Mechanisms of laser-induced damage in absorbing glasses with nanosecond pulses.

The propagation of 355-nm, nanosecond pulses in absorbing glasses is investigated for the specific case examples of the broadband absorbing glass SuperGrey and the Ce3+-doped silica glass. The study involves different laser irradiation conditions and material characterization methods to capture the transient material behaviors leading to laser-induced damage. Two damage-initiation mechanisms were identified: (1) melting of the surface as a result of increased temperature; and (2) self-focusing caused by a transient change in the index of refraction. Population of excited states greatly affects both mechanisms by increasing the transient absorption cross section via excited-state absorption and introducing a change of the refractive index to support the formation of graded-index lensing and self-focusing of the beam inside the material. The governing damage-initiation mechanism depends on the thermodynamic properties of the host glass, the electronic structure characteristics of the doped ion, and the laser-spot size.

Mechanisms of surface contamination in fused silica by means of laser-induced electrostatic effects.

We demonstrate that fused-silica samples exposed to nanosecond laser pulses at 355 nm and 1064 nm develop long-lived electrostatic charges on their surfaces. These charges extend well beyond the area exposed to the laser beam. The results suggest this effect is dependent on laser fluence and wavelength. In addition, ejected particles generated during laser-induced breakdown are electrostatically charged. Experiments indicate that such electrostatic charges can produce forces that can support the transport of dielectric and metallic microspheres between surfaces. This in turn can promote increased contamination of optical components during operation at relevant excitation conditions.

Particle damage sources for fused silica optics and their mitigation on high energy laser systems.

High energy laser systems are ultimately limited by laser-induced damage to their critical components. This is especially true of damage to critical fused silica optics, which grows rapidly upon exposure to additional laser pulses. Much progress has been made in eliminating damage precursors in as-processed fused silica optics (the advanced mitigation process, AMP3), and very high damage resistance has been demonstrated in laboratory studies. However, the full potential of these improvements has not yet been realized in actual laser systems. In this work, we explore the importance of additional damage sources-in particular, particle contamination-for fused silica optics fielded in a high-performance laser environment, the National Ignition Facility (NIF) laser system. We demonstrate that the most dangerous sources of particle contamination in a system-level environment are laser-driven particle sources. In the specific case of the NIF laser, we have identified the two important particle sources which account for nearly all the damage observed on AMP3 optics during full laser operation and present mitigations for these particle sources. Finally, with the elimination of these laser-driven particle sources, we demonstrate essentially damage free operation of AMP3 fused silica for ten large optics (a total of 12,000 cm2 of beam area) for shots from 8.6 J/cm2 to 9.5 J/cm2 of 351 nm light (3 ns Gaussian pulse shapes). Potentially many other pulsed high energy laser systems have similar particle sources, and given the insight provided by this study, their identification and elimination should be possible. The mitigations demonstrated here are currently being employed for all large UV silica optics on the National Ignition Facility.

The role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: I. Damage morphology.

Laser-induced damage with ps pulse widths straddles the transition from intrinsic, multi-photon ionization and avalanche ionization-based ablation with fs pulses to defect-dominated, thermal-based damage with ns pulses. We investigated the morphology of damage for fused silica and silica coatings between 1 ps and 60 ps at 1053 nm. Using calibrated laser-induced damage experiments, in situ imaging, and high-resolution optical microscopy, atomic force microscopy, and scanning electron microscopy, we show that defects play an important role in laser-induced damage down to 1 ps. Three types of damage are observed: ablation craters, ultra-high density pits, and smooth, circular depressions with central pits. For 10 ps and longer, the smooth, circular depressions limit the damage performance of fused silica and silica coatings. The observed high-density pits and material removal down to 3 ps indicate that variations in surface properties limit the laser-induced damage onset to a greater extent than expected below 60 ps. Below 3 ps, damage craters are smoother although there is still evidence as seen by AFM of inhomogeneous laser-induced damage response very near the damage onset. These results show that modeling the damage onset only as a function of pulse width does not capture the convoluted processes leading to laser induced damage with ps pulses. It is necessary to account for the effects of defects on the processes leading to laser-induced damage. The effects of isolated defects or inhomogeneities are most pronounced above 3 ps but are still discernible and possibly important down to the shortest pulse width investigated here.

Role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: II. Scaling laws and the density of precursors.

We investigate the role of defects in laser-induced damage of fused silica and of silica coatings produced by e-beam and PIAD processes which are used in damage resistant, multi-layer dielectric, reflective optics. We perform experiments using 1053 nm, 1-60 ps laser pulses with varying beam size, number of shots, and pulse widths in order to understand the characteristics of defects leading to laser-induced damage. This pulse width range spans a transition in mechanisms from intrinsic material ablation for short pulses to defect-dominated damage for longer pulses. We show that for pulse widths as short as 10 ps, laser-induced damage properties of fused silica and silica films are dominated by isolated absorbers. The density of these precursors and their fluence dependence of damage initiation suggest a single photon process for initial energy absorption in these precursors. Higher density precursors that initiate close to the ablation threshold at shorter pulse widths are also observed in fused silica, whose fluence and pulse width scaling suggest a multiphoton initiation process. We also show that these initiated damage sites grow with subsequent laser pulses. We show that scaling laws obtained in more conventional ways depend on the beam size and on the definition of damage for ps pulses. For this reason, coupling scaling laws with the density of precursors are critical to understanding the damage limitations of optics in the ps regime.

Laser-induced Hertzian fractures in silica initiated by metal micro-particles on the exit surface.

Laser-induced Hertzian fractures on the exit surface of silica glass are found to result from metal surface-bound micro particles. Two types of metal micro-spheres are studied (stainless-steel and Al) using ultraviolet laser light. The fracture initiation probability curve as a function of fluence is obtained, resulting in an initiation threshold fluence of 11.1 ± 4.7 J/cm2 and 16.5 ± 4.5 J/cm2 for the SS and Al particles, accordingly. The modified damage density curve is calculated based on the fracture probability. The calculated momentum coupling coefficient linking incident laser fluence to the resulting plasma pressure is found to be similar for both particles: 32.6 ± 15.4 KN/J and 28.1 ± 10.4 KN/J for the SS and Al cases accordingly.

The effect of laser pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair.

Growth of laser damage on SiO(2) optical components used in high power lasers can be reduced or eliminated by pre-exposure to pulses of a few hundred ps in duration. Such pre-exposure would cause weak locations on the optics surface to self-identify by initiating very small damage sites. The sites which initiate will be only a few microns in diameter and will have a very low probability of growing even without any further treatment. Repairing damage sites when small is important because both laser mitigation and acid etching are very successful in preventing such small sites from growing.

Predicting laser-induced bulk damage and conditioning for deuterated potassium dihydrogen phosphate crystals using an absorption distribution model.

We present an empirical model that describes the experimentally observed laser-induced bulk damage and conditioning behavior in deuterated potassium dihydrogen phosphate (DKDP) crystals. The model expands on an existing nanoabsorber precursor model and the multistep absorption mechanism to include two populations of absorbing defects, one with linear absorption and another with nonlinear absorption. We show that this model connects previously uncorrelated small-beam damage initiation probability data to large-beam damage density measurements over a range of nanosecond pulse widths. In addition, this work predicts the damage behavior of laser-conditioned DKDP.

Effect of multiple wavelengths on laser-induced damage in KH(2-x)DxPO4 crystals.

Laser-induced damage is a key factor that constrains the ways in which optical materials are used in high-power laser systems. We study the size and density of bulk laser-induced damage sites formed during frequency tripling in a DKDP crystal. The characteristics of the damage sites formed during tripling, for which 1053, 526, and 351 nm light is simultaneously present, are compared to those of damage sites formed by 351 nm light alone. The fluence of each wavelength is calculated as a function of depth with a full 4D(x,y,z,t) frequency conversion code and compared with measured damage density and size distributions. The density of damage is found to be governed predominantly by 351 nm light with some lesser, though nonnegligible, contribution from 526 nm light. The 1053 nm light does not appear to contribute to the damage density. The morphology of the damage sites, however, is seen to be relatively insensitive to wavelength and to depend only on total fluence of all wavelengths present. The strong and negligible wavelength dependences of initiation density and damage morphology, respectively, indicate that the dominant energy deposition mechanism varies during the damaging pulse.

Radiation produced by femtosecond laser-plasma interaction during dielectric breakdown.

Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.

Multiwavelength investigation of laser-damage performance in potassium dihydrogen phosphate after laser annealing.

The laser-induced damage performance of potassium dihydrogen phosphate and deuterated potassium dihydrogen phosphate nonlinear optical crystals after pre-exposure to lower-energy laser pulses (laser annealing, also known as laser conditioning) is investigated as a function of wavelength for both the damaging and conditioning pulses. We obtain a quantitative evaluation of the bulk damage performance of these materials by measuring the density of damage events as a function of laser parameters. This new method allows for a detailed assessment of the improvement of material performance from laser conditioning and reveals the key parameters for optimizing performance depending on the operational wavelength.

Localized dynamics during laser-induced damage in optical materials.

Laser-induced damage in wide band-gap optical materials is the result of material modifications arising from extreme conditions occurring during this process. The material absorbs energy from the laser pulse and produces an ionized region that gives rise to broadband emission. By performing a time-resolved investigation of this emission, we demonstrate both that it is blackbody in nature and that it provides the first direct measurement of the localized temperature of the material during and following laser damage initiation for various optical materials. For excitation using nanosecond laser pulses, the plasma, when confined in the bulk, is in thermal equilibrium with the lattice. These results allow for a detailed characterization of temperature, pressure, and electron densities occurring during laser-induced damage.

Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms.

A novel experimental approach is employed to understand the mechanisms of laser induced damage. Using an OPO (optical parametric oscillator) laser, we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from the near ultraviolet into the visible. Distinct steps, whose width is of the order of k(B)T, are observed in the damage threshold at photon energies associated with the number of photons (3-->2 or 4-->3) needed to promote a ground state electron across the energy gap. The wavelength dependence of the damage threshold suggests that a primary mechanism for damage initiation in DKDP is a multiphoton process in which the order is reduced through excited defect state absorption.