It is well known that stimulated Brillouin scattering (SBS) and stimulated Raman scattering SRS are two kinds of parametric instabilities in laser–plasma interaction (LPI) of inertial confinement fusion (ICF)[
High Power Laser Science and Engineering, Volume. 1, Issue 2, 02000094(2013)
Experimental observation of backscattered light based on different coherence between incident laser beams
Recent experimental results on NIF revealed a much higher stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) backscatter than expected; one possible reason was due to the coherence between incident laser beams. In our research, two laser beams (, in each one) with different coherent degrees between them from the SG-II facility were employed to irradiate an Au plate target; the backscatter of SBS and SRS in the range of the given solid angle had been measured. The results showed that it could change dramatically corresponding to the difference of the coherent degree between the two laser beams, and there was usually more intense backscatter the higher the coherent degree between the incident beams.
1. Introduction
It is well known that stimulated Brillouin scattering (SBS) and stimulated Raman scattering SRS are two kinds of parametric instabilities in laser–plasma interaction (LPI) of inertial confinement fusion (ICF)[
2. Experimental condition and setup
The experiment was carried out on the SG-II laser facility; Figure
Because the wavelengths of SBS and SRS are very different (the former is similar to the wavelength (527 nm) of the incident laser, while the latter has a broad band (covers 1–2 times the wavelength of the incident laser)), with peak wavelength near 800 nm, it is feasible to separate the backscatter of SBS and SRS by a spectroscope. Figure
In order to study the influence of the coherent condition on backscatter in our experiment, the key is to form different coherent degrees between the incident beams. Fortunately, the driving laser beams 1# and 3# are all well polarized linearly, so it is possible to achieve different coherence degrees between them by altering their polarization directions. In fact, it was realized by rotating the frequency doubling crystal. The frequency doubling efficiency of the crystal remains as it rotates , but the polarization direction of the driving laser appears a quarter turn.
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Figure
3. Experimental results and analysis
Table
Of course, two factors may have affected the experimental result somewhat. On the one hand, the experimental data are insufficient due to the limited shots. On the other hand, sampling in a small aperture may be vulnerable to the difference in distributions of laser intensity from different shots. However, the experiment data from different beams reveal a consistent trend. That is, although insufficient data and sampling in a small aperture may have some effect on the experimental result, it is credible that the backscatter of SBS and SRS becomes more intense along with the increase of coherence degree between incident beams.
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For beam 3#, the spectrum of SRS was measured with a grating spectrometer, as shown in Figure
4. Conclusion
Experimental results prove that the impact of coherence between laser beams on backscattered light is real, and that there is usually more intense backscatter the higher the coherent degree between the incident beams. Meanwhile, the conclusion above suggests a possible way to understand the increase of SBS and SRS backscatter in recent related experiments on NIF.
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Xiangfu Meng, Chen Wang, Honghai An, Guo Jia, Huazhen Zhou, Sizu Fu. Experimental observation of backscattered light based on different coherence between incident laser beams[J]. High Power Laser Science and Engineering, 2013, 1(2): 02000094
Received: Jun. 27, 2012
Accepted: May. 3, 2013
Published Online: Nov. 19, 2018
The Author Email: Xiangfu Meng (mengxiangfu07@163.com)