Two Raman cells are used: one as a seed cell and a second as the pump cell. The cells contain deuterium at approximately 300 psi. As the figure shows, a beam splitter directs thirty percent of the original laser beam into the seed cell with the remainder going to the pump cell. A plano-convex lens (with a 100 cm focal length) focuses the light into the center of the seed cell. The backscattered Stokes beam passes back through the lens and is reflected by dichroic mirrors into the pump cell. A plano-convex lens (with a 100 cm focal length) is used to focus the seed Stokes beam into the center of the pump cell. The pump laser pulse is focused by a plano-convex lens (with a 75 cm focal length) and enters the pump cell at the same time as the seed cell Stokes beam; the two beams propagate in opposite directions through the pump cell. An output Raman pulse is generated which is composed of both an amplified seed beam and the Stokes beam backscattered from the laser beam. The distance from the laser to the seed cell is approximately 4 m and from the laser to the pump cell is approximately 6 m.
The dual Raman cell system is a unique system, developed at NRL and described by a NRL Patent. The system can used at high power (generating 200 mJ/pulse), operate at 50 Hz, provide short pulses (3 ns), and high beam quality (100 microradians).
Careful alignment of both the focused seed and pump beams is essential. During the alignment procedure, the pump cell is removed and the Nd:YAG is operated at relatively low power. Both the Stokes seed beam and the pump laser beam are aligned so that they pass through a pinhole that is positioned at the center of the pump cell and a second alignment guide located further down the beam path.