Laser damage to production- and research-grade KDP crystals

Abstract

We present the results of laser damage measurements conducted on potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (KD*P) crystals that were grown recently for both production and research applications by several sources. We have measured extrinsic damage thresholds that cover wavelengths from 1064 nm to 266 nm at pulse durations in the 3-to I 0-ns regime.Many of the samples were extracted from boules grown specifically to yield large-area crystals, up to 32-cm square, for laser fusion applications. These crystals were the result of efforts, both by the Lawrence Livermore National Laboratory (LLNL) and commercial crystal-growth companies, to yield high-threshold KDP. In particular we have established that such crystals can reliably survive fluences exceeding 15 J/cm2 at 355 nm and 20 J/cm2 at 1064 nm when irradiated with 3-ns pulses. We present details of how bulk and surface damage to these crystals scale with pulse duration and wavelength as well as of morphological effects due to laser conditioning.

Historical perspective on fifteen years of laser damage thresholds at LLNL

“…7 shows that at 1caconditioning of KDP improved the threshold by a factor of 2.1 over unconditioned values. 36 We observed even greater improvement by thermally annealing KDP crystals at temperatures ranging from 135°C to 175°C. 37 On the other hand, some of the newest KDP crystals had such inherently high thresholds that they requh-ed no conditioning at all.…”

Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion

“…The plates cut from these boules were then diamond turned to a surface smoothness of ϳ40 -140 Å rms ͑recent improvements in KDP diamond turning have reduced the surface roughness to 20 -50 Å rms͒. The crystals were laser conditioned 23 on line by a slow increase in the incident fluence. Off-line tests show that conditioning increases the 1054-nm damage threshold from 34 to 43 J͞cm 2 at 3 ns for the KDP Pockels cell and doubler crystals.…”

“…Laser conditioning refers to the process of increasing the damage threshold of an optical material by exposing it to a series of laser shots with monotonically increasing fluence. 23 Following conditioning, a series of experiments was conducted to characterize the 3 conversion performance. One of our major goals was to demonstrate Ͼ70% conversion efficiency at high peak power ͑2.5-3.5 GW͞cm 2 ͒.…”

Effect of temporal pulse shape on optical damage

“…Because damage data has historically been taken with pulses whose temporal shapes were (more or less) Gaussian [2,[6][7][8][9][10], it is desirable to base our predictions on data taken with Gaussian pulses of different pulse widths. To do so, we must adopt some parametric model of the damage process.…”

“…A large body of work exploring the damage behavior of bulk damage behavior exists [1,2,9,[16][17][18][19][20][21][22], but for the purpose of testing the model we need only state that for damage initiated with third harmonic light at high fluence? precursors are known to be present in excess of 10 4 per mm 3 and that the measured pulse scaling is ~t 0.35 +/-0.05 .…”