A review of research into stimulated Brillouin scattering (SBS) phase conjugation and beam cleanup conducted at the
Air Force Institute of Technology is presented. CW phase conjugation was demonstrated using SBS in short lengths of
multi-mode, step-index fiber. A fidelity of 0.8 was achieved using a 0.06-NA fiber of 40 m length and a 0.13-NA fiber
at 15 m length. In modeling and experiment, the fidelity declined as fiber length or numerical aperture increased. A
relationship was established empirically between step-index fiber parameters and the phase conjugation fidelity. In
addition, increased fidelity was related to an increase in the effective Brillouin gain coefficient in step-index fibers.
Beam cleanup was observed in both graded-index and step-index fibers. The Stokes beam generated in long, multimode,
graded-index fibers was analyzed using spatial and phase methods and found to be a low-order mode of the fiber instead
of a phase conjugate of the pump. SBS in long, graded-index fibers was used to combine multiple beams and to improve
the beam quality of a single aberrated beam.
We report on the performance of seeded Raman fiber amplifiers based on multimode gradient index fiber. The "beam
cleanup" improvement in beam quality of the Stokes beam over that of input pump beam (previously observed in
unseeded fiber amplifier geometries using stimulated Raman scattering in multimode graded index fiber) is shown here
to be limited by the beam quality of the input seed beam for seeded amplifier configurations. The amplifiers are
characterized in terms of their capacity for beam-cleanup, their ability to amplify the seed and in terms of their output
spectra. The advantages and disadvantages of a backward-pumped geometry versus a forward-pumped geometry are
discussed. Depending on the geometry, amplified power can be readily distributed to a cascade of Stokes frequencies
(unseeded forward-pumped geometry) or can be mostly contained in the seed frequency (seeded backward-pumped
Stimulated Brillouin Scattering (SBS) is a polarization-dependent, nonlinear process that is often the limiting factor for high-power fiber laser applications. We report the results of experiments measuring the SBS thresholds and the SBS gain bandwidths in several passive optical fibers. Fibers with nearly identical mode-field diameters and loss coefficients from different manufacturers were selected. Observations from these experiments indicate that the SBS gain coefficient for fibers from different manufacturers varied significantly resulting in a 70% deviation in SBS threshold. Also, polarization-maintaining (PM) fiber exhibited a significant increase in the SBS threshold for a linearly polarized pump beam that is launched into the PM fiber at 45° relative to the fiber's slow axis. This increase in threshold was not mirrored in non-PM fiber. These results suggest that the polarization multiplier in the SBS threshold equation may be highest when a PM fiber is used with the appropriate launch conditions, rather than a non-PM conventional single-mode fiber. We will present the experimental results and a theoretical model demonstrating the polarization dependent gain properties in both PM and non-PM fiber.
The beam-cleanup properties of stimulated Raman scattering in a long multi-mode fiber and its application to laser brightness enhancement have been investigated. We show that in a multi-mode fiber that supports propagation of a highly aberrated pump beam with the beam quality (M2) as high as 20, a clean Gaussian-like Stokes beam is generated. The dramatic enhancement in beam quality achieved in the present investigation clearly demonstrates the utility of fiber-based Raman beam cleanup devices for laser brightness enhancement. (Summary only available)