Conical wavefront pumping enabling miniaturized gaseous Raman laser with high beam quality
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摘要: 为了实现拉曼激光装置的小型化并抑制激光诱导击穿现象，利用锥透镜将泵浦激光调制成贝塞尔光束以实现受激拉曼变频。实验结果表明，增益介质的气压，泵浦光的直径，锥透镜的底角均对光子转化效率产生影响。在3.5 MPa甲烷中，
1064 nm波长、366 mJ脉冲能量的泵浦光能够产生128 mJ的1543 nm前向拉曼激光，光子转化效率达到50.7%，且有望在更高气压和更高泵浦能量下实现更高转化效率。遮挡锥透镜中心圆角尖端，仍可保留97 mJ的拉曼激光脉冲能量，此时光束质量β=2.19。实验验证了拉曼池可设计为长度0.4 m而不损坏窗口。综合多个实验结果可以推论，在不牺牲转化效率的前提下，拉曼池可以进一步缩短至0.3 m。通过轴向移动锥透镜在长拉曼池内的位置，可调节前后向斯托克斯光的输出比例。Abstract:Background Different applications require lasers of different wavelengths, and Raman laser is one of effective methods to expand spectral range of lasers. Raman lasers have advantages of high conversion efficiency, excellent beam quality, excellent scalability and wide range coverage etc. However, the cumbersome size of Raman cell (especially the long length of Raman cell) deteriorates the application of Raman laser. To reduce the length of Raman cell, a short-focus lens is required, and this would lead laser-induced breakdown (LIB).Purpose To realize miniaturization of Raman laser devices while suppressing LIB, this work proposed a method to modulate the pump laser into a Bessel beam to achieve stimulated Raman frequency conversion using an axicon. The goal is to achieve high photon conversion efficiency (PCE) and beam quality in a compact system.Methods By the comparison of intensity at focus and depth of focus of f = 0.5 m focal lens and axicon, axicon with bottle angle of 2° could effectively reduce laser intensity at focus and increase the depth of focus. In this work, a pulsed1064 nm laser was used as pump source, pressurized methane was used as Raman medium, and axicon with bottle angle of 2° was used to focus pump laser. Pressure of methane, pump laser divergence angles and diameter of pump beam were optimized to achieve the maximum conversion efficiency.Results In 3.5 MPa methane and 366 mJ energy of1064 nm pump laser, 128 mJ forward Raman laser at1543 nm was generated; the corresponding photon conversion efficiency was 50.7%, and higher output energy and conversion efficiency were expected under higher pressure and at higher pump energy. By blocking the central rounded apex of the axicon, the Raman laser pulse energy of 97 mJ can still be retained with the beam quality β=2.19. An experiment verified that the Raman cell can be designed to be 0.4 m without damaging the window. Based on the results of multiple experiments, it can be inferred that the Raman cell can be further shortened to 0.3 m without sacrificing the conversion efficiency. By axially translating the axicon within an extended cell, the forward/backward Stokes light ratio became tunable.Conclusions This study demonstrates the viability of Bessel beams for compact, high-efficiency gaseous Raman lasers. The conical wavefront pumping strategy mitigate LIB risks and enable system miniaturization, offering a promising pathway for practical applications.-
Key words:
- lasers and laser optics /
- nonlinear optics /
- axicon /
- miniaturization /
- stimulated Raman scattering
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图 1 锥透镜及其用于产生贝塞尔光束的示意图
Figure 1. Schematic diagram of the axicon and schematic diagram of generating a Bessel beam using an axicon to modulate the wavefront
图 2 使用不同方案聚焦400 mJ/8 ns泵浦光（Φ8 mm）在光轴上功率密度分布的模拟结果
Figure 2. Simulation results of the power density distribution along the optical axis for the 400 mJ/8 ns pump light (Φ8 mm) focused by different schemes
图 4 泵浦光在不同气压下，经位置1处锥透镜会聚后，FS1 PCE随着泵浦脉冲能量的变化曲线（Φ8 mm）；并与3.5 MPa下经
$ f=0.5\;\mathrm{m} $ 透镜聚焦的情形作对比Figure 4. Curves of FS1 PCE versus pump pulse energy (Φ8 mm) with the pump light focused through an axicon at Position 1 under different pressures, compared with the case using an
$ f=0.5\;\mathrm{m} $ lens under 3.5 MPa pressure
图 5 锥透镜置于位置1时，在不同光斑直径或光束发散角下，FS1 PCE随着泵浦脉冲能量的变化曲线（3.5 MPa）
Figure 5. when the axicon was placed at Position 1, curves of FS1 PCE versus pump pulse energy under different pump spot diameters or pump beam divergence angles (3.5 MPa)
图 6 不同等效锥透镜底角下，S1 PCE随着泵浦脉冲能量的变化曲线（3.5 MPa，Φ8 mm）
Figure 6. Curves of S1 PCE versus pump pulse energy under different equivalent axicon base angles (3.5 MPa, Φ8 mm)
图 7 凸透镜聚焦和不同等效底角锥透镜聚焦的情形下，输出3 mJ FS1对应泵浦能量阈值的实验值和理论值
Figure 7. In the cases of focusing with a convex lens and axicons of different equivalent base angles, experimental and theoretical value of pump energy threshold corresponding to 3 mJ FS1 output
图 8 紧凑拉曼池设计的模拟实验中，S1 PCE随着泵浦脉冲能量的变化曲线（3.5 MPa，Φ6 mm）
Figure 8. Curves of S1 PCE versus pump pulse energy in the simulation experiment of compact Raman cell design (3.5 MPa, Φ6 mm)
图 9 在锥透镜的最优空间姿态下，S1 PCE随着泵浦脉冲能量的变化曲线（3.5 MPa，Φ8 mm）。
Figure 9. Curves of S1 PCE versus pump pulse energy under the optimal spatial configuration of the axicon (3.5 MPa, Φ8 mm)
图 10 用刀口法测定的FS1束腰数据（取Φ10 mm处为
$ z=0\;\mathrm{mm} $ ）Figure 10. Data of measuring FS1 beam waist by the knife-edge method (the position of Φ10 mm was set as
$ {\textit{z}}=0\;\mathrm{mm} $ )表 1 分别使用
$ f=0.5\;\mathrm{m} $ 透镜和不同底角锥透镜聚焦泵浦光(Φ8 mm，400 mJ，8 ns)，模拟出的最高功率密度和焦深Table 1. Simulated maximum power density and depth of focus by focusing pump light (Φ8 mm, 400 mJ, 8 ns) using an
$ f=0.5\;\mathrm{m} $ lens and axicons with different base-angles, respectivelyf/m base-angles/
(°)maximum power
density/(TW cm−2)depth of focus /m
(50% power density)gain length /m
(10% power density)convex lens 0.5 0.897 0.030 0.050 axicon 0.5 0.066 0.367 0.961 1 0.130 0.208 0.471 2 0.244 0.118 0.217 3 0.346 0.084 0.156 5 0.541 0.054 0.094 10 1.084 0.027 0.046 
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