John Honig

National Ignition Facility wavefront requirements and optical architecture

With the first four of its eventual 192 beams now executing shots, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the worlds largest and most energetic laser. The optical system performance requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures. These mission requirements have led to a design strategy for achieving high quality focusable energy and power from the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass architecture with a single large amplifier type that provides high gain, high extraction efficiency and high packing density. We have taken a systems engineering approach to the practical implementation of this design that specifies the wavefront parameters of individual optics in order to achieve the desired cumulative performance of the laser beamline. This presentation provides a detailed look at the causes and effects of performance degradation in large laser systems and how NIF has been designed to overcome these effects. We will also present results of spot size performance measurements that have validated many of the early design decisions that have been incorporated in the NIF laser architecture.

Mitigation of laser damage on National Ignition Facility optics in volume production

The National Ignition Facility has recently achieved the milestone of delivering over 1.8 MJ and 500 TW of 351 nm laser energy and power on target, which required average fluences up to 9 J/cm2 (3 ns equivalent) in the final optics system. Commercial fused silica laser-grade UV optics typically have a maximum operating threshold of 5 J/cm2. We have developed an optics recycling process which enables NIF to operate above the laser damage initiation and growth thresholds. We previously reported a method to mitigate laser damage with laser ablation of the damage site to leave benign cone shaped pits. We have since developed a production facility with four mitigation systems capable of performing the mitigation protocols on full-sized (430 mm) optics in volume production. We have successfully repaired over 700 NIF optics (unique serial numbers), some of which have been recycled as many as 11 times. We describe the mitigation systems, the optics recycle loop process, and optics recycle production data.

Cleanliness improvements of National Ignition Facility amplifiers as compared to previous large-scale lasers

Prior to the recent commissioning of the first National Ignition Facility (NIF) beamline, full-scale laser-amplifier-glass cleanliness experiments are performed. Aerosol measurements and obscuration data acquired using a modified flatbed scanner compare favorably to historical large-scale lasers and indicate that NIF is the cleanest large-scale laser built to date.

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.

Diode-pumped Nd:YAG laser with 38 W average power and user-selectable, flat-in-time subnanosecond pulses

A diode-pumped injection-seeded Nd:YAG laser system with an average output power of 38 W is described. The laser operates at 300 Hz with pulse energies up to 130 mJ. The temporal pulse shape is nominally flat in time and the pulse width is user selectable from 350 to 600 ps. In addition, the spatial profile of the beam is near top hat with contrast <10%.

Shape dependence of laser–particle interaction-induced damage on the protective capping layer of 1ω high reflector mirror coatings

Abstract. The response of a potential candidate protective capping layer (SiO2 or Al2O3) to laser exposure of 1ω (1053 nm) to high-reflector silica-hafnia multilayer coatings in the presence of variously shaped Ti particles is investigated by combining laser damage testing and numerical modeling. Each sample is exposed to a single oblique angle (45 deg) laser shot (p-polarization, ∼10  J/cm2, 14 ns) in the presence of spherically or irregularly shaped Ti particles on the surface. The two capping layers show markedly different responses. For the spherical particles, the Al2O3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. The different response of the capping layer is attributed to the large difference in the thermal expansion coefficient of the materials, with that of the Al2O3 about 15 times greater than that of the SiO2 layer. For the irregular particles, the Al2O3 capping layer displays minimal to no damage while the SiO2 capping layer is significantly damaged. The difference is due to the disparity in mechanical strength with Al2O3 possessing approximately 10 times higher fracture toughness.

Impact of particle shape on the laser-contaminant interaction induced damage on the protective capping layer of 1ω high reflector mirror coatings

We report an investigation on the response to laser exposure of a protective capping layer of 1ω (1053 nm) high-reflector mirror coatings, in the presence of differently shaped Ti particles. We consider two candidate capping layer materials, namely SiO2 and Al2O3. They are coated over multiple silica-hafnia multilayer coatings. Each sample is exposed to a single oblique (45°) shot of a 1053 nm laser beam (p polarization, fluence ~ 10 J/cm2, pulse length 14 ns), in the presence of spherically or irregularly shaped Ti particles on the surface. We observe that the two capping layers show markedly different responses. For spherically shaped particles, the Al2O3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. In contrast, the SiO2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. For irregularly shaped Ti filings, the Al2O3 capping layer displays minimal to no damage while the SiO2 capping layer is significantly damaged. In the case of the spherical particles, we attribute the different response of the capping layer to the large difference in thermal expansion coefficient of the materials, with that of the Al2O3 about 15 times greater than that of the SiO2 layer. For the irregularly shaped filings, we attribute the difference in damage response to the large difference in mechanical toughness between the two materials, with that of the Al2O3 being about 10 times stronger than that of the SiO2.

High‐power, high‐beam‐quality solid‐state lasers for materials processing applications

The Laser Science and Technology Department at Lawrence Livermore National Laboratory is developing solid‐state lasers with high average power and high beam quality. Specific systems include a laser to generate 1.0–1.4 nm X‐rays for proximity print lithography, a 400‐mJ, 500‐Hz laser for 13.0 nm projection lithography and unique systems for speckle imaging, laser radars, and medical treatments.

Advanced laser driver for soft x-ray projection lithography

A diode-pumped Nd:YAG laser for use as a driver for a soft x-ray projection lithography system is described. The laser will output 0.5 to 1 J per pulse with about 5 ns pulse width at up to 1.5 kHz repetition frequency. The design employs microchannel-cooled diode laser arrays for optical pumping, zigzag slab energy storage, and a single frequency oscillator injected regenerative amplifier cavity using phase conjugator beam correction for near diffraction limited beam quality. The design and initial results of this lasers activation experiments are presented.

Phase modulation in high power optical systems caused by pulsed laser-driven particle ablation events

Surface modification of fused silica windows caused by the laser ablation of surface-bound microparticles is investigated. Using optical and electron microscopies between laser pulses, we detail the ablation, fragmentation and dispersal of 2-150 μm diameter particles of various materials. Following complete ablation and ejection of all debris material, surface pitting was found to be highly dependent on material type and particle size. Subsequent light propagation modeling based on pit morphology indicates up to ~4x intensification. Understanding this class of non-local, debris-generated damage is argued to be important for effective design of high-power optical windows and debris-mitigation strategies.