FWHM Pulse Length Research Articles

Stable fundamental and dual-pulse mode locking of red-emitting VECSELs

We present a thorough characterization of a red-emitting mode-locked semiconductor disk laser producing the first stable fundamental mode-locked pulse train in the red spectral range. The achievable temporal FWHM pulse length of the Lorentzian-shaped pulses is just below 3 which represents the shortest pulses obtained in a fully-resonant absorber and gain structure design. We also include an intracavity birefringent filter to stabilize and tune the emission wavelength between and . This, in combination with a real-time sampling of the emitted pulse train, allows us to distinguish three regimes according to their round-trip dynamics. The first regime resembles stable fundamental mode locking and is observed for wavelengths below while for an emission wavelength of the continuous wave regime is observed. These two operating regimes enclose the dual-pulse mode locking regime appearing between and . This regime is characterized by an energy exchange between the individual pulses of the dual-pulse mode locking regime and we clearly relate the side-peaks in the radio frequency spectrum to the dual-pulse energy-exchange frequency and the temporal interpulse spacing to the individual arm lengths of the gain-folded external cavity. Overall, we attribute these different operating regimes to near-bandgap operation of the SESAM which allows for dispersion of saturation fluence and modulation depth.

Laser ignition of a multi-injector LOX/methane combustor

This paper reports the results of a test campaign of a laser-ignited combustion chamber with 15 shear coaxial injectors for the propellant combination LOX/methane. 259 ignition tests were performed for sea-level conditions. The igniter based on a monolithic ceramic laser system was directly attached to the combustion chamber and delivered 20 pulses with individual pulse energies of $${33.2 \pm 0.8 \,\text{ mJ }}$$ at 1064 nm wavelength and 2.3 ns FWHM pulse length. The applicability, reliability, and reusability of this ignition technology are demonstrated and the associated challenges during the start-up process induced by the oxygen two-phase flow are formulated. The ignition quality and pressure dynamics are evaluated using 14 dynamic pressure sensors distributed both azimuthally and axially along the combustion chamber wall. The influence of test sequencing on the ignition process is briefly discussed and the relevance of the injection timing of the propellants for the ignition process is described. The flame anchoring and stabilization process, as monitored using an optical probe system close to the injector faceplate connected to photomultiplier elements, is presented. For some of the ignition tests, non-uniform anchoring was detected with no influence onto the anchoring at steady-state conditions. The non-uniform anchoring can be explained by the inhomogeneous, transient injection of the two-phase flow of oxygen across the faceplate. This characteristic is verified by liquid nitrogen cold flow tests that were recorded by high-speed imaging. We conclude that by adapting the ignition sequence, laser ignition by optical breakdown of the propellants within the shear layer of a coaxial shear injector is a reliable ignition technology for LOX/methane combustors without significant over-pressure levels.

Proton emission from thin hydrogenated targets irradiated by laser pulses at 1016 W/cm2

The iodine laser at PALS Laboratory in Prague, operating at 1315 nm fundamental harmonics and at 300 ps FWHM pulse length, is employed to irradiate thin hydrogenated targets placed in vacuum at intensities on the order of 10(16) W∕cm(2). The laser-generated plasma is investigated in terms of proton and ion emission in the forward and backward directions. The time-of-flight technique, using ion collectors and semiconductor detectors, is used to measure the ion currents and the corresponding velocities and energies. Thomson parabola spectrometer is employed to separate the contribution of the ion emission from single laser shots. A particular attention is given to the proton production in terms of the maximum energy, emission yield, and angular distribution as a function of the laser energy, focal position, target thickness, and composition. Metallic and polymeric targets allow to generate protons with large energy range and different yield, depending on the laser, target composition, and target geometry properties.

Production of Nanopowders Using Nanosecond Electron Beam

Frequency nanosecond electron accelerators type URT has been developed for use in radiation technologies. The accelerator includes a thyratron, a pulse transformer and a semiconductor opening switch. The accelerating voltage is up to 0.9 MV, FWHM pulse length is up to 60 ns, and the repetition rate is up to 250 Hz. A metal ceramic cathode, which is used in the accelerator, provides an electron beam cross section up to 16 cm in diameter or rectangular with the dimensions of 45 * 5cm2 at a maximum density of the pulse current equal to 2 A/cm2. Thus, a family of nanosecond electron accelerators has been developed for use in radiation technologies applied to layers up to 0.4 g/cm2 thick, such as production of nanopowders from gas and liquid precursors. Experiments on the irradiation of silver nitrate solutions in various liquids by a URT-0.5 accelerator facilitated the development of technology for producing silver nanopowders (NPs). The powder yield and particle size as a function of the irradiation mode has been established. An increase in the adsorbed dose results in an increased powder yield; however, the particle size decreases and the powder agglomeration rate increases. Therefore, the targeted production of silver NPs with a mean particle size in the range of 90–1100 nm is possible. Besides to produce TiO2 and SiO2 NPs the gas precursors were used.

Fast mechanical shutter for pulsed ion beam generation

A pneumatically actuated mechanical shutter has been developed for pulsed ion beam operation. The mechanical shutter provides vacuum isolation between the gas filled ion source (nominally at ∼10mTorr) and the high vacuum accelerator chamber (nominally <μTorr), which is essential for applications involving high voltage gradients. In the present design, the FWHM pulse length of the ion beam was measured to be approximately 1.5ms with rise and decay times of ∼1ms. Steady-state pressure measurements show an order of magnitude decrease in the main chamber pressure by incorporating the shutter.

High harmonic generation as a seed source for free electron lasers

The design of a high harmonic generation (HHG) source for seeding the proposed New Light Source free electron laser (FEL) is presented. The seed source is designed to cover the energy range from 50 to 100 eV with a peak power of 400 kW and a 20 fs FWHM pulse length and is tuneable to provide continuous coverage over the full 50–100 eV range. The seed pulses are produced by HHG in rare gases in the loose-focusing regime, driven by a high-power kHz laser system. Continuous tunability is achieved by varying the wavelength of the drive laser over ±25 nm from the 800 nm centre wavelength. Future prospects for increasing the HHG yield and short wavelength cut-off by using multi-wavelength generation schemes and longer drive wavelengths are considered, as is the potential for FEL seeding at higher repetition rates.

First experimental implementation of pulse shaping for neutron diffraction on pulsed sources

One of the central issues in the design and the use of pulsed neutron sources is the control of pulse length in elastic scattering experiments, most significantly diffraction on crystalline matter. On the existing short pulse spallation sources the strongly wavelength dependent source pulse length that determines the resolution is permanently fixed on each beam line by the type of the moderator it faces. We have experimentally implemented for the first time the wavelength frame multiplication (WFM) multiplexing chopper method, an earlier proposed variant of the by now fully tested repetition rate multiplication technique for inelastic scattering spectroscopy on pulsed neutron sources. We have operated the time-of-flight diffractometer at the continuous reactor source at BNC in an unconventional multiplexing mode that emulates a pulsed source. As a full proof of principle of the WFM method we have experimentally demonstrated the extraction from each source pulse a series of polychromatic, chopper shaped neutron pulses, which can continuously cover any wavelength band. The achieved 25 μs FWHM pulse length is shorter than that can be obtained at all at short pulse spallation sources for cold neutrons. The method allows us to build efficient, high and variable resolution diffractometers at long pulse spallation sources.

Pulsed interfermetric holography of laser-produced air breakdown

Air breakdown at pressures of 300 and 753 torr was produced by a 28J, 40 ns FWHM neodymium glass laser focused with a 100 mm focal length lens. The breakdown region was observed by recording infinite fringe, interferometric, diffuse holograms with a single-mode ruby laser of 100 ns FWHM pulse length. The two lasers were synchronized to examine the time region from < 1–38 μs after initiation. Shock wave velocities of about Mach 2 were typical at these times and the shock front was virtually spherical. The R-t diagram was found to be in excellent agreement with the weak spherical blast wave theory of Sakurai. The radial density profile of the shock wave was determined by an Abel inversion of the fringe pattern. Both the R-t function and the density profile were calculated using a one-dimensional, hydrodynamic model and good agreement was obtained.