Justin E. Wolfe

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%.

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

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°.

Improved method for laser damage testing coated optics

The laser damage test for qualifying a coating run of anti-reflection coated optics consists of scanning a pulsed 1064 nm laser to illuminate approximately 2400 sites over a 1 cm x 1 cm area on a test sample. Scans are repeated at 3 J/cm2 increments until the fluence specification for the optic is reached. In the past, initiation of 1 or more damage sites was classified as a failed coating run, requiring the production optics in the corresponding coating lot be reworked and recoated. Recent laser damage growth tests of 300 repetitive pulses performed on numerous damage sites revealed that all were stable up to 20 J/cm2. Therefore the acceptance criteria has been modified to allow a moderate number of damage sites, as long as they are smaller than the allowed dig size and are stable (do not grow). Consequently many coating runs that previously would have been rejected are now accepted, resulting in higher yield, lower cost, and improved delivery schedule. The new test also provides assurance that initiated damage sites are stable during long term operation.

Fabrication of mitigation pits for improving laser damage resistance in dielectric mirrors by femtosecond laser machining

Femtosecond laser machining is used to create mitigation pits to stabilize nanosecond laser-induced damage in multilayer dielectric mirror coatings on BK7 substrates. In this paper, we characterize features and the artifacts associated with mitigation pits and further investigate the impact of pulse energy and pulse duration on pit quality and damage resistance. Our results show that these mitigation features can double the fluence-handling capability of large-aperture optical multilayer mirror coatings and further demonstrate that femtosecond laser macromachining is a promising means for fabricating mitigation geometry in multilayer coatings to increase mirror performance under high-power laser irradiation.

Impact of substrate surface scratches on the laser damage resistance of multilayer coatings

Substrate scratches can limit the laser resistance of multilayer mirror coatings on high-peak-power laser systems. To date, the mechanism by which substrate surface defects affect the performance of coating layers under high power laser irradiation is not well defined. In this study, we combine experimental approaches with theoretical simulations to delineate the correlation between laser damage resistance of coating layers and the physical properties of the substrate surface defects including scratches. A focused ion beam technique is used to reveal the morphological evolution of coating layers on surface scratches. Preliminary results show that coating layers initially follow the trench morphology on the substrate surface, and as the thickness increases, gradually overcoat voids and planarize the surface. Simulations of the electrical-field distribution of the defective layers using the finite-difference timedomain (FDTD) method show that field intensification exists mostly near the top surface region of the coating near convex focusing structures. The light intensification could be responsible for the reduced damage threshold. Damage testing under 1064 nm, 3 ns laser irradiation over coating layers on substrates with designed scratches show that damage probability and threshold of the multilayer depend on substrate scratch density and width. Our preliminary results show that damage occurs on the region of the coating where substrate scratches reside and etching of the substrate before coating does not seem to improve the laser damage resistance.

Modeling of light intensification by conical pits within multilayer high reflector coatings

Removal of laser-induced damage sites provides a possible mitigation pathway to improve damage resistance of coated multilayer dielectric mirrors. In an effort to determine the optimal mitigation geometry which will not generate secondary damage precursors, the electric field distribution within the coating layers for a variety of mitigation shapes under different irradiation angles has been estimated using the finite difference time domain (FDTD) method. The coating consists of twenty-four alternating layers of hafnia and silica with a quarter-wave reflector design. A conical geometrical shape with different cone angles is investigated in the present study. Beam incident angles range from 0° to 60° at 5° increments. We find that light intensification (square of electric field, |E|2) within the multilayers depends strongly on the beam incident direction and the cone angle. By comparing the field intensification for each cone angle under all angles of incidence, we find that a 30° conical pit generates the least field intensification within the multilayer film. Our results suggest that conical pits with shallow cone angles (≤ 30°) can be used as potential optimal mitigation structures.

Automated laser damage test system with real-time damage event imaging and detection

An automated laser damage test system has been developed by the National Ignition Facility small optics metrology group. The Small Optics Laser Damage (SOLD) system measures the fluence at which laser damage occurs in optical coatings and substrates following the requirements of MEL01-013-OD. Irradiation of the sample is by a 1064nm, 8ns pulse with a 1mm 1/e2 diameter. The test protocol requires raster scanning of a 1cm2 area at increasing fluence levels. Real-time high-resolution imaging of the surface during raster scanning enables automated detection and sizing of defects to 10 microns. Improved imaging resolves actual size of damage events while the automated damage detection removes the subjectivity of the human operator in thresholding damage events. In addition, a map is created enabling additional functions such as excluding damage sites on future scans and to returning to the damage site for growth testing.

Defect insensitive 100 J/cm2 multilayer mirror coating process

Multilayer mirrors are fluence-limited by nodular defects. Such defects originate from the deposition source, inadequate cleaning, transport, pump down, heating, shedding from rotating hardware, etc. These overcoated inclusions behave as micro-lenses resulting in light intensification within the multilayer structure. To minimize the impact of these defects, a planarization process has been developed to reduce geometric-induced light intensification. By exploiting the angledependent etching rate of materials, a deposit-and-etch process reduces nodular defect height and diameter. Planarized defects demonstrate a greater than 20x increase in laser resistance at a wavelength of 1064 nm and pulse length of 10 ns. Process parameters were explored such as planarization efficiency of the coating materials, discrete versus continuous etching, thick planarization layers for substrate defects, and etching throughout the multilayer to planarize coating defects.

Laser damage resistant pits in dielectric coatings created by femtosecond laser machining

Replacing growing damage sites with benign, laser damage resistant features in multilayer dielectric films may enable large mirrors to be operated at significantly higher fluences. Laser damage resistant features have been created in high reflecting coatings on glass substrates using femtosecond laser machining. These prototype features have been damage tested to over 40 J/cm2 (1064nm, 3ns pulselength) and have been shown not to damage upon repeated irradiation at 40J/cm2. Further work to optimize feature shape and laser machining parameters is ongoing.

Improved Optical Quality for Ti:Sapphire Using MRF

Magneto-rheological finishing (MRF) imprinting techniques have been applied to Ti:sapphire crystals to compensate for submillimeter distortions, thereby, improving the transmitted wavefront and increasing the availability of large aperture parts.