Abstract
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.
Highlights
KH2PO4 (KDP) crystals are important nonlinear optical materials in many applications, such as frequency converters and Pockel cells for high-power large-aperture laser systems[1]
The KDP plates, labeled as CF-1 to CF-6, were cut in a Type I second harmonic generation (SHG) orientation from the same boule prepared by rapid growth with continuous filtration
The density of bulk defects is the number of scattering sites (‘micron size’ and ‘submicron size’ defects) divided by the illuminated volume of the KDP crystal
Summary
KH2PO4 (KDP) crystals are important nonlinear optical materials in many applications, such as frequency converters and Pockel cells for high-power large-aperture laser systems[1]. In the 1990s, the rapid growth technique was developed to grow large-scale (40–50 cm) KDP boules at a rate of 10–20 mm/day[2, 3], which was one of the seven wonders during NIF laser construction[4]. Due to a growth rate an order of magnitude faster than conventional growth, a greater number of bulk defects were formed, degrading the performance of the crystals by causing pinpoint damage and optical losses, which adversely affected the quality of the downstream beam[1, 5]. It was found that typical damage pinpoints were caused by defects, which efficiently absorbed laser energy, leading to a temperature rise and subsequent ‘micro-explosion’[5,6,7].
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