Threshold and conversion experiments on seeded stimulated Raman scattering

Abstract

Abstract. High energy-conversion efficiencies in Stimulated Raman Scattering (SRS) are demonstrated both in experiments and by simulations for pump powers below SRS threshold. The scattering is induced by a short seed pulse at the Stokes frequency, the pulse width of which is much shorter than the pump pulse width and which is comparable with the medium's dephasing time. PACS: 42.50.Md; 42.55.Ye; 42.65.Dr In a Raman-active medium, high conversion efficiencies to the Stokes wave can be achieved by Stimulated Raman Scattering (SRS). Due to the low gain for scattering to Stokes waves in the infrared, high pump powers and long interaction paths are necessary. Additionally, when the scattering becomes transient for pump pulses with a pulse duration comparable to the medium's dephasing time, the Raman gain is further reduced. One method to overcome the low Raman gain is by seeding. The scattering is then made efficient by an initially applied Stokes field rather than started up from the Stokes-noise field. The seed power easily exceeds the Stokes-noise power by many orders of magnitude, and substantially lowers the gain necessary to obtain Stokes powers comparable to the pump power. In previously performed experiments [1], the conversion efficiency in transient scattering was increased by a seed pulse with a pulse duration comparable to that of the pump pulse. ExperimentsIn the experiments described in this paper, the SRS efficiency of a pump pulse in the infrared is increased by adding a seed pulse at the Stokes frequency which is much shorter than the pump pulse, while the medium's dephasing time is of the same order of magnitude as the seed pulse. It is demonstrated both in experiments and in simulations that this short seed pulse is able to induce SRS with high energy-conversion efficiency, where unseeded scattering would otherwise generate Stokes energies below detection levels.In these experiments, a 10.26 gm pump pulse from a TEA CO2 laser with a pulse width of 70 ns at half maximum and a pump energy in the gain-switched peak of maximum 1.4 J was scattered by SRS in para-hydrogen (p-H2). The pump beam was nearly TEM0o but the pulse shape generally showed small modulation due to mode beating. The So(0) rotational transition was used as the Raman transition. For this transition, the pressure dependence of the dephasing time T2 of the Raman polarizability can be expressed as T2 = (~Bp)-i with B = 97.3 MHz/bar [2] for pressures p, where the Raman line width is pressure broadened, i.e., for pressures above approximately 0.3 bar [3]. The Raman shift of 354 cm-1 shifts the pump wavelength to a Stokes wavelength of 16.11 gm. To increase the small Raman gain at these long Stokes wavelengths, a Multi-Pass Cell (MPC) [4] was used. The MPC consisted of two spherical mirrors separated by 3.7 m in a near-concentrical, stable-cavity setup in which the pump and Stokes beams were focused at each pass with a confocal parameter of 72 cm. In the MPC, the number of passes could be varied with an axially mounted rotatable perisco...