Threshold energy measurements of transient rotational Stimulated Raman Scattering are compared to Raman conversion calculations from semiclassical theories using a simple concept of a gain reduction factor which expresses the reduction of the gain from its steady-state value due to transient scattering. Excellent agreement was found with theoretically derived gain reduction in the transient and highly transient case.
We demonstrate a miniaturized broadband spectrometer employing a reconstruction algorithm for resolution enhancement. We use an opto-digital co-design approach, by firstly designing an optical system with certain residual aberrations and then correcting these aberrations with a digital algorithm. The proposed optical design provides an optical resolution less than 1.7 nm in the VIS-channel (400-790 nm) and less than 3.4 nm in the NIR-channel (760-1520 nm). Tolerance analysis results show that the components are within a commercial class, ensuring a cost-efficient design. We build the prototype with a size of 37x30x26 mm3 and demonstrate that by applying a restoration algorithm, the optical resolution can be further improved to less than 1.3 nm (VIS-channel) and less than 2.3 nm (NIR-channel).
The generation of long Stokes pulses by stimulated Raman scattering, which is made efficient by a short seed pulse at the Stokes frequency, is demonstrated both in experiments and in simulations for pump powers below the threshold power for unseeded scattering. It is observed that a second seed pulse with appropriate phase and intensity can extend, switch-off or briefly interrupt the Stokes pulse.
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...
In transient stimulated Raman scattering (SRS), efficient conversion of short pump pulses to Stokes radiation is difficult to obtain. A short seed pulse at the Stokes frequency increases the efficiency and may induce full depletion of the pump pulse, even when the pump power is too low to scatter efficiency without seed. However, for low pump powers the Raman polarizability induced by the pump and seed pulse decays, which results in a terminating Stokes pulse, even when the pump pulse itself is long. Nevertheless, the Stokes pulse can be wider than the dephasing time of the Raman polarizability or the width of the seed pulse by an order of magnitude. Applying a second seed may once again induce a sufficiently large Raman polarizability but only if the phase of the Stokes seed matches the phase difference between the pump and remaining Raman polarizability. If not, the polarizability induced by the first pulse and consequently the conversion efficiency for the Stokes radiation decreases. Therefore, it is possible to switch off the Stokes pulse by applying a second seed with appropriate phase and intensity.
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