Multiseed stimulated rotational Raman scattering for wave-front control

We apply a new technique, multiseed stimulated rotational Raman scattering, to the wave-front control of Raman-converted light when the Raman gain-length product is large enough to produce significant conversion beyond the first-Stokes order. For rotational Raman scattering, such as that which dominates in atmospheric propagation, the polarizations of adjacent Stokes orders may be made circular and opposite in order to eliminate Stokes-anti-Stokes coupling and thereby minimize losses that are due to amplified noise. A parametric process that couples four distinct waves recursively generates a fourth wave when any three waves are incident. Thus at least two seeds (e.g., first and second Stokes) are necessary to start the process. Computer simulations then show that, within limitations imposed by diffraction and competition from noise, the wave fronts of higher-order Stokes fields are determined by those of the incident fields. Starting from a 250-nm laser and using subatmospheric H₂ as a Raman medium, we generate multiple seeds with alternate polarization helicities on the S(1) rotational transition. Multiseed experiments showed that a good-beam-quality core was converted and amplified but that a poor-quality background beam was amplified as well. With the aid of computer simulations, we interpret this effect as resulting from the competition between seeded light and noise, the noise being favored by its buildup during the delay until the arrival of the leading edges of the seed pulses. The beam’s additional loss of quality is attributable to the growth of aberrations from smaller aberrations and intensity variations that are present in the incident beams.