Primary operating wavelength for the target shots on NIF is the third harmonic of the 1053-nm fundamental wavelength at 351 nm. Efficient conversion of the amplified 1ω light to its third harmonic is accomplished by a pair of non-linear potassium dihydrogen phosphate (KDP) and potassium di-deuterium phosphate (DKDP) crystals installed in the beamline FOAs (Figure 4-4). The first crystal combines two 1ω photons into a second harmonic photon at 527 nm (second harmonic generator or SHG) and the second crystal combines a 1ω photon and a 2ω photon into a 3ω photon at 351 nm (third harmonic generator or THG). The crystal thicknesses and cut angles are chosen to optimize peak power conversion efficiency. However, since the conversion efficiency varies with intensity and experiments often require shaped pulses (such as those shown in Figure 4-11), there is a considerable amount of unconverted light remaining on NIF beamlines, particularly for shaped pulses. Most of the unconverted light is in 1ω.
The frequency conversion at the NIF FOAs results in all three harmonics (1ω, 2ω, and 3ω) entering the Target Chamber. The final focusing lens for each beam is wedged slightly to separate the three harmonics at TCC. Figure 4-12 shows the pattern of 1ω, 2ω, and 3ω light from a single quad when looking in the 3ω focal plane. The chromatic dispersion of the focus lens combined with the wedge angle of the lens gives a separation of ~2.9 mm between the closest 2ω beam edge from the 3ω aim-point and ~4.8 mm between the closest 1ω edge and the 3ω aim-point. The overlap becomes more complicated when multiple beams are focused to a given point. Figure 4-12 shows an example of the distribution of 1ω footprints from 96 NIF beams (upper hemisphere beams, for example) pointed and focused at TCC.
