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IEEE Journal of Quantum Electronics

Improved short-pulse pumping of stimulated Raman scattering in lead vapor

Larry R. Marshall, James A. Piper

Published Jan 1, 1990• 5 citations
Publisher

Abstract

Improved efficiency for short pulse stimulated Raman scattering (SRS) in Pb vapor is obtained using accumulation of gain between a closely spaced pulse pair. The peak power conversion efficiency of the delayed pulse is increased to 40%, in contrast to 15% for a single pulse. Numerical modeling gives good agreement with observation, and suggests that this enhancement is caused by the continued presence of coherent excitation produced by the first pulse. The dependence of the enhancement in delayed-pulse efficiency on Xe buffer gas pressure in the Pb-vapor cell is attributed to changes in the transient response time T/sub 2/ of the Pb medium with Xe pressure.>

DOI
10.1109/3.60908

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Annual Meeting Optical Society of America•Jan 1, 1985•0 citations
Improved efficiency for short pulse stimulated Raman scattering (SRS) in Pb vapor is obtained using accumulation of gain between a closely spaced pulse pair. The peak power conversion efficiency of the delayed pulse is increased to 40%, in contrast to 15% for a single pulse. Numerical modeling gives good agreement with observation, and suggests that this enhancement is caused by the continued presence of coherent excitation produced by the first pulse. The dependence of the enhancement in delayed-pulse efficiency on Xe buffer gas pressure in the Pb-vapor cell is attributed to changes in the transient response time T/sub 2/ of the Pb medium with Xe pressure.>
The short-pulse conversion efficiency of stimulated Raman scattering in Pb vapor is increased from 15% to 35% by using a novel technique that employs a closely spaced pair of pump pulses. The second pulse scatters off the coherent excitation induced in the medium by the first pulse, with a resultant enhancement in efficiency. To our knowledge these results give the first observation of such long-lived cooperative phenomena in stimulated Raman scattering. We have also observed this phenomenon in H(2) and show that this technique is readily applicable to other Raman-active media.
The stimulated collision-induced fluorescence (SOF) at wavelengths of 472.6nm, 582.6nm and 648.3nm, as well as Raman-shifted radiation at wavelength of 475nm were observed in barium vapor pumped by broadband pulse of the XeOl laser. The SCF was found to be self-termination short pulses with FWHM of 8 ns, and its internsity inoreased with buffer-gas pressure. There existed time delay aa long as 20 ns between the front edges of the pump pulse and of the Raman-shifted pulse, and the Raman conversion efficiency decreased significantly as the buffer-gas pressure increasing.
The high pulse repetition rates (~100 MHz) of radio-frequency free-electron lasers make possible the investigation of molecular response to a rapid pulse train.1 When the molecular decay time T2 is much larger than the time between pulses T, molecular memory is carried from pulse to pulse. For stimulated Raman scattering (SRS) the memory effect can lead to an increase in the size of the Raman dipole resulting in enhanced scattering. We analyze the 1-D SRS equations in the condition that the scattering under an individual pulse occurs in the transient regime (~10 ps). We obtain a general expression for the behavior of SRS which reduces to the following long-time limits: (1) When T2 ≪ T, the SRS from each pulse in the train develops independently from the others; (2) When T2 ≫ T, the transient SRS response is equivalent to the steady-state response to a constant pump at the time-averaged pump intensity. We also perform this analysis for a medium such as the atmosphere where the density of scatterers and Raman bandwidth decrease exponentially with distance.
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.
Investigations were made of the accumulation effect exhibited by transient electronic stimulated Raman scattering in atomic barium vapor pumped by a train of ultrashort pulses with a controlled time delay between the pulses. The essential feature of the accumulation effect is that the Stokes energy of a pulse is contributed to not only by its own exciting pump pulse, but also by the previous pump pulse, the influence of the latter depending strongly on the ratio of the transverse relaxation time T2 to the interpulse time interval. If one is to observe the accumulation effect in electronic stimulated Raman scattering, it is important to minimize the influence of depletion of the number of atoms. A method based on the accumulation effect for determination of T2 in one laser shot was developed.
We consider transient stimulated Raman amplification of trains of picosecond pulses. In this regime the gain of a single pulse is reduced from its steady-state value because of the inability of the nonlinear polarization to follow the time variations of the pump pulse. In addition oscillatory behavior of the pump and Stokes pulse amplitudes have been predicted in the transient regime in conditions of high pump depletion. When the pump radiation is in the form of a train of pulses whose separation is comparable to or shorter than the dephasing time of the nonlinear medium, increased gain can be experienced by later pulses in the train because they can grow from the material polarization created by earlier pulses. We have analyzed this type of amplification as a function of pulse spacing, pump energy, and pump intensity in the regime of high pump depletion. In addition we have considered the effects of Stark shifts of the two-photon resonant Raman level on the Stokes conversion efficiency. We find that at constant pulse intensity the conversion efficiency increases with time through the pulse train, gradually approaching steady state at low conversion efficiencies at times long compared to T2. The maximum conversion efficiency increases as the pulse spacing is decreased. If the pump depletion is too high the peak conversion of the Stokes oscillates through the train in a manner similar to the oscillations of the intensity of a single pulse. A Stark shift of the resonant level can cause reduced gain as well as introduce phase shifts on the Stokes and pump beams. The effects of these processes on the conversion efficiency are discussed.