Small-scale self-focusing of divergent beam in nonlinear media with loss

Bespalov-Talanov theory on small-scale self-focusing is extended to include medium loss for a divergent beam. Gain spectrum of small-scale perturbation is presented in integral form, and based on the derived equations we find that the cutoff spatial frequency for perturbation keeps a constant value. The larger the medium loss is, the smaller the fastest growing frequency and the maximum gain of perturbation with defined propagation distance are. For a given medium loss the maximum gain of perturbation becomes larger, while the fastest growing frequency becomes smaller as the propagation distance becomes longer. Furthermore, physical explanations for the appearance of these features are given.

Study of beam divergence and thrust vector eccentricity characteristics of the Hall thruster based on dual Faraday probe array planes and its applications

The accurate knowledge of the thrust vector eccentricity and beam divergence characteristics of Hall thrusters are of significant engineering value for the beneficial integration and successful application of Hall thrusters on spacecraft.For the characteristics of the plume bipolar diffusion due to the annular discharge channel of the Hall thruster,a Gaussian-fitted method for thrust vector deviation angle and beam divergence of Hall thrusters based on dual Faraday probe array planes was proposed in respect of the Hall thruster beam characteristics.The results show that the ratios of the deviation between the maximum and minimum values of the beam divergence angle and the thrust vector eccentricity angle using a Gaussian fit to the optimized Faraday probe dual plane to the mean value are 1.4%and 11.5%,respectively.The optimized thrust vector eccentricity angle obtained has been substantially improved,by approximately 20%.The beam divergence angle calculated using a Gaussian fitting to the optimized Faraday probe dual plane is approximately identical to the non-optimized one.The beam divergence and thrust vector eccentricity angles for different anode mass flow rates were obtained by averaging the beam divergence and thrust vector eccentricity angles calculated by the dual-plane,Gaussian-fitted ion current density method for different cross-sections.The study not only allows for an immediate and effective tool for determining the design of thrust vector adjustment mechanisms of spacecraft with different power Hall thrusters but also for characterizing the 3D spatial distribution of the Hall thruster plume.

High-Power Single-Spatial-Mode GaSb Tapered Laser around 2.0 μm with Very Small Lateral Beam Divergence

We report high-power single-spatial-mode type-I GaSb-based tapered lasers fabricated on the InGaSb/AlGaAsSb material system. A straight ridge and three different tapered waveguide structures with varying flare angles are fabricated to optimize the output power and spatial-mode performance. The best devices exhibit single-spatial-mode operation with room-temperature output power up to 350？mW in continuous-wave mode at an emission wavelength around 2.0？μm with a very small far-field lateral divergence angle, which is beyond state of the art in terms of single-spatial-mode output power.

Theoretical analyses on improved beam properties of GaSb-based 2.X-μm quantum-well diode lasers with no degradation in laser parameters

An asymmetric laser heterostructure is developed to improve the beam properties of GaSb-based diode lasers with no degradation in laser parameters. Employing the semivectorial finite difference method, the dependences of beam divergence and optical confinement factor on waveguide width and refractive index step are investigated theoretically. After carefully design, a particular asymmetric laser structure is proposed. Its beam divergence in the fast axis is reduced from 61° to 34° compared with that of the broad-waveguide structure. The optical confinement factor is approximately equal to 0.0362 and comparable to that of the conventional broad-waveguide structure.

Beam divergence effects on high power optical parametric oscillation

The beam divergence effects of the input pump laser on a high power nanosecond optical parametric oscillator （OPO） have been numerically simulated. The OPO conversion efficiency is affected due to the angular deviation of real laser beams from ideal phase matching conditions. Our theoretical model is based on the decomposition of the Gaussian beam and assumes each component has a single deviation angle and thus a particular wave vector mismatch. We take into account the variable intensity profile in the spatial and temporal domains of the Gaussian beam, the pump depletion effects for large-signal processes as well as the oscillatory effects of the three waves. Two nonlinear crystals β-BaB2O4 （BBO） and LiB305 （LBO） have been investigated in detail. The results indicate that the degree of beam divergence strongly influences the maximum pump intensity, optimum crystal length and OPO conversion efficiency. The impact of beam divergence is much more severe in the case of critical phase-matching for BBO than in the case of non-critical phase-matching for LBO. The results provide a way to choose the optimum parameters for a high power ns OPO such as the nonlinear material, the crystal length and the pump intensity, etc. Good agreement is obtained with our experimental results.

Investigation on the effect of beam divergence angle upon output waveform based on stimulated Brillouin scattering optical limiting

This paper investigates the effect of beam divergence angle on output waveform based on stimulated Brillouin scattering optical limiting.Output waveforms in the case of different pump divergence angles are numerically simulated,and validated in a Nd：YAG seed-injected laser system.The results indicate that a small pump divergence angle can lead to good interaction between pump and Stokes,and a platform can be easily realized in the transmitted waveform.In contrast,a peak followed by the platform appears when the divergence angle becomes large.

Thermal analysis of EAST neutral beam injectors for long-pulse beam operation

Two sets of neutral beam injectors(NBI-1 and NBI-2) have been mounted on the EAST tokamak since 2014. NBI-1 and NBI-2 are co-direction and counter-direction, respectively. As with indepth physics and engineering study of EAST, the ability of long pulse beam injection should be required in the NBI system. For NBIs, the most important and difficult thing that should be overcome is heat removal capacity of heat loaded components for long-pulse beam extraction. In this article, the thermal state of the components of EAST NBI is investigated using water flow calorimetry and thermocouple temperatures. Results show that(1) operation parameters have an obvious influence on the heat deposited on the inner components of the beamline,(2) a suitable operation parameter can decrease the heat loading effectively and obtain longer beam pulse length, and(3) under the cooling water pressure of 0.25 MPa, the predicted maximum beam pulse length will be up to 260 s with 50 keV beam energy by a duty factor of 0.5. The results present that, in this regard, the EAST NBI-1 system has the ability of long-pulse beam injection.

Study on direct point spread function measurement of capillary plate collimators with divergent X-ray beams

Purpose The Filed of View(FOV)of eXTP/LAD is limited by lead-glass capillary plate collimators placed in front of the Silicon Drift Detectors(SDDs)for reducing the background contamination caused by the photons of Cosmic X-ray Background(CXB)leaking from outside the FOV.The core quality parameters of lead-glass capillary plate collimators are Open Area Ratio(OAR)and FOV.The performance of lead-glass capillary plate collimators is determined by its design specification such as pore-to-pore misalignment,pore perpendicularity with collimator surface and pore diameter.Since the design specification is the result of a large number of micropores,we use the full width at half maximum(FWHM)and peak value of point spread function(PSF)to characterize the capillary plate collimator performance.In order to quickly evaluate the performances of collimators,we developed a direct PSF measurement method using divergent beams.Methods In this paper,the simulation package of the divergent beamsmeasurement setup is developed based on Geant4.The simulation parameters of scintillator absorption length are corrected by experimental data.We simulate the PSF of collimators with different distribution of pore diameter and misalignment by parallel beams and divergent beams.By comparing the peak value and FWHM of collimators PSF,the feasibility in replacing parallel beams measurement with divergent beams measurement is studied.Results It is verified that the influence of the geometric size of 33μm radius of the micro-focus of X-ray tube can be ignored.The results show that the FWHM of collimators is increased with error root mean square error(RMS)of the distribution of collimators pore diameter and pore inclination angle.The peak value of PSF is mainly influenced by pore non-parallelism of the pores,and it decreases with an increase in pore inclination angle.Comparison of parallel beams PSF and divergent beams PSF shows well consistency of peak value and FWHM of collimator.But,the peak value of the divergent beams PSF is mainly affected by the defects of the pores close to the optical axis and result in difference from the parallel beams results consequently.We have developed capillary plate collimators response function based on the projected area function of a cylindrical pore and the fluorescence broadening of the scintillator and fitted PSF by response function.The capillary plate collimator open area ratio is calculated by using the micropores radius and collimator frame thickness obtained by fitting parameter,and the result is close to the parallel beams PSF.Conclusion The feasibility divergent beams measurement is verified.

Optimizing depth of field in 3D light-field display by analyzing and controlling light-beam divergence angle

A concept of divergence angle of light beams(DALB)is proposed to analyze the depth of field(DOF)of a 3D light-field display system.The mathematical model between DOF and DALB is established,and the conclusion that DOF and DALB are inversely proportional is drawn.To reduce DALB and generate clear depth perception,a triple composite aspheric lens structure with a viewing angle of 100°is designed and experimentally demonstrated.The DALB-constrained 3D light-field display system significantly improves the clarity of 3D images and also performs well in imaging at a 3D scene with a DOF over 30 cm.

Narrow vertical beam divergence angle for display applications of 645 nm lasers

We design a 645 nm laser diode(LD)with a narrow vertical beam divergence angle based on the mode expansion layer.The vertical beam divergence of 10.94°at full width at half-maximum is realized under 1.5 A continuous-wave operation,which is the smallest vertical beam divergence for such an LD based on the mode expansion layer,to the best of our knowledge.The threshold current and output power are 1.07 A and 0.94 W,limited by the thermal rollover for the 100μm wide and 1500μm long broad area laser,and the slope efficiency is 0.71 W/A.The low coherence device is fabricated with the speckle contrast of 3.6%and good directional emission.Such 645 nm LDs have promising applications in laser display.

Impact of pre-pulse current and delay time on 46.9 nm laser with larger inner diameter alumina capillary

In this Letter, we firstly, to the best of our knowledge, demonstrated the influence of pre-pulse current and delay time on the intensity of a discharge pumped Ne-like Ar soft X-ray laser operating at 46.9 nm by employing an alumina capillary having an inner diameter of 4.8 mm. Specifically, the delay time was changed from 8 to 520 μs in small intervals. The pre-discharge current was increased from 25 A to 250 A through small steps, while keeping the main discharge current constant. Usually, a small pre-discharge current is applied to an Ar-filled capillary to attain a plasma column having sufficient pre-ionization before the injection of the main current. The predischarge current of 140 A was declared the best current to obtain lasing with a 4.8 mm diameter capillary.The laser spots were captured at best time delays for the pre-discharge currents of 25, 45, 80, 140, and250 A, which support the experimental results. We observed that by applying the pre-discharge current of140 A, the laser spot exhibits small divergence, higher symmetry, and uniformity, which is clear evidence of strong amplification. The laser spot obtained at 140 A is cylindrically symmetric and has a better structure than those reported by all other groups in the literature. Hence, the laser spot indicates that the laser beam is highly focusable and beneficial for the applications of the 46.9 nm laser. Results of this Letter might open a new way to enhance applications of a 46.9 nm capillary discharge soft X-ray laser.