Adaptive-Optics Compensation through a High-Gain Raman Amplifier

The applicability ofRaman amplification to the problem ofhigh-power laser propagation through a turbulent atmosphere was investigated in a two-part experimental program. The goal of the program wasto determine the extent ofphasefront preservation possible in a high-gain Raman amplifier. The utility ofan unconventional approach to adaptiveoptics compensation, known as "Raman look-through," was examined. The Raman look-through technique, which combines conventional adaptive optics using a deformable mirror and wavefront sensor with the nonlinear process of stimulated Raman scattering, performs a wavefront correction on the low-power input to a Raman amplifier, and then aniplifies the corrected beam. The first part of the experimental program determined the degree of phasefront preservation in a high-gain Raman amplifier. The phasefront of the input beam to a Raman amplifier was deliberately aberrated, and the phasefront of the amplified output beam was measured and compared to the input beam. The comparisons indicate that, under the proper conditions, a high degree of phasefront preservation can be achieved. The second part of the experimental program studied adaptive-optics compensation through a Raman amplifier. Atmospheric turbulence was simulated by specially . fabricated quartz plates that produced turbulence-like phasefront distortion on a transmitted beacon. The beacon phasefront was measured by a 69-channel wavefront . sensor that commanded a deformable mirror to impose the conjugate phasefront on the .. Stokes seed to a high-gain large-Fresnel-number Raman amplifier. Mter amplification, '" the output Stokes beam retraced the path of the beacon through the simulated turbulence. Measurements of the Stokes beam quality indicated a dramatic improvement with near-diffraction-limited performance. The results of both experiments are in good agreement with theory.