Effect of thermal annealing and second harmonic generation on bulk damage performance of rapid-growth KDP type-I doublers at 1064 nm
Laser-Induced Damage in Optical Materials: 2000
This paper discusses the results of thermal annealing and in-situ second harmonic generation (SHG) damage tests performed on six rapid growth KDP type 1 doubler crystals at 1064 nm (1 m) on the Zeus automated damage test facility. Unconditioned (S/1) and conditioned (R/l) damage probability tests were performed before and after thermal annealing, then with and without SHG on six doubler crystals from the NIJ?-size, rapid growth KDP boule F6. The tests revealed that unannealed, last-grown
... l tlom the boule in either prismatic or pyramidal sectors exhibited the highest damage curves. After thermal annealing at 160°C for seven days, the prismatic sector samples increased in performance ranging fi-om 1.6 to 2.4X, while material from the pyramidal sector increased only modestly, ranging fi-om 1.0 to 1.4x. Second harmonic generation decreased the damage fluence by an average of 20 percent for the S/1 tests and 40 percent for R/l tests. Conversion efficiencies under test conditions were measured to be 20 to 30 percent and compared quite well to predicted behavior, as modeled by LLNL ftequency conversion computer codes. The damage probabilities at the 1co NIF redline fluence (scaled to 10 ns via t0"5)for S/1 tests for the unannealed samples ranged from 20 percent in one sample to 90-100 percent for the other 5 samples. Thermal annealing reduced the damage probabilities to less than 35 percent for 3 of the poor-performing crystals, while two pyramidal samples remained in the 80 to 90 percent range. Second harmonic generation in the annealed crystal increased the S11 damage probabilities on all the crystals and ranged from 40 to 100 percent. In contrast, R/l testing of an unannealed crystal resulted in a damage probability at the NIF redline fluence of 16%. Annealing increased the damage performance to the extent that all test sites survived NIF redline fluences without damage. Second harmonic generation in the R/l test yielded a damage probability of less than 2 percent for the annealed crystal. These results indicate that a combination of thermal and laser conditioning would virtually eliminate bulk damage at the highest NIF fluences and indicate the need for laser conditioning of NIF SHG crystals.