Growth of laser damage on SiO2 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.
©2011 Optical Society of America
The output of both large-aperture and table top lasers is often constrained by the need to avoid inducing damage in the laser’s optics. This is particularly true in the case of Inertial Confinement Fusion (ICF) class lasers such as the National Ignition Facility (NIF) in the United States and the Laser Mega-Joule (LMJ) in France. In the past a number of efforts have been made to reduce laser induced damage on the exit surface of SiO2 optics by pre-initiating damage and then repairing it as a final processing step before the optics are used [1–3]. This method has the advantage of allowing weak locations on the surface of the optic to self-identify by initiating damage under controlled circumstances. Because the process is done before installation, the localized damage on the optic can be repaired after a single initiation pulse, rather than after being initiated and grown for some number of shots as would likely be the case if the damage were to initiate after installation [4–6]. Off-line pre-initiation reduces the logistical burden of a large laser facility by reducing the number of times optics must be installed and removed from the system. This strategy would be significantly enhanced if sites can be initiated small enough to ease repair [1–3,7,8]. It should be noted that pre-initiation is a distinctly different procedure than conditioning, though the terms are occasionally used interchangeably. The term conditioning has historically been used to describe a stepwise increase in laser irradiation which makes a material or component more resistant to laser-induced damage without actually producing observable damage and therefore eliminating the need for repair. For example, laser conditioning is routinely employed to increase the bulk damage threshold of KDP and DKDP materials for frequency harmonic generation [9,10].
Past pre-initiation work has utilized XeF lasers because of their excellent beam quality and reliability and access to the ~351 nm (3ω) wavelength used in ICF lasers (see for example the work by Prasad et al. or Bertussi et al.). The disadvantage to using a XeF laser is that the pulse shape has an effective temporal duration much longer than the pulses used on Fusion class laser systems . Additionally the concept of pre-initiation relies on an implicit assumption that the precursors initiated with one pulse are the same as those initiated by the pulses used in the system. The duration of the single-pass XeF pulses could be described as ~9 ns with a Full Width at Half Maximum (FWHM) measure, or as ~16 ns, or even 30 ns in duration for 1/e2 and 1/e3 measurements, respectively. Though the effects of pulse duration have been explored previously  we will elucidate here that the pulse shape (and hence duration) is relevant to the pre-initiation process in two ways. Laser induced damage in the ns regime has been shown to be comprised of a rapid series of events starting with an initial energy absorption by an extrinsic precursor [3,13–17]. After the initial first few hundred picoseconds (the exact amount of time depending on laser intensity) laser energy is deposited on a temperature activated absorption front in the bulk silica . Because the mechanisms for the initial energy deposition are different from those governing the final damage site size , it is not surprising that different measures of the pulse are more relevant when discussing initiation density (or probability) and size of initiated damage sites . In this work we show that the temporal characteristics of a laser pulse are equally, if not more important parameters than spatial laser parameters to achieve controlled pre-initiation of SiO2 optics.
2. Damage initiation
The Optical Sciences Laser (OSL) Facility at Lawrence Livermore National Laboratory (LLNL) was used to perform the SiO2 surface damage testing experiments . The OSL laser can produce 3ω (351 nm) laser pulses of near arbitrary temporal shape with durations up to 30 ns and is used in these experiments to initiate damage on the exit surface of SiO2 optics with pulses of Gaussian shape with different FWHM as well as more complex pulse shapes, similar to those output from an XeF laser system and to those used for early ignition experiments on the NIF [20–22] (see Fig. 1 inset). The techniques used to generate and analyze damage testing data were reported elsewhere . In brief, samples are contained in a vacuum chamber with a pressure of 10−5 torr at room temperature. Spatially separated sub-apertures on the same test sample are sequentially exposed to a single pulse. In this way multiple pulse shapes can be tested on a single sample to reduce any effect sample variability may have on the measurements. Similarly, the spatial contrast in the test beam enables a single sub-aperture to be tested with one pulse shape with a range of laser fluences. The local density of damage sites are measured with an automated microscope and correlated to the local fluence. The relationship between laser fluence and damage density is typically referred to as a ρ(ϕ) or “rho of phi” measurement or curve . Figure 1 shows two example ρ(ϕ) curves.
3. Effect of pulse shape on damage initiation density
Though the focus of this work is the effect of pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair, a brief discussion on the effects of pulse shape on the number of sites initiated is warranted to put this work in context. The effect of pulse duration (in the ns regime) on the propensity of damage initiation is well known and is often described by pulse scaling for both density and probabilistic measurements [25–27]. Equation (1) demonstrates the use of pulse scaling to damage density measurements.11,12,25,26].
To illustrate the effect of pulse duration (and shape) on damage initiation, ρ(ϕ) curves for pulse shapes typical for XeF lasers as well as one similar to a proposed ‘ignition’ pulse for use in inertial confinement fusion experiments are shown in Fig. 1. The data show that the fluence of the XeF pulse must be 170% that of the ignition-like pulse in order to achieve the same damage density. More detailed examinations of the effects of pulse shape on the density of damage sites are available elsewhere [12,28].
4. Effect of pulse duration on damage site initiation size
After the damage was initiated the samples were then coated with a few mono-layers of Au (for enhanced conductivity) before the acquisition of SEM images of individual damage sites. Figures 2(a) and 2(b) show damage sites initiated on the exit surface of SiO2 optics with Gaussian shaped pulses of various lengths and one site initiated with a XeF pulse, respectively. All damage sites are from regions with damage densities of ~10 sites per mm2. Damage sites produced by the ignition-like pulse are indistinguishable to those produced by a 3 ns Gaussian pulse. By comparison damage sites produced by a typical pulse shape from a XeF laser are 4 times larger than those produced by a 3 ns pulse and more than 20 times larger than sites produced by a 350 ps pulse. Similar pulse duration dependence to damage site size has been previously reported for bulk sites in KDP crystals . It should be noted that because sites were chosen from regions with similar densities of sites, the fluence which created each site is necessarily different from one another. The fluence used to create the XeF sites (~35 J/cm2) is 1.7 times higher than that needed for the 3 ns Gaussian pulse and 8 times higher than that needed for the 350 ps sites.
5. Effect of site size on damage growth
Measurements of the probability that a damage site will grow under subsequent laser exposure as a function of size (see Fig. 3 ) show that smaller site have a smaller likelihood of growth on a given shot. The techniques used for theses measurements and a detailed exploration of this effect is the topic of another publication . In brief, samples are prepared by initiating individual sites in arrays with spacing of a few millimeters, sufficient to preclude the sites from affecting each other. After each site is imaged with an automated microscope the several hundred sites are then simultaneously exposed with 5-ns FIT pulses in the OSL facility. The sites are re-imaged and the before and after (laser exposure) micrographs are compared to ascertain if each site grew. A change in diameter of 2 microns is reliably detectable and is treated as our noise floor .
It is not clear what the dominant mechanism is for the correlation between site size and probability of growth, though the amount and morphology of sub-surface fracture is a likely candidate for large sites. The density of atomic scale defects, and stressed and densified material surrounding the site may also contribute. The micrographs used in determining growth suggest that the presence of micron scale sub-surface cracks are not, in general, correlated to growth for sites smaller than about 30 microns. The size dependence of the probability of growth and the above discussed dependence of initiation size on pulse duration raises the intriguing possibility of initiating damage sites small enough that they will be very unlikely to grow. Pulses as short as a few tens of ps (or shorter) will produce sites as small as a micron or less. Even the roughly 4-micron sites initiated with 500-ps pulses are expected to have a probability of growth of only 10−4 when irradiated with 9 J/cm2.
In this work we have verified that the effects of pulse shape on initiation density previously reported for bulk damage in KDP are also present on exit surface SiO2. In addition to the effects on the density of initiations we show that the duration of the pulse strongly affects the size at which damage sites initiate. Size is important both because of the previously reported effects on the success of optics repair, but also because small sites are less likely to grow, even without repair. These findings are applicable to optics preparation for fusion class lasers though care should be taken to ensure the pre-initiation pulse initiates the same population of precursors as the future use pulse. By replacing the traditional XeF type systems, historically used for pre-initiation and conditioning, with lasers with shorter pulse durations damage sites can be initiated with a much smaller initial size, reaping the benefits associated with smaller sites mentioned above. In addition, as shown in the present work, by using short enough laser pulse durations to pre-initiate sites there is the possibility of producing sites so small that they do not grow at all under exposure to laser conditions typical to fusion class laser systems.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.
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