Zhi M. Liao (Lawrence Livermore National Laboratory)
Keywords: Lasers
Abstract:
Diode-pumped solid-state lasers are one of the potential driver technologies for inertial fusion energy power production. The Mercury Laser at Lawrence Livermore National Laboratory is a diode-pumped solid-state laser that will deliver 100 J of energy at 1047 nm with a repetition rate up to 10 Hz and is a scalable prototype of a fusion energy laser driver. As the Mercury laser undergoes proof-of-principal design and activation, high-repetition wavefront correction is one of the advanced components that will be tested within its unique four-pass architecture.
High average power operation of the Mercury Laser induces dynamic aberrations to the laser beam wavefront. Analysis of recent data indicates that up to 4 waves of low order aberration (mainly focus error or power, with spatial resolution < 0.5 cm-1) could be expected at each pass. The Mercury laser system uses a custom-designed high-repetition-rate adaptive optics system that consists of a 100-mm diameter bimorph deformable mirror (DM) and a four-way shearing interferometric wavefront sensor capable of running at 10 Hz with very high resolution (100 x 100 sample points). The DM is based on lead zirconate titanate (PZT) technology and is coated with a high-damage threshold antireflection (AR) coating (> 10 J/cm2). The DM has a 5 x 8 actuator configuration with an additional large actuator for power correction (up to 20 waves) for a total of 41 actuators.
The DM has a flatness of 0.9 waves peak to valley (PV), which with full correction reduces to 0.17 waves PV and 0.03 waves root mean square (RMS). Placing the deformable mirror (DM) at an image relay point on the laser system between passes allows the DM to be twice as effective in correcting the wavefront. The AO system was able to successfully close the loop on the laser; reducing the wavefront error from 7.2 waves PV and 1.9 waves RMS to 0.83 waves PV and 0.15 waves RMS. Analysis of the residual wavefront shows that all but 0.1 waves PV and 0.04 waves RMS of the residual wavefront errors are high spatial frequency distortions arising from crystal imperfections (which is above and beyond what the current DM can correct).
Date of Conference: September 10-14, 2006
Track: Lasers