To increase the spectral range covered by laser systems output frequencies, different active media materials are used or the laser radiation is converted using nonlinear optical methods. Stimulated Raman scattering in solid state crystals is a powerful technique that enables to downshift the radiation frequency and generate one or more new lines, which are separated by a Raman frequency. The first Raman-active crystal used was CaCO₃ and among other crystals used for Raman conversion belong NaNO₃, Ba(NO₃)₂ and diamond [1]. In our experiment, we investigated Stimulated Raman scattering of picosecond laser pulses in three tungstate crystals – KGd(WO₄)₂, KY(WO₄)₂ and BaWO₄. Laser system comprising Nd:YAG actively-passively mode-locked oscillator, two telescopes, pulse selector, and three Nd:YAG amplifiers was used as a source of laser radiation. The laser system generated 1064-nm, 50-ps pulses of up to 100 mJ of energy. KDP II nonlinear crystal was used for second harmonic generation. In each crystalline medium, we measured the stimulated Raman scattering threshold and generated energy in each spectral Stokes component.

The threshold of the stimulated scattering process was firstly measured for second harmonic pumping. The threshold intensity values allowed us to calculate the experimental threshold gain coefficient G=25/(I*l) (I is the threshold intensity and l is the length of the crystal) to be equal to 14.4 cm/GW for BaWO₄, 11.5 cm/GW for KGd(WO₄)₂ and 18.7 cm/GW for KY(WO₄)₂ under 532-nm pumping. For 1064 nm pump wavelength the threshold measurements were also performed and the gain values were obtained to be 3.8 cm/GW for BaWO₄, 3.0 cm/GW for KGd(WO₄)₂ and 4.7 cm/GW for KY(WO₄)₂. Maximum conversion efficiencies of the pump radiation to the first and second Stokes components were measured to be 30 % and 15% for BaWO₄ and 18% and 15% for KGd(WO₄)₂, respectively. The conversion to the third Stokes component reached 2 % in BaWO₄ and 5 % in KGd(WO₄)₂ . When scattering to the higher-order Stokes components appears, the saturation of the lower order Stokes was observed and their conversion efficiency was decreased.

We can conclude that efficient stimulated Raman scattering has been observed in all investigated tungstate crystals, BaWO₄ being the most promising of them due to the high value of gain in both picosecond and nanosecond [2] temporal regions.

References

[2] Zverev,P.G. - Basiev,T.T. - Sobol,A.A. - Skornyakov,V.V.: Stimulated Raman scattering in alkali - earth tungstate crystals, Quantum Electronics, 29, N12, (1999)

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