Pulse Peak Power Research Articles

Independent component analysis for pulse compressed frequency modulated thermal wave imaging for inspection of mild steel

Non-destructive testing & evaluation plays a crucial role in various sectors for testing the reliability of materials. Of the different available non-destructive examination techniques, thermal non-destructive testing provides fast and remote inspection of the materials. Among various widely used thermal non-destructive testing techniques, frequency modulated thermal wave imaging gained its importance due to its higher test sensitivity and resolution. The adopted matched filter approach on the obtained temporal temperature distribution further concentrates supplied excitation energy into a narrow duration high peak power pulse. In this paper, the merits of the reconstructed high peak power pulsed data have been considered and emphasized in the context of independent component analysis. The obtained results clearly indicate that pulse compressed data improves defect detectability, reliability, memory usage, and computational complexity.

Broadband metasurface design for terahertz quantum‐cascade VECSEL

A broadband active metasurface is designed around quantum-cascade (QC) gain material for use in a broadband tunable QC vertical external-cavity surface-emitting laser (VECSEL). The metasurface is based on a unit cell containing two resonant waveguide elements that couple with the periodicity of the metasurface to produce a multi-resonant reflection spectrum. Simulated reflectance spectra exhibit full-width half-maximum bandwidths up to 50% of the centre frequency. The tested QC-VECSEL demonstrates up to 15 mW of peak pulse power at 77 K and supports multimode lasing from 2.85 to 3.9 THz (>30% fractional bandwidth around 3.37 THz). The frequency coverage is limited by how short the cavity can be made, rather than the metasurface bandwidth. Consistent multimoding results from non-uniform distribution of spectral energy across the surface.

Formations of a giant “running” pulse in the process of a quasi-regular amplification of a long wave signal by a slipping electron bunch

It is shown that the process of a quasi-regular amplification of a long input wave pulse by a short “slipping” electron bunch can be transformed into a different short-pulse process. Namely, a short wave pulse is formed on one of the fronts of the electron bunch and amplifies as it “runs” along the bunch. The peak power of the wave pulse formed in this way can be several orders of magnitude higher than the generation power at the saturation stage of the quasi-regular amplification stage. Estimates show that based on the existing electronic technique, it is possible to implement ultra-short sub-terahertz pulse sources based on this principle with a gigawatt level of the peak power.

Design of Nanosecond Pulse Driver Circuit

The laser can achieve high peak power pulse output by electro-optic Q-switch technology. Aiming at the problem of how to achieve Q-switch output with ns level front and kV level amplitude on 100kΩ large load, a method of pulse output by using avalanche conduction state of triode is proposed. By using the 11 stages pulse forming circuit, the DC power supply voltage can be greatly reduced, and the selected devices can work normally under the condition of meeting the first stage derating standard, in order to improve the stability. The key factors affecting the parameters of the output pulse, such as amplitude, pulse width, frequency, leading edge and trailing edge, are analyzed and verified by experiments. Using +150V DC power supply, the experimental results show that the pulse driving source circuit designed in this paper can output a half height width of 12.6ns and an amplitude of 1500V Q-switch regulating signal on a 100kΩ load.

High-peak-power 786 nm and 452 nm lasers based on 1064 nm intracavity-driven cascaded nonlinear optical frequency conversion.

We demonstrated high-peak-power 786 nm and 452 nm lasers based on 1064 nm intracavity-driven cascaded nonlinear optical frequency conversion (CNOFC). The 1064 nm fundamental wave generated from the LD-side-pumped Nd:YAG was first intracavity converted to 1572 nm by a noncritically phase-matched KTP OPO. Then a LBO-based second harmonic generation of 1572 nm was served as cascaded process to produce 786 nm laser radiation. The maximum average output power at 786 nm was 1.34 W, corresponding to a pulse peak power of 14.2 kW with 11.2 ns pulse width and 8 kHz pulse repetition rate. Furthermore, a third stage of sum frequency mixing between 786 nm and 1064 nm was designed to achieve the blue emission at 452 nm. The 452 nm blue laser delivers 263 mW, 6.2 ns pulses with a peak power of 5.3 kW, paving the way for achieving high-peak-power blue lasers.

Stability Analysis and Transient Attraction Dynamics of Flat-Top Solitons via Modulated Driving Fields in Normal Dispersion Microcavity

We theoretically and numerically investigate the stability and transient attraction dynamics of flat-top solitons (or platicons) in normal group velocity dispersion (GVD) microcavity via modulation-assisted four wave mixing instead of mode interaction. The spatial bifurcation structure of the stationary states depending on modulation power, pump power and cavity detuning is mapped, indicating a significantly extended existence range by the modulation-assisted approach. We explore the transformation between discrete energy spectrum of dark solitons and a quasi-continuous spectrum of platicons with and without a third-order dispersion and identify the connections between these two states. Robust trapping and manipulation of temporal platicons is also verified by implementation of phase modulation, and explained by transient attraction dynamics. One may manipulate the duration of the pulse by varying modulation power, pump power, or cavity detuning. Specifically, the effect of high-order dispersion on Kerr frequency comb generation is studied, and the results show that different orders of GVD term affect the 3dB comb bandwidth, repetition rate, spectral position of dispersive waves, as well as temporal profile and pulse peak power.

Isolated single-cycle extreme-ultraviolet pulses from undulator radiation.

We propose a method to generate an isolated single-cycle pulse in the extreme-ultraviolet spectral region using a broadband conventional laser. The uncompressed laser pulse imprints a chirped sinusoid current profile onto a relativistic electron beam. As the beam propagates along a specifically tailored magnetic field of an undulator, it produces an isolated single-cycle pulse. For moderate laser intensities (0.2 mJ per pulse) and typical operating parameters of current electron accelerators, we predict a 26 as, 5 GW peak-power pulse spanning wavelengths down to 15 nm.

High-Speed Chemical Imaging by Dense-Net Learning of Femtosecond Stimulated Raman Scattering.

Hyperspectral stimulated Raman scattering (SRS) by spectral focusing can generate label-free chemical images through temporal scanning of chirped femtosecond pulses. Yet, pulse chirping decreases the pulse peak power and temporal scanning increases the acquisition time, resulting in a much slower imaging speed compared to single-frame SRS using femtosecond pulses. In this paper, we present a deep learning algorithm to solve the inverse problem of getting a chemically labeled image from a single-frame femtosecond SRS image. Our DenseNet-based learning method, termed as DeepChem, achieves high-speed chemical imaging with a large signal level. Speed is improved by 2 orders of magnitude with four subcellular components (lipid droplet, endoplasmic reticulum, nuclei, cytoplasm) classified in MIA PaCa-2 cells and other cell types which were not used for training. Lipid droplet dynamics and cellular response to dithiothreitol in live MIA PaCa-2 cells are demonstrated using this computationally multiplex method.

Dissipative-Soliton-Resonance generation in anomalous-dispersion fiber lasers based on spectral filtering effect

In this article, we numerically research the generation of Dissipative-Soliton-Resonance (DSR) pulse in anomalous dispersion regime based on spectral filtering effect. We find that a strong spectral filtering effect can limit the spectrum broadening and avoid the pulse breaking up in the high effective gain of the cavity, which leads to the formation of DSR pulse. Meanwhile, we also find that the peak power of DSR pulse increases with the saturation power, which is in line with the case in normal dispersion regime. As a comparison, the DSR pulse cannot be obtained in the absence of spectral filtering effect, which proves that the generation of DSR pulse needs strong spectral filtering effect in anomalous dispersion regime. We believe that our work can be served as a theoretically guideline to investigate the generation of DSR pulse in anomalous dispersion regime.

Microfiber-Knot-Resonator-Induced Energy Transferring From Vector Noise-Like Pulse to Scalar Soliton Rains in an Erbium-Doped Fiber Laser

We report on scalar soliton rains (SRs) shedding from vector square-wave noise-like pulse (NLP) by introducing a microfiber-knot-resonator (MKR) into an erbium-doped fiber (EDF) laser. The EDF laser has an ultralong cavity of ∼3.07 km and a large anomalous cavity-dispersion, facilitating the formation of NLP. Once an MKR was further incorporated, comb-like filtering effect could be clearly observed on the emission spectrum. Apart from that, SRs shedding from the NLP were observed, which resulted from the MKR-induced energy transferring effect from the NLP to SRs. It was further found that the SRs existed only along a particular polarization direction, whereas the NLPs could be found in any pair of orthogonal polarization directions, despite some differences in pulse peak power. These suggest that the SRs possess scalar feature, whereas the co-circulating NLP is a package of vector pulses. Our results represent the first observation of MKR-induced pulse energy transferring effect and also the first observation of the co-existence of scalar and vector pulses in a single fiber system. Besides enriching the dynamics and interactions of optical pulses in fiber lasers, our results extend the applications of MKR to the area of pulse manipulations through spectral tailoring and local nonlinearity engineering.

Strengthening effect of microwave on spark ignited spherical expanding flames of methane-air mixture

The Microwave Assisted Spark Ignition system is a novel and potential ignition method to improve efficiency of spark ignition engines. To better understand the coupling mechanisms/characteristics between microwaves and spark plasma, experiments were conducted to visualize spark discharge and flame development with microwave emitted in a Constant Volume Combustion Chamber. The operation parameters varied included equivalence ratio, ambient pressure, microwave pulse repetition frequency, and peak power. To clearly illuminate the coupling mechanism between microwave and spark plasma, a small-diameter antenna device was specifically designed to minimize heat loss. The microwave assisted spark ignition and flame propagation characteristics of methane-air premixed spherical flames were studied first. Subsequently, non-reactive discharge was experimented to exclude the effects from combustion. Results showed that microwave increased the development speed of an early flame kernel by 60% with 1 kHz microwave pulse repetition frequency and 1000 W peak power under the condition of equivalence ratio 0.6 and ambient pressure 0.1 MPa. The enhancement was believed to be the result of enhanced chemical reaction and the induced flame deformation by collisions between high energy electrons and other particles. As the ambient pressure increased, the enhancement effect of microwaves on flame kernels was found to diminish. In this situation, non-reactive discharge and related MAI experiments showed that a higher pulse repetition frequency or peak power was beneficial to strengthen the effect of microwave as much higher energy absorption efficiency was achieved.

Theoretical research of time dependent pulsed diode-pumped Rb vapor laser

A time dependent 3D model for pulsed diode-pumped Rb vapor laser is established by using the methods of circular cutting and alternating-direction implicit (ADI). The model is validated by comparing with Zhao’s experiment. The spatiotemporal distributions of intensities of pump light and laser, and population densities of different energy levels are presented and discussed in a single-pulse diode-pumped Rb vapor laser. The lasing dynamics and characteristics of a multi-pulse diode-pumped Rb vapor laser are also calculated and analyzed, and results show that higher peak power and average power of each laser pulse can be obtained if the pump power is greater. Moreover, for constant pump pulse width, the peak power of subsequent laser pulses (in addition to the first pulse) increases with increasing time interval between two adjacent pump pulses. For constant time interval between two adjacent pump pulses, the large pump pulse width is not conducive to improving the peak power of subsequent laser pulses.

Wide-Range Wavelength-Tunable Mode-Locked Fiber Laser Based on Fiber Bragg Grating

We demonstrated a wide-range wavelength-tunable passively mode-locked fiber laser based on the fiber Bragg grating (FBG) fabricated by femtosecond laser as the wavelength selection element. A ring fiber cavity with the nonlinear-optical loop mirror as its saturable absorber was employed for mode-locking operation. The peak wavelength of the fiber laser was 1547.9 nm with a spectral full-width-at-half-maximum of 0.90 nm at room temperature. The pulse duration of the fiber laser was about 2.94 ps closed to the Fourier-limited pulse duration of 2.80 ps by assuming sech <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pulse shape. The repetition rate was 860.5 kHz and the pulse peak power was about 56.2 W. The wavelength tuning range of the fiber laser achieved 14.2 nm by varying the FBG temperature from 25 °C to 1000 °C.

All-fiberized very-large-mode-area Yb-doped fiber based high-peak-power narrow-linewidth nanosecond amplifier with tunable pulse width and repetition rate* *Project supported by the National Natural Science Foundation of China (Grant No. 61675009) and the Beijing Natural Science Foundation Program, China, and Scientific Research Key Program of Beijing Municipal Education Commission, China (Grant No. KZ201910005006).

We demonstrate an all-fiberized narrow-linewidth nanosecond amplifier with high peak power, tunable pulse width, and repetition rate. A fiber-coupled narrow-linewidth laser diode operating at 1064.1 nm is employed as the seed source, which is gain-switched to generate nanosecond pulses with tunable pulse widths of 1–200 ns and tunable repetition rates of 10 Hz–100 kHz. By utilizing a very-large-mode-area Yb-doped fiber with a core diameter of 50 μm in the power amplifier, thresholds of the stimulated Brillouin scattering at different pulse widths and repetition rates are increased. The maximum average power reaches 30.8 W at the pulse width of 4 ns and a repetition rate of 100 kHz, corresponding to an optical-to-optical conversion efficiency of ∼55.2%. Pulse energy and peak power are calculated to be 0.2 mJ and 50 kW, respectively, which are limited by stimulated Brillouin scattering. The 3-dB spectral linewidth remains around 0.05 nm during the power scaling process. The stimulated Brillouin scattering limited output powers at different pulse widths and repetition rates are investigated. Peak power of 47.5 kW (0.19 mJ) is obtained for the 4 ns pulses at a repetition rate of 50 kHz, which is nearly the same as that of 4 ns pulses at 100 kHz. When the pulse width of the seed source is increased to 8 ns, peak powers/pulse energies are decreased to 19.6 kW/0.11 mJ and 13.3 kW/0.08 mJ at repetition rates of 50 kHz and 100 kHz, respectively.

Continuous wave and SnSe2/PdSe2 passively Q-switched Nd:GdNbO4 laser under direct pumping

In this paper, a continuous wave (CW) and two-dimensional (2D) tin diselenide (SnSe2) and palladium diselenide (PdSe2) passively Q-switched Nd:GdNbO4 laser under direct pumping was demonstrated for the first time. The CW laser performance under the pump at 879 nm was compared to that using traditional 808 nm pumping. The 2D SnSe2 and PdSe2 nanosheets were synthesized by chemical vapor deposition (CVD) technique. The optical properties and microstructures of two nanosheets were investigated. In the passively Q-switched operation, the highest repetition rate, narrowest pulse width, maximum pulse energy and peak power of SnSe2/Nd:GdNbO4 laser were 160 kHz, 555 ns, 2.3 μJ and 4.2 W, respectively. And for PdSe2/Nd:GdNbO4 laser, the corresponding values were 143 kHz, 530 ns, 3.7 μJ and 7.0 W, respectively.

Analysis of actively Q-switched infrared Raman lasers with crystalline media of multi-Raman-modes

Actively Q-switched intracavity Raman lasers can generate multi-wavelength infrared laser based on rich modes of crystalline Raman media. Assuming the intracavity photon densities and initial population inversion densities to be of Gaussian spatial distributions, the space-dependent rate equations of actively Q-switched intracavity Raman lasers with two Raman modes are deduced. The rate equations are solved numerically to investigate the influences of normalized variables on pulse energy, peak power, pulse duration, and conversion efficiency of Raman laser outputs, respectively. There are obvious discrepancies between the results of two Raman modes. The dependence of Raman laser conversion efficiency at 1103 nm on normalized initial population inversion density is numerically calculated, the experimental results are consistent with the numerical ones, and the optimum conditions are proposed theoretically in the end.

34.7 W passive peak power Q-switch Ho:Sc2SiO5 laser operating at 2 μm with a few-layer molybdenum disulfide saturable absorber

Laser emission performances at 2 μm obtained from a Ho:Sc2SiO5 crystal was reported in this work. A few-layer MoS2 saturable absorber was used to obtain passively Q-switched regime. The laser beam quality was characterized by an M2 factor of 1.13 × 1.11. In continuous-wave mode the laser delivered 3.28 W output power for the pump with 12.58 W at 1.94 μm, which corresponds to an overall optical-to-optical efficiency of 0.26. The slope efficiency was 0.50. The maximum average power was 1.11 W and the laser run at 128.4 kHz repetition rate, indicating pulse energy of 8.62 μJ. The shortest laser pulse duration was measured to be 248 ns from which pulse peak power was determined as 34.77 W.

High peak power, multi-wavelength, passively Q-switched Raman lasers based on Yb:YAG/Cr4+:YAG composite crystal

Stable, multi-wavelength, high peak power, passively Q-switched Raman lasers based on Yb:YAG/Cr4+:YAG composite crystal and c-cut YVO4 crystal have been demonstrated under quasi-continuous-wave laser diode end-pumping. A Raman laser with a pulse energy of 50 μJ and a pulse width of 0.88 ns is obtained. The corresponding pulse peak power reaches 56.9 kW, which to our best knowledge is the highest peak power achieved in intracavity Raman lasers with Yb3+ ion-doped crystals as the gain medium. Stable pulse trains of the fundamental lasers and Raman lasers were achieved at different pump pulse energies. These findings have potential applications in nonlinear frequency conversion, photodynamic therapy, industrial processing and terahertz wave generation.

Measuring self-steepening with the photon-conserving nonlinear Schrödinger equation.

We propose an original, simple, and direct method to measure self-steepening (SS) in nonlinear waveguides. Our proposal is based on results derived from the recently introduced photon-conserving nonlinear Schrödinger equation (NLSE) and relies on the time shift experienced by soliton-like pulses due to SS upon propagation. In particular, a direct measurement of this time shift allows for a precise estimation of the SS parameter. Furthermore, we show that such an approach cannot be tackled by resorting to the NLSE. The proposed method is validated through numerical simulations, in excellent agreement with the analytical model, and results are presented for relevant spectral regions in the near infrared, the telecommunication band, and the mid infrared, and for realistic parameters of available laser sources and waveguides. Finally, we demonstrate the robustness of the proposed scheme against deviations expected in real-life experimental conditions, such as pulse shape, pulse peak power, pulsewidth, and/or higher-order linear and nonlinear dispersion.

Numerical simulation of two-soliton and three-soliton molecules evolution in passively mode-locked fiber laser

In this paper, we have studied the dynamic evolution and output characteristics of soliton molecules in the passively mode-locked Yb-doped fiber laser through numerical simulation. Under two different intra-cavity average dispersion values (5.142 ps2/km, -4.461 ps2/km), with the increase of small signal gain (g0) or gain saturation energy (Esat). Soliton molecules will not always maintain the state of two-soliton molecules or three-soliton molecules, but will occur at intervals. We have obtained specific multi-soliton molecules partitions, and studied the effects of g0 and Esat on the pulse duration, peak power and pulse energy of soliton molecules in these regions. Simulation results have theoretical significance for the practical application of soliton communication.