High-energy Broadband Research Articles

Differentiating between artificial and natural sources of electromagnetic radiation at a seismogenic fault

Ultralow frequency (ULF) to low frequency (LF) electromagnetic radiation represents one of the most promising effects of brittle rock strain and microcracking that might be potentially helpful for short term earthquake forecasting. In this study the results of a six month monitoring campaign are presented from Obir Cave in the eastern Alps. Direct experimental observations of electromagnetic radiation have been made using a customised broadband data logger installed next to the Obir Fault - this seismogenic fault near the Periadriatic Lineament is known to be related to at least three large prehistoric earthquakes. On the basis of these measurements it has been possible to characterise a number of distinct signals: artificial constant narrowband signals at discrete frequencies; short serial broadband impulses; high energy broadband impulses; and low energy broadband impulses. The narrowband artificial signals were removed from the electromagnetic radiation time series analysis so that the natural signals were enhanced and could be compared more easily to meteorological parameters and rock strain indicators. Critically, the high energy broadband impulses show a strong correlation with lightning activity across much of central Europe and the eastern Mediterranean while the low energy broadband impulses appear to be associated with local rock strain in and around Obir Cave. Unfortunately, it seems certain that some of the essential strain related impulses are likely to have been overprinted by the larger lightning related impulses.

High-energy broadband Nd: Glass regenerative amplifier with coated birefringent filter for the high-power laser facilities

The regenerative amplifier is an important component of the high-power laser facility. A high-energy broadband Nd: glass regenerative amplifier with an energy of 34.1 mJ and a bandwidth of 4.3 nm was demonstrated using an intra-cavity specially coated birefringent filter (CBRF). This letter proposes a design scheme for a CBRF that can control the flat-top area of gain spectrum of the regenerative amplifier, which has been studied theoretically and experimentally. The transmittance of the specially coated BRF was tested in this experiment. The amplifier output spectrum had a flexible control ability and long-term stability. It can be applied to support high-energy broadband amplification in inertial confinement fusion (ICF) facilities, and can also be extended to other broadband amplification system to support high-energy picosecond petawatt laser facilities.

Accurate model and performance analysis of broadband pulsed amplification in picosecond petawatt laser system

In order to accurately analyze the broadband pulsed amplification performances of the domestic picosecond petawatt laser system, which uses large aperture N<sub>31</sub> or N<sub>41</sub> neodymium glass as gain medium, the broadband pulsed amplification model is improved by introducing the actual stimulated emission cross section (SECS) of neodymium glass. Comparing with the SECS under Gaussian approximation, the amplified pulsed spectrum gain narrowing effect with different SECSs are analyzed. It is found that in the actual SECS of N<sub>31</sub> neodymium glass laser, the gain-narrowing effect is enhanced, the output energy decreases, gain’s saturation effect weakens, system’s accumulated <i>B</i> integral augments, but the laser system turns insensitive to the center wavelength simultaneously. Based on the Shenguang II high energy picosecond petawatt laser system which uses N<sub>31</sub> neodymium glass, the spectral shape, center wavelength, and energy stability of amplified output pulse are simulated by using different SECSs. It is shown that the super-Gaussian spectral shape narrows more greatly than Gaussian spectral shape, the spectrum bandwidth narrows from 10 to about 3 nm with gain larger than 10<sup>7</sup>, and the accumulated <i>B</i> integral increases to 1.7. Additionally, the gain-narrowing effect makes the output spectrum (with 1054 nm of center wavelength) less affected by changing the inputted center wavelength from 1052 to 1056 nm, and the gain saturation effect can improve output energy stability to less than 2% (root mean square (RMS)) with about 3% (RMS) inputted energy stability, which are beneficial to the subsequent pulse compression and physical experiment. Based on the above analysis, a broadband pulsed amplified experiment is conducted by using Shenguang II petawatt laser system, the injected seed is about 10 nm (full width at half maximum (FWHM)) with 5 order super Gaussian shape at 1054-nm center wavelength, and 1.2 mJ with 3% (RMS) energy stability from optical parametric chirped pulse amplification. The amplified pulse with 1900 J at 1054.2 nm (3 nm FWHM) and stability < 2% (shot to shot) is achieved, and the spectral shapes and bandwidths after bar and disk amplifiers are measured, which are consistent with theoretical analysis results. The results can provide a necessary reference for constructing high energy broadband laser system and improving its performances in the future.

Spectral shaping of picosecond petawatt laser system based on lithium niobate birefringent crystal

In recent years, chirped pulse amplification (CPA) technology injects vitality into the development of ultra-strong and ultra-short lasers. However, in the CPA based gain media, the gain narrowing effect limits the higher output of ultrashort pulse in energy, power, signal-to-noise ratio. In order to compensate for the gain narrowing caused by the broadband amplification of Nb:glass in picosecond pewter laser system, a method of high-energy spectral shaping is proposed based on LiNbO<sub>3</sub> birefringent crystal, and the spectral phase introduced by the crystal is analysed for the first time. Based on the strict Jones matrix, the transmittance function of birefringent crystal and the spectral phase introduced by the crystal are obtained. Further, three kinds of birefringent crystals are compared among each other, and the results show that the higher birefringence and the smaller thickness are required to achieve the same intensity modulation. For the laser pulse at 1053 nm, LiNbO<sub>3</sub> is selected as the spectral shaping crystal due to its high birefringence, large diameter, and non-deliquescent. The influences of crystal thickness, tilt angle, and in-plane rotation angle on the spectral intensity modulation are simulated theoretically, and the results show the above parameters affect the modulation bandwidth, center wavelength, and modulation depth of the shaping. By analyzing the spectral phase introduced by the crystal, it is found that the dispersion of each order changes with the thickness of the crystal, the tilt angle, and the in-plane rotation angle, and it is the most sensitive to the change of thickness. In addition, by controlling the dispersion of each order, the influence on the pulse signal-to-noise ratio can be weakened during spectrum shaping. On the basis of theoretical analysis, the shaping experiment with a center wavelength of 1053 nm, modulation bandwidth of 10 nm, and modulation depth of 80% is carried out. And the phase introduced by the LiNbO<sub>3</sub> is measured. The experimental results are consistent with the theoretical analysis. For the Shenguang Ⅱ high-energy petawatt laser system, by the above-mentioned shaping scheme, a high-energy broadband laser output of 1700 J and 6 nm (FWHM) is realized for the first time in China, which is 2 times that at 3.2 nm when it is not shaped. The research effectively compensates for the Nb:glass gain narrowing effect, and will provide references for the parameter design, material selection and spectral phase compensation in the birefringent spectral shaping.

High energy redshifted and enhanced spectral broadening by molecular alignment.

We demonstrate an efficient approach for enhancing the spectral broadening of long laser pulses and for efficient frequency redshifting by exploiting the intrinsic temporal properties of molecular alignment inside a gas-filled hollow-core fiber (HCF). We find that laser-induced alignment with durations comparable to the characteristic rotational time scale TRotAlign enhances the efficiency of redshifted spectral broadening compared to noble gases. The applicability of this approach to Yb lasers with (few hundred femtoseconds) long pulse duration is illustrated, for which efficient broadening based on conventional Kerr nonlinearity is challenging to achieve. Furthermore, this approach proposes a practical solution for high energy broadband long-wavelength light sources, and it is attractive for many strong field applications.

X-Ray Phase Contrast Imaging of Liquid Film and Spray Development Inside an Aircraft Engine Swirler

Abstract Modern aircraft engines combine liquid fuel and air using an intricate flow device with many fuel and air flow passages. To date, the process by which the fuel atomizes within this swirler set has not been examined directly due to optical access limitations. In this work, high-speed X-ray phase-contrast imaging of a liquid spray inside a gas turbine engine swirler geometry is presented. Measurements were carried out at the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory using the high-energy broadband X-ray beam. The synchrotron X-ray source provides the necessary photon energy and flux to capture time-resolved fluid phenomena within the confines of the relevant geometry while liquid and air are flowing. Spray nozzle hardware and geometries were provided by the National Jet Fuels Combustion Program (NJFCP), allowing for characterization of the spray using a commercially relevant configuration. Modified swirlers were three-dimensional printed with acrylic to improve imaging access while maintaining influential internal features. Water was used as a surrogate fluid for these studies to demonstrate the visualization capabilities. The experiments were conducted at atmospheric exit pressure conditions with a pressure drop of 6% across the swirler. High-speed imaging of the pilot spray cone revealed sheet breakup several millimeters downstream of the orifice exit, upon interaction with the radial assist air flow. These droplets and ligaments were observed to impinge on the inner filming surface of the swirler and flow toward the exit while developing a tangential flow. Under these conditions, the liquid film grows up to several hundred microns in thickness on the filming surface, and subsequently forms ligaments up to several millimeters in length before breaking up. This work demonstrates the capability of X-ray diagnostics in visualizing liquid flows within solid geometries of technical relevance. Furthermore, the spatial quantification of filming flows and liquid interaction with the swirler air provides validation data for modeling of the multiphase flows and surface interactions within the swirler.

High peak and average power Ti:sapphire thin disk amplifier with extraction during pumping.

The combination of the extraction during pumping (EDP) amplification scheme and the thin disk (TD) technology has been successfully applied to the Ti:sapphire (Ti:sa) laser medium for the first time, to the best of our knowledge. In a proof-of-principle experiment, we demonstrate high energy broadband amplification in a room temperature water cooled EDP-TD head of stretched femtosecond pulses at a 10 Hz repetition rate, instead of performing a cryogenically cooled traditional multi-pass scheme. Hence, the EDP-TD combination can overcome the limits associated with thermal effects and transverse amplified spontaneous emission, enabling Ti:sa laser systems to have a petawatt peak and hundreds of watts of average power.

Generation of high-energy broadband femtosecond deep-ultraviolet pulses by highly nondegenerate noncollinear four-wave mixing in a thin transparent solid

We present a simple and efficient technique for the generation of ultrashort deep-ultraviolet pulses based on four-wave mixing of noncollinear laser pulses in a thin solid. Sub-30-fs pulses (Fourier-limit of 13 fs) centered at 270 nm, with energies up to 6 μJ, were obtained by mixing the fundamental and the second harmonic of a Ti:sapphire amplifier in fused silica. Temporal characterization was performed with a dispersionless self-diffraction FROG setup. Spectra as broad as 20 nm were also obtained that can in principle support sub-4-fs deep-ultraviolet pulses.The results are well described by two-dimensional numerical simulations.

High-power top-hat pulses from a Yb master oscillator power amplifier for efficient optical parametric amplifier pumping

We demonstrate shaping of high-energy broadband Yb amplifier pulses for the generation of a (sub)picosecond top-hat temporal pulse profile that significantly improves pumping efficiency of an optical parametric amplifier (OPA). Phase-only modulation is applied by an acousto-optic programmable dispersion filter. This simple scheme is scalable to a high average power due to a relatively broad bandwidth of the Yb:CaF(2) gain medium used in the amplifier that supports a sub-150-fs transform-limited pulse duration. Additionally we show that OPA seeding with supercontinuum remains possible because top-hat-shaped pulses passed through a glass block recompress to ≈200 fs with minimum satellite production.

The metallicity of gamma-ray burst environments from high-energy observations

Gamma-ray bursts (GRBs) and their early afterglows ionise their circumburst material. Only high-energy spectroscopy therefore, allows examination of the matter close to the burst itself. Soft X-ray absorption allows an estimate to be made of the total column density in metals. The detection of the X-ray afterglow can also be used to place a limit on the total gas column along the line of sight based on the Compton scattering opacity. Such a limit would enable, for the first time, the determination of lower limits on the metallicity in the circumburst environments of GRBs. In this paper, we determine the limits that can be placed on the total gas column density in the vicinities of GRBs based on the Compton scattering. We simulate the effects of Compton scattering on a collimated beam of high energy photons passing through a shell of high column density material to determine the expected lightcurves, luminosities, and spectra. We compare these predictions to observations, and determine what limits can realistically be placed on the total gas column density. The smearing out of pulses in the lightcurve from Compton scattering is not likely to be observable, and its absence does not place strong constraints on the Compton depth for GRBs. However, the distribution of observed luminosities of bursts allows us to place statistical, model-dependent limits that are typically <~1e25 cm^{-2} for less luminous bursts, and as low as ~1e24 cm$^{-2} for the most luminous. Using the shape of the high-energy broadband spectrum, however, in some favourable cases, limits as low as ~5e24 cm^{-2} can placed on individual bursts, implying metallicity lower limits from X- and gamma-rays alone from 0 up to 0.01 Z/Zsun. At extremely high redshifts, this limit would be at least 0.02 Z/Z_sun, enough to discriminate population III from non-primordial GRBs.

高损伤宽带啁啾镜

A high-energy broadband chirped mirror is designed for sub-10-fs high-energy chirped-pulse oscillators and amplification systems. This mirror consists of TiO2/SiO2 quarter-wave coatings for a broad bandwidth and optimized HfO2/SiO2 chirped coatings for increasing the laser-induced damage threshold (LIDT). The calculated results show that the mirror has low group delay dispersion (GDD) oscillation and can withstand high-energy chirped pulse laser in a wide range of 720−880 nm. OCIS codes: 310.6860, 320.5520, 140.3330. doi: 10.3788/COL201008S1.0156.

High-energy broadband four-wave optical parametric amplification in bulk fused silica

Efficient broadband four-wave optical parametric amplification in bulk Kerr medium (fused silica) is demonstrated by means of noncollinear phase matching and cylindrical focusing geometry without the onset of beam breakup and filamentation. Amplified signal energy as high as 180 microJ with 1 ps, 1.8 mJ pumping at 1,055 nm is achieved with pump-to-signal energy conversion close to 10%. More than 70 nm FWHM parametric gain bandwidth around 740 nm is demonstrated without imposing angular dispersion on the frequency components of the broadband seed signal.

Development of a high-contrast, high-peak power, Ti:Sapphire/OPCPA laser system

We report on a high-contrast, high-peak power laser system, which combines both conventional laser amplification and optical parametric chirped pulse amplification (OPCPA) techniques. By employing an OPCPA system after a regenerative amplifier, we have enhanced the prepulse contrast by six orders of magnitude. A prepulse contrast of better than 4.4 × 10−11 has been measured with a high-energy broadband pulse of 24 mJ at a 10 Hz repetition rate from the OPCPA system. With a subsequent four-pass Ti:sapphire amplifier, we have achieved an amplified energy of 279 mJ and an ultrashort recompressed amplified pulse duration of 23.5 fs. We also discuss the basic design aspects and present our preliminary experimental investigations for higher energy operation to a multijoule.

Optical stability of the Cr3+ centres in codoped stoichiometric and congruent LiNbO3:Cr:Mg

The optical stability of the R-lines and broad band associated with the Cr3+ ions has been studied in codoped congruent and near stoichiometric LiNbO3:Cr:Mg crystals. It has been found that the 2E→4A2 transition of the Cr3+ ions located in Nb5+ sites, [Cr]Nb, can be bleached under optical excitation of the high energy broad absorption band luminescence whereas the emission from the same transition but ascribed to Cr3+ ions located in Li+ sites, [Cr]Li, is optically stable. At variance, the broad emission associated with the 4T2→4A2 vibronic transition ascribed to these centres is optically stable.The recovery temperature for the optical bleaching has been found to be 150K, and has been correlated to Nb4+ electron traps located in Nb5+ antisite position.The different optical stability found for the R-lines and broad band emission associated with the [Cr]Nb sites confirm the existence of two [Cr]Nb centres with low and high crystal field, respectively.

Prediction Method for Broadband Noise from Unsteady Flow in a Slat Cove

Noise from high-lift devices such as slats and flaps can contribute significantly to the overall aircraft sound pressure levels, particularly during approach. The acoustic spectrum of the noise radiated from slats exhibits two distinct features. There is a high-frequency tonal noise component and a high-energy broadband component ranging from low- to midfrequencies. The objective is to predict the broadband slat noise. The broadband noise is predicted using a two-step process. First, the noise sources are modeled based on the local turbulence information. Then, the sound from these sources is propagated by assuming that the flow past the wing is uniform. A boundary element method is used to find Green’s function for wave propagation in a moving medium in the presence of the wing. The noise in the far field is then predicted by forming a convolution of Green’s function with the modeled sources. The attractive feature of this prediction scheme is the quick computational time, which makes it suitable for new design and control strategies.

Prepulse-free, multi-terawatt, sub-30-fs laser system

We have built a prepulse-free, multi-terawatt, ultrashort pulse laser system, which combines both conventional laser amplification and optical parametric chirped pulse amplification (OPCPA) techniques. By employing an OPCPA system after the regenerative amplifier in a Ti:sapphire chirped pulse amplification laser chain, we have dramatically enhanced the prepulse contrast by 6 orders of magnitude. A prepulse contrast of better than 4.4 x 10-11 has been measured with a high energy broadband pulse of 24 mJ at 10 Hz repetition rate from the OPCPA system. Using a subsequent four-pass Ti:sapphire amplifier, we have achieved an amplified energy of 279 mJ and an ultrashort recompressed amplified pulse duration of 23.5 fs, corresponding to the peak powers for OPCPA and four-pass amplifier of 0.5 TW and 5.9 TW, respectively.

Unusual heat transport in underdoped cuprates

Within the t– J model, the heat transport of the underdoped cuprates is studied based on the fermion–spin theory. It is shown that at low temperatures the energy dependence of the thermal conductivity spectrum consists of two bands. The higher-energy band shows a weak peak, while the low-energy peak is located at a finite energy. This high-energy broad band is severely suppressed with increasing temperatures, and vanishes at higher temperature. It is also shown that the temperature dependence of the thermal conductivity increases monotonously with increasing temperatures, in agreement with experiments.

Indications of c-axis Charge Transport in Hole Doped Triangular Antiferromagnets**The project supported by National Natural Science Foundation of China under Grant Nos. 10074007, 10125415, and 90103024

The c-axis charge transport of the hole doped triangular antiferromagnet is investigated within the t-J model by considering the incoherent interlayer hopping. It is shown that the c-axis charge transport of the hole doped triangular antiferromagnet is essentially determined by the scattering from the in-plane fluctuation. The c-axis conductivity spectrum shows a low-energy peak and the unusual high-energy broad band, while the c-axis resistivity is characterized by a crossover from the high temperature metallic-like behavior to the low temperature insulating-like behavior, which is qualitatively consistent with those of the hole doped square lattice antiferromagnet.

CORRELATION OF CRITICAL TEMPERATURES AND ELECTRICAL PROPERTIES IN TITANIUM FILMS

Recently transition-edge sensors (TES) have obtained an increasing interest as light detectors due to their high energy resolution and broadband response. Titanium (Ti), with transition temperature up to 0.5 K, is among the suitable materials for TES application. In this work we investigate Ti films obtained from two materials of different purity deposited by e-gun on silicon nitride. Films with different thickness and deposition substrate temperature have been measured. Critical temperatures, electrical resistivities and structural properties obtained from x-ray are related to each other.

Underwater high energy acoustic communications device

An underwater high energy acoustic communications device has inner shell-like structure having an outer surface connected to first ends of tuning springs and outer shell-like structure having an inner surface connected to second ends of the tuning springs to concentrically locate and space the inner and outer structures apart. Explosive units mounted on the inner surface of the outer structure each includes an explosive squib, rigid tube, and projectile. A micro-controller feeds firing signals over electrical leads to the explosive units to detonate them and fire their projectiles to strike the inner structure that emits ringing high-energy broadband acoustic signals that are propagated into surrounding seawater. A transducer connected to the micro controller receives acoustic signals from a remote source to activate the micro-controller to generates preprogrammed sequences of firing signals to propagate coded messages by the high-energy broadband, acoustic signals.