonic generation (SHG) in Collisionless magnetoplasma is explored in present work. Whenever the input beam propagates along external magnetic magnetic field direction, then there are two propagation modes viz. extraordinary mode and ordinary mode. The modification in magnetic field strength causes redistribution of carriers. The density gradients get established in plasma in a normal direction to input wave due to ponderomotive force. Further, there is production of electron plasma wave (EPW) at input wave frequency due to density gradients. EPW nonlinearly interacts with pump wave causing generation of 2nd harmonics. The 2nd order differential equation (ODE) for beam width of and efficiency of 2nd harmonics are derived through well-known paraxial theory approach. RK4 method is employed for carrying out numerical calculation of nonlinear ODE along with efficiency of 2nd harmonics. Impact of change in selective laserplasma parameters and externally applied magnetic field on beam waist of input wave and efficiency of SHG are also explored.

Low-noise 2-GHz figure-9 fiber laser based on passive harmonic mode-locking.

We have proposed and demonstrated the generation of a high-repetition-rate ultrashort pulse with long-term stability and low noise based on a harmonic mode-locked (HML) figure-9 fiber laser. Different HML orders from the 2nd to the 13th are generated by adjusting the pump power, net dispersion, and wave plate angles. A 2-GHz HML pulse is obtained with a 155-MHz fundamental repetition rate in a Yb-doped fiber laser, and the corresponding supermode suppression level is as high as 50 dB. The average power of the output pulse is nearly 200 mW, and the compressed pulse duration is 535 fs. To the best of our knowledge, 2 GHz and 196 mW represent, respectively, the highest repetition rate and output power in a figure-9 fiber laser. This work offers a potential pathway to achieve high-repetition-rate ultrashort pulses in the GHz range with high power and low noise levels.

Parametric Instability of Alfvén Waves and Wave Packets in Periodic and Open Systems

The parametric decay instability of Alfvén waves has been widely studied, but few investigations have examined wave packets of finite size and the effect of different boundary conditions on the growth rate. In this paper, we perform a linear analysis of circular and arc-polarized wave trains and wave packets in periodic and open boundary systems in a low-β plasma. We find that both types of wave are 3–5 times more stable in open boundary conditions compared to periodic. Additionally, once the wave packet width ℓ becomes smaller than the system size L, the growth rate decreases nearly with a power law γ ∝ ℓ/L. This study demonstrates that the stability of a pump wave cannot be separated from the laboratory settings, and that the growth rate of daughter waves depends on the conditions downstream and upstream of the pump wave and on the fraction of volume it fills. Our results can explain simulations and experiments of localized Alfvén waves. They also suggest that Alfvénic fluctuations in the solar wind, including sharp impulses known as switchbacks, can be more stable than traditional theory suggests depending on wind conditions.

Performance evaluation of ∼2.1 μm microchip laser operation in Ho3+ doped germanate glass

An in-band pumped continuous wave (CW) ∼2.1 μm microchip laser is presented for the first time based on a short cavity Ho3+ doped germanate glass (GeO2-PbO-Ga2O3–Na2O: GPGN). A 1.94 μm, 5 W Tm3+ fiber laser was employed for the excitation of the Ho3+ ions. A 19% laser slope efficiency was achieved in a simple, unoptimized plane parallel Fabry-Perot cavity configuration. A positive thermal lens was estimated in the laser cavity with a sensitivity factor of S ∼31 m−1W−1 and an optical path distortion value exceeding 6 μm. The laser results along with the analysis of the thermal lens indicate that with improved thermal management and an optimized cavity configuration GPGN is a promising gain medium for microchip laser operation around 2.1 μm.

Low-threshold CW eye-safe vortex generation from an intracavity pump-wave off-axis pumped OPO.

A low-threshold efficient setup for CW wavelength versatile vortex generation is proposed. The intracavity optical parametric oscillator is utilized for low-threshold operation. By using the pump-wave off-axis pumping for the signal-wave oscillator, high-order Hermite-Gaussian modes at the eye-safe region can be obtained, which can further generate a vortex via an astigmatism mode converter. The overall laser performance is additionally verified to be better by comparing it with the diode off-axis pumping method where the TEM1,0 pump wave was generated first for the signal-wave TEM1,0 mode oscillation. This method is expected to be further applied for more complicated structured light generation.

Understanding of topological mode and skin mode morphing in 1D and 2D non-Hermitian resonance-based meta-lattices

Recent advances have demonstrated that the non-Hermitian skin effect (NHSE), induced by system non-Hermiticity, can manipulate the localization of in-gap topological edge modes (TEMs) within mechanical topological insulators. This study introduces a straightforward analytical framework to elucidate the competition between NHSE and TEM localization in a classical mechanical meta-lattice, highlighting its impact on the dynamic behavior of TEMs within separate Bragg scattering band gaps (BSBGs). We propose a 1D non-Hermitian meta-lattice featuring a locally resonant system with active feedback control, characterized by a real-valued transfer function. This local resonance creates two separate BSBGs, each hosting a TEM defined by non-Hermitian bulk-edge correspondence. Our theoretical and numerical analyses reveal that the NHSE, with its asymmetric localization within the two BSBGs, can shift the localization of TEMs in distinct ways. This leads to an asymmetric phase transition, wherein one TEM can be delocalized and relocalized by tuning the transfer function, while the other maintains its initial localization. Moreover, we extend the mechanism of 1D asymmetric TEM delocalization to the non-Hermitian morphing of TEMs, showcasing notable examples such as temporal and spatial topological wave pumping with space- and time-dependent transfer functions in 1D time-varying and 2D stacked meta-lattices. This research bridges a gap between non-Hermitian mechanical constructs and their potential applications in classical mechanics, reinterpreting known topological wave control in 1D and uncovering new mechanisms in 2D.

Vortex Motion on the Surface of Shallow and Deep Water

The attenuation of vortex motion on the surface of shallow and deep water is investigated. The vortex flow was formed by two mutually perpendicular waves excited by plungers at a frequency of 6 Hz (wavelength λ = 5.6 cm) on the surface of water measuring 70 × 70 cm and depth h. After reaching a stationary state in the vortex system, the wave pumping was switched off, and the attenuation of the surface flow was recorded. On the surface of shallow water, λ/2π ≈ h, when the characteristic size of the vortices L exceeds the depth of the liquid, L h, the time dependence of the energy of the vortex flow Е(t) is described by an exponential function at all pumping levels, and the dependence of the enstrophy Ф(t) = Ω2(t) has an exponential dependence only in the range of wave vectors 0–0.3 см–1. On the surface of deep water λ/2π h, L ≈ h dependence Е(t) and Ф(t) are far from exponential in all ranges of wave numbers. At high pumping levels, the dependencies Е(t) and Ф(t) are nonmonotonic, which can be attributed to the influence of volumetric flows.

Measurement and assignment of J = 5 to 9 rotational energy levels in the 9070-9370cm-1 range of methane using optical frequency comb double-resonance spectroscopy.

We use optical-optical double-resonance spectroscopy with a continuous wave (CW) pump and a cavity-enhanced frequency comb probe to measure the energy levels of methane in the upper part of the triacontad polyad (P6) with higher rotational quantum numbers than previously assigned. A high-power CW optical parametric oscillator, tunable around 3000 cm-1, is consecutively locked to the P(7, A2), Q(7, A2), R(7, A2), and Q(6, F2) transitions in the ν3 band, and a comb covering the 5800-6100cm-1 range probes sub-Doppler ladder-type transitions from the pumped levels with J' = 6 to 8, respectively. We report 118 probe transitions in the 3ν3 ← ν3 spectral range with uncertainties down to 300kHz (1 × 10-5cm-1), reaching 84 unique final states in the 9070-9370cm-1 range with rotational quantum numbers J between 5 and 9. We assign these states using combination differences and by comparison with theoretical predictions from a new abinitio-based effective Hamiltonian and dipole moment operator. This is the first line-by-line experimental verification of theoretical predictions for these hot-band transitions, and we find a better agreement of transition wavenumbers with the new calculations compared to the TheoReTS/HITEMP and ExoMol databases. We also compare the relative intensities and find an overall good agreement with all three sets of predictions. Finally, we report the wavenumbers of 27 transitions in the 2ν3 spectral range, observed as V-type transitions from the ground state, and compare them to the new Hamiltonian, HITRAN2020, ExoMol, and the WKMLC line lists.

Robust super-resolution classifier by nonlinear optics.

Spatial-mode projective measurements could achieve super-resolution in remote sensing and imaging, yet their performance is usually sensitive to the parameters of the target scenes. We propose and demonstrate a robust classifier of close-by light sources using optimized mode projection via nonlinear optics. Contrary to linear-optics based methods using the first few Hermite-Gaussian (HG) modes for the projection, here the projection modes are optimally tailored by shaping the pump wave to drive the nonlinear-optical process. This minimizes modulation losses and allows high flexibility in designing those modes for robust and efficient measurements. We test this classifier by discriminating one light source and two sources separated well within the Rayleigh limit without prior knowledge of the exact centroid or brightness. Our results show a classification fidelity of over 80% even when the centroid is misaligned by half the source separation, or when one source is four times stronger than the other.

1 kHz, 10 mJ, sub-200 fs regenerative amplifier utilizing a dual-crystal configuration of Yb:CaGdAlO4 featuring exceptional beam quality

In order to boost the energy of the femtosecond regenerative amplifier (RA), we adopt the chirped pulse amplification technique to stretch the seed pulses through the Martinez stretcher and inject them into our specially designed dual-crystal regenerative cavity based on the Yb:CaGdAlO4 (Yb:CGA) crystals. To avoid damaging the component coating, we meticulously regulate the size of the cavity mode on the surface of each component within the cavity to ensure that the energy density remains below the damage threshold. The final output of 11.3 mJ pulse energy was obtained at a 1 kHz repetition rate with a power stability of 0.35% over 1 hour, which is the highest energy we know of for the regenerative output of Yb:CGA crystals. Additionally, leveraging the wide gain spectrum of the Yb:CGA crystal, we achieve a spectrum full width at half maximum (FWHM) of 9 nm along with a compressed pulse duration of 198 fs. The combination of the dual-crystal setup, superior thermal properties of the Yb:CGA crystals, quasi-continuous wave pumping approach, and the thermal-insensitive design of the regenerative cavity effectively minimize the thermal impact on the crystals. The output beam exhibits near-diffraction-limited performance, with an M2 value of only 1.05 × 1.06.

Evaluation of compressive damage in concrete using ultrasonic nonlinear coda wave interferometry

This study investigates the feasibility of nonlinear coda wave interferometry (NCWI) for evaluating compressive damage in concrete, with a particular focus on the interference caused by the compressive stress-induced slow dynamics. Slow dynamics refers to a phenomenon in which the stiffness of concrete immediately decreases after moderate mechanical conditioning and then logarithmically evolves back to its initial value over time. A series of experiments were conducted to validate this concept. The experimental findings indicate that slow dynamics following the unloading of concrete specimen significantly interfere with NCWI testing. The changes in dv/v caused by the slow dynamics are opposite to those induced by the pump wave in NCWI. After the slow dynamics have been eliminated, an evaluation indicator, defined as the efficient nonlinear level αdv/v, demonstrates an excellent correlation with compressive damage. The value of the indicator decreases with increasing compressive stress. Furthermore, the coda wave interferometry (CWI) and direct wave interferometry (DWI) are performed as comparisons. In summary, the feasibility and superiority of NCWI are demonstrated in concrete compressive damage evaluation.

Six-wave interaction in multimode waveguides with Kerr nonlinearity with allowance for the Gaussian structure of pump waves

The quality of wavefront conjugation in the case of six-wave interaction in two-dimensional multimode waveguides with Kerr nonlinearity is analyzed under the condition that one of the pump waves excites a zero waveguide mode and the amplitude distribution of the other pump wave at the waveguide end facet varies according to the Gauss law. It is shown that in a waveguide with infinitely conducting walls, the half-width of the modulus of the point spread function of a six-wave radiation converter is completely determined by the transverse size of the waveguide and weakly depends on the width of the Gaussian pump wave. In a parabolic-index profile waveguide, a decrease in the width of the Gaussian pump wave at the waveguide ends commonly leads to a monotonic decrease in the half-width of the point spread function modulus.

Experimental and numerical investigation on detection of BVID in CFRP composite plates using vibro-acoustic modulation

ABSTRACT Carbon fibre reinforced plastic (CFRP) composite has gained popularity due to the advantages such as its high strength-to-weight ratio and high corrosion resistance, but susceptibility to barely visible impact damage limits their use in engineering. This study experimentally and numerically investigates the detection of BVID in CFRP composite plates using Vibro-acoustic modulation (VAM) technique. An experimental system of VAM was constructed for measuring sidebands induced by the nonlinear effects of BVID. An integrated finite element (FE) model is developed to further explore the nonlinear interaction between the BVID, low-frequency pump, and high-frequency probe waves. To address this issue of the mesh size and computational time step mismatch caused by the large difference in frequency between the pump and probe waves, a regionally refined mesh was employed at the BVID region. Furthermore, the effects of BVID’s size and depth on the modulation were discussed. Through the Hilbert-Huang Transform (HHT), the amplitude modulation (AM) and frequency modulation (FM) indexes were extracted from the VAM responses to quantitatively characterise the impact damages. The results demonstrate that the MIA index increases steadily with the exciting voltage and damage degree, making it a suitable indicator to accurately assess the impact damage in CFRP composites.

Characterization of parametrically amplified spin wave resonances in thin YIG films

We have experimentally shown that the perpendicular pumping geometry is optimal for parametric amplification of thermal magnons, which form Spin Wave Resonances (SWR) modes in in-plane magnetized Yttrium Iron Garnet (YIG) films. Above the parametric instability threshold, the parametric SWR spectrum exhibits a second order nonlinearity: the effect of modulational instability at Continuous Wave (CW) pumping. The parametric SWR modes possess a high quality factor, Qf = 2.0 × 106 that is two orders of magnitude more than the Q factor of the Ferromagnetic Resonance (FMR) in high quality YIG films.

Low-frequency parametric amplification in saturable ytterbium-doped fibers at 1064 nm

Experimental results on parametric optical processes in saturable ytterbium-doped fibers (YDFs) at the wavelength of 1064 nm are reported. Significant photoinduced change of the refractive index under saturation of the fiber-optical absorption in the near-infrared range enables us to consider this medium as an effective optical Kerr (χ′′′) medium. For pumping, it needs continuous-wave Nd:YAG laser power of 10-mW scale, but the saturation process has the characteristic time ≤1ms, governed by the metastable level lifetime in YDF. We investigate the nearly degenerate collinear configuration of the optical parametric amplification (OPA). Here we focus on unidirectional transformation of the initial amplitude modulation in the incident pump wave to the output phase modulation as a manifestation of OPA in a narrowband case. This effect results from the conventional coupled-wave equations for the signal and idler waves—if one considers them as the modulation sidebands. It can also be treated as a result of the self-phase modulation of the periodically modulated pump in the Kerr medium. OPA gain of about 0.64 with the net gain ≈0.29 was experimentally estimated for the utilized 2-m-long YDF in the frequency band ≈200Hz.

Record bandwidth waveband-shift-free optical phase conjugation in nonlinear fiber optical loop mirror.

We present a novel configuration for broadband, wavelength-shift-free optical phase conjugation (OPC) utilizing four-wave mixing in a nonlinear fiber optical loop mirror (NOLM). In the proposed configuration, the input signals and the pump wave return to the input port of the NOLM whereas the phase-conjugated signals generated in the NOLM loop are transmitted through the output port. This allows the phase-conjugated copies to occupy the same wavelength band as the input signals, in line with the requirements for practical deployment of OPC in communication links. The demultiplexing of the phase conjugates from the input signals sharing the same band is achieved by imparting an asymmetric phase shift on the pump via a fiber Bragg grating. We experimentally demonstrate waveband-shift-free OPC with an extinction ratio between signals and conjugated copies at the NOLM output of 17 dB to 25 dB across a band of 35 nm. Whilst a 7-nm wide performance gap exists in the middle of the band, this is the record bandwidth for waveband-shift-free OPC in an all-fiber setup. We compare the experimental results with numerical simulations of the OPC-NOLM, identify the reason for the observed performance gap, and justify the route for further performance improvement.

Brillouin expanded time-domain analysis based on dual optical frequency combs

Brillouin Optical Time-Domain Analysis (BOTDA) is a widely-used distributed optical fiber sensing technology employing pulse-modulated pump waves for local information retrieval of the Brillouin gain or loss spectra. The spatial resolution of BOTDA systems is intrinsically linked to pulse duration, so high-resolution measurements demand high electronic bandwidths inversely proportional to the resolution. This paper introduces Brillouin Expanded Time-Domain Analysis (BETDA) as a modified BOTDA system, simultaneously achieving high spatial resolution and low detection bandwidth. Utilizing two optical frequency combs (OFCs) with different frequency intervals as pump and probe, local Brillouin gain spectra are recorded by their spectral beating traces in an expanded time domain. A 2-cm-long hotspot located in a 230 m single-mode fiber is successfully measured in the time domain with a detection bandwidth of less than 100 kHz using dual OFCs with tailored spectral phase, line spacing, and bandwidth.

On possibility of anomalous microwave power absorption and gyrotron frequency sub-harmonics emission in X2-mode ECRH experiments at the TCV tokamak

In this paper, we analyze theoretically the low-threshold parametric decay instability (PDI) that can be excited at the tokamak à configuration variable (TCV tokamak, Lausanne, Switzerland) during X2 electron cyclotron resonance heating experiments producing a two-dimensionally localized upper hybrid (UH) wave and a daughter extraordinary wave running from the decay layer outward to the plasma edge. The primary instability is then saturated due to the cascade of secondary decays into two-dimensionally localized UH and ion Bernstein waves. The level of plasma microwave emission in the frequency range substantially below half the pump wave frequency and of anomalous power losses of the pump are predicted.

Double wavefront reversal during six-wave interaction on Kerr nonlinearity in a waveguide with infinitely conducting surfaces

Background. Generation of a wave with a double reversed wavefront in multimode waveguides increases the efficiency of six-wave radiation converters and expands the possibilities of its use in adaptive optics problems and the conversion of complex spatially inhomogeneous waves. Aim. The quality of double wavefront reversal during six-wave interaction in a waveguide with infinitely conducting surfaces with Kerr nonlinearity is analyzed for the ratio of the wave numbers of the pump waves equal to 2 and 0,5, and the condition that one of the pump waves excites the zero mode of the waveguide, and the amplitude distribution of the other pump wave excites the edges of the waveguide are described by a Gaussian function. Methods. The influence of pump wave parameters on the half-width and contrast of the amplitude modulus of the object wave was studied using numerical methods. A wave from a point source located on the front face of the waveguide was used as a signal wave. Results. The dependences of the half-width and contrast of the amplitude modulus of the object wave on the ratio between the width of the waveguide and the width of the Gaussian pump wave are obtained. Conclusion. It is shown that the maximum change in the characteristics of a wave with a double reversed wavefront is observed when the width of the Gaussian pump waves changes in the range from 0,3 to 2 half-widths of the waveguide.