Abstract
In Raman conversion systems employing large optics or moderate to high repetition rates, active aberration correction can be performed using phase conjugation. This technique is applicable to Raman amplifier geometries. A Stokes seed beam, generated in a Raman oscillator, was propagated through the Raman cell without amplification. It was observed to accumulate the component phase errors as well as the gain medium flow and turbulence aberrations present in the system and was phase-conjugated by stimulated Brillouin scattering (SBS). The seed beam was then retrodirected back into the Raman cell simultaneously with the arrival of the pump beam(s). Both experiments and analyses showed that the ability of the conjugated return beam to correct aberrations in the presence of gain can be influenced by the degree of aberration present, Fresnel number of the cell, Stokes seed injection level (which is a function of SBS reflectivity), and the time scale during which the aberrations change in the medium. The Doppler frequency change introduced by SBS can, depending on the choice of medium, shift the phase-conjugated seed beam out of the Raman linewidth. This frequency shift can be precorrected by pressure tuning in the oscillator or preamplifier stage to compensate with an opposing shift before the zero-gain pass through the amplifier.
© 1985 Optical Society of America
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