Short-pulse Source Research Articles

Terahertz pulsed spectroscopy of paraffin-embedded brain glioma.

The refractive indices, absorption coefficients, and complex dielectric constants of paraffin-embedded brain glioma and normal brain tissues have been measured by a terahertz time-domain spectroscopy (THz-TDS) system in the 0.2- to 2.0-THz range. The spectral differences between gliomas and normal brain tissues were obtained. Compared with normal brain tissue, our results indicate that paraffin-embedded brain gliomas have a higher refractive index, absorption coefficient, and dielectric constant. Based on these results, the best THz frequencies for different methods of paraffin-embedded brain glioma imaging, such as intensity imaging, coherent imaging with continuum THz sources, and THz pulsed imaging with short-pulsed THz sources, are analyzed.

Tunable WDM sampling pulse streams using a spatial phase modulator in a biased pulse shaper

We generate transform-limited WDM optical sampling pulse bursts by filtering ultrashort pulses from a mode-locked laser. A phase spatial light modulator (SLM) is used in a biased pulse shaper to circumvent the need to modulate with 2π phase wraps, which are known to limit the phase response. The arrangement compresses and retimes user-selectable bandwidths from the optical short pulse source with precise control of pulse bandwidth, pulse stream rates, and duty cycle.

Recent advances in development of vertical-cavity based short pulse source at 1.55 μm

This paper reviews and discusses recent developments in passively mode-locked vertical external cavity surface emitting lasers (ML-VECSELs) for short pulse generation at 1.55 μm. After comparing ML-VECSELs to other options for short pulse generation, we reviewed the results of ML-VECSELs operating at telecommunication wavelength and point out the challenges in achieving sub-picosecond operation from a ML-VECSEL at 1.55 μm. We described our recent work in the VECSELs and semiconductor saturable absorber mirrors (SESAMs), their structure design, optimization and characterization, with the goal of moving the pulse width from picosecond to sub-picosecond.

Development of Au–In–Cd decoupler by a hot isostatic pressing (HIP) technique for short pulsed spallation neutron source

In order to utilize the Au–In–Cd alloy as a decoupler for the MW-class spallation neutron sources, it is required to bond Au–In–Cd alloy to an aluminum alloy (A5083), which from the structural material of the moderator in the Japan Spallation Neutron Source. The hot isostatic pressing (HIP) technique is adopted to bond the Au–In–Cd alloy to the A5083 alloy in the form of the curved shape of the moderator vessel. We obtained an adequate bonding strength of 95.2MPa, which was three times larger than the required design strength (30MPa) under the conditions of a temperature of 535°C, pressure of 100MPa and a holding time of 1h.

High power THz sources and applications at ENEA-Frascati

ENEA has a long term expertise in the development of powerful short-pulse mm-wave and THz radiation sources driven by free electrons. Various electron-wave interaction schemes were successfully tested in the past, including Cerenkov and Smith-Purcell radiators as well as undulator devices. Two THz-FEL sources are currently available, covering altogether the spectral range from 90 GHz to 0.7 THz. Recently a novel Electro-Magnetic pulser, capable of providing both nanosecond THz electromagnetic (EM) radiation pulses and electrostatic (ES) pulses with identical time duration in one device has been designed and is currently under development. The pulser will allow, for the first time, direct comparison of EM and ES pulses on biological systems with peak electric fields that are significantly higher than any reported to date.

Evaluating Local Elastic Anisotropy of Rocks and Sediments by Means of Optoacoustics While Drilling Oil and Gas Boreholes

The optoacoustic method is used to evaluate local elastic anisotropy of rocks and sediments. The method is based on laser sound generation by irradiating a spot on the wall of the oil or gas borehole. The optoacoustic method offers an advantage of precise non-contact placing of a short-pulse point sound source. Pulses of a compression wave, shear waves, and a surface wave are induced in the formation as a result of optoacoustic conversion. The surface trace of the bulk compression wave propagating along the borehole surface has a velocity corresponding to that of a bulk wave. Hence, measurements of the trace propagation time along several predetermined paths on the surface of a borehole provide evaluation of local elastic anisotropy in situ. The pick-up is provided with a piezoelectric ceramic transducer positioned at a predetermined point on the surface of the borehole. The optoacoustic conversion regime parameters were chosen to provide separation of the trace pulse of another surface perturbance at the travel distance of about 0.1 m. The local measurements on the borehole wall are aimed to support accurate depth imaging of seismic data. Understanding these common anisotropy effects is important when interpreting seismic data where they are present.

P195 Plume characteristics of fluticasone propionate/formoterol pMDI compared with fluticasone propionate/salmeterol pMDI

BackgroundIt has been suggested that aerosol inhaler characteristics such as fine particle size distribution, low plume velocity, and long duration of the aerosol cloud may assist coordination of inhalation with...

Experimental Electrically Reconfigurable Time-Domain Spectral Amplitude Encoding/Decoding in an Optical Code Division Multiple Access System

An electrically reconfigurable time-domain spectral amplitude encoding/decoding scheme is proposed herein. The setup is based on the concept of temporally pulse shaping dual to spatial arrangements. The transmitter is based on a short pulse source and uses two conjugate dispersive fiber gratings and an electro-optic intensity modulator placed in between. Proof of concept results are shown for an optical pulse train operating at 1.25 Gbps using codes from the Hadamard family with a length of eight chips. The system is electrically reconfigurable, compatible with fiber systems, and permits scalability in the size of the codes by modifying only the modulator velocity.

Nanosecond pulsed laser welding of high carbon steels

Abstract The present paper deals with the possibility to exploit low-cost, near infra-red, nanosecond pulsed laser sources in welding of high carbon content thin sheets. The exploitation of these very common sources allows to achieve sound weld beads with a good depth-to-width ratio and very small heat affected zones when the proper process parameters are involved. In particular the role of pulse frequency, pulse duration, peak power and welding speed on the characteristics of the weld beads is studied and the advantage of the application of short-pulse laser sources over traditional long-pulse or continuous wave one is assessed.

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Simulations of Dynamic Laser/Plasma X-Ray Production

Intense laser beams focused onto thin high-atomic-number targets can generate short intense bursts of MeV X-rays from a small area of the target. Such systems are being developed as short-pulse point-projection X-ray sources for imaging high-density objects. Here, large-scale (400-million macroparticles and 15-million grid cells) 3-D particle-in-cell simulations are described that model the dynamic interaction between the laser beam, a blowoff plasma layer, and the solid-density target. The simulations self-consistently treat the nonlinear interaction between the incident laser pulse and the blowoff plasma layer where a relativistic electron beam is generated. This beam propagates into the solid-density high-atomic-number target where MeV bremsstrahlung is generated. The model tracks the generation, propagation, and self-absorption of radiation in the blowoff plasma, target, and beyond. Radiation production (fluence and energy spectrum) is characterized in the simulations as a function transverse target size, laser-injection angle, and laser energy. The simulated X-ray fluence for the case of a 45 °-angle-of-incidence 100-J 0.5-ps laser pulse with a 6- μm FWHM focus produces a peak dose in excess of 0.2 rad from a 10-μm-thick square gold target, consistent with experimental measurements.

X-rays inspire electron movies

The advent of high-energy, short-pulse X-ray sources based on free-electron lasers, laser plasmas and high-harmonic generation is now making it possible to probe the dynamics of electrons within molecules.

Free-space nitrogen gas laser driven by a femtosecond filament

We report an experimental proof and full characterization of laser generation in molecular nitrogen in an argon-nitrogen gas mixture remotely excited at a distance above 2 m in a femtosecond laser filament. Filamentation experiments performed with near-infrared, 1-\ensuremath{\mu}m-wavelength and midinfrared, 4-\ensuremath{\mu}m-wavelength short-pulse laser sources show that mid-IR laser pulses enable radical enhancement of filamentation-assisted lasing by N${}_{2}$ molecules. Energies as high as 3.5 \ensuremath{\mu}J are achieved for the 337- and 357-nm laser pulses generated through the second-positive-band transitions of N${}_{2}$, corresponding to a 0.5$%$ total conversion efficiency from midinfrared laser energy to the energy of UV lasing.

Comparison of the thermal effects of the transport of a short-pulse laser and a multi-pulse laser through a participating medium

Temporal signals resulting from the boundaries of a participating medium subjected to a short-pulse laser source are investigated. Cases of both a single as well as a pulse-train consisting of multiple pulses are considered. Temporal variation of the incident laser pulse can be a step or a Gaussian function of time. Temporal evolutions of transmittance and reflectance signals, and incident radiation at various locations in the planar medium are studied for a wide range of the values of the extinction coefficient. The problem is analyzed using both the finite volume method and the discrete ordinates method. In all cases, results of both the methods are compared. An excellent agreement has been found.

Using short pulses to enhance the production rate of vibrationally excited hydrogen molecules in hydrogen discharge

Hydrogen discharges driven by the combined radio-frequency (rf)/short pulse sources are investigated using the particle-in-cell method. The simulation results show that the discharge driven additionally by the short pulse can enhance the electron density and modulate the electron energy to provide a better condition for negative hydrogen ion production than the discharge driven by the rf-only source.

A new method for measuring the response time of the high pressure ionization chamber

Time response is an important performance characteristic for gas-pressurized ionization chambers. To study the time response, it is especially crucial to measure the ion drift time in high pressure ionization chambers. In this paper, a new approach is proposed to study the ion drift time in high pressure ionization chambers. It is carried out with a short-pulsed X-ray source and a high-speed digitizer. The ion drift time in the chamber is then determined from the digitized data. By measuring the ion drift time of a 15atm xenon testing chamber, the method has been proven to be effective in the time response studies of ionization chambers.

Widely Wavelength-Tunable Two-Colored Solitons and Small Spectral Component for Broadband Mid-Infrared Wavelength Generation in a Highly Birefringent Photonic Crystal Fiber

With a highly birefringent photonic crystal fiber (HB-PCF) specially designed and fabricated in our laboratory, the two-colored solitons and small spectral component formed by the balance between the dispersion and birefringence effect are generated above 2000 nm when the polarization direction of pump pulse is aligned along the fast and slow axis of HB-PCF. When the average power of the pump at 870 nm increases from 100 to 300 mW, the broadband solitons at the fast and slow axis and small spectral component can be efficiently generated from 2120 to 2230 nm, 2070 to 2180 nm, and 2260 to 2370 nm, and the tunable wavelength range can be up to 300 nm. To the best of our knowledge, the largest span of soliton wavelength shift over 1300 nm is achieved. The widely tunable soliton pulse and small spectral component can provide the particularly useful short-pulse source for the mid-infrared photonics and spectroscopy.

Optimization of gas utilization efficiency for short-pulsed electron cyclotron resonance ion source

Numerical analysis of (6)He atoms utilizing efficiency in the ion source with powerful gyrotron heating is performed in present work using zero-dimensional balanced model of ECR discharge in a magnetic trap. Two ways of creation of ion source with high gas utilization efficiency (up to 60%-90%) are suggested.

A Neutron Diffractometer for a Long Pulsed Neutron Source

Long pulsed neutron sources are being actively developed as small university based sources and are being considered for the next generation of high powered sources, such as the European Neutron Source (ESS) and the Spallation Neutron Source (SNS) second target station. New instrumentation concepts will be required to effectively utilize the full spectrum of neutrons generated by these sources. Neutron diffractometers, which utilize time-of-flight (TOF) techniques for wavelength resolution, are particularly problematic. We describe an instrument for a long pulsed source that provides resolution comparable to that obtained on short pulsed sources without the need of long incident flight paths. We accomplish this by utilizing high speed choppers to impose a time structure on the spectrum of incident neutrons. By strategically positioning these choppers the response matrix assumes a convenient form that can be deconvoluted from the measured TOF spectrum to produce the diffraction pattern of the sample. We compare the performance of this instrument to other possible diffraction instruments that could be utilized on a long pulsed source.

Time-of-Flight Neutron Diffraction for Long Pulse Neutron Sources

The Time-of-Flight (TOF) method for neutron diffraction was proposed half a century ago by Buras and Leciejewicz [1] as a very efficient alternative to the crystal monochromator technique most often used at that time. It was the advent of the first pulsed spallation sources that has shown how farsighted this proposal was– the diffractometers naturally based on the TOF method have become most productive powerhouses at these sources. High performance in diffraction is a most basic goal and requirement for all neutron sources now or in the future and a technique to achieve this is of central importance for the new generation of long pulse sources pioneered by ESS. The central challenge for such a technique is to produce short pulsed white neutron beams from the long pulses in order to meet all resolution requirements in diffraction. No efficient method for such pulse shaping could be successfully developed at the short pulse sources. In the second half of the 1990s, in collaboration with HZB (then HMI), the Budapest Neutron Center (BNC) undertook the development of powerful TOF diffraction for both its own use and with the perspective of establishing at a continuous source the foundations of TOF diffraction techniques for future long pulse sources.