Abstract

We have confirmed, for the first time to our knowledge, the nonexponential character of Raman amplification at the phase matching angle ϕ for which 2k_L = k_s + k_A. The experiments reported here used spatially filtered pump and probe beams at 532 and 633 nm, respectively, of 30-ps duration interacting in a one-half meter cell filled with deuterium at a pressure of 600 psi. The pump and probe beams were crossed at various small angles in the cell and the gain experienced by the probe pulse was measured. At angles below 5 mrad the Stokes amplification was exponential in character and reached levels of 10⁷. At an angle of 7 mrad, the phase matching angle for our conditions, there was no observed amplification of the Stokes pulse. At yet larger angles (~9 mrad) the gain was again exponential in character and reached levels of 10⁷. We have also observed a double ring pattern in the anti-Stokes radiation produced in our experiment. This pattern was predicted by Bloembergen¹ with the explanation that a single anti-Stokes ring is split by a region of gain suppression at the phase matching angle. We have verified that the diameter of the dark band in the anti-Stokes emission grows as the square root of the pressure, exactly as would be predicted assuming that the dark region is caused by gain suppression at phase matching.

© 1986 Optical Society of America