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Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats &amp; Spheres

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial ...

Lawrence Livermore National Laboratory

Laser-induced pinpoint bulk damage of deuterated potassium dihydrogen phosphate at 351 nm is shown to depend on the propagation direction relative to the crystallographic axes and on growth temperature in addition to the previously reported dependence on continuous filtration. Pulse-length scaling is also consistent with earlier reports. The leading hypothesis for the cause of pinpoint damage is absorbing nanoparticle impurities, and our results are consistent with but not conclusive for that model. Advances in technology have led to greatly improved damage resistance.

Laser-induced damage in deuterated potassium dihydrogen phosphate.

The initiation of laser damage within optical coatings can be better understood by electric-field modeling of coating defects. The result of this modeling shows that light intensification as large as 24x can occur owing to these coating defects. Light intensification tends to increase with inclusion diameter. Defects irradiated over a range of incident angles from 0 to 60 deg tend to have a higher light intensification at a 45 deg incidence. Irradiation wavelength has a significant effect on light intensification within the defect and the multilayer. Finally, shallow, or in the case of 45 deg irradiation, deeply embedded inclusions tend to have the highest light intensification.

Laser intensification by spherical inclusions embedded within multilayer coatings.

Laser-induced damage at near operational laser excitation conditions can limit the performance of potassium dihydrogen phosphate (KH(2)PO(4), or KDP) and its deuterated analog (DKDP) which are currently the only nonlinear optical materials suitable for use in large-aperture laser systems. This process has been attributed to pre-existing damage precursors that were incorporated or formed during growth that have not yet been identified. In this work, we present a novel experimental approach to probe the electronic structure of the damage precursors. The results are modeled assuming a multi-level electronic structure that includes a bottleneck for 532 nm excitation. This model reproduces our experimental observations as well as other well-documented behaviors of laser damage in KDP crystals. Comparison of the electronic structure of known defects in KDP with this model allows for identification of a specific class that we postulate may be the constituent defects in the damage precursors. The experimental results also provide evidence regarding the physical parameters affecting the ability of individual damage precursors to initiate damage, such as their size and defect density; these parameters were found to vary significantly between KDP materials that exhibit different damage performance characteristics.

Investigation of the electronic and physical properties of defect structures responsible for laser-induced damage in DKDP crystals.

Growing laser damage sites on multilayer high-reflector coatings can limit mirror performance. One of the strategies to improve laser damage resistance is to replace the growing damage sites with predesigned benign mitigation structures. By mitigating the weakest site on the optic, the large-aperture mirror will have a laser resistance comparable to the intrinsic value of the multilayer coating. To determine the optimal mitigation geometry, the finite-difference time-domain method was used to quantify the electric-field intensification within the multilayer, at the presence of different conical pits. We find that the field intensification induced by the mitigation pit is strongly dependent on the polarization and the angle of incidence (AOI) of the incoming wave. Therefore, the optimal mitigation conical pit geometry is application specific. Furthermore, our simulation also illustrates an alternative means to achieve an optimal mitigation structure by matching the cone angle of the structure with the AOI of the incoming wave, except for the p-polarized wave at a range of incident angles between 30° and 45°.

Searching for optimal mitigation geometries for laser-resistant multilayer high-reflector coatings.

Surface laser damage limits the lifetime of optics for systems guiding high fluence pulses, particularly damage in silica optics used for inertial confinement fusion-class lasers (nanosecond-scale high energy pulses at 355 nm/3.5 eV). The density of damage precursors at low fluence has been measured using large beams (1-3 cm); higher fluences cannot be measured easily since the high density of resulting damage initiation sites results in clustering. We developed automated experiments and analysis that allow us to damage test thousands of sites with small beams (10-30 µm), and automatically image the test sites to determine if laser damage occurred. We developed an analysis method that provides a rigorous connection between these small beam damage test results of damage probability versus laser pulse energy and the large beam damage results of damage precursor densities versus fluence. We find that for uncoated and coated fused silica samples, the distribution of precursors nearly flattens at very high fluences, up to 150 J/cm2, providing important constraints on the physical distribution and nature of these precursors.

Extracting the distribution of laser damage precursors on fused silica surfaces for 351 nm, 3 ns laser pulses at high fluences (20-150 J/cm2).

Pitch button blocking (PBB), involving attaching small pitch buttons between the back of a thin workpiece (i.e., optic) and a blocking plate, enables noncompliant convergent polishing in which the workpiece stiffness and block interface strength are maintained. This process has been optimized, and practical design criteria (number, size, and spacing of pitch buttons) have been determined both experimentally and theoretically using a thermoelastic model. The optimized PBB process has been successfully implemented on 100-265 mm sized workpieces with aspect ratios up to 45, resulting in maximum peak-to-valley heights of <|0.1| μm after blocking and polishing.

Optimized pitch button blocking for polishing high-aspect-ratio optics.

The light emission produced near the surface of fused silica following laser-induced breakdown on the exit surface was spatially and spectrally resolved. This signal is in part generated by ejected particles while traveling outside the hot ionized region. The thermal emission produced by the particles can be separated from the plasma emission near the surface and its spectral characteristics provide information on the temperature of the particles after ejection from the surface. Assuming the emission is thermal in origin, data suggest an initial average temperature on the order of at least 0.5 eV.

Characterization of ejected fused silica particles following surface breakdown with nanosecond pulses.

The self-focusing characteristic of 355 nm, 3.3 ns pulses propagating through phosphate glass samples is found to significantly change during repeated exposure. The results indicate this change is related to the formation of color centers in the material as well as the generation of a transient defect population during exposure to the laser pulses. A model is used to fit the experimental data and obtain an estimated range of values for the modified linear and nonlinear indices of refraction.

Michael Feit

Lasers,
Optics,
& Targets for the National Ignition Facility,
Lasers, Optics, & Targets for the National Ignition Facility

Lawrence Livermore National Laboratory

Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats & Spheres

Michael Feit

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Lawrence Livermore National Laboratory

Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats & Spheres

Lasers,
Optics,
& Targets for the National Ignition Facility,
Lasers, Optics, & Targets for the National Ignition Facility