Richard P. Hackel

NIF Optical Materials and Fabrication Technologies: An Overview

The high-energy/high-power section of the NIF laser system contains 7360 meter-scale optics. Advanced optical materials and fabrication technologies needed to manufacture the NIF optics have been developed and put into production at key vendor sites. Production rates are up to 20 times faster and per-optic costs 5 times lower than could be achieved prior to the NIF. In addition, the optics manufactured for NIF are better than specification giving laser performance better than the design. A suite of custom metrology tools have been designed, built and installed at the vendor sites to verify compliance with NIF optical specifications. A brief description of the NIF optical wavefront specifications for the glass and crystal optics is presented. The wavefront specifications span a continuous range of spatial scale-lengths from 10 μm to 0.5 m (full aperture). We have continued our multi-year research effort to improve the lifetime (i.e. damage resistance) of bulk optical materials, finished optical surfaces and multi-layer dielectric coatings. New methods for post-processing the completed optic to improve the damage resistance have been developed and made operational. This includes laser conditioning of coatings, glass surfaces and bulk KDP and DKDP and well as raster and full aperture defect mapping systems. Research on damage mechanisms continues to drive the development of even better optical materials.

High-average-power dye laser at Lawrence Livermore National Laboratory.

The copper-laser-pumped dye laser system developed at the Lawrence Livermore National Laboratory (LLNL) is now capable of sustained, efficient, and reliable operation at total powers exceeding 2500 W and single amplifier chain powers exceeding 1300 W. Wavelength center frequency stability is maintainable to < 50 MHz. Laser dyes developed at LLNL permit tunability from 550 to 650 nm. Wave-front quality is < lambda/4 peak to valley. The system is operated remotely with the aid of a comprehensive set of diagnostics. Besides supporting its primary atomic-vapor-laser-isotope-separation mission, the system is being used in alternate applications such as materials processing and the generation of artificial guide stars.

Growth of laser damage in fused silica: diameter to depth ratio

Growth of laser initiated damage plays a major role in determining optics lifetime in high power laser systems. Previous measurements have established that the lateral diameter grows exponentially. Knowledge of the growth of the site in the propagation direction is also important, especially so when considering techniques designed to mitigate damage growth, where it is required to reach all the subsurface damage. In this work, we present data on both the diameter and the depth of a growing exit surface damage sites in fused silica. Measured growth rates with both 351 nm illumination and with combined 351 nm and 1054 nm illumination are discussed.

Growth of laser damage in SiO2 under multiple wavelength irradiation

In laser systems using frequency conversion, multiple wavelengths will be present on optical components. We have investigated the growth of laser initiated damage in fused silica in the presence of multiple wavelengths. In particular, we measured growth at 351 nm in the presence of 1053 nm near the threshold of growth for 351 nm alone. The data shows that the sum fluence determines the onset of growth as well as the growth rate. The measured growth coefficient is consistent with all the energy being delivered at 351 nm. Additionally, we measured growth at 527 nm in the presence of 1053 nm near the threshold of growth at 527 nm alone. In this case, the sum fluence also determines the growth coefficient but the rate is consistent with all the energy being delivered at 1053 nm. We present the measurements and discuss possible reasons for the behavior.

Growth of laser damage on the input surface of SiO2 at 351 nm

Growth of laser initiated damage is a potential lifetime limiter of laser optics. While laser initiated damage occurs most often on the exit surface of optical components, some damage sites can occur on the input surface. We have investigated the growth of laser initiated damage in fused silica when the damage occurs on the input surface of the optic. We have measured both the threshold for growth as well as the lateral growth rate at 351 nm. The lateral growth of damage on the input surface is best described as having a linear dependence on shot number. The rate of growth has a linear dependence on fluence, with an extrapolated threshold of approximately 6 J/cm2. This behavior will be contrasted to growth of damage when located on the exit surface. The behavior will be compared to growth of input surface damage when the irradiation wavelength is 1053 nm or 527 nm.

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.

Picosecond laser damage performance assessment of multilayer dielectric gratings in vacuum

Precise assessment of the high fluence performance of pulse compressor gratings is necessary to determine the safe operational limits of short-pulse high energy lasers. We have measured the picosecond laser damage behavior of multilayer dielectric (MLD) diffraction gratings used in the compression of chirped pulses on the Advanced Radiographic Capability (ARC) kilojoule petawatt laser system at the Lawrence Livermore National Laboratory (LLNL). We present optical damage density measurements of MLD gratings using the raster scan method in order to estimate operational performance. We also report results of R-on-1 tests performed with varying pulse duration (1-30 ps) in air, and clean vacuum. Measurements were also performed in vacuum with controlled exposure to organic contamination to simulate the grating use environment. Results show sparse defects with lower damage resistance which were not detected by small-area damage test methods.

In situ monitoring of surface postprocessing in large-aperture fused silica optics with optical coherence tomography

Optical coherence tomography (OCT) is explored as a method to image laser-damage sites located on the surface of large aperture fused silica optics during postprocessing via CO2 laser ablation. The signal analysis for image acquisition was adapted to meet the sensitivity requirements for this application. A long-working-distance geometry was employed to allow imaging through the opposite surface of the 5 cm thick optic. The experimental results demonstrate the potential of OCT for remote monitoring of transparent material processing applications.

Mitigation of growth of laser initiated surface damage in fused silica using a 4.6-micron wavelength laser

Surface damage caused by high fluence, 351 nm light to fused silica optics can adversely affect the performance of fusion class laser systems like that of the National Ignition Facility (NIF). It is typically initiated as a small pit and grows in both diameter and depth during normal operation with cracks that extend into the bulk. Mitigation of this growth has been previously reported using a 10.6 micron CO2 laser. Here, we report growth mitigation with the 4.6 micron light from a frequency-doubled, 9.2 micron CO2 laser. The motivation for using 4.6 microns is >25 times longer absorption length in fused silica at room temperature compared to that at 10.6 micron. Mitigation of subsurface cracks at 10.6 micron required ablation of material to the depth of the cracks. In contrast, it was possible to mitigate the subsurface cracks using 4.6 micron light without significant ablation of material. Damage sites as large as 500 microns in diameter with cracks extending to 200 microns in depth were successfully mitigated with 4.6 microns.

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.