High Repetition Rate Laser Research Articles

Simultaneous 10-kHz PLIF and Chemiluminescence Imaging of OH Radicals in a Microwave Plasma-Enhanced Flame

This paper examines the structure of microwave (MW)-enhanced flames through 10-kHz imaging. High repetition rate laser diagnostic methods are used to simultaneously record 2-D images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma-enhanced flame. Collecting both OH planar laser-induced fluorescence and chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume-integrated excited state emission. A tunable, MW waveguide plasma source-operating at 2.45 GHz and delivering 90-130 W to the flowfield-ignites and sustains a CH4/air flame, whereas laser-induced fluorescence and chemiluminescence are acquired at a sustained framing rate of 10 kHz, using two intensified CMOS cameras and a synchronized laser. Multiple geometries and flames (premixed and nonpremixed) are studied by adjusting gas flow compositions and the plasma applicator nozzle components. A stoichiometric premixed flame configuration produces a divergent flame with large-scale fluctuations and vortex shedding into ambient air and is capable of feedstock flow velocities for combustion-to-plasma power ratios . Another arrangement produces plasma along the initial mixing layer of a nonpremixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with a little flamefront fluctuation. The high-speed diagnostics capture OH signals in cinematic sequences, providing new understanding of the plasma-assisted flame holding mechanism and allowing for the tracking of individual flow feature development.

Electron densities and energies of a guided argon streamer in argon and air environments

In this study we report the temporally and spatially resolved electron densities and mean energies of a guided argon streamer in ambient argon and air obtained by Thomson laser scattering. The plasma is driven by a positive monopolar 3.5 kV pulse, with a pulse width of 500 ns and a frequency of 5 kHz which is synchronized with the high repetition rate laser system. This configuration enables us to use the spatial and temporal stability of the guided streamer to accumulate a multitude of laser/plasma shots by a triple grating spectrometer equipped with an ICCD camera and to determine the electron parameters. We found a strong initial ne-overshoot with a maximum of 7 × 1019 m−3 and a mean electron energy of 4.5 eV. This maximum is followed by a fast decay toward the streamer channel. Moreover, a 2D distribution of the electron density is obtained which exhibits a peculiar mushroom-like shape of the streamer head with a diameter significantly larger than that of the emission profile. A correlation of the width of the streamer head with the expected pre-ionization channel is found.

Direct laser writing of buried waveguide in As_2S_3 glass using a helical sample translation

We report the fabrication and the characterization of buried waveguide in As(2)S(3) glass. It is well known that the interaction of femtosecond pulses with this material at high laser repetition rates results in a mainly negative refractive index variation, due to heat accumulation effect. However, we show here that a helical translation of the sample parallel to the laser beam, allows the inscription of a core of positive refractive variation, with full control over its magnitude and diameter. An example demonstrating the high symmetry of the guided mode is given.

Direct laser writing of microstructures on optically opaque and reflective surfaces

Direct laser writing (DLW) based on ultra-localized polymerization is an efficient way to produce three-dimensional (3D) micro/nano-structures for diverse applications in science and industry. It is attractive for its flexibility to materialize CAD models out of wide spectrum of materials on the desired substrates. In case of direct laser lithography, photo-crosslinking can be achieved by tightly focusing ultrashort laser pulses to a photo- or thermo-polymers. Selectively exposing material to laser radiation allows creating fully 3D structures with submicrometer spatial resolution. In this paper we present DLW results of hybrid organic–inorganic material SZ2080 on optically opaque and reflective surfaces, such as silicon and various metals (Cr, Ti, Au). Our studies prove that one can precisely fabricate 2D and 3D structures with lower than 1μm spatial resolution even on glossy or rough surfaces (surface roughness rms 0.068–0.670μm) using sample translation velocities of up to 1mm/s. Using femtosecond high pulse repetition rate laser, sample translation velocity can reach over 1mm/s ensuring repeatable submicrometer structuring resolution.

Carrier-envelope phase stable few-cycle pulses at 400 kHz for electron-ion coincidence experiments

Coincident electron-ion detection after photoionization in a "reaction microscope" is a very powerful tool to study atomic and molecular dynamics. However, the implementation of this tool in the field of attosecond science has so far been rather limited, due to the lack of high repetition rate laser sources capable of delivering few-cycle pulses with sufficient energy per pulse. In this article, the development of a Non-collinear Optical Parametric Amplifier (NOPA) capable of delivering Carrier-Envelope Phase (CEP) stable pulses with sub-6 fs duration and pulse energies in the few-µJ range is presented. The potential of combining the high repetition rate source and a reaction microscope operating at this high frequency is demonstrated in a proof-of-principle experiment on strong field ionization of Ar atoms.

Beam collimation and energy spectrum compression of laser-accelerated proton beams using solenoid field and RF cavity

This paper presents a new method of laser produced proton beam collimation and spectrum compression using a combination of a solenoid field and a RF cavity. The solenoid collects laser-driven protons efficiently within an angle that is smaller than 12 degrees because it is mounted few millimeters from the target, and collimates protons with energies around 2.3MeV. The collimated proton beam then passes through a RF cavity to allow compression of the spectrum. Particle-in-cell (PIC) simulations demonstrate the proton beam transport in the solenoid and RF electric fields. Excellent energy compression and collection efficiency of protons are presented. This method for proton beam optimization is suitable for high repetition-rate laser acceleration proton beams, which could be used as an injector for a conventional proton accelerator.

SU-E-T-52: Evaluation of EBT2 and EBT3 Films for Dosimetry in Laser-Driven Ion Accelerators

Purpose: Laser‐driven ion accelerators could be a future option for a compact medical ion accelerator for radiation therapy. Laser ion accelerators can deliver a typical dose of few Gy in a single ion pulse of ns duration. Their dose rate, thus, exceeds the typical dose rate of a medical accelerator by several orders of magnitude. Dosimetry at high repetition rate laser ion accelerators requires 2D dosimeters with dose‐rate independent response. Both requirements can be fulfilled by Gafchromic EBT2 or EBT3 films. Energies for application of laser accelerated ions (e.g. in cell irradiation experiments) are still limited to about 10 MeV. Dose response and energy dependence of Gafchromic EBT2 or EBT3 films are, therefore, evaluated in the low proton energy regime as delivered by a laser ion accelerator and compared to clinical photon and proton beams. Methods: The dose response of EBT2 and EBT3 film has been characterized for a wide proton energy range. 6 MV photon irradiations at a clinical Linac provide a reference curve. The energy dependence of both films, has been studied in particular for low proton energies as predominating in the vicinity of the Bragg peak. Mono‐energetic beams of 3 MeV, 11 MeV and 20 MeV but also up to a maximum energy of 200 MeV were used for these studies. Results: In general, EBT2 and EBT3 have a comparable performance and show similar response to photons and protons down to a proton energy of 10 MeV. A dose under‐estimation by up to 20 % in the Bragg peak region is visible in depth dose measurements of both types of film. Conclusion: EBT 2 and EBT3 films can be applied as 2D dosimeter in the low proton energy regime of a laser ion accelerator. Dose under‐response corrections are required for proton energies below 10 MeV. This work was funded by the DFG cluster of excellence Munich Centre for Advanced Photonics (MAP).

Bright betatron X-ray radiation from a laser-driven-clustering gas target

Hard X-ray sources from femtosecond (fs) laser-produced plasmas, including the betatron X-rays from laser wakefield-accelerated electrons, have compact sizes, fs pulse duration and fs pump-probe capability, making it promising for wide use in material and biological sciences. Currently the main problem with such betatron X-ray sources is the limited average flux even with ultra-intense laser pulses. Here, we report ultra-bright betatron X-rays can be generated using a clustering gas jet target irradiated with a small size laser, where a ten-fold enhancement of the X-ray yield is achieved compared to the results obtained using a gas target. We suggest the increased X-ray photon is due to the existence of clusters in the gas, which results in increased total electron charge trapped for acceleration and larger wiggling amplitudes during the acceleration. This observation opens a route to produce high betatron average flux using small but high repetition rate laser facilities for applications.

Parasitic heating effects in high frame rate laser imaging experiments

The number of applications using high frame rate imaging in combustion research has grown rapidly in recent years. Enabled by continuous improvements in laser power, a wide range of diagnostics have been developed to measure velocity, species concentration, and temperature. Growing attention is focused on measurements near surfaces, e.g., to gain better insight into transient boundary layer flows in internal combustion engines. During such experiments, laser light is used to illuminate the gas phase region above the surface, but often the laser beam is terminated into the surface directly. Thus, laser operation at several kilohertz and power levels in the range of 10–100 W raise concerns about heating the surface and altering the conditions in the gas phase. In other words, the non-intrusive properties of laser diagnostics might not be guaranteed under such conditions. We have investigated the effect of heating by high repetition rate lasers by measuring the temperature of an exposed metal surface with an infrared sensor and by various simulation approaches. The current results show a modest but noticeable influence of laser heating.

Nanoindentation studies on waveguides inscribed in chalcogenide glasses using ultrafast laser

Optical straight waveguides are inscribed in GeGaS and GeGaSSb glasses using a high repetition-rate sub-picosecond laser. The mechanical properties of the glasses in the inscribed regions, which have undergone photo induced changes, have been evaluated by using the nano-indentation technique. Results show that the hardness and elastic modulus of the photo-modified glasses are significantly lower as compared to the other locations in the waveguide, which tend to be similar to those of the unexposed areas. The observed mechanical effects are found to correlate well with the optical properties of the waveguides. Further, based on the results, the minimum threshold values of hardness and elastic modulus for the particular propagation mode of the waveguide (single or multi), has been established.

The effect of long timescale gas dynamics on femtosecond filamentation

Femtosecond laser pulses filamenting in various gases are shown to generate long- lived quasi-stationary cylindrical depressions or 'holes' in the gas density. For our experimental conditions, these holes range up to several hundred microns in diameter with gas density depressions up to ~20%. The holes decay by thermal diffusion on millisecond timescales. We show that high repetition rate filamentation and supercontinuum generation can be strongly affected by these holes, which should also affect all other experiments employing intense high repetition rate laser pulses interacting with gases.

Ultrastable bonding of glass with femtosecond laser bursts

We report on the welding of fused silica with bursts of ultrashort laser pulses. By optimizing the burst frequency and repetition rate, we were able to achieve a breaking resistance of up to 96% of the bulk material, which is significantly higher than conventional high repetition rate laser bonding. The main reason for this stability increase is the reduced stress in the surroundings of the laser induced weld seams, which is proven by measurements of the stress-induced birefringence. A detailed analysis of the shape of the molten structures shows elongated structures in the burst regime. This can be attributed to stronger heating, which is supported by our thermodynamic simulations of the laser melting and bonding process.

Coherent beam combination laser system using SBS-PCM for high repetition rate solid-state lasers

In this paper, the coherent beam combination technique using SBS-PCM for a high repetition rate laser will be demonstrated. It is a novel concept capable of achieving a high energy and high repetition rate laser. A phase conjugation mirror and beam combination laser using a self-phase-controlling technique for coherent beam combination is introduced in this paper. And, a practical demonstration of the coherent beam combination laser using SBS-PCM is presented. With a wave-front dividing scheme, the relative phases are measured to be less than λ/26 even when the amplifier is operating. The coherent beam combination laser using SBS-PCMs has the additional advantage of providing accurate high speed/high rate injection of the fusion target in the LIFE reactor system. The coherent beam combination of 25 100J/10Hz laser modules for the 2.5kJ/10Hz laser beam line for the fusion energy driver is suggested, which is available with the current laser technologies.

32 W百兆赫兹高重频可调谐单频激光光源

32 W百兆赫兹高重频可调谐单频激光光源

HIDEX: A new high resolution x-ray spectrometer for detailed line profile measurements

We present preliminary obtained with a new spectrometer based on the Johann configuration of cylindrically bent crystals, the HIDEX. The aim of this instrument is to provide detailed line shape and shift measurements of transitions originating from high intensity laser/matter interaction, especially when matter is in extreme conditions of temperature and density. The HIDEX provides two new features. First, its alignment procedure has been improved being now based on an accurate motorized rotation stage that provides a robust and fast way to position the main components in the desired geometrical configuration. Second, there is the option to mount a Charge Coupled Device (CCD) as detector, allowing the instrument to be operated in high repetition rate laser facilities where opening the chamber migh be a critical issue. Here, we report about the test of the prototype at PALS kilo-joule laser facility, Prague, that demonstrated the new alignment procedure concept. First results are discussed.

Implementation of High Dynamic Raman Lidar System for 3D Map of Particulate Optical Properties and Their Time Evolution

Fast scanning lidar system can provide aerosol volume distribution and time evolution in atmosphere. Usually fast scanning lidar has a relative simple configuration. Multiwavelength depolarization and Raman measurements gave us more information about the particulate shape, type and dimension. The most Raman lidars use high power laser source in order to get enough Raman scattering signal. One of a challenge for Raman lidar system is to have enough dynamic range to satisfy the measurements both for pure molecule Rayleigh scattering and high dene aerosol, such as dust storm, volcano emitted aerosol and high polluted urban aerosol. The estimation of microphysical properties requires independent measurements of both backscatter and extinction coefficient at several wavelengths (multiwavelength Raman lidar). Additional information can be retrieved from simultaneous measurements of the depolarization signal and water vapor mixing ratio, since those measurements are particularly useful to correlate aerosol optical properties with their shape and hygroscopicity. A multi-wavelength depolarization and Raman lidar system has been designed to perform volume scanning of the atmosphere and to retrieve high quality 3D map of particulate optical properties and their time evolution. This system is equipped with a doubled and tripled Nd:YAG diode-pumped laser that is specifically designed for this device, with a repetition rate of 1KHz and average optical power of 0.6W at 355nm, 1.5W at 532nm and 1W at 1064nm. The relative high repetition rate laser source can increase the detectable signal dynamic range. The receiving system is based on a 25cm modified Cassegrain telescope. The spectral selection of the backscatter elastic and Raman signals is made through a system of dichroic beam splitters and narrow band (0.5 nm) interferential filters. Fast single photon counting photomultipliers are used to collect the selected radiation. Each detected signal is acquired by multichannel scalers with a raw spatial resolution varying from 30cm to 30m. Moreover, polarization purity of laser line allows to perform polarization measurements at both 355 and 532nm. This device is installed in the Beijing city area, which is strongly affected from anthropogenic pollution and sand dust from Gobi desert.

Double-crystal RbTiOPO4 for simultaneous 200 kHz Q-switching and second-harmonic generation in a direct-pumped Nd:YVO4 laser

We report on electro-optic (EO) Q-switched intracavity second-harmonic generation (SHG) based on double-crystal RbTiOPO4 (RTP) in an 880 nm diode-end-pumped Nd:YVO4 laser for the first time. The double-crystal RTP performed simultaneously as a high repetition rate laser Q switch and a second-harmonic generator. At a repetition rate of 200 kHz, a maximum average output power of 2.51 W at 532 nm wavelength (, 1) and a shortest pulse width of 15.7 ns were achieved under an absorbed pump power of 21 W, corresponding to an optical–optical conversion efficiency of 11.9% and a slope efficiency of 12.8%, respectively.

Absolute absorption cross sections for two selected lines of formaldehyde around 6625 cm−1

Absolute absorption cross sections of formaldehyde, CH2O, have been measured at total pressures of 14.5 and 66mbar Helium by cw-CRDS for two selected lines at around 6625cm−1. Absolute CH2O concentrations have been determined in situ by measuring the pseudo-first order decays of OH radicals using high repetition rate laser induced fluorescence (LIF). OH radicals have been generated by laser photolysis of H2O2 in the presence of CH2O and from the well-known rate constant of the reaction of CH2O with OH radicals, the absolute CH2O concentration has been determined. Concentrations between 1.5 and 4×1014cm−3 have been used. The line strengths for the lines at 6624.779cm−1 and 6625.248cm−1 have been found to be (9.1±1.8)×10−24cm and (5.3±1.0)×10−24cm, respectively. The broadening coefficients in Helium have also been determined for both lines. Converting the line strengths to absorption cross sections it is found that these values are a factor of 2 smaller than the only known determination of these absorption cross sections in the literature (Staak et al., J. Molec. Spectroscopy 229 (2005) 115–121).

Performance of a high repetition pulse rate laser system for in-gas-jet laser ionization studies with the Leuven laser ion source @ LISOL

The laser ionization efficiency of the Leuven gas cell-based laser ion source was investigated under on- and off-line conditions using two distinctly different laser setups: a low-repetition rate dye laser system and a high-repetition rate Ti:sapphire laser system. A systematic study of the ion signal dependence on repetition rate and laser pulse energy was performed in off-line tests using stable cobalt and copper isotopes. These studies also included in-gas-jet laser spectroscopy measurements on the hyperfine structure of 63Cu. A final run under on-line conditions in which the radioactive isotope 59Cu (T1/2=81.5s) was produced, showed a comparable yield of the two laser systems for in-gas-cell ionization. However, a significantly improved time overlap by using the high-repetition rate laser system for in-gas-jet ionization was demonstrated by an increase of the overall duty cycle, and at the same time, pointed to the need for a better shaped atomic jet to reach higher ionization efficiencies.

High speed imaging in fundamental and applied combustion research

This overview presents examples of applications of high frame rate imaging diagnostics in fundamental and applied combustion research. Progress in the performance of high frame rate digital cameras and high repetition rate lasers enabled the development of a range of new imaging diagnostics for measurements of velocities, concentrations and temperatures. Camera frame rates and storage capacities are now adequate to resolve and follow time scales spanning six orders of magnitude while camera chip size limitations restrict the spatial dynamic range to about three orders of magnitude. High-speed imaging studies of mixing processes, flame stabilization, ignition and extinction and the coupling of acoustic and chemical processes in turbulent flames and internal combustion engines have produced a wealth of new understanding, contributing also to the development of predictive models. Future progress in designing and operating cleaner and more efficient combustion device hinges on our ability to push operating conditions to leaner mixtures and often to higher pressures. There, small variations in boundary conditions, e.g. flow patterns or the formation of a fuel spray, might lead to combustion failures that can range from acoustic noise in a jet flame, a misfire in an automobile engine, to lean-blow-out of an aircraft gas turbine engine; from nuisance to catastrophe. High-speed imaging in turbulent flames and internal combustion engines allowed capturing and identifying detrimental conditions that might be rare in occurrence and defining in leading to failure. The examples presented in this review illustrate the status of diagnostic capabilities, show sample results, and examine some future directions.