Influence of laser array performance on spectrally combined beam

Abstract

Incoherent spectral beam combining (SBC) of multiple laser beams is accomplished along the emitters’ arraying direction. Considering that the output beams from a laser array (LA) usually have deflection angles, positional displacements and divergence angles even after being collimated, a propagation model of SBC systems based on multilayer dielectric gratings has been built up. On the basis, properties of the spectrally combined beam affected by parameters of the LA have been discussed in detail. Simulation results show that with the increase in the deflection angle, both the power and the beam quality of the combined beam degrade dramatically. The positional displacement has little impact on the intensity distribution and the beam quality of combined beam but change the wavelength composition of the combined beam. The divergence angle strongly affects the intensity distribution and the beam quality of the combined beam. Additionally, the effect of the deflection angle on the output beam quality is more obvious and may shift the beam spot when comparing with that of the divergence angle.     

Saturable-absorber-based spatial filtering of high-power laser beams

A saturable-absorber-based technique for spatial filtering of high-average-power laser beams is described. For a focused, radially symmetric beam having its highest intensity at the center, this saturable absorber behaves like a soft aperture with gradually increasing attenuation toward the beam edges, thus selectively transmitting the low divergence components that are confined close to the central axis of the propagating laser beam. This technique has been successfully used to reduce the divergence of a high-power, high-repetition-rate, tunable, narrowband, pulsed dye laser. Our results demonstrate how a judicious choice of operating parameters allows spatial filtering to be achieved with the introduction of a minimum absorption loss of the laser beam in the saturable absorber. Following a time-dependent analysis of a rate equation model describing the propagation and interaction of the laser beam with the saturable absorber, we have also obtained theoretical estimates for the extent of spatial filtering. Our theoretical estimates have been found to be in good agreement with our experimental observations.     

On the Global Efficiency of the Laser Optical Excitation of a Fast Atomic Beam

Abstract

The interaction of a fast atomic beam and a laser beam that crosses at right angles has been considered. We have studied the competition between the Doppler effect, due to the angular divergence of the atomic beam and the effect of the laser light intensity distribution. For low laser power values, an optimum waist size can be determined. For higher laser power values, the conditions for a maximum global efficiency are outlined.     

X-ray imaging with parametric X-rays (PXR) from a lithium fluoride (LiF) crystal

X-ray imaging produced by parametric X-rays (PXR) is reported. The PXR are generated using an electron beam with energy of 56 MeV and currents up to 6.4 μA, that interacted with a 1.5-mm thick lithium fluoride (LiF) crystal target in Laue geometry. The LiF (2 2 0) crystallographic plane was used with a Bragg angle (θ_B) near 15° to produce PXR with energy 16.6 keV and FWHM of 3%. This intense, directional, and tunable X-ray source was then used to image a small fish, an integrated circuit, and an electrical switch with an intensified CCD camera. The bremsstrahlung from the target crystal is characterized as a source of noise and compared to the PXR.     

Investigation in the propagation of non-paraxial TE vector Gaussian beam from vectorial structure

Based on the vectorial structure of non-paraxial electromagnetic beam and the non-paraxial vectorial moment theory of light beam propagation, the beam waists, the divergence angles and the beam propagation factors of TE and TM terms have been presented for non-paraxial TE vector Gaussian beam. The formulae obtained are further discussed at the highly non-paraxial and paraxial cases. Their respective maximum divergence angles are given at the highly non-paraxial case. When reducing to the paraxial case, the beam propagation factors of TE and TM terms are smaller than unity, which results from their special energy flux distribution. As TE and TM terms can be detached in the far field, they can be applied in the optical storage and collimation domains.     

轴向受压中间支承梁的横向振动和稳定性

研究了一端固支且自由端轴向受压具有中间支承梁的横向振动和稳定性。利用边界条件推导了此种梁频率方程及分段振型函数的解析表达式。根据频率方程讨论了中间支承位置变化对梁固有频率的影响。应用Ritz-Galerkin截断方法,采用梁的前四阶振型对梁的运动微分方程进行离散化处理,讨论了梁在各个中间支承位置处的失稳形式。发现了在梁上存在一个特殊的中间支承位置ξl,当中间支承位置ξbξl时,随着压力p从零开始增加,梁先发生颤振失稳,当中间支承位置ξbξl时,则梁先发生发散失稳,而在中间支承位置ξl处,梁由颤振失稳跳跃到发散失稳。     

Decreasing the total divergence of polycapillary X-ray microbeams

The divergence characteristics of intense quasi-parallel X-ray microbeams have been studied at the output of the “microfocus X-ray generator-cylindrical polycapillary structure” system by scanning the beam with a knife edge. It us established that the total microbeam divergence decreases in the region near the output edge of the polycapillary structure (divergence quasi-decrease effect).     

Using mosaic crystals for the generation of intense X-ray beams

The parametric X-ray (PXR) yield due to 500-MeV electrons in a 2-mm-thick diamond crystal with a mosaicity angle of ～0.2 mrad has been studied. It is shown that the mosaic crystal structure leads to a significant (about fourfold) increase in the PXR yield doe to the contribution of diffracted bremsstrahlung radiation. Advantages of using mosaic crystals for the generation of intense X-ray beams are discussed.     

Simulation of X-ray beamlines with the new ray tracing tool XTrace

A novel X-ray tracing simulation tool XTrace has been developed recently in order to calculate the beam properties from the X-ray source (bending magnet, undulator, wiggler, etc.) through all optical elements (slit, filter, window, mirror, crystal monochromator, multilayer, etc.) and the sample to the detector. In XTrace, the diffraction by a perfect crystal is described by the dynamical theory taking into account refraction and absorption effects. The code has been used to simulate the X-ray beamline of the Synchrotron Laboratory for Environmental Studies (Synchrotron Umwelt-Labor (SUL)) at ANKA. XTrace has been successfully used to simulate precisely the beam parameters such as position, size, divergence, photon flux, transmission, heat load, etc. at any distance from the source. For a better insight it is also possible to visualize the beam profile, energy spectrum, transmission spectrum, intensity distribution, power density, heat load, foot print, DuMond diagram, ray propagation diagram, etc. An excellent agreement between measured and simulated flux intensities over the whole energy range at the sample position for the X-ray beamline SUL has been found. XTrace has been proven to be a reliable, powerful, fast and easy to use tool for describing existing and designing and optimizing new X-ray beamlines in the future.     

Runaway electron beam generation by a plasma cathode in atmospheric air discharge

Electron beam generation in atmospheric air discharge with a flat cathode has been experimentally studied. The discharge was excited by voltage pulses of negative polarity with an amplitude of 220 kV, a full width at half maximum of 2 ns, and a leading front width of 0.7 ns. The electron beam was monitored using luminescence of a layer of ZnS-CdS:Ag phosphor placed behind a 20-μm-thick foil anode. It was found that the intensity and homogeneity of luminescence of the phosphor layer increased when a dielectric tube with a length smaller than half of the interelectrode distance was placed in the near-cathode part of the air gap. It is concluded that a plasma cathode is formed within the volume confined by the tube and electrons emitted from this region are accelerated in the open part of the gap. In addition, the dielectric tube decreases the divergence of the electron beam.     

Multiple-return-pass beam divergence and the linewidth equation

A multiple-return-pass equation for beam divergence that is applicable to multiple-prism grating tunable laser oscillators is derived by use of ray-transfer matrices. This beam divergence is then incorporated into the multiple-return-pass dispersive linewidth equation. A discussion of the application of the equation is given.     

Propagation modes and regimes of intense laser beam in magnetized plasma

This paper presents an analytical and numerical investigation of a circularly polarized beam propagating along the static magnetic field parallel to oscillating magnetic field in plasma at relativistic intensities. In the high intensity regime, such a magnetic field is created by the pulse itself. The authors have identified three regimes of propagation taking into account the relativistic mass correction. Based on WKB and paraxial ray theory, an appropriate expression for a dielectric tensor has been evaluated in the presence of an externally applied magnetic field. The natural electromagnetic modes are circularly polarized. Consequently, extraordinary and ordinary modes propagate, which are significantly affected due to the relativistic mechanism. The regimes are characterized by dimensionless power and beamwidth, characterizing the nature of propagation as steady divergence, oscillatory divergence and self-focusing. Numerical computations are presented and discussed for typical parameters of laser plasma interaction; defined through critical parameters, namely cyclotron-to-beam frequency (Ω _c ), plasma-to-beam frequency (Ω _p ) and beam power for arbitrary large intensities.     

Characterization of a periodically poled lithium niobate optical parametric oscillator pumped by a Bessel beam

Abstract

A Bessel beam pumped periodically poled lithium niobate optical parametric oscillator has been characterized. A slope efficiency of 5.4 ± 0.1% was obtained and a threshold pulse energy of 472 ± 12 μJ was measured, which corresponds to a threshold fluence of 0.207 ± 0.005 J cm^?2. The signal output was tuned over ～20 nm by changing the crystal temperature by 50°C. The FWHM bandwidth at each temperature was observed to be 0.3 ± 0.1 nm at each wavelength. The beam profiles of the Bessel pump and Gaussian signal beams were studied in both the near and far field. A beam divergence of 1.14 ± 0.02 mrad was measured.     

Super-Gaussian resonators for long-pulse XeCl lasers

Near-diffraction-limited laser beams of high energy have been achieved with a long-pulse (110-ns) XeCl laser fitted with a super-Gaussian unstable cavity of magnification M = 2.7. It has been found that when the super-Gaussian output coupler is replaced with a hard-edge aluminized mirror of radius equal to the super-Gaussian mirror spot size, the output laser energy is reduced by 10%, whereas the beam divergence increases more than 50%.     

Electrically pumped waveguide lasing from ZnO nanowires

Ultraviolet semiconductor lasers are widely used for applications in photonics, information storage, biology and medical therapeutics. Although the performance of gallium nitride ultraviolet lasers has improved significantly over the past decade, demand for lower costs, higher powers and shorter wavelengths has motivated interest in zinc oxide (ZnO), which has a wide direct bandgap and a large exciton binding energy. ZnO-based random lasing has been demonstrated with both optical and electrical pumping, but random lasers suffer from reduced output powers, unstable emission spectra and beam divergence. Here, we demonstrate electrically pumped Fabry-Perot type waveguide lasing from laser diodes that consist of Sb-doped p-type ZnO nanowires and n-type ZnO thin films. The diodes exhibit highly stable lasing at room temperature, and can be modelled with finite-difference time-domain methods.     

Fundamental Parameters Line Profile Fitting in Laboratory Diffractometers

The fundamental parameters approach to line profile fitting uses physically based models to generate the line profile shapes. Fundamental parameters profile fitting (FPPF) has been used to synthesize and fit data from both parallel beam and divergent beam diffractometers. The refined parameters are determined by the diffractometer configuration. In a divergent beam diffractometer these include the angular aperture of the divergence slit, the width and axial length of the receiving slit, the angular apertures of the axial Soller slits, the length and projected width of the x-ray source, the absorption coefficient and axial length of the sample. In a parallel beam system the principal parameters are the angular aperture of the equatorial analyser/Soller slits and the angular apertures of the axial Soller slits. The presence of a monochromator in the beam path is normally accommodated by modifying the wavelength spectrum and/or by changing one or more of the axial divergence parameters. Flat analyzer crystals have been incorporated into FPPF as a Lorentzian shaped angular acceptance function. One of the intrinsic benefits of the fundamental parameters approach is its adaptability any laboratory diffractometer. Good fits can normally be obtained over the whole 20 range without refinement using the known properties of the diffractometer, such as the slit sizes and diffractometer radius, and emission profile.     

Measurement of neutron flux and beam divergence at the cold neutron guide system of the new Munich research reactor FRM-II

A sophisticated neutron guide system has been installed at the new Munich neutron source FRM-II to transport neutrons from the D₂ cold neutron source to several instruments, which are situated in a separate neutron guide hall. The guide system takes advantage of supermirror coatings and includes a worldwide unique “twisted” guide for a desired phase space transformation of the neutron beam. During the initial reactor commissioning in summer 2004, the integral and differential neutron flux as well as the distribution of beam divergence at the exit of two representative and the twisted neutron guide were measured using time-of-flight spectroscopy and gold-foil activation. The experimental results can be compared to extensive simulation calculations based on MCNP and McStas. The investigated guides fulfill the expectations of providing high neutron fluxes and reveal good quality with respect to the reflective coatings and the installation precision.     

Construction and characterization of a continuous atom beam interferometer

In this article, we investigate both theoretically and experimentally the method of Ramsey interference applied to cold atoms being emitted from a 2-dimensional magneto-optical trap (2D-MOT) crossing continuous wave (cw) laser fields that drive Raman transitions between ground-state hyperfine sublevels. In our apparatus, the atomic beam divergence is large (56 mrad half-angle), necessitating the inclusion in our model of both the atomic divergence and the laser beam divergence as well as averaging over the transit time of the atoms through the laser beam. We discuss the effects of the divergence of the atomic and laser beams, and then we describe our apparatus and qualitatively compare our experimental results to the theoretical results. We find good agreement between our model and our experimental results.     

Time evolution of reflective thermal lenses and measurement of thermal diffusivity in bulk solids

A simple method for optically measuring the thermal diffusivity of solids is demonstrated. The thermal displacement created on a substrate by a focused laser beam is determined from the divergence that it induces in a weak probe beam. The dynamics of the surface lens and the amplitude of the probe beam's divergence are then used to determine the thermal diffusivity of the substrate. Several materials that span a wide range of thermal properties are studied.     

Optimization of a silicon X-ray phase plate

We report that X-ray phase plates give optimal performance when operated, not at the exact Bragg angle but at the angle for which the phase shift is reduced by π. This operating point maximizes the output of circular polarization by accepting larger beam divergence.