The Design of Precision Mounts for Optimizing the Conversion Efficiency of KDP Crystals for the National Ignition Facility*

A key design challenge for the National Ignition Facility (NIF), being constructed at Lawrence Livermore National Laboratory (LLNL), [Hibbard, R. L., 1998], is the frequency converter consisting of two KDP crystals and a focusing lens. Frequency conversion is a critical performance factor for NIF and the optical mount design for this plays a key role in meeting design specifications. The frequency converter, Figure 1, is a monolithic cell that mounts the optics and is the point on the beamline where the frequency conversion crystals are optimally aligned and the cell is focused on target. The lasing medium is neodymium in phosphate glass with a fundamental frequency (1ω) of 1.053 µm. Sum frequency generation in a pair of conversion crystals (KDP/KD*P) produces 1.8 MJ of the third harmonic light (3ω or λ=0.35 µm). The phase-matching scheme on NIF is type I second harmonic generation followed by type II sum-frequency-mixing of the residual fundamental and the second harmonic light. This laser unlike previous laser system designs, must achieve high conversion efficiency, 85%, which is close to the 90.8% theoretical maximum. As a result, this design is very sensitive to angular variations in beam propagation and in the crystal axes orientation. Factors that influence the phase matching angle include crystal inhomogeneity, residual and induced stress in the crystals, the crystals’ natural and mounted surface figure, mounting imperfections and gravity sag. These angular variations need to be controlled within a 40 µrad error budget. The optical mount contributions to the angular error budget are 20 µrad and are what make the frequency converter in the Final Optics Cell (FOC) such a challenging precision design.