John J. Adams

Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation

Characterizing laser-induced damage in optical materials is important for laser design and operation. Previous methods of evaluating optical materials damage resistance to high-power laser irradiation have typically suffered from shot-to-shot uncertainties in laser energy output and/or have insufficient sensitivity. More importantly, such methods do not address the aspects of laser-induced damage important to laser beam propagation, namely the amount of light scattered by the damage. We present a method for the quantitative correlation of material modification on the surface or in the bulk of optical materials to laser parameters, which deconvolutes the effects of laser output instability. In image analysis, two images, one a fluence spatial profile and the other a visible light scatter image of the damage, are directly compared to extract scatter as a function of fluence. An automated microscope is used to record the location and number of bulk damage sites and determine a calibration factor between the scatter signal observed and damage density. We illustrate the method with a determination of both bulk damage density as a function of laser fluence and of a representative size distribution in a DKDP crystal. Our method is capable of determining damage densities with an absolute uncertainty of ±0.3 pinpoint damage sites per cubic millimetre (pp mm−3) in the range 1–100 pp mm−3 with our minimum detectable density being 0.01 pp mm−3. We also determined the pps produced by laser pulses of 351 nm, 3 ns light to have a mean diameter of 5.5 ± 2.5 µm (1/e2).

Nonlinear optical properties of LaCa(4)O(BO(3))(3).

We have grown LaCa(4)O (BO(3))(3) (LaCOB), an isostructural member of GdCa(4)O(BO(3))(3) (GdCOB) family and characterized its nonlinear optical properties. At 1064nm, d(eff) of 0.52+/-0.05 pm /V and an angular sensitivity of 1224+/-184(cm rad)(-1) for type I frequency doubling in LaCOB were determined relative to those of KTiOPO(4) , beta-BaB(2)O(4) , KD(2)PO(4) , LiB(3)O(5) , YCa(4)O(BO(3))(3) (YCOB), and GdCOB. The d(alphabetabeta) and d(gammabetabeta) coefficients of the nonlinear optical tensor for LaCOB, GdCOB, and YCOB were determined to be equivalent within the experimental uncertainty and have values of ?0.26+/-0.04?pm/V and ?1.69+/-0.17?pm /V , respectively. From phase-matching angle measurements at 1064 and 1047nm, we predict that LaCOB is noncritically phase matched at 1042+/-1.5 nm .

Monitoring annealing via CO 2 laser heating of defect populations on fused silica surfaces using photoluminescence microscopy

Photoluminescence (PL) microscopy and spectroscopy under 266 nm and 355 nm laser excitation are explored as a means of monitoring defect populations in laser-modified sites on the surface of fused silica and their subsequent response to heating to different temperatures via exposure to a CO(2) laser beam. Laser-induced temperature changes were estimated using an analytic solution to the heat flow equation and compared to changes in the PL emission intensity. The results indicate that the defect concentrations decrease significantly with increasing CO(2) laser exposure and are nearly eliminated when the peak surface temperature exceeds the softening point of fused silica (approximately 1900K), suggesting that this method might be suitable for in situ monitoring of repair of defective sites in fused silica optical components.

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.

High laser-resistant multilayer mirrors by nodular defect planarization [Invited]

Substrate defect planarization has been shown to increase the laser resistance of 1053 nm mirror coatings to greater than 100  J/cm2, an increase of 20-fold, when tested with 10 ns laser pulses. Substrate surface particles that are overcoated with optical interference mirror coatings become nodular defects, which behave as microlenses intensifying light into the defect structure. By a discrete process of angle-dependent ion etching and unidirectional ion-beam deposition, substrate defects can be reduced in cross-sectional area by over 90%.

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.

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.

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.

Engineering meter-scale laser resistant coatings for the near IR

Laser resistant coatings are needed for beam steering (mirrors), pulse switching (polarizers), and high transport efficiency on environmental barriers (windows / lenses) on large laser systems. A range of defects limit the exposure fluence of these coatings. By understanding the origin and damage mechanisms for these defects, the deposition process can be optimized to realize coatings with greater laser resistance. Electric field modeling can provide insight into which defects are most problematic. Laser damage growth studies are useful for determining a functional laser damage criteria. Mitigation techniques such as micro-machining with a single-crystal diamond cutting tool or short pulse laser ablation using the burst technique can be used to arrest growth in damage sites to extend optic lifetime.

Laser damage mechanisms in conductive widegap semiconductor films

Laser damage mechanisms of two conductive wide-bandgap semiconductor films - indium tin oxide (ITO) and silicon doped GaN (Si:GaN) were studied via microscopy, spectroscopy, photoluminescence (PL), and elemental analysis. Nanosecond laser pulse exposures with a laser photon energy (1.03 eV, 1064 nm) smaller than the conductive films bandgaps were applied and radically different film damage morphologies were produced. The laser damaged ITO film exhibited deterministic features of thermal degradation. In contrast, laser damage in the Si:GaN film resulted in highly localized eruptions originating at interfaces. For ITO, thermally driven damage was related to free carrier absorption and, for GaN, carbon complexes were proposed as potential damage precursors or markers.