Laser Propagation Path Research Articles

Precision prediction of beacon center spot in atmospheric turbulent environments

Atmospheric turbulence in the laser propagation path induces phase distortion and adversely affects the acquisition of the beacon spot center for satellite–ground laser links. We introduce an optical flow method to track multiple feature points and obtain the overall motion state of the light spot to predict the inter-frame offset and offset direction of the center position of the light spot. We combined rough and specific selections to improve the feature point selection and proposed a feature point exit and supplement mechanism suitable for an atmospheric turbulence environment. The algorithm outperformed traditional algorithms in terms of stability and accuracy in experimental verification, providing a simple and high-precision solution for predicting beacon spot centers under turbulent atmospheric conditions.

Polarization and phase control of electron injection and acceleration in the plasma by a self-steepening laser pulse

We describe an interplay between two injection mechanism of background electrons into an evolving plasma bubble behind an intense laser pulse: one due to the overall bubble expansion, and another due to its periodic undulation. The two mechanisms occur simultaneously when an intense laser pulse propagating inside a plasma forms a shock-like steepened front. Periodic undulations of the plasma bubble along the laser propagation path can either inhibit or conspire with electron injection due to bubble expansion. We show that carrier-envelope-phase (CEP) controlled plasma bubble undulation induced by the self-steepening laser pulse produces a unique electron injector—expanding phase-controlled undulating bubble (EPUB). The longitudinal structure of the electron bunch injected by the EPUB can be controlled by laser polarization and power, resulting in high-charge (multiple nano-Coulombs) high-current (tens of kilo-amperes) electron beams with ultra-short (femtosecond-scale) temporal structure. Generation of high-energy betatron radiation with polarization- and CEP-controlled energy spectrum and angular distribution is analyzed as a promising application of EPUB-produced beams.

Supercontinuum Induced by Filamentation of Bessel-Gaussian and Laguerre-Gaussian Beams in Water

In this paper, we study the characteristics of the supercontinuum (SC) induced by the filamentation of two typical vortex beams (i.e., Laguerre-Gaussian (LG) and Bessel-Gaussian (BG) beams) in water. By moving the cuvette filled with water along the laser propagation path, we measure the SC induced by the filamentation of the two vortex beams at different positions in water. The results show that the degree of spectral broadening induced by the filamentation of LG beams hardly changes with the change of position, while for BG beams, the spectral broadening induced by filamentation is weak on both sides and strong in the middle. The value of topological charge (TC) affects the length of the filament formed by BG beams; however, its effect on the spectral broadening induced by the filamentation of LG and BG beams is negligible.

Self-correction of air refractive index in distance measurement using two-color sinusoidal phase modulating interferometry

A two-color sinusoidal phase modulating (SPM) interferometer based on electro-optic modulator (EOM) is devised for self-correction of air refractive index in distance measurement. Two laser sources with high frequency stability and close wavelengths (633 nm and 782 nm) are used in this scheme. High-accuracy phase demodulation was realized using phase generated carrier (PGC) demodulation techniques with an FPGA-based signal processing board. Experimental results of nanometer displacement measurements show that the resolution and accuracy of PGC demodulation is better than 1°, and the nonlinear error is less than 0.2°. Performance tests of two-color SPM interferometer compared with one-color interferometers under dynamic environment were implemented with an arm length difference of 4 m. The results show that, the measured distance of one-color interferometer drifts 4.8 μm in 600 s, which is caused by the deviation between the measured value of air refractive index and the actual value along the laser propagation path, and this drift reduces to ±1.5 μm using two-color interferometer without environmental sensing.

Synthesis of colloidal aluminum nanoparticles by nanosecond pulsed laser and the effect of external electric field and laser fluence on ablation rate

In this paper, the ablation of aluminum was induced by a Q-switched Nd:YAG laser (1064 nm, ~10 ns and 10 Hz) in ethanol at room temperature for producing colloidal nanoparticles. Two different experimental schemes (i.e. the electric field parallel, and perpendicular to the laser propagation path) were used to investigate the dependence of ablation rate on the external electric field characteristics and the laser fluence. The results show that at a constant laser fluence, regardless of the type of scheme, the ablation rate is increased in the presence of electric field. However, the characteristics of craters strongly depend on the direction of the electric field. The results also show that at both schemes with a constant electric field, the ablation rate rises when the laser fluence is increased. According to the results, it can be concluded that the ablation rate enhanced by applying electric field and increasing laser pulse energy.

Polarization-orthogonal filament array induced by birefringent crystals in air

We demonstrate the generation of filament array with orthogonal polarizations in air by using specifically designed wedge-type birefringent quartz plates. Experimental results show that the number of the generated filaments can be expressed as N = 2n wherenis the number of quartz plates inserted in the laser propagation path. By manipulating the optic axis of the quartz plates with respect to the polarization direction of the input laser pulse, the generated filaments can be separated into two parts with the polarization directions perpendicular with each other. The separation distance between the adjacent filaments is found to be linearly dependent on the focal length of external focusing lens. Our results provide a simple and efficient way to generate regular and reproductive femtosecond filament array in air.

Refraction Correction of Airborne LiDAR Bathymetry Based on Sea Surface Profile and Ray Tracing

Water depth can be measured using airborne LiDAR bathymetry (ALB). However, when the green laser beam passes through the air–water interface, the sea surface slope greatly affects the laser propagation path, significantly influencing the accuracy of the measured seafloor topography. To reduce its influence, a refraction correction method at the air–water interface based on the sea surface profile and ray tracing is proposed. First, the 3-D sea surface profile is fit based on the least-squares criteria and the wave spectrum, using the laser point data reflected by the sea surface. Then, on the basis of the sea surface slope, the geolocation biases of the laser points are corrected by tracing every laser transmission path at the air–water interface. The developed method is used to correct the ALB data collected in the South China Sea, and verified by the topography data captured by a ship-borne multibeam echo sounder. Before the refraction correction, the mean absolute error (MAE) is 14.2 cm, and the root-mean-square error (RMSE) is 17.5 cm. After the refraction correction, the MAE and RMSE decrease to 7.2 and 8.3 cm, respectively. The developed method can effectively improve the bathymetric accuracy of the ALB data.

Experimental demonstration of low laser-plasma instabilities in gas-filled spherical hohlraums at laser injection angle designed for ignition target.

Octahedral spherical hohlraums with a single laser ring at an injection angle of 55^{∘} are attractive concepts for laser indirect drive due to the potential for achieving the x-ray drive symmetry required for high convergence implosions. Laser-plasma instabilities, however, are a concern given the long laser propagation path in such hohlraums. Significant stimulated Raman scattering has been observed in cylindrical hohlraums with similar laser propagation paths during the ignition campaign on the National Ignition Facility (NIF). In this Rapid Communication, experiments demonstrating low levels of laser-driven plasma instability (LPI) in spherical hohlraums with a laser injection angle of 55^{∘} are reported and compared to that observed with cylindrical hohlraums with injection angles of 28.5^{∘} and 55^{∘}, similar to that of the NIF. Significant LPI is observed with the laser injection of 28.5^{∘} in the cylindrical hohlraum where the propagation path is similar to the 55^{∘} injection angle for the spherical hohlraum. The experiments are performed on the SGIII laser facility with a total 0.35-μm incident energy of 93 kJ in a 3 nsec pulse. These experiments demonstrate the role of hohlraum geometry in LPI and demonstrate the need for systematic experiments for choosing the optimal configuration for ignition studies with indirect drive inertial confinement fusion.

Femtosecond laser ionization and fragmentation of molecules for environmental sensing

Recent studies have demonstrated that femtosecond laser pulses have high potential in application to environmental science. Because of the properties of ultrafast, broadband and high power, the propagation of femtosecond laser pulses in air can lead to the generation of a strong field of 1013–1014 W/cm2 with a large distance range from meter to kilometers. The strong laser field induces ionization and fragmentation of molecules in the laser propagation path, resulting in characteristic fingerprint emissions. This paper mainly focuses on recent research advances in environmental sensing by using femtosecond laser pulses through strong-field-induced ionization and fragmentation of molecules. The fingerprint emissions of molecules in strong laser fields are discussed based on the understanding of strong-field–molecule interactions in atmospheric as well as in vacuum environments. This is followed by a comprehensive review of several recently developed optical methods for coherent control of fingerprint emissions of molecules. Lastly, both current challenges and a future perspective of this dynamic field are discussed.

Optical turbulence parameters characterized via optical measurements over a 2.33 km free-space laser path

Optical turbulence research contributes to improved laser communications, adaptive optics, and long-range imaging systems. This paper presents experimental measurements of scintillation and focal spot displacement to obtain optical turbulence information along a near-horizontal 2.33 km free-space laser propagation path. Calculated values for the refractive index structure constant (C(n)(2)) and Fried parameter (r0) are compared to scintillometer-based measurements for several cases in winter and spring. Optical measurements were investigated using two different laser sources for the first and second parts of the experiment. Scintillation index estimates from recorded signal intensities were corrected to account for aperture averaging. As a result, we found that an earlier calculation algorithm based on analysis of log-amplitude intensity variance was the best estimator of optical turbulence parameters over the propagation path considered.

Longitudinal Ion Acceleration From High-Intensity Laser Interactions With Underdense Plasma

Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed. The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations have been used to investigate the acceleration mechanism. The time varying magnetic field associated with the fast electron current provides a contribution to the accelerating electric field as well as providing a collimating field for the ions. A strong correlation between the plasma density and the ion acceleration was found. A short plasma scale-length at the vacuum interface was observed to be beneficial for the maximum ion energies, but the collimation appears to be improved with longer scale-lengths due to enhanced magnetic fields in the ramp acceleration region.

Collimated Multi-MeV Ion Beams from High-Intensity Laser Interactions with Underdense Plasma

A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high-intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He2+ of (40(+3)(-8)) MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space-charge fields normal to its boundary.