Design and performance of the Nova frequency conversion system

The Nova laser is equipped with ten large-aperture (74-cm) potassium dihydrogen phosphate (KDP) crystal arrays capable of converting the fundamental wavelength (1.054 μm) to the visible (0.527-μm) or ultraviolet (0.351-μm) light. Each array assembly consists of a 3 x 3 deep-web aluminum eggcrate, which supports two crystal arrays configured in the quadrature 2ω/cascade 3ω architecture. The individual crystals are precisely machined so that the phase-matched direction is accurately located with respect to the crystal surface normal (+-30 μrad). The eggcrate supports each crystal so that the surface normals are parallel to within +-30 μrad. The completed assembly is angle tuned for peak conversion efficiency to +-30 μrad relative to the input beam. The Nova crystal arrays are installed just before the target focus lens where the beams are linearly polarized, collimated (+-50 μrad), and 74 cm in diameter. Angle tuning is facilitated by a precision two-axis gimbal which can also rotate the array about the beam direction. Each of the ten gimbals is controlled from a local control console or the main control room. Infrared operation is achieved by detuning the array so that no conversion takes place. Operation at 0.527 μ is achieved by angle tuning the assembly so that both crystals convert the fundamental to the second harmonic. The assembly is rotated about the beam axis to achieve the correct input polarization. Operation at 0.351 μm is achieved by tipping the assembly approx. =5 mrad and rotating approx. =100 so as to tune the second crystal for mixing. The Nova frequency conversion subsystem has recently been activated, and preliminary measurements made on the optical quality and overall conversion efficiency are reported