Lawrence Livermore National Laboratory



IR Laser Microshaping of Silica Optics: Rapid Ablation Mitigation (RAM)

Precisely-shaped cone micro-machined into the surface of a fused silica optic, replacing a UV laser induced damage site which is prone to further absorption and damage.

PLS scientists and engineers, along with development teams in NIF, have successfully developed laser-based techniques to repair optical damage on fused silica optics used in the NIF laser. LLNL expertise in CO2 laser processing of glass optics dates back to the early 80's, and more recently, detailed models of the laser-material interaction and response has led to a better understanding of the physics involved. To overcome structural relaxation and Marangoni flow problems associated with long laser exposures, the group shifted laser parameters towards shorter pulse, small beam diameter to effectively 'machine' away material, leaving a precise micro-shaped mitigation site. The process, Rapid Ablation Mitigation, or RAM, is currently used in the NIF for damage repair.


Laser-based Chemical Vapor Deposition

Left: Simulation of laser-based chemical vapor deposition (L-CVD) showing velocity contours associated with the CVD precursor flow from a 3-mm diameter nozzle and the temperature field induced by laser heating at the air-glass interface. In experiments, laser light is coupled into the nozzle through an optical window (not shown) and directed co-axially with the flow and through the nozzle orifice. Right: example of 'in-filling' of surface damage using L-CVD, before fill (top) and after (bottom). The initial pit width and depth were 200 and 8 μm respectively, and following infilling the depth was reduced to ~500 nm.

Top: Simulation of laser-based chemical vapor deposition (L-CVD) showing velocity contours associated with the CVD precursor flow from a 3-mm diameter nozzle and the temperature field induced by laser heating at the air-glass interface. In experiments, laser light is coupled into the nozzle through an optical window (not shown) and directed co-axially with the flow and through the nozzle orifice. Bottom: example of 'in-filling' of surface damage using L-CVD, before fill (top) and after (bottom). The initial pit width and depth were 200 and 8 μm respectively, and following infilling the depth was reduced to ~500 nm.

Scientists and engineers in PLS have developed methods to replenish material lost to optical damage thereby refurbishing the optical component, using laser-based chemical vapor deposition (L-CVD). While damage repair using laser ablation (e.g. RAM) has been successful, the removal of glass necessarily modulates the light passing through the optic, albeit at very low levels. Ideally, material lost to the original damage event is replenished thereby preventing loss of light and introduction of light modulation. Because of the non-equilibrium conditions driving by rapid laser heating, L-CVD is also being explored as a means to produce ultra-hard materials for defense applications and NIF target capsules.


Laser processing of micro- and nano-scale metal structures

Precise and controlled laser heating of metal surfaces can drive microstructural evolution and surface coarsening for a wide range of applications. For example, the large thermal gradients and cooling rates can be exploited to reduce grain size and increase mechanical hardness. Short exposures of laser light in nanostructured materials can be used to tune the structures through surface diffusion-driven coarsening. More recently, PLS scientists working in laser material processing have applied in situ diagnostics techniques and thermal modeling to understand the material response involved in metal-based additive manufacturing. Through the combination of validated hydrocode and experimentation of laser-heated metal powders, researchers hope to unravel the complex dynamics involved in Selective Laser Melting and ultimately improve the mechanical properties of additively manufactured metal components.

A 532 nm laser was used to evolve nano-porous Au thin films by exploiting the temperature-dependent surface diffusivity.