Walter D. Sell

Growth of laser-initiated damage in fused silica at 351 nm

The effective lifetime of optics in the UV is limited both by laser induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser induced damage in fused silica in both air and vacuum. The data shows exponential growth in the lateral size of the damage site with shot number above threshold fluence. The concurrent growth in depth follows a linear dependence with shot number. The size of the initial damage influences the threshold for growth; the morphology of the initial site depends strongly on the initiating fluence. We have found only a weak dependence on pulse length for growth rate. Low fluence conditioning in air may delay the onset of growth. Most of the work has been on bare substrates but the presence of a sol-gel AR coating has no significant effect.

Pulse length dependence of laser conditioning and bulk damage in KD2PO4

An experimental technique has been developed to measure the damage density ρ(Φ) variation with fluence from scatter maps of bulk damage sites in plates of KD2PO4 (DKDP) crystals combined with calibrated images of the damaging beams spatial profile. Unconditioned bulk damage in tripler-cut DKDP crystals has been studied using 351 nm (3ω) light at pulse lengths of 0.055, 0.091, 0.30, 0.86, 2.6, and 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulse lengths. The results also show that for all the pulse lengths the scatter due to damage is a strong function of the damaging fluence. It is determined that the pulse length scaling for bulk damage scatter in unconditioned DKDP material varies as τ0.24±0.05 over two orders of magnitude of pulse lengths. The effectiveness of 3ω laser conditioning at pulse lengths of 0.055, 0.096, 0.30, 0.86, 3.5, and 23 ns is analyzed in term of damage density ρ(Φ) at 3ω, 2.6 ns. The 860 ps conditioning to a peak irradiance of 7 GW/cm2 had the best performance under 3ω, 2.6 ns testing. It is shown that the optimal conditioning pulse length appears to lies in the range from 0.3 to 1 ns with a low sensitivity of 0.5 J/cm2/ns to the exact pulse length.

Results of pulse-scaling experiments on rapid-growth DKDP triplers using the Optical Sciences Laser at 351 nm

Results are reported from recently performed bulk-damage, pulse-scaling experiments on DKDP tripler samples taken from NIF-size, rapid-growth boule BD7. The tests were performed on LLNLs Optical Sciences Laser. A matrix of samples was exposed to single shots at 351 nm (3(omega) ) with average fluences from 4 to 8 J/cm2 for pulse durations of 1, 3 and 10 ns. The damage sites were scatter-mapped after testing to determine the damage evolution as a function of local beam fluence. The average bulk damage microcavity (pinpoint) density varied nearly linearly with fluence with peak values of approximately 16,000 pp/mm3 at 1 ns, 10,000 pp/mm3 at 3 ns and 400 pp/mm3 at 10 ns for fluences in the 8-10 J/cm2 range. The average size of a pinpoint was 10(+14,-9) micrometers at 1 ns, 37+/- 20 micrometers at 3 ns and approximately 110 micrometers at 10 ns, although all pulse durations produced pinpoints with a wide distribution of sizes. Analysis of the pinpoint density data yielded pulse-scaling behavior of t0.35. Significant planar cracking around the pinpoint as was observed for the 10 ns case but not for the 1 and 3 ns pulses. Crack formation around pinpoints has also been observed frequently for Zeus ADT tests at approximately 8 ns. The high pinpoint densities also lead to significant eruption of near-surface bulk damage. Measurements of the damage site area for surface and bulk gave ratios (Asurf/Abulk) of 2:1 at 1 ns, 7:1 at 3 ns and 110:1 at 10 ns.

Wavelength and pulselength dependence of laser conditioning and bulk damage in doubler-cut KH2PO4

An experimental technique has been utilized to measure the variation of bulk damage scatter with damaging fluence in plates of KH2PO4 (KDP) crystals. Bulk damage in unconditioned and laser-conditioned doubler-cut KDP crystals has been studied using 527 nm (2ω) light at pulselengths of 0.3 - 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulselengths. In particular, there is ~4X increase in fluence for equivalent scatter for damage at 2ω, 10 ns as compared to 0.30 ns in unconditioned KDP. The results for the unconditioned and conditioned KDP show that for all the pulselengths the scatter due to the bulk damage is a strong function of the damaging fluence (θ~5). It is determined that the 2ω fluence pulselength-scaling for equivalent bulk damage scatter in unconditioned KDP varies as τ0.30±0.11 and in 3ω, 3ns ramp-conditioned KDP varies as τ0.27±0.14. The effectiveness of 2ω and 3ω laser conditioning at pulselengths in the range of 0.30-23 ns for damage induced 2ω, 3 ns is analyzed in terms of scatter. For the protocols tested (i.e. peak conditioning irradiance, etc.), the 3ω, 300 ps conditioning to a peak fluence of 3 J/cm2 had the best performance under 2ω, 3 ns testing. The general trend in the performance of the conditioning protocols was shorter wavelength and shorter pulselength appear to produce better conditioning for testing at 2ω, 3 ns.

Results of sub-nanosecond laser-conditioning of KD2PO4 crystals

Previous work [1] has shown the optimum pulse length range for laser-conditioning tripler-cut DKDP with 355 nm (3ω) light lies between 200 ps and 900 ps for damage initiated at 3 ns. A 3ω, 500 ps (500 ps) table-top laser system has been built at Lawrence Livermore National Laboratory (LLNL) [2] to take advantage of this optimal conditioning pulse length range. This study evaluates parameters important for practically utilizing this laser as a raster-scan conditioning laser and for determining the effectiveness of various conditioning protocols. Damage density vs. test fluence (ρ(Φ) was measured for unconditioned and 500-ps laser-conditioned (conditioned) DKDP with 3ω, 3 ns (3 ns) test pulses. We find a 2.5X improvement in fluence in the 3 ns ρ(Φ) after conditioning with 500 ps pulses to 5 J/cm2. We further determine that the rate of improvement in ρ(Φ)decreases at the higher conditioning fluences (i.e. 3.5 - 5 J/cm2). Single-shot damage threshold experiments at 500 ps were used to determine the starting fluence for our 500 ps conditioning ramps. We find 0%, 70%, and 100% single-shot damage probability fluences of 4, 4.5, and 5 J/cm2, respectively at 500 ps. Bulk damage size distributions created at 3 ns are presented for unconditioned and conditioned DKDP. The range of diameters of bulk damage sites (pinpoints) in unconditioned DKDP is found to be 4.6 ± 4.4 µm in agreement with previous results. Also, we observe no apparent difference in the bulk damage size distributions between unconditioned and conditioned DKDP for testing at 3 ns.

Overview of recent KDP damage experiments and implications for NIF tripler performance

Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements of inertial confinement fusion lasers at LLNL. The maximum fluence requirement for KD*P triplers on the NIF is 14.3 J/cm2 at 351 nm in a 3 ns pulse. Currently KD*P cannot meet this requirement without laser (pre)conditioning. In this overview, recent experiments to understand laser conditioning and damage phenomena in KDP and KD*P are discussed. These experiments have led to a fundamental revision of damage test methods and test result interpretation. In particular, the concept of a damage threshold has given way to measuring performance by damage distributions using beams of millimeter size. Automated R/1 damage test have shown that the best rapidly grown KDP crystals exhibit the same damage distributions as the best conventionally grown KD*P. Continuous filtration of the growth solution and post-growth thermal annealing are shown to increase the damage performance as well.

Spatial filter issues

Experiments and calculations indicate that the threshold pressure in spatial filters for distortion of a transmitted pulse scales approximately as I-0.2 and (F#)2 over the intensity range from 1014 to 2 X 1015 W/cm2. We also demonstrated an interferometric diagnostic that will be used to measure the scaling relationships governing pinhole closure in spatial filters.

Surface damage growth mitigation on KDP/DKDP optics using single-crystal diamond micromachining

A process whereby laser-initiated surface damage on KDP/DKDP optics is removed by spot micro-machining using a high-speed drill and a single-crystal diamond bit, is shown to mitigate damage growth for subsequent laser shots. Our tests show that machined dimples on both surfaces of an AR coated doubler (KDP) crystal are stable, for 526 nm, ~3.2 ns pulses at ~12 J/cm2 fluences. Other tests also confirmed that the machined dimples on both surfaces of an AR coated tripler (DKDP) crystal are stable, for 351 nm, ~3 ns pulses at ~8 J/cm2. We have demonstrated successful mitigation of laser-initiated surface damage sites as large as 0.14 mm diameter on DKDP, for up to 1000 shots at 351 nm, 13 J/cm2, ~11 ns pulse length, and up to 10 shots at 351 nm, 8 J/cm2, 3 ns. Details of the method are presented, including estimates for the heat generated during micromachining and a plan to implement this method to treat pre-initiated or retrieved-from-service, large-scale optics for use in high-peak-power laser applications.

Correlation of Laser-Induced Damage to Phase Objects in Bulk Fused Silica

The Optical Sciences Laser (OSL) Upgrade facility, described in last years proceedings, is a kJ-class, large aperture (100cm2) laser system that can accommodate prototype optical components for large-scale inertial confinement fusion lasers. High-energy operation of such lasers is often limited by damage to the optical components. Recent experiments on the OSL Upgrade facility using fused silica components at 4 J/cm2 (351-nm, 3-ns) have created output surface and bulk damage sites that have been correlated to phase objects in the bulk of the material. Optical Path Difference (OPD) measurements of the phase defects indicate the probability of laser-induced damage is strongly dependent on OPD.

Performance of Thin Borosilicate Glass Sheets at 351-nm

Previously, we reported preliminary results for commercial thin borosilicate glass sheets evaluated for use as a frequently-replaced optic to separate the radiation and contamination produced by the inertial confinement fusion experiments in the National Ignition Facility target chamber from the expensive precision laser optics which focus and shape the 351-nm laser beam. The goal is identification of low cost substrates that can deliver acceptable beam energy and focal spots to the target. The two parameters that dominate the transmitted beam quality are the transmitted wave front error and 351-nm absorption. Commercial materials and fabrication processes have now been identified which meet the beam energy and focus requirements for all of the missions planned for the National Ignition Facility. We present the first data for use of such an optic on the National Ignition Facility laser.