Conclusion
In this work, experiments of D2 SRS with multi-pass configuration were conducted at infrared nanosecond pumping. The
lowest S1 threshold is 5 mJ (1 MW) only, which is an order of magnitude lower than the one in previous report [14]. The maximum conversion efficiency of S1 is also improved. The effects of
the D2 pressure and pass number on the threshold and conversion efficiency of S1 and S2 have been investigated. The thresholds of S1 and S2 were decreased with the increasing pressure of D2 and
pass number, but the effects on the conversion efficiency was much more complicated. AS1 and S2 both contribute to the consumption of S1, but S2 dominates due to strong cascade processes.Therefore, only when S2 is suppressed, the S1 conversion efficiency increases with the pump energy. The maximum S1 conversion efficiency reached (56.174.4)% with the configuration of
9-pass and at a pressure of 42 bar D2. High order of Stokes conversion demonstrated in this work may be a potential way to generate IR above 2 μm. The threshold for S2 generation was only
10.9 mJ (2.2 MW), and the maximum S2 conversion efficiency was estimated to be (32.973.8)%. To the best of our knowledge, these values are the lowest threshold and the highest conversion efficiency of SRS in D2 pumped by the nanosecond laser radiation at
1064 nm. With the improvement of the reflectivity of the cavity
mirrors and optimization of the pass number and pressure of D2,the conversion efficiency of S2 could increase significantly. Indirect proof indicated that a substantial amount of S3 at 23 mm was probably generated in this experiment, and the threshold of S3
was expected to be less than 20 mJ (4.0 MW). Finally, this work
has demonstrated an efficient method to reduce the threshold of SRS and to optimize the conversion efficiency of S1 and S2. This process makes the application of SRS to gases with relatively smaller Raman gain coefficients feasible. This configuration may also be applied to other types of Raman gain media, e.g., to methane to reduce the power of the pump laser beam, avoiding laser induced break-down and carbon contamination. By the different combination of pump source, Raman gas and order of Stokes, more near IR light sources could be obtained.