High robustness single-shot wavefront sensing method using a near-field profile image and fully-connected retrieval neural network for a high power laser facility.

This paper proposes a single-shot high robustness wavefront sensing method based on deep-learning for wavefront distortion measurement in high power lasers. This method could achieve fast and robust wavefront retrieval by using a single-shot near-field profile image and trained network. The deep-learning network uses fully-skip cross connections to extract and integrate multi-scale feature maps from various layers and stages, which improves the wavefront retrieval speed and enhances the robustness of the method. The numerical simulation proves that the method could directly predict the wavefront distortion of high power lasers with high accuracy. The experiment demonstrates the residual RMS between the method and a Shack-Hartmann wavefront sensor is less than 0.01 µm. The simulational and experimental results show that the method could accurately predict the incident wavefront distortion in high power lasers, exhibiting high speed and good robustness in wavefront retrieval.

Deformable mirror resolution-matching-based two-stage wavefront sensorless adaptive optics method.

In high-power laser facilities, the application of a traditional wavefront control method is limited under the influence of a continuous phase plate (CPP). In order to obtain a satisfactory far-field intensity distribution at the target of the beamline with the CPP, a novel deformable mirror (DM) resolution-matching-based two-stage wavefront sensorless adaptive optics method is proposed and demonstrated. The principles of the DM resolution-matching method and two-stage wavefront sensorless adaptive optics method are introduced, respectively. Based on the numerical model, the matching relationship between the actuator space of the DM and the spatial period of the CPP is investigated. By using the resolution-matched DM, the feasibility of the two-stage wavefront sensorless adaptive optics method is numerically and experimentally verified. Both the numerical and the experimental results show that the presented DM resolution-matching-based two-stage wavefront sensorless adaptive optics method could achieve the target focal spot control under the influence of the CPP, and the profile and the intensity uniformity of the corrected focal spot are optimized close to the designed ideal focal spot.

Numerical analysis of a novel two-stage enlargement and adaptive correction approach for the annular aberration compensation.

The annular laser beam (ALB) is widely used in many fields, which could be affected by laser power and beam quality. To effectively and flexibly improve the beam quality of high-power large-aperture thin-wall ALB, a two-stage enlargement and adaptive correction configuration (TEACC) consisting of a novel outer-surface tubular deformable mirror (OTDM) and two extra prism groups (EPGs) is proposed in this paper. The correction principle and design principle of the TEACC are derived and analyzed. Based on the principle, a typical OTDM prototype and EPG structure are designed. Annular aberrations are compensated by applying the OTDM's influence functions and the least-square algorithm in simulation. The results show that the TEACC could perfectly compensate the wavefront distortions described by the 2nd to 36th order Zernike annular aberrations.

Theoretical research on the novel adaptive optics configuration based on the tubular deformable mirror for the aberration correction of the annular laser beam.

The annular laser beam (ALB) has been widely used in many fields for its unique intensity distribution. Especially, in the materials processing, the power and the beam quality of the large-aperture thin-wall ALB are of vital. However, limited by the aperture, the actuators' spacing or the damage threshold, the existing deformable mirrors (DMs) are not suitable for the correction of the ALB. Considering the stretching effect of the oblique incidence, in this paper, by using the tubular DM (TDM), a novel adaptive optics (AO) configuration is promoted to increase the number of the effective actuators covered by the input ALB. The coordinate transformation equations and correction principle of the novel AO configuration are derived based on the ray tracing. A typical TDM prototype is designed based on the coordinate transformation equations. The influence function characteristics of the TDM is analyzed using the finite element method, and the correction ability of the novel AO configuration based on the TDM is verified. Simulation results show that the TDM could perfectly compensate the wavefront distortions described by the 2th to 15th order Zernike annular aberrations.

First Octahedral Spherical Hohlraum Energetics Experiment at the SGIII Laser Facility.

The first octahedral spherical hohlraum energetics experiment is accomplished at the SGIII laser facility. For the first time, the 32 laser beams are injected into the octahedral spherical hohlraum through six laser entrance holes. Two techniques are used to diagnose the radiation field of the octahedral spherical hohlraum in order to obtain comprehensive experimental data. The radiation flux streaming out of laser entrance holes is measured by six flat-response x-ray detectors (FXRDs) and four M-band x-ray detectors, which are placed at different locations of the SGIII target chamber. The radiation temperature is derived from the measured flux of FXRD by using the blackbody assumption. The peak radiation temperature inside hohlraum is determined by the shock wave technique. The experimental results show that the octahedral spherical hohlraum radiation temperature is in the range of 170-182 eV with drive laser energies of 71 kJ to 84 kJ. The radiation temperature inside the hohlraum determined by the shock wave technique is about 175 eV at 71 kJ. For the flat-top laser pulse of 3 ns, the conversion efficiency of gas-filled octahedral spherical hohlraum from laser into soft x rays is about 80% according to the two-dimensional numerical simulation.

Enhancement of UV Second-Harmonic Radiation at Nonlinear Interfaces with Discontinuous Second-order Susceptibilities.

We investigate the generation of ultraviolet (UV) second-harmonic radiation at the boundary of a UV transparent crystal, which is derived from the automatic partial phase matching of the incident wave and the total internal reflection. By adhering to another UV non-transparent crystal with a larger second-order nonlinear coefficient χ(2), a nonlinear interface with large disparity in χ(2) is formed and the enhancement of UV second-harmonic radiation is observed experimentally. The intensity of enhanced second harmonic wave generated at the nonlinear interface is up to 11.6 times that at the crystal boundary. As a tunable phase-matching method, it may suggest potential applications in the UV, and even vacuum-UV region.

Polarization smoothing for single beam by a nematic liquid crystal scrambler.

Polarization smoothing (PS) is a key approach to suppress laser plasma instabilities (LPI) in inertial confinement fusion (ICF) experiments. Here, we propose a liquid crystal (LC) PS element to realize single beam smoothing and demonstrate its smoothing effect, in principle, with a 2×2 LC polarization checkerboard, which reduces the laser intensity variation in the focal spot to 78.4%. LC PS elements, which have potential applications in high-power ICF laser drivers, have many advantages because they are easy to fabricate, cost effective, flexible, and large.

Surface enhanced nonlinear Cherenkov radiation in one-dimensional nonlinear photonic crystal.

We study the configuration of efficient nonlinear Cherenkov radiation generated at the inner surface of a one-dimensional nonlinear photonic crystal, which utilizes the combination of both quasi-phase matching and total internal reflection by the crystal surface. Multi-directional nonlinear Cherenkov radiation assisted by different orders of reciprocal vectors is demonstrated experimentally. At specific angles, by associating with quasi-phase matching, the incident fundamental wave and total internal reflection wave format completely the phase-matching scheme, leading to great enhancement of harmonic wave intensity.

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.

Optical zooming based on focusing grating in direct drive ICF.

In direct drive ICF, optical zooming is an effective way to mitigate cross-beam energy transfer and increase the hydrodynamic efficiency, by reducing the spot size of the laser beams while target compressing. In this paper, a novel optical zooming scheme is proposed, which employs a focusing grating to focus the broadband laser pulse, changing the spot size on the target within single beamlet. Experimentally, a focusing grating with clean aperture of 40-mm × 40-mm placed after the collimated light successfully realized the peak-valley of defocusing wavefront distribution of 0.73 µm as the wavelength ranging from 1052.43 nm to 1053.23 nm. Extended to the full-sized focusing grating with laser beam of 360-mm × 360-mm, it is derived that the focal spot reduction reaches to 21.8% with the 3rd harmonic light ranging from 350.81 nm to 351.08 nm, decreasing from 375 µm to 294 µm with 300 µm shaping continuous phase plate.

Nonlinear Cherenkov radiation at the interface of two different nonlinear media.

We discuss the nonlinear response due to the spatial modulation of the second-order susceptibility at the interface between two nonlinear media, and experimentally demonstrate that the nonlinear Cherenkov radiation is enhanced by the interface of two nonlinear crystals with a large disparity in χ(2). In our experiment, the intensity of the nonlinear Cherenkov radiation generated at the nonlinear interface was approximately 4 to 10 times that at the crystal boundary. This result suggests potential applications to efficient frequency conversion.

Shannon entropy method of small-scale self-focusing assessment in high-power laser systems.

Through analysis of near-field beam profiles, we propose a method using Shannon entropy to assess the development of small-scale self-focusing during laser propagation and amplification in high-power laser systems. In this method, the entropy curve that corresponds to increasing B integral displays an evident turning point at which small-scale self-focusing starts to rapidly develop. In contrast to classical methods using contrast, modulation, or power spectral density, the proposed method provides the B integral criterion more clearly and objectively. This approach is an optimization method that can be utilized in the design and operation of high-power laser systems.

Sum-frequency nonlinear Cherenkov radiation generated on the boundary of bulk medium crystal.

We demonstrated experimentally a method to generate the sum-frequency Nonlinear Cherenkov radiation (NCR) on the boundary of bulk medium by using two synchronized laser beam with wavelength of 1300 nm and 800 nm. It is also an evidence that the polarization wave is always confined to the boundary. Critical conditions of surface sum-frequency NCR under normal and anomalous dispersion condition is discussed.

Experimental demonstration of polarization smoothing in a convergent beam.

We report an experimental demonstration of polarization smoothing by using a specially cut KDP plate in the convergent ultraviolet (351 nm wavelength) section of a high-power laser system. A 410 mm×410 mm×10 mm sized KDP plate placed after the final focus lens successfully reduced the focal spot intensity contrast to 75.6%, which is in good agreement with the theoretical expectation. In our experiment, no obvious transverse stimulated scattering was observed with the maximum 351 nm laser energy of 4032 J/3 ns, thus indicating the operation safety margin of our laser system.

Spatial modulation of second-harmonic generation via nonlinear Raman-Nath diffraction in an aperiodically poled lithium tantalite.

We propose and experimentally demonstrate a colorful nonlinear Raman-Nath second-harmonic generation by engineering the quadratic nonlinearity χ((2)) in an aperiodically poled lithium tantalite. The engineered nonlinear structure allows multicolored Raman-Nath second-harmonic signal outputs along a uniform direction, which cannot be achieved in a uniform nonlinear grating. The diffraction angles are independent of the beam waist and the position of incidence. This verifies that nonlinear Raman-Nath diffraction does not depend on the local superlattice structure where the fundamental frequency beam locates, but on the whole nonlinear χ((2)) crystal.

Enhanced nonlinear Cherenkov radiation on the crystal boundary.

We demonstrate a method to generate enhanced nonlinear Cherenkov radiation (NCR) in the bulk medium by utilizing the total reflection on the physical boundaries inside the crystal. This is the experimental demonstration of enhanced NCR by a sharp χ(2) modulation from 0 to 1, and a new way for generating enhanced NCR in addition to using the waveguide structures and the ferroelectric domain walls, which also possesses a better beam quality for applications.

Nonlinear Cherenkov radiation in an anomalous dispersive medium.

By using ultrathin nonlinear media, the phase velocity of nonlinear polarization waves can be continuously adjusted. We realized nonlinear Cherenkov radiation (NCR) in an anomalous dispersion medium experimentally, which breaks the minimum speed limit of NCR. This modified nonlinear Cherenkov radiation has two new special states with respect to its prior, i.e., the degenerate state and extreme state. Also, periodically arranged ultrathin nonlinear media can enhance NCR and realize the nonlinear Smith-Purcell effect.

Domain wall characterization in ferroelectrics by using localized nonlinearities.

In this paper, a method of domain wall characterization in ferroelectrics through Cherenkov second harmonic generation by localized nonlinearities is proposed. By this method, domain wall width is estimated to be less than 10nm. High spatial angular resolution of about 10 mrad in the experiment reveals the fine structures of the domain walls. Combined with scanning techniques, this method can reconstruct domain wall patterns with high resolution. This method has advantages of being nondestructive, noncontact, in situ as well as of high resolution.

Soliton formation and collapse in tunable waveguide arrays by electro-optic effect.

Light propagation in periodically poled lithium niobate (PPLN) waveguide arrays is numerically investigated. By employing the concept of a linear focusing effect, we propose a brand new method to control discrete diffraction and spatial solitons in PPLN waveguide arrays by electrical field. Simulated results are presented, and the principle is discussed to demonstrate the feasibility of the method.