A frequency converter is usually used in high-power laser facilities to triple the frequency of the incident laser. It consists of a KDP crystal and a KD*P crystal in series. The measurement of the detuning angles of the frequency converter is essential for improving the frequency conversion efficiency. In this paper, we propose a measurement method that uses a diverging beam to illuminate the frequency converter and get the detuning angles by analyzing the light intensity distribution of the third harmonic laser. The simulated results show that there is a strict linear relationship between the characteristic parameters and the detuning angles of frequency converter, verifying the feasibility of this method. Compared with existing measurement methods, its optical path is simple, and the single measurement time can be shortened to a few minutes from a few hours.
Polarization smoothing is a means to increase the uniformity of the focal spot by using polarization characteristics of light. It can theoretically reduce the contrast of focal spot to 0.707 times, and because of its real-time modulation characteristics, it can significantly suppress backscattering and improve the beam-target coupling performance of high-power laser facilities. However, in the traditional polarization smoothing scheme, the crystal used for polarization smoothing (largeaperture KDP crystal) will produce transverse stimulated Raman scattering in the high-power ultraviolet band, which will cause crystal damage. To prepare for the exploration of new single beamlet polarization smoothing schemes, we conduct a simulation study on the scheme of modulating the full Poincaré beam with a single transmission element. The results show that a single waveplate with a fast axis that rotates with polar angle and a phase retardation that increases with radius can modulate a full Poincaré beam, and we analyze the properties of this full Poincaré beam. Then combined with the engineering parameters, we analyzed the polarization smoothing performance of the full Poincaré beam with square aperture laser input. The performance was quantified by several target focal spot intensity evaluation indexes, and the results show that the use of the full Poincaré beam for polarization smoothing can reduce the contrast of the focal spot effectively, and the reduction range is close to the theoretical maximum value. And there are advantages in suppressing high energy density hot spots and optimizing the spatial frequency of the focal spot.
Aiming to minimize the surface distortion of large-aperture laser transport mirrors in high-power laser facilities, an assembly design and mounting method are proposed for the mirror. First, a theoretical model on the mirror surface deformation is established. With a new assembly design, the mirror is fastened on its neutral plane and its optical surface distortion can be precisely compensated through several adjustable forces on the sides, which will generate bending moments on the mirror body. Furthermore, a dynamic kinematic joint is designed, in which a corresponding relationship between spring compression and screw rotation ensures the accurate control of the magnitude of mirror preload. Finally, the performance of the presented method has been validated through field experiments and numerical simulations. This transport mirror assembly and mounting design have obvious technical advantages on simple mechanical structure, high operational efficiency, and precise preload control. The results show that the assembly design and optimized mounting strategy can keep the total surface distortion of the mirror within 350 nm (peak-valley).
Surface control and phase matching of large laser conversion optics are urgent requirements and huge challenges in high-power solid-state laser facilities. A self-adaptive, nanocompensating mounting configuration of a large aperture potassium dihydrogen phosphate (KDP) frequency doubler is proposed based on a lever-type surface correction mechanism. A mechanical, numerical, and optical model is developed and employed to evaluate comprehensive performance of this mounting method. The results validate the method’s advantages of surface adjustment and phase matching improvement. In addition, the optimal value of the modulation force is figured out through a series of simulations and calculations.
This paper puts forward an adaptive optics (AO) mounting method of large aperture KDP frequency doubler used in high power solid state lasers. Integrated optomechanical theory is proposed and applied to verify the mechanical and optical performances of this AO method particularly. According to the thin plate theory and nonlinear optics theory, optomechanical model is developed. Then, the finite element method is employed to establish the numerical model and simulate the distortion process of the crystal plates under various boundary conditions. The results indicate that this AO method could correct the deformed surface and modify the phase matching condition significantly, which means the second harmonic generation (SHG) efficiency will be improved as well.
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