CC BY
2*
ALT'23
The 30th International Conference on Advanced Laser Technologies
LD-O-6
Stimulated Raman scattering in the ultrasound wave field - high conversion efficiency
A. A.Skrabatun, A.A.Matrokhin, A.D.Kudryavtseva, A.N.Maresev, T.V.Mironova, M.A.Shevchenko, N.V.Tcherniega, S.F.Umanskaya
The P.N.Lebedev Physical Institute of tht RAS, Leninskii pr., 53, Moscow 119991, Russia
In numerous applications using the stimulated Raman scattering (SRS) process, the key issue is the energy efficiency of the process. One of actively applied ways to improve the efficiency of SRS is the use of distributed feedback, first considered in [1] and actively used at present [2]. SRS efficiency can be also significantly increased by a local increase in pressure caused by a shock wave [3].
This work presents the results of experimental studies of the effect of ultrasonic waves on the efficiency of the SRS process for a number of Raman-active liquids. SRS was excited by the second harmonic of a mode-locked Nd:YAG laser (532 nm, 30 ps, 10 mJ, 10 Hz). The experiments were carried out for two different geometries. In the first case, the radiation entered the sample from above, through the free surface. In this case, the SRS was registered in backward direction. In the second case, the exciting radiation was focused through the window of the cell and SRS was registered in forward and backward directions. The cell with the sample was placed in an ultrasonic bath (200 W, 40 kHz).
The experimental results showed that when the pump is focused into the open surface of the liquid, the backward SRS intensity increases in water by 1.5 times, in heavy water by more than 2 times, in ethanol for the first Stokes component by 15 times, and for the second Stokes component by 4 times (Fig. 1).
a)
b)
Fig. 1. SRS spectra when radiation is focused from above through the free surface of a liquid with ultrasonic impact switched off and on. a) in heavy water (pump energy 10 mJ), b) in alcohol (pump energy 2 mJ).
An increase in the power of ultrasound leads to a significant increase in the intensity of the scattered radiation. When the radiation was focused through the cell window into the bulk of the liquid, the forward SRS intensity also increased significantly when ultrasound was turned on. In water, the inclusion of ultrasound led to an increase in the SRS intensity by more than an order of magnitude.
An increase in the SRS intensity under ultrasonic impact is due to the arising of distributed random feedback caused by the phase inhomogeneities formation in the liquid. The reasons for the occurrence of these inhomogeneities are the acoustic currents of vortex flows that occur under the action of ultrasound, as well as cavitation - the formation and collapse of bubbles in a liquid.
[1] S.A.Akhmanov and G.A.Lyakhov, Effects of inhomogeneity of optical pumping in lasers and stimulated scattering. Self-excitation due to distributed feedback, JETP 66, 96-108 (1974).
[2] 5. S. Loranger and R. Kashyap, Efficiency increase of distributed feedback Raman fiber lasers by dynamic control of the phase shift, Optics Letters 43, Issue 23, 5705-5708 (2018).
[3] C.Wang, Y.Wang, X.Cao, S.Wang, C.Sun, and Z.Men, Shock compression-induced enhancement of stimulated Raman scattering in heavy water, Optics Communications, 501, 127394 (2021).
