Phase Conjugate Fidelity of Focused Stimulated Brillouin Scattering

摘要

The first measurements of the effect of the focal intensity on the fidelity of phase conjugation in focused stimulated Brillouin scattering (SBS) are reported. Regardless of the strength of aberration, laser beams with focal intensities above 5 times the SBS threshold were conjugated accurately. As the focal intensity declined below this value, the phase conjugate fidelity declined. The maximum correctable aberration strength was that which reduced the focal intensity to near the SBS threshold. Higher laser powers corrected stronger aberrations. The pump laser was a single axial mode, tripled, Q-switched Neodymium:YAG, with a 6 nm pulse-width, operating at 355 nm. A computerized far-field image analysis system directly measured the focal intensity reduction due to aberrations. The focal intensity of the brightest far-field spot controlled both the reflectivity and the fidelity. A pair of laminar airflow, hot-wire array aberrators provided continuously variable, nearly sinusoidal phase modulation in the transverse axes. Lateral shearing interferometry and energy in the bucket measurements gave the Strehl ratio of the backscattered (reconstructed) beam, and hence the phase conjugate fidelity. A phenomenoligical theory for the phase conjugate fidelity of focused SBS was developed using resonator boundary conditions and a Gauss-Hermite mode decomposition. Analytical solutions for the case of no pump depletion were in qualitative agreement with the data. Keywords: Theses; Stimulated Brillouin scattering; Optical phase conjugation; Phase reconstruction; Nonlinear optics. (JHD)