1.Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, China 2.Key Laboratory of Nondestructive Test (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 41776111, 41666004, 61865013, 41576033, 61665008), the Natural Science Foundation of Jiangxi Province, China(Grant Nos. 20171BAB202039, 20161BBH80036), the Distinguished Young Fund of Jiangxi Province, China (Grant No. 20171BCB23053) and the Aeronautical Science Foundation of China (Grant Nos. 2016ZD56007, 2016ZD56006).
Received Date:17 August 2018
Accepted Date:24 December 2018
Available Online:01 February 2019
Published Online:20 February 2019
Abstract:Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two kinds of emblematic inelastic scattering processes resulting from the interaction of high-intensity laser with matter. Generally, competition between SBS and SRS is a common phenomenon in many substances. In liquid or high-pressure gas, if a single longitudinal mode laser is used as a pump source, both SBS and SRS can be excited, but the SBS will become very strong due to higher gain and optical phase conjugation. In comparison, the SRS gain is typically 2 orders of magnitude smaller than the SBS gain so that most of the pump laser energy is spent on the SBS and the SRS is greatly suppressed. To improve the output energy of SRS in liquid medium, a method of suppressing the SBS process by controlling temperature of medium is proposed. The SRS generation system using broadband pulse laser of 532 nm in wavelength as a pumping source is designed, the output energy of forward SRS (FSRS) and backward SBS (BSBS) in water with different temperatures are measured, and the physical mechanisms of the influences of water temperature, pumping linewidth and thermal defocusing on the output energy of SRS are analyzed. The experimental results indicate that by reducing the water temperature, the SBS process can be significantly suppressed, and the beam distortion caused by thermal defocusing effect can be reduced, thus effectively improving the output energy of SRS. Unlike the single longitudinal mode laser, when the pump source is handled in multiple longitudinal modes with a wide linewidth, the gain of FSRS is higher than that of the backward SRS (BSRS). Meanwhile, since the SBS gain coefficient is restricted by the linewidth of the pump laser, the FSRS process is dominant and both backward SBS and BSRS are significantly suppressed. It is necessary to state that none of the influence of backward SRS, self-focusing, optical breakdown and other non-linear effects on the output energy of SRS is considered in this paper, and only the effectiveness of reducing temperature to improve the energy output of forward SRS is verified from the perspective of temperature change. The results are of great significance for the multi-wavelength conversion of SRS in liquid medium. Keywords:stimulated Raman scattering/ stimulated Brillouin scattering/ energy amplification/ thermal defocusing
3.实验结果与分析泵浦光脉冲时域轮廓如图2(a)所示, 脉宽约为8 ns. 图2(b)所示为532 nm激光脉冲泵浦下水的归一化SRS光谱, 水的温度为23 °C, 泵浦能量为100 mJ, 光谱由分辨率为0.4 nm的光纤光谱仪(AvaSpec-ULS2048, Avantes)在水槽出光口采集获得. 可以看出, 在泵浦光波长两侧分布着发生“红移”(~649 nm)和“蓝移”(~436 nm)的SRS特征峰, 分别对应纯水中SRS的斯托克斯和反斯托克斯分量. 其中, 斯托克斯频移是由水分子对称分布的O—H键拉伸振动引起, 其强度远远大于反斯托克斯. 图中插图为单反相机拍摄记录的经二向色镜分光后的SRS光斑, 可明显看出发生“红移”的斯托克斯信号. 图 2 (a)泵浦光时域轮廓; (b)水的受激拉曼散射归一化光谱图, 泵浦能量为100 mJ/Pulse Figure2. (a) Temporal profile of pump leaser; (b) normalized SRS spectrum of distilled water at pump energy of 100 mJ/Pulse.
图3所示为不同温度下FSRS输出能量随入射泵浦光能量的变化关系. 从图3可以看出, 在相同温度下, SRS能量随入射光能量增加而增加, 且在低温时趋于线性变化; 而在相同泵浦能量不同温度条件下, 温度越高, SRS能量越低, 在水温为5 °C时, 最大输出能量为26.2 mJ, 水温35 °C时的最大输出能量为8.5 mJ. 图 3 不同温度下SRS输出能量随入射泵浦光能量变化 Figure3. Output energy of SRS versus the incident pump energy at different temperatures.
为了分析及解释SRS能量随温度变化关系, 实验同步测量了不同温度下BSBS输出能量变化. 图4为同一实验条件下, BSBS输出能量随入射泵浦能量的变化关系. 从图4中可看出, 在相同温度下, SBS 能量随泵浦能量增加而增加, 当入射能量超过一定值时, 增加斜率减小. 在相同泵浦能量下, 温度越高, SBS输出能量越高, 在水温为35 °C时最大输出能量为16 mJ, 在水温为5 °C时, SBS输出能量小于1 mJ. 图 4 不同温度下SBS输出能量随入射泵浦光能量变化 Figure4. Output energy of SBS versus the incident pump energy at different temperatures.
因此, 随着温度的增加折射率减小, 会产生介质的热散焦效应. 依据文献[26]报道, 当介质温度降到2 °C时, 可以近似认为${{{\rm{d}}n} / {{\rm{d}}T}} = 0$, 即可以不考虑温度对折射率的影响. 这就说明可以通过降低介质的温度来减小热散焦效应对水中SRS过程的影响, 从而提高SRS的输出能量. 图5显示的是CCD相机测量的不同温度下出光口剩余泵浦光束强度远场分布轮廓. 入射泵浦光能量为50 mJ, 出光口剩余泵浦光经过衰减器(40 dB)及聚焦透镜(f = 150 cm)后入射到CCD阵列, CCD的象元尺寸为6.5 μm. 图5(a)显示的水温5 °C时剩余泵浦光的强度分布轮廓, 其横截面高斯拟合相似度在X, Y方向分别为77.1%和80.2%, 近似于高斯光束. 图5(b)显示的是水温为35 °C时的强度分布轮廓, 能很明显地看出光束出现强度分布畸变. 从SRS的光斑分布上也可以看出热散焦效应带来的光束畸变不利于SRS的有效产生, 如图6所示. 图中显示的是入射光泵浦能量为150 mJ时, 用单反相机记录的经过二向色镜分光后的SRS输出光斑分布, 从图6中可以看出, 在5 °C时SRS的光斑能量更集中, 而在35 °C时光斑出现扩散现象, 并且强度明显减弱, 这也印证了我们的分析结果. 图 5 入射泵浦能量为50 mJ时, 不同温度下水池出光口剩余泵浦光强度的远场分布轮廓 (a) 5 °C; (b) 35 °C Figure5. Far-field profiles of intensity distribution of residual pump beam at the exit of the cell window at different temperatures,when the incident pump energy is 50 mJ/Pulse: (a) 5 °C; (b) 35 °C.
图 6 入射泵浦能量为150 mJ时, 不同温度下的SRS输出光斑分布 (a) 5 °C; (b) 35 °C Figure6. Facula profiles of SRS at different temperatures when the incident pump energy is 150 mJ/Pulse: (a) 5 °C; (b) 35 °C.
图 1 实验测量原理图
图 2 (a)泵浦光时域轮廓; (b)水的受激拉曼散射归一化光谱图, 泵浦能量为100 mJ/Pulse
图 3 不同温度下SRS输出能量随入射泵浦光能量变化
图 4 不同温度下SBS输出能量随入射泵浦光能量变化
图 5 入射泵浦能量为50 mJ时, 不同温度下水池出光口剩余泵浦光强度的远场分布轮廓 (a) 5 °C; (b) 35 °C
图 6 入射泵浦能量为150 mJ时, 不同温度下的SRS输出光斑分布 (a) 5 °C; (b) 35 °C