Pulse Steepening Research Articles

Pulse shaping, high average power narrow linewidth Er, Yb co-doped fiber amplifier

We demonstrate a high repetition rate, high average power narrow-linewidth fiber amplifier at 1.55 μm eye-safe waveband by using a very-large-mode-area active fiber doping Er and Yb, having a 60-μm-diameter core and a 600-μm-diameter inner cladding. A maximum average power reaches 16 W at a 10 kHz repetition rate for 600 ns pulses in the stimulated Brillouin scattering (SBS) free operation, which calculates to a 1.6 mJ pulse energy and 2.7 kW peak power. In addition, the peak power rises to 4.9 kW with 148 ns pulses at the onset of SBS. The signal peak is located at 1,551.15 nm with a 36 dB signal-to-noise ratio (SNR). In order to eliminate the pulse steepening, the pulse shaping technology using a triangular-like wave with a slower rising edge is applied on the input pulses. The half-width of the frequency-intensity spectrum from high-average-power fiber amplifier is about 1.02 MHz, approaching the Fourier transform limit.

Science and technologies in targets for laser accelerators

<p indent="0mm">Laser plasma acceleration (also referred to as laser acceleration) employs ultra-strong electric fields generated when an ultra-intense laser pulse interacts with the plasma to accelerate charged particles. As a promising novel acceleration method, its acceleration gradient can be 10³–10⁶ times that of conventional accelerators. Laser accelerators based on the laser acceleration process can not only generate ultra-short pulse particle/photon beams for fundamental research, but also provide high-energy protons/heavy ions for tumor radiotherapy devices, benefiting people’s lives and health. Under the strong national investment around the world, basic and applied research related to laser accelerators has developed rapidly in recent years. Unlike conventional accelerators, which use macroscopic electromagnetic fields to accelerate particles in a vacuum, the laser acceleration process is realized by shooting targets with laser pulses. The scientific and technical problems related to the “target” are the core problems of the laser accelerator. In order to build an accelerator that can stably output high-energy particle beams, on the one hand, developing new target systems and deeply studying laser acceleration physics is highly necessary. On the other hand, overcoming a large number of key technical issues related to the preparation and characterization of targets and shooting is also very crucial. This paper gives an introduction to the key problems in the science and technologies for laser accelerators and highlights the progress in this field made by our team at the Institute of Heavy Ion Physics of Peking University. By exploiting nanotechnology and nanomaterials, we developed novel targets made of carbon nanotubes, which can serve as long-awaited perfect near-critical-density (NCD) targets. With such targets, we observed the laser focusing and pulse steepening in NCD plasmas for relativistic laser pulses for the first time. We also successfully achieved cascaded ion acceleration scheme by designing and building double-layer CNT targets, which solves the long-standing dilemma between the ionization and long-time acceleration of heavy ions acceleration. By collaborating with the Institute for Basic Sciences (IBS, Korea), we generated record-breaking <sc>580 MeV</sc> carbon ions and <sc>1.2 GeV</sc> Au ions in experiments. Recently, we numerically and experimentally studied how an ultra-intense femtosecond laser pulse interacts with a nanowire array target and eventually creates a high-energy-density plasma. The generation of soft-X-rays/EUV radiations and fusion neutrons from such a plasma were measured and studied in detail. Micro-channel targets are very interesting targets for us as well. We revealed how a laser pulse propagates through a micro-channel, peels off and accelerates electrons to GeV level, and produces corpus gamma rays with very small divergence angles. Experiments collaborating with the China Academy of Engineering Physics confirmed our prediction. Besides the above breakthroughs in acceleration sciences, we solved many key technical problems in laser ion accelerators. For example, we developed some mass-preparation methods for high-quality targets, including nanometer-thin diamond-like-carbon foils and metal foils, measured the target damage thresholds of a majority of existing targets in laser plasma experiments, figured out how to precisely locate transparent targets with high-precision. For non-stop running of the laser ion accelerators, we recently developed liquid sheets, which is highly promising to realize continuous beam delivery for weeks.

Research on the Picoseconds Gating Pulse of Framing Camera Using Pulse Steepening Technique

Research on the Picoseconds Gating Pulse of Framing Camera Using Pulse Steepening Technique

Third-order Riemann pulses in optical fibers.

We introduce the concept of third-order Riemann pulses in nonlinear optical fibers. These pulses are generated when properly tailored input pulses propagate through optical fibers in the presence of higher-order dispersion and Kerr nonlinearity. The local propagation speed of these optical wave packets is governed by their local amplitude, according to a rule that remains unchanged during propagation. Analytical and numerical results exhibit a good agreement, showing controllable pulse steepening and subsequent shock wave formation. Specifically, we found that the pulse steepening dynamic is predominantly determined by the action of higher-order dispersion, while the contribution of group velocity dispersion is merely associated with a shift of the shock formation time relative to the comoving frame of the pulse evolution. Unlike standard Riemann waves, which exclusively exist within the strong self-defocusing regime of the nonlinear Schrödinger equation, such third-order Riemann pulses can be generated under both anomalous and normal dispersion conditions. In addition, we show that the third-order Riemann pulse dynamics can be judiciously controlled by a phase chirping parameter directly included in the initial chirp profile of the pulse.

High power microsecond fiber laser at 1.5 μm.

In this work, we demonstrate a single frequency, high power fiber-laser system, operating at 1550 nm, generating controllable rectangular-shape μs pulses. In order to control the amplified spontaneous emission content, and overcome the undesirable pulse steepening during the amplification, a new method with two seed sources operating at 1550 nm and 1560 nm are used in this system. The output power is about 35 W in CW mode, and the peak power is around 32 W in the pulsed mode. The repetition rate of the system is tunable between 50 Hz to 10 kHz, and the pulse duration is adjustable from 10 μs to 100 μs, with all on the fly electronically configurable design. The system demonstrates excellent long and short time stability, as well as spectral and spatial beam quality.

Pulse evolution and plasma-wave phase velocity in channel-guided laser-plasma accelerators.

The self-consistent laser evolution of an intense, short-pulse laser exciting a plasma wave and propagating in a preformed plasma channel is investigated, including the effects of pulse steepening and energy depletion. In the weakly relativistic laser intensity regime, analytical expressions for the laser energy depletion, pulse self-steepening rate, laser intensity centroid velocity, and phase velocity of the plasma wave are derived and validated numerically.

Optically isolated, 2 kHz repetition rate, 4 kV solid-state pulse trigger generator.

This paper presents the design and operation characteristics of a solid-state high voltage pulse generator. Its primary utilization is aimed at triggering a gaseous spark gap with high repeatability. Specifically, the trigger generator is designed to achieve a risetime on the order of 0.1 kV/ns to trigger the first stage, trigatron spark gap of a 10-stage, 500 kV Marx generator. The major design components are comprised of a 60 W constant current DC-DC converter for high voltage charging, a single 4 kV thyristor, a step-up pulse transformer, and magnetic switch for pulse steepening. A risetime of <30 ns and pulse magnitude of 4 kV is achieved matching the simulated performance of the design.

An exact solution of a Burgers' equation governing the nonlinear propagation of infrasound in a range-dependent, windy atmosphere

We present the derivation of an exact solution of an inviscid Burgers' equation that governs the nonlinear propagation of infrasound in a range-dependent, windy atmosphere with arbitrary sound speed, wind speed, and density profiles. The Burgers' equation is a reduced form of a nonlinear transport equation obtained using weakly, nonlinear ray theory. The solution extends known solutions for a homogeneous, windless medium to include the effects of wind and the inhomogeneity in the properties of the atmosphere. Analytical expressions for the shock velocity and period lengthening are readily obtained from the solution. The exact solution is used to validate our numerical algorithm which uses a spectral method to integrate the Burgers' equation. These models are compared with observed infrasound arrivals that clearly demonstrate nonlinear pulse steepening and stretching while propagating in the upper atmosphere.

Exponential pulse steepening in degenerate two-photon propagation

The resonant propagation of an optical pulse in a two-photon absorber with a coherent initial excitation of the medium may lead to a pulse steepening growing exponentially in space. In addition another example is given wherein the steepening is followed by flattening. These results are new although, principally, they could be obtained from the general solution of the respective Goursat problem of an amplitude-modulated initial wave (H Steudel and D J Kaup 1996 J. Mod. Opt. 43 1851). In addition, we also give a simple theorem whereby it is demonstrated that the spatial derivative of the phase of the electric field will always be bounded. Furthermore, it is also shown that there can be certain critical points at which the atomic state, as well as the electric field, can become stationary in time and the spatial direction.

Quantum vacuum radiation in optical glass

A recent experimental claim of the detection of analogue Hawking radiation in an optical system [PRL 105 (2010) 203901] has led to some controversy [PRL 107 (2011) 149401, 149402]. While this experiment strongly suggests some form of particle creation from the quantum vacuum (and hence it is per se very interesting), it is also true that it seems difficult to completely explain all features of the observations by adopting the perspective of a Hawking-like mechanism for the radiation. For instance, the observed photons are emitted parallel to the optical horizon, and the relevant optical horizon is itself defined in an unusual manner by combining group and phase velocities. This raises the question: Is this really Hawking radiation, or some other form of quantum vacuum radiation? Naive estimates of the amount of quantum vacuum radiation generated due to the rapidly changing refractive index --- sometimes called the dynamical Casimir effect --- are not encouraging. However we feel that naive estimates could be misleading depending on the quantitative magnitude of two specific physical effects: "pulse steepening" and "pulse cresting". Plausible bounds on the maximum size of these two effects results in estimates much closer to the experimental observations, and we argue that the dynamical Casimir effect is now worth additional investigation.

Half-mJ All-Fiber-Based Single-Frequency Nanosecond Pulsed Fiber Laser at 2-$\mu{\rm m}$

We report a high energy, high peak power, all-flber-based single-frequency ~15-ns pulsed laser source at ~1918.4 nm in master oscillator-power amplifier configuration. The pulsed fiber laser seed was achieved by directly modulating a continuous- wave single-frequency fiber laser using an electro-optic modulator driven by an arbitrary waveform generator to pre-shape the pulses before amplification to mitigate pulse steepening and dynamic gain saturation. One piece of large (25-μm) core, polarization-maintaining highly thulium-doped germanate glass fiber was used in the power amplifier stage to boost the energy and the peak power of the 15-ns pulses to >;0.5 mj and >;33 kW without any nonlinearities.

High peak-power single-frequency pulses using multiple stage, large core phosphate fibers and preshaped pulses

We have developed a monolithic high power pulsed fiber laser in a master oscillator power amplifier (MOPA) configuration, which is capable of reaching 0.38 mJ pulse energy and 128 kW peak power for 3 ns pulses at ~1550 nm while maintaining transform-limited linewidth. The fiber laser pulse seed was achieved by directly modulating a CW single-frequency fiber laser using an electro-optic modulator. We used an arbitrary waveform generator to preshape the fiber laser pulses before amplification to avoid pulse steepening and dynamic gain saturation. Single-mode, polarization maintaining highly Er/Yb codoped large core phosphate fibers were used in the power amplifier stages to scale the transform-limited fiber laser pulses, avoiding any nonlinearities.

Nonlinear coherent Thomson scattering from relativistic electron sheets as a means to produce isolated ultrabright attosecond x-ray pulses

A new way to generate intense attosecond x-ray pulses is discussed. It relies on coherent Thomson scattering (CTS) from relativistic electron sheets. A double layer technique is used to generate planar solid-density sheets of monochromatic high-$\ensuremath{\gamma}$ electrons with zero transverse momentum such that coherently backscattered light is frequency upshifted by factors up to $4{\ensuremath{\gamma}}^{2}$. Here previous work [H.-C. Wu et al., Phys. Rev. Lett. 104, 234801 (2010)] is extended to the regime of high-intensity probe light with normalized amplitude ${a}_{0}&gt;1$ leading to nonlinear CTS effects such as pulse contraction and steepening. The results are derived both by particle-in-cell (PIC) simulation in a boosted frame and by analytic theory. PIC simulation shows that powerful x-ray pulses (1 keV, $&lt;10\text{ }\mathrm{\text{attosecond}}$, and $&gt;10\text{ }\mathrm{\text{gigawatt}}$) can be generated. They call for experimental verification. Required prerequisites such as manufacture of nanometer-thick target foils is ready and ultrahigh contrast laser pulses should be within reach in the near future.

Nonlinear Pulse Propagation and Phase Velocity of Laser-Driven Plasma Waves

Laser evolution and plasma wave excitation by a relativistically intense short-pulse laser in underdense plasma are investigated in the broad pulse limit, including the effects of pulse steepening, frequency redshifting, and energy depletion. The nonlinear plasma wave phase velocity is shown to be significantly lower than the laser group velocity and further decreases as the pulse propagates owing to laser evolution. This lowers the thresholds for trapping and wave breaking and reduces the energy gain and efficiency of laser-plasma accelerators that use a uniform plasma profile.

Breakthroughs in Silicon Photonics 2009

Silicon photonics research published in 2009 exhibited considerable dynamism. Significant progress was made in research on optomechanics, novel phenomena in Bragg gratings, bio-sensing, photo-detectors, and silicon-organic hybrid structures. Non-linearity was successfully exploited for third-harmonic generation, pulse steepening, waveform compression, and high-speed sampling. All three solid states of silicon-single-crystal, polycrystalline, and amorphous-featured.

Self-compression of 2 μm laser filaments

We numerically study the filamentation of ultrashort laser pulses at 2 microm carrier wavelength in noble gases (argon, xenon) and in air. Compared with filamentation in the near-visible domain (800 nm), mid-infrared optical sources with durations close to a single cycle can be generically produced at various pressures and powers near the self-focusing threshold. The mechanism by which self-compression takes place mainly involves optical self-focusing, pulse steepening and plasma defocusing. On-axis spectra and spectral phases are discussed. Delivering single-cycled pulses at long wavelengths has important applications in the generation of high-order harmonics and isolated attosecond pulses.

Modeling of laser wakefield acceleration atCO2laser wavelengths

The upgraded Accelerator Test Facility (ATF) ${\mathrm{CO}}_{2}$ laser located at Brookhaven National Laboratory offers a unique opportunity to investigate laser wakefield acceleration (LWFA) with a $10.6\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{m}$ laser, a wavelength where little experimental work exists. While long laser wavelengths have certain advantages over short wavelengths, our modeling analysis has uncovered another important effect. The upgraded ATF ${\mathrm{CO}}_{2}$ laser will have a pulse length as short as 2 ps. At a nominal plasma density of $\ensuremath{\sim}{10}^{16}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$, this pulse length would normally be considered too long for resonant LWFA, but too short for self-modulated LWFA. However, our model simulations indicate that a well-formed wakefield is nevertheless generated with electric field gradients of ${E}_{z}\ensuremath{\gtrsim}2\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ assuming 2.5 TW laser peak power. The model indicates pulse steepening is occurring due to various nonlinear effects. It is possible that this intermediate laser pulse length mode of operation may permit the creation of well-formed, regular-shaped wakefields, which would be needed for staging the LWFA process. Discussed in this paper are the model, its predictions for an LWFA experiment at the ATF, and the pulse steepening effect.

Self-modulated wakefield and forced laser wakefield acceleration of electrons

The interaction of intense laser pulses (power&amp;gt;30 TW) with underdense plasmas has been studied. In the regime where the pulse length is much longer than the plasma period (τl≫2πωp−1), the laser pulse is found to be self-modulated at the plasma frequency by the forward Raman scattering instability. Wavebreaking of the resulting plasma wave results in energetic electrons being accelerated to more than 100 MeV. Reducing the pulse length so that τl∼2πωp−1, but retaining the same power, also leads to wavebreaking. This is a direct result of a combination of laser beam self-focusing, front-edge laser pulse steepening and relativistic lengthening of the plasma wave wavelength, which can result in a forced growth of the wakefield plasma wave, even for initially nonresonant laser pulses (τl≠πωp−1). Since, in this forced laser wakefield regime, the interaction of the plasma wave and the bunch of accelerated electrons with the laser pulse is reduced, this can result in higher energy gain (to beyond 200 MeV) and better beam quality.

Unconditional temporal instability and self-pulse structuration in the phase-matched degenerate backward optical parametric oscillator

Stability analysis of the degenerate backward optical parametric oscillator in the phase-matching decay interaction, between a CW-pump and a sub-harmonic counter-propagating signal, proves that the inhomogeneous stationary solutions are always temporally unstable whatever the cavity length and pump power above threshold. The nonlinear dynamics exhibits self-pulse structuration of the backward signal with unlimited amplification and compression. Above a critical pulse steepening, dispersion saturates this singular behavior leading to strongly modulated solitary structures.

Asymmetric ultra-short pulse splitting measured in air using FROG

Frequency-resolved optical gating is used to measure non-linear ultra-short pulse propagation in air over a distance of up to 4.35 m. During spatial collapse of the pulse, both pulse steepening and asymmetric pulse splitting are observed to occur at a power level of about 14 GW and at a propagation distance less than the diffraction length.