To reduce the seed length while maintaining the advantages of the cuboid KDP-type crystal, a long-seed KDP crystal with size $471~\text{mm}\times 480~\text{mm}\times 400~\text{mm}$ is rapidly grown. With almost the same high cutting efficiency to obtain third harmonic generation oriented samples, this long-seed KDP-type crystal can be grown with a shorter seed than that of the cuboid KDP-type crystal. The full width at half maximum of the high-resolution X-ray diffraction of the (200) crystalline face is 28.8 arc seconds, indicating that the long-seed KDP crystal has good crystalline quality. In the wavelength range of 377–1022 nm, the transmittance of the long-seed KDP crystal is higher than 90%. The fluence for the 50% probability of laser-induced damage (LID) is $18.5~\text{J}/\text{cm}^{2}$ (3 ns, 355 nm). Several test points survive when the laser fluence exceeds $30~\text{J}/\text{cm}^{2}$ (3 ns, 355 nm), indicating the good LID performance of the long-seed KDP crystal. At present, the growth of a long-seed DKDP crystal is under way.

In this paper, a highly deuterated potassium dihydrogen phosphate (DKDP) crystal with sizes up to $318~\text{mm}\times 312~\text{mm}\times 265~\text{mm}$ was grown by the rapid-growth method. The synthesis tank device was specially designed to synthesize a higher deuterium concentration and high-purity DKDP solution. The deuterium content of the as-grown crystal, which was 97.9%, was determined by two methods, including infrared (IR) spectroscopy and thermo-gravimetric analysis (TGA) measurements. The performances of the 97.9% DKDP crystal, including transmission, absorption coefficient, and laser-induced damage threshold (LIDT) were measured. The results indicate that, in the near-infrared band, the transmission of the 97.9% DKDP crystal is higher than that of KDP and 70% DKDP crystals, and the absorption coefficient is lower. The LIDT of the crystal reached $23.2~\text{J}\cdot \text{cm}^{-2}$ (R-on-1, 1064 nm, 3 ns), which meets the engineering requirements for use in optical applications.

Rapid growth processing of KDP crystals was improved by employing continuous filtration to eliminate bulk defects. The performances of the KDP crystals, including scattering defects, laser damage resistance and transmittance, were measured and analyzed. Compared with rapid-grown KDP without continuous filtration, the transmittance in the near-infrared was increased by at least 2%, almost all of ‘micron size’ defects were eliminated and ‘sub-micron size’ defects were decreased by approximately 90%. Laser damage testing revealed that the laser-induced damage thresholds (LIDTs), as well as the consistency of the LIDTs from sample to sample, were improved greatly. Moreover, it identified that ‘micron size’ defects were the precursors which initiated laser damage at relative lower laser fluence (4–6 J cm−2), and there was a lower correlation between smaller size scattering defects and laser damage initiation. The improved consistency in the LIDTs, attributed to elimination of ‘micron size’ defects, and LIDT enhancement originated from the decreased absorption of the KDP crystals.