Wanjun Dai

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.

Process-oriented adaptive optics control method in the multi-pass amplifiers

In this talk, we propose and demonstrate the process-oriented adaptive optics (AO) wavefront control method, for optimizing the beam quality in the multi-pass amplifiers. Different from the conventional target-oriented wavefront control approach, the novel method divides the aberration correction process into several steps, to optimize the wavefront quality in time during the courses of the beams transport and amplification. The experimental results show that the proposed method can effectively prevent the beam quality from worsening and ensure the successful reality of multi-pass amplification, so it has obvious advantages both in efficiency and accuracy over the traditional target-oriented method.

Beam alignment based on two-dimensional power spectral density of a near-field image

Beam alignment is crucial to high-power laser facilities and is used to adjust the laser beams quickly and accurately to meet stringent requirements of pointing and centering. In this paper, a novel alignment method is presented, which employs data processing of the two-dimensional power spectral density (2D-PSD) for a near-field image and resolves the beam pointing error relative to the spatial filter pinhole directly. Combining this with a near-field fiducial mark, the operation of beam alignment is achieved. It is experimentally demonstrated that this scheme realizes a far-field alignment precision of approximately 3% of the pinhole size. This scheme adopts only one near-field camera to construct the alignment system, which provides a simple, efficient, and low-cost way to align lasers.

Research on the shooting accuracy of ICF laser device based on radiation fluid

The shooting accuracy of cluster laser is an important indicator to evaluate the performance of ICF laser devices. By measuring the distribution of the X-ray generated from interaction between the third-harmonic beam and the target, the position information of the third-harmonic beam to the target can be obtained, along with the shooting accuracy. In the beam transmission process, the fundamental, second-harmonic beams and the third-harmonic beams approach to the target at the same time generating spurious X-ray. Based on the radiation fluid, the present paper is to assess the effect of the stray light on the performance of the shooting accuracy. The intensity distribution and power density of the fundamental, second-harmonic and third-harmonic beams at the target position were calculated for the SG-III laser device using SG-99 software. The characteristics of X-ray generated by the different beams radiation are simulated by one-dimensional radiation fluid program MULTI 1D. The results show that the power density of the fundamental, the second-harmonic and third-harmonic beams at the target position are, under the condition of typical shooting precision test (infused fundamental energy of 50J and pulse width is 200ps) 0.28GW / s / cm2 , 0.14GW / s / cm2 , 99GW / s / cm2 , respectively. The X-ray energy intensity radiated from the interaction between the third-harmonic beam and target is 104 times of that from the fundamental, second-harmonic beam. In the current optical system configuration conditions of SG-III laser device, the effects of the fundamental and second-harmonic beams on the target accuracy test can be ignored.

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.

Automatic alignment technology in high power laser system

The high power solid laser system is becoming larger and higher energy that requires the beam automatic alignment faster and higher precision to ensure safety running of laser system and increase the shooting success rate. This paper take SGIII laser facility for instance, introduce the basic principle of automatic alignment of large laser system. The automatic alignment based on digital image processing technology which use the imaging of seven-classes spatial filter pinholes for feedback to working. Practical application indicates that automatic alignment system of cavity mirror in SGIII facility can finish the work in 210 seconds of four bundles and will not exceed 270 seconds of all six bundles. The alignment precision promoted to 2.5% aperture from 8% aperture. The automatic alignment makes it possible for fast and safety running of lager laser system.

The entire beam wavefront control of high power laser facility

Experiment of entire beam wavefront compensation was carried out in a beamline of a high power laser facility, and two adaptive optics systems with different intentions were applied in the chosen beamline. After correction, the far-filed irradiance distribution is concentrated evidently and the entrance rate of 3ω focal spot to a 500-μm hole is improved to be about 95% under number kilojoules energy.

High-accuracy arithmetic for cavity mirror automatic alignment in multi-pass beam system

Beam alignment of multi-pass amplification is based on cavity mirror alignment. To optimize multi-segmental parallel cavity mirror alignment arithmetic of high power solid-state lasers, propose a new type of arithmetic of multi-pass beam path cavity mirror based on diffraction symmetry, and the accuracy of multi-pass amplification beam path alignment is improved by 10μrad up to 3.96μrad. The arithmetic avoids low accuracy of CM alignment caused by poor image quality, It makes SG-Ⅲ facility operate long term and properly.

Beam phase-distortion correction in a high power laser based on the stochastic parallel gradient descent technique

The feasibility of the closed-loop adaptive optic system based on laser far-field information was experimentally demonstrated, which can significantly increase the working efficiency of the multi-beamline high power solid laser facility. A stochastic parallel gradient descent algorithm with mean-radius cost function was applied in the adaptive optics that was installed in one chosen beamline in the SG-III laser facility. The far-field diameter from a reference laser was corrected from about 275 to 100??m, very close to the ideal limit of 75??m. Multiple-wavefront correction can be performed with this far-field detection based adaptive optics system, and the experimental result validates the practicability of the proposed system in the multi-beamline high power solid laser facilities.

Optical limiting characteristics of zinc phthalocyanine

In this paper, it is the first time to experimentally demonstrate the dependence of optical limiting performance of zinc phthalocyanine (ZnPc) dimethylformamide (DMF) solution on concentration and optical length. The optical limiting characteristics are measured at various concentrations and optical lengths. By employing a reverse saturable absorption (RSA) equation, the physical mechanism of such optical limiting is interpreted, leading to excellent agreement with the experimental results. The obtained results imply that the optical limiting effects are quite sensitive to both concentration and optical length, giving parameter guidance for designing optical limiter in practice.