[en] Based on the fractal theory, this paper analyzed statistical distribution features of middle-high frequency phase distortion of the optics for Shenguang-Ⅲ laser facility. The middle-high frequency phase distribution is found to be consistent with the model of fractional Brownian random field. The fractal dimension was calculated by structure function. The fractal dimension of large-aperture neodymium-doped phosphate amplifier glass is distributed between 2.34 and 2.43. A method of phase simulation by random midpoint-displacement was presented. In this method, the fractal dimension can be viewed as a parameter for estimating phase distortion, which helps to evaluate the high laser optics. (authors)

[en] In high power laser facilities, the GRMS is a key specification to evaluate the low frequency wavefront distortion. While the deformable mirror is widely used to correct the distortion, the raw GRMS specification should include the effect of deformable mirror. The article presents the method to decompose the influence of deformable mirror and the partial coherent characteristic of corrected wavefront. The relation between the GRMS and the size of laser focus of corrected wavefront is verified. Based on the research, the procedure of setting up the GRMS specification of the optics is established. (authors)

[en] In the SG-Ⅲ prototype laser facility, if the laser's narrow bandwidth is adjusted to broadband while maintaining the main configuration, the energy output will decrease. This article evaluates the kilojoule energy output capability of tens-nm broadband laser, using numerical simulation softwares SG99 and CPAP respectively for narrowband and broadband laser propagation and amplification, and the program of one-dimension reverse propagation and amplification. The result shows the growing of gain-narrowing effect under broadband condition. The compensation of gain-narrowing is studied. With compensation, while losing 40% of the input energy, the output energy can reach 2000∼3000 J under 3 J injection. Hence there should be at least 5 J energy output after the preamplifier. This research will conduct the implementation plans for tens-nm broadband 2ω laser system. (authors)

[en] The diffraction character is redefined based on Fresnel number. This new definition is extended to focusing or diffusing optical path. The sign of the new diffraction character is defined for each optical element in a laser system. According to these definitions and extending, the effect of phase aberration on near-field modulation and the effect of linear diffraction on FM-to-AM are studied. The linear diffraction effect on high power laser facility can be evaluated by the combination of the new diffraction character and the energy fluence. (authors)

[en] Based on the analytical solution of the dependence of phase matching angle on the crystal temperature, we illustrate the thermo-optic physical mechanism of temperature influence on the frequency conversion. The thermal sensitivity of the doubler and tripler crystals are numerically simulated respectively. Due to such sensitivity discrepancy among various crystals, the phase matching angle curve regarding the crystal temperature variation in our laser facility is obtained in experiment. The influence of PM angular detune on frequency conversion is theoretically analyzed under different input intensities, which is demonstrated by the conversion efficiency decline induced by temperature variation. According to the thermal sensitivity parameter obtained in experiments, we compensate the conversion efficiency with the crystal angle following, resulting in stable output of third harmonic energy. (authors)

[en] The transmission characteristics of phase modulation pulse transmitted through the filter in the power amplifier are investigated theoretically and experimentally. The narrow bandpass filter can induce large temporal modulation depth for the phase modulation pulse and induce double amplitude modulation (AM) if the frequency shift is lower than half bandwidth of the signal spectrum. We should choose a wider bandwidth filter to minimize the impact of the filter on the output pulse and suppress the amplified spontaneous emission (ASE) for the power fiber amplifier. These results are of benefit to the design of the fiber front end system. (authors)

[en] This research establishes a polarization converter consisting of biaxial cascade crystals and a 4f system. The polarization manipulation of vector beams is investigated theoretically. This paper develops an analytical model for the vector beams created by biaxial cascade crystals based on Belsky–Khapalyuk–Berry paraxial theory. The transformation of the electric field caused by conical refraction is verified to be a unitary transformation, resulting in the inhomogeneous state of polarization with the intensity distribution remaining invariant. The numerical simulation results show that the state of polarization varies with incident polarization and the angle between cascade crystals. In particular, for the angle of zero, the exit beam has the state of polarization covering the whole Poincaré sphere. Moreover, the part-Poincaré beams are generated by rotating the crystal around the optics axis. The details of the state of polarization are mapped on the Poincaré sphere to analyze the distribution rule. This method has the potential to extend the coding degree of freedom for optical communication and the diversity of light and matter interactions. The unitary transformation may have great value in research about optical neural networks. (paper)