In order to obtain the intensity distribution of a 351 nm focal spot and smoothing by spectral dispersion (SSD) focal plane profile of a SGII-upgraded facility, a type of off-axis imaging system with three spherical mirrors, suitable for a finite distance source point to be imaged near the diffraction limit has been designed. The quality factor of the image system is 1.6 times of the diffraction limit tested by a 1053 nm point source. Because of the absence of a 351 nm point source, we can use a Collins diffraction imaging integral with respect to λ=351  nm, corresponding to a quality factor that is 3.8 times the diffraction limit at 351 nm. The calibration results show that at least the range of ±10  mrad of view field angle and ±50  mm along the axial direction around the optimum object distance can be satisfied with near diffraction limited image that is consistent with the design value. Using this image system, the No. 2 beam of the SGII-upgraded facility has been tested. The test result of the focal spot of final optics assembly (FOA) at 351 nm indicates that about 80% of energy is encompassed in 14.1 times the diffraction limit, while the output energy of the No. 2 beam is 908 J at 1053 nm. According to convolution theorem, the true value of a 351 nm focal spot of FOA is about 12 times the diffraction limit because of the influence of the quality factor. Further experimental studies indicate that the RMS value along the smoothing direction is less than 15.98% in the SSD spot test experiment. Computer simulations show that the quality factor of the image system used in the experiment has almost no effect on the SSD focal spot test. The image system can remarkably distort the SSD focal spot distribution under the circumstance of the quality factor 15 times worse than the diffraction limit. The distorted image shows a steep slope in the contour of the SSD focal spot along the smoothing direction that otherwise has a relatively flat top region around the focal spot center.

Large-aperture ultrashort ultrahigh intensity laser systems are able to achieve unprecedented super-high peak power. However, output power from a single laser channel is not high enough for some important applications and it is difficult to improve output power from a single laser channel significantly in the near future. Coherent beam combining is a promising method which combines many laser channels to obtain much higher peak power than a single channel. In this work, phase effects of coherent beam combining for large-aperture ultrashort laser systems are investigated theoretically. A series of numerical simulations are presented to obtain the requirements of spatial phase for specific goals and the changing trends of requirements for different pulse durations and number of channels. The influence of wavefront distortion on coherent beam combining is also discussed. Some advice is proposed for improving the performance of combining. In total, this work could help to design a practical large-aperture ultrashort ultrahigh intensity laser system in the future.

Phase mismatching for third-harmonic generation (THG) in barium metaborate (BBO) crystal is investigated in detail. Upon using BBO crystal in the Type I (oo→e) scheme, in the two independent planes (principal section of the crystal and the plane normal to principal section), when the input second-harmonic beam deviates from the expected direction, phase mismatching occurs and the angle deviation produces different effects on the conversion efficiency. We numerically simulate these two cases of phase mismatching and identify the relation between the conversion efficiency and the deviation angle in the air. The computational results indicate that the phase matching tolerance in the principal section is 0.2• (in the air), while the phase matching tolerance in the plane normal to principal section is 4.5• (in the air). After conducting a lab experiment, the results agree perfectly with the computational results, indicating that the deviation angle in the principal section has a greater effect on the conversion efficiency.OCIS codes: 190.0190, 190.2620, 320.7100. doi: 10.3788/COL20100806.0612. Barium metaborate (BBO) crystal is a negative uniaxial crystal, whose large nonlinear efficiency and damage threshold make it a widely used material in many fields, including ultrashort pulse measuring, optical parametric process, photon metrology, etc.[1−5] Improving the frequency conversion efficiency to promote the authenticity of the signal has been the general focus of most research; of these, the key issue is the phase matching angle optimum design [3,6] . However, other aspects should also be considered so as to ensure high frequency conversion efficiency in practice. Likewise, there is a need to ensure that the beams enter the cutting crystal in the expected direction. If the input second-harmonic beam deviates from the expected direction in two independent planes, namely, the principal section of the crystal and the plane normal to principal section, phase mismatching is likely occur and the influence of the degree of deviation angle could have different results in the two cases. Based on momentum conservation and the refractive index ellipsoid, phase mismatching for type I (oo→e) third-harmonic generation (THG) in BBO crystal is investigated to illustrate the issue. In this letter, we aim to identify how the angle deviation affects the conversion efficiency and determine the adjusting accuracy in the two abovementioned cases.We analyze the phase mismatching of type I THG [7] in BBO crystal with a size of 4×4×2 (mm). Firstly, the wave vectors of the three frequencies involved must have the same direction, and the phase matching condition must be completely satisfied aswhere k iω (i = 1,2,3) are the wave vectors of the fundamental, second-harmonic, and third-harmonic frequencies, respectively, as indicated bySince the three frequencies are collinear, the phase matching condition can be written aswhere θ is the phase matching angle, the subscripts o and e refer to ordinary and extraordinary beams, and n e (3ω, θ) = 1...

We reveal a variety of nonlinear Cerenkov radiation (NCR) patterns that occur in a single photonic crystal modulated by domain walls, which manifest themselves as normal, degenerated, and anomalous-dispersion-like NCR type sum-frequency generation. The phase-matching geometries of the evolution of Cerenkov radiation varying with the dispersion relationship among the interaction waves are exploited, respectively.

The experimental study of laser-driven material state equation puts forward extremely high requirements for the uniformity and stability of the target spot intensity distribution, and these two characteristics greatly determine the accuracy and repeatability of the experimental results. In this paper, a beam smoothing scheme combining diffraction-weakened lens array (LA) with induced spatial incoherent (ISI) technique based on low-coherence laser is proposed to solve the problems, that is, the uniformity and stability of the target spot intensity distribution in the material state equation experiments driven with narrow-band coherent laser. The super-Gaussian soft aperture used in our scheme can improve the intensity fluctuation caused by the hard-edge diffraction of the lens elements, and the temporal smoothing technique, ISI, can reduce the interference effect between the lens array elements. The speckle patterns of target spot, which are caused by interference between beamlets and determine the high nonuniformity, will randomly reconstruct after each coherent time. The high-frequency components are further smoothed by the time-average effect. In broadband high-power laser devices, ISI can be combined with LA by making the lens elements with different thickness values. This scheme can enhance the focal spot uniformity and improve the tolerance of the system to the wavefront phase distortion. The influence of wavefront phase distortion on target surface uniformity and stability are analyzed. The simulation results show that this smoothing scheme significantly reduces the target spot nonuniformity, improves the tolerance of random wavefront phase distortion, and presents a uniform and stable target spot intensity distribution. The nonuniformity of target spot will be reduced to about 10% after 10 ps, and about 3% after 100 ps. In addition, statistical analysis shows that the peak-to-valley value and the nonuniformity of the target spot intensity distribution are strongly correlated with the gradient of root-mean-square of the wavefront phase distortion. Using this method, the tolerance range of the wavefront phase distortion can be given according to the requirements of the experiments, which has reference value for designing and optimizing the laser driver parameters in the state equation experiment.