1.School of Science, Tianjin University of Technology, Tianjin 300384, China 2.The 18th Research Institute of China Electronics Technology Group Corporation, Tianjin 300384, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11504268) and the Scientific Research Program of Tianjin Municipal Education Commission, China (Grant No. 2017KJ240)
Received Date:17 August 2020
Accepted Date:17 February 2021
Available Online:19 April 2021
Published Online:05 May 2021
Abstract:Ultra-short pulse (picosecond) anti-Stokes laser can be obtained by using Raman frequency converter in a crystal medium by the coherent anti-Stokes Raman scattering effect. The crystalline Raman frequency converter based on the pump-probe method can realize the collinear interaction of coherent anti-Stokes Raman scattering, thus effectively improving the conversion efficiency of the anti-Stokes light. Theoretical simulation is an important means to study laser operation. Coupled wave equation is widely used to study the characteristics of Raman laser and anti-Stokes laser. Although the coupling wave theory of anti-Stokes Raman frequency shifter reported previously can reflect the operation law of the frequency shifter, the optimization of the frequency shifter and the influence of the frequency shifter parameters on the output characteristics of anti-Stokes laser have not been reported so far. In this paper, the picosecond anti-Stokes Raman frequency converter based on the pump-probe method is studied theoretically. Considering the generation of the first Stokes light in the probe channel and the second Stokes light in the pump channel, the coupled wave equation of the collinear picosecond anti-Stokes Raman frequency converter is established under the plane wave approximation. Without loss of generality, four dimensionless comprehensive parameters are introduced to normalize the equations. A set of universal theoretical curves describing the operation of the Raman frequency converter is obtained. The numerical solutions of the equations show that the performance of the Raman frequency converter mainly depends on three parameters: the normalized phase mismatch parameter ΔK, the normalized Raman gain coefficient G, and the energy ratio of the probe light to the fundamental light rprobe. The reasonable values of normalized variables are determined when the high efficiency anti-Stokes conversion is realized. Experimental data are used to verify the correctness of the theoretical model. The theoretical value of the anti-Stokes conversion efficiency is basically consistent with the literature data. The normalized coupled wave theory proposed in this paper is helpful in understanding the operation law of the picosecond anti-Stokes Raman frequency shifter, and has guiding significance for the design of the frequency converter. Keywords:coherent anti-Stokes scattering/ collinear four-wave mixing/ coupled wave equation/ normalization theory
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2.1.相互作用原理
考虑探测通道中一阶斯托克斯分量的产生和抽运通道中二阶斯托克斯分量的产生, 忽略高阶斯托克斯光和反斯托克斯光的产生, 各分量之间的相互作用原理如图1所示. 图1(a)为抽运通道SSRS过程的能级图. 频率为ωpump的抽运光入射到拉曼晶体中, 与物质分子相互作用, 一个抽运光光子转化成一个频率为ω1s的一阶斯托克斯光光子, 当一阶斯托克斯光强度大于二阶斯托克斯光阈值时, 作为抽运光产生频率为ω2s的二阶斯托克斯光. 探测通道的CARS过程是抽运光、探测光、一阶斯托克斯光和反斯托克斯光的四波混频过程, 如图1(b)所示. 在这个过程中, 产生一个频率为ωa的反斯托克斯光子的同时, 消耗一个一阶斯托克斯光子和一个频率为ωprobe的探测光光子, 产生一个抽运光光子. 当反斯托克斯光强度足够大时, 还可以作为抽运光通过SSRS向探测光转化, 如图1(c)所示. 图 1 抽运通道和探测通道中的拉曼散射能级图 (a) 抽运通道的SSRS能级图; (b) 探测通道的CARS能级图; (c) 探测通道的SSRS能级图 Figure1. Raman scattering energy levels in pump and probe channels: (a) SSRS in pump channel; (b) CARS in probe channel; (c) SSRS in probe channel.
表2不同情况下反斯托克斯转化效率的理论值与实验数据的对比结果 Table2.Comparisons of theoretical and experimental results of anti-Stokes conversion efficiency under different conditions.
为进一步验证本文理论的正确性, 下面考虑一种更接近实际的情况. 基频光光强在横截面上近似为高斯分布, 且在传播方向上各处的光束半径R均相等. 因此, 基频光的缓变振幅$ E _{\rm{p}}^{\rm{g}}$是径向坐标r和t的函数,