Simultaneous Laser Pulses Research Articles

Laser-Assisted Field Evaporation of Chromia with Deep Ultraviolet Laser Light.

In this work, samples of chromia (Cr2O3) scale have been prepared for atom probe tomography and field evaporated with deep ultraviolet laser light (258 nm wavelength). The investigated range of laser energies spans more than three orders of magnitude between 0.03 and 90 pJ. Furthermore, the effects of detection rate and temperature were investigated. Simultaneous voltage and laser pulses were employed on additional needle specimens to reduce the standing voltage and minimize background noise during the measurement. Smooth evaporation with minimal mass spectrum peak tails was maintained over the whole range of measurement parameters. High laser energies result in significant underestimation of the oxygen content. Only laser energies below 1 pJ resulted in measured values near the expected oxygen content of 60 at%, the closest being about 58 at%.

Doppler-free spectroscopy of an atomic beam probed in traveling-wave fields

The authors propose and demonstrate a method for precision spectroscopy of atoms in a beam using two sequential rather than simultaneous laser pulses, thereby overcoming systematic challenges associated with standing-wave beams that plague other methods. Their results show that this method can effectively suppress the first-order Doppler effect and reduce systematic uncertainty.

Dual-signal-resonant optical parametric oscillator intracavity driven by a coaxially end-pumped laser with compound gain media.

A flexible method of generating stable dual-wavelength laser pulses with tunable power ratio and pulse interval is proposed, through integrating a coaxially end-pumped laser with compound gain media and an intracavity pumped optical parametric oscillator (IOPO). A theoretical model was built by a set of time-domain coupling wave equations containing both the generation of two fundamental waves from a shared pump source and the conversion to signal waves through the parametric process. Simulations showed that by simply varying the pump focal position or pump wavelength, the gains in two laser crystals could be changed, leading to simultaneous change in average power ratio and time interval between two wavelengths. Experimental verifications were performed with combined laser crystals (Nd:YAG and a-cut Nd:YLF) and a nonlinear crystal (KTA), which enabled dual-wavelength signal output in the 1.5-1.6 μm eye-safe region and demonstrated coincident conclusions with theoretical results. As there was no gain competition between two fundamental waves, stable signal output was obtained. Moreover, various wavelength pairs in any wavelength ranges are possible by using different laser crystals and nonlinear crystals. It is believed that this is a promising method for generating simultaneous dual-wavelength laser pulses for applications in lidar, remote sensing, nonlinear frequency conversion, etc.

3D LAND COVER CLASSIFICATION BASED ON MULTISPECTRAL LIDAR POINT CLOUDS

Abstract. Multispectral Lidar System can emit simultaneous laser pulses at the different wavelengths. The reflected multispectral energy is captured through a receiver of the sensor, and the return signal together with the position and orientation information of sensor is recorded. These recorded data are solved with GNSS/IMU data for further post-processing, forming high density multispectral 3D point clouds. As the first commercial multispectral airborne Lidar sensor, Optech Titan system is capable of collecting point clouds data from all three channels at 532nm visible (Green), at 1064 nm near infrared (NIR) and at 1550nm intermediate infrared (IR). It has become a new source of data for 3D land cover classification. The paper presents an Object Based Image Analysis (OBIA) approach to only use multispectral Lidar point clouds datasets for 3D land cover classification. The approach consists of three steps. Firstly, multispectral intensity images are segmented into image objects on the basis of multi-resolution segmentation integrating different scale parameters. Secondly, intensity objects are classified into nine categories by using the customized features of classification indexes and a combination the multispectral reflectance with the vertical distribution of object features. Finally, accuracy assessment is conducted via comparing random reference samples points from google imagery tiles with the classification results. The classification results show higher overall accuracy for most of the land cover types. Over 90% of overall accuracy is achieved via using multispectral Lidar point clouds for 3D land cover classification.

3D LAND COVER CLASSIFICATION BASED ON MULTISPECTRAL LIDAR POINT CLOUDS

Multispectral Lidar System can emit simultaneous laser pulses at the different wavelengths. The reflected multispectral energy is captured through a receiver of the sensor, and the return signal together with the position and orientation information of sensor is recorded. These recorded data are solved with GNSS/IMU data for further post-processing, forming high density multispectral 3D point clouds. As the first commercial multispectral airborne Lidar sensor, Optech Titan system is capable of collecting point clouds data from all three channels at 532nm visible (Green), at 1064 nm near infrared (NIR) and at 1550nm intermediate infrared (IR). It has become a new source of data for 3D land cover classification. The paper presents an Object Based Image Analysis (OBIA) approach to only use multispectral Lidar point clouds datasets for 3D land cover classification. The approach consists of three steps. Firstly, multispectral intensity images are segmented into image objects on the basis of multi-resolution segmentation integrating different scale parameters. Secondly, intensity objects are classified into nine categories by using the customized features of classification indexes and a combination the multispectral reflectance with the vertical distribution of object features. Finally, accuracy assessment is conducted via comparing random reference samples points from google imagery tiles with the classification results. The classification results show higher overall accuracy for most of the land cover types. Over 90% of overall accuracy is achieved via using multispectral Lidar point clouds for 3D land cover classification.

(LIBS) A PROMISING TECHNIQUE, ITS LIMITATIONS AND A PROPOSED METHOD

Laser Induced Breakdown spectroscopy (LIBS) is an extremely potential spectroscopic analytical tool. A highly focused laser bean stiochiometerically ablates the surface of the material in the form of a plasma plume. Excited species in the plasma plume emit their characteristic wavelengths upon de-excitation which are collected, dispersed and analyzed for qualitative and quantitative analyses. Basic LIBS setup includes a laser, target sample, optical fiber and a spectrometer. However it has been used in different configurations like single–pulse and double pulse configurations. LIBS has several advantages over other currently practiced analytical techniques in terms of higher resolution, better limit of detection (LOD), negligible sample preparation etc. Despite of all these advantages it is suffering from poor accuracy and reproducibility of results due to uncontrolled atmosphere around the targeted sample and variations in other experimental parameters. In order to improve reliability of LIBS in terms of accuracy and reproducibility we have designed a methodology for experimentation under controlled environmental conditions inside an especially designed ablation chamber. We will make use of multiple simultaneous laser pulses, which are supposed to play a significant role in improving the analytical accuracy of LIBS particularly for non homogeneous samples. In this article we will briefly review the basics of LIBS, its types, common instrumentations, advantages, limitations applications and at the end our proposed methodology.

Active Q-Switching of the Diode-Pumped Two-Frequency ${\rm Yb}^{3+}{:}{\rm KGd}({\rm WO}_{4})_{2}$ Laser

Taking advantage of the spectroscopic anisotropy of the Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped KGd(WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> crystal and its anisotropic laser gain, we have obtained two-frequency and two-polarization lasing in the 1-3 THz range of frequency difference. Inside the laser crystal, the ordinary and extraordinary waves are spatially separated and pumped with an optical bifurcated fiber. The two output waves are merged in a collinear beam suitable for future frequency downconversion. The key contribution of this paper is that two-frequency lasing is demonstrated in active Q-switch regime from an acoustooptic modulator up to 10 kHz repetition rate and leading to simultaneous laser pulses when adequate experimental conditions are fulfilled.

Optical injection of charge current in quantum wires: Oscillations induced by excitonic effects

We investigate the charge current that is optically injected by interference between one- and two-photon excitation in the presence of excitonic effects. We consider a realistic V-shaped quantum wire excited slightly below the band gap by two simultaneous femtosecond laser pulses of frequency $2\ensuremath{\omega}$ and $\ensuremath{\omega}$ that interact, respectively, with the lowest ${B}_{1}$ and ${A}_{1}$ excitons. Using effective multiband Bloch equations for two-photon transitions, including the Coulomb interaction within the Hartree-Fock approximation, we show that, because of the different symmetry properties of the involved excitons, the generated charge current displays oscillations due to the quantum interference between the excitonic coherences.

Resonant two-photon dissociation of the in the presence of two-colour pulsed laser fields

We present a theoretical study of resonant two-photon dissociation (TPD) of the molecule in the presence of pulsed two-colour laser fields using the density matrix formalism. The study includes the calculation of dissociation probabilities and decay rates as functions of time and the variation of total dissociation probabilities with detuning when both fields are assumed to have the same temporal shape and duration. Results are reported for two simultaneous laser pulses as well as for two pulses with a time delay. The investigations were done with different values of peak intensities , for the initial and intermediate levels as v = 0, J = 0 and v'=17, J'=1, respectively, of the ground electronic state and the final continuum energy reached by photons was taken to be . The time variation and delay between pulses lead to interesting features in the time-dependent decay rates and spectral shapes of total dissociation probabilities.

Multiphoton ionization of molecules: A comparison between femtosecond and nanosecond laser pulse ionization efficiency

Multiphoton ionization mass spectra of nonvolatile molecules laser desorbed into a supersonic beam are recorded. It is shown by indirect measurements that the laser desorption of neutrals is not mass limited, but lead to the formation of neutrals with intesities large enough for intense signals. To investigate the efficiency of the multiphoton ionization process with varying laser pulse durations, simultaneous laser pulses of 500 fs and 5 ns or 100 fs and 5 ns have been applied to the neutral beam. The energies of both femtosecond and nanosecond laser pulses are held in a comparable magnitude, and thus produce, in the resulting ion intensity, very large differences up to 4 orders of magnitude. For larger evaporated molecules (> 500 u) the ionization efficiency from nanosecond laser pulses drops significantly in comparison to femtosecond laser pulse excitation. A variety of possible reasons for the different ionization and dissociation behavior in femtosecond and nanosecond laser pulse excitations are discussed in this paper. It is rationalized that even with very short laser pulses and large molecules the "ladder switching model" for ionization and fragmentation is valid.

Elementary quanta of open systems

It is argued that elementary quanta of an open system are determined by the spectral decomposition of a thermodynamic Hamiltonian, not an ordinary Hamiltonian. Recently observed spectral shifts in semiconductors irradiated with two simultaneous laser pulses may be manifestation of this fundamental role of the thermodynamic Hamiltonian.

Effects of pulse modulation on multistep laser excitation

Through numerical solution of the Schrodinger equation we illustrate the influence of laser modulation upon the excitation of anN-level system irradiated by simultaneous laser pulses. We show that amplitude modulation, applied to a Gaussian statistical distribution of detunings, creates a sideband structure to the excitation-reaction profile which can enhance the excitation over that for monochromatic excitation.

Laser produced plasma heated by two simultaneous laser pulses

A single laser pulse plasma heating and that of two simultaneous pulses with different frequencies and the same front pulse shaping are compared in a one-dimensional model.

Plasma Production with a Multibeam Laser System

A multibeam laser system has been used to produce spherical plasmas whose energies and growth rates have been compared with the results of machine computations. The laser system is designed to provide twelve simultaneous laser pulses that can be focused symmetrically on a common point in a gas-filled target chamber. Under typical operating conditions, a total energy of about 2 J in approximately 10 nsec was directed at targets of deuterium, hydrogen, and helium at atmospheric pressure. A multiple pulse schlieren system, utilizing light from the laser oscillator, was used to measure the growth rates of the shock waves associated with the expanding plasmas. The data were compared with computational results obtained using a one-dimensional machine code which treats the formation, heating, and hydrodynamic development of laser-produced plasmas. The measured growth rates are found to be in quite good agreement with the computed results and with the behavior expected on the basis of Taylor's blast wave theory.

Four-wave parametric interaction in potassium vapor

Two simultaneous laser pulses incident on a cell of potassium vapor at 350°C generate an intense (1-kW) beam of violet light at about 4000 Å. The incident pulses are a 2-MW ruby beam and a 300-kW ruby-generated Stokes beam nearly resonant with the potassium 4P <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3/2</sup> - 4S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> transition. A simplified theory is presented which treats the problem as a four-wave parametric interaction, and accounts for such experimental details as the doublet stucture of the violet light and the fact that the components of the doublet have different intensities.