This paper describes the modelling and design development of an optical coating that is suitable for broad bandwidth high reflection (BBHR) at 45° angle of incidence (AOI), P polarization (Ppol) and fs-class laser pulses whose frequencies correspond to wavelengths from 800 to 1000 nm, and that can eventually be produced uniformly on meter-class optical substrates. The coating design process was guided by specifications of not only high reflection but also high laser-induced damage threshold (LIDT) as well as low group delay dispersion (GDD) for reflected light over the broad, 200 nm bandwidth in order to minimize temporal broadening of the fs pulses upon reflection. The coating is based on TiO2/SiO2layer pairs by means of e-beam evaporation with ion-assisted deposition (IAD). We used OptiLayer Thin Film Software to explore coating designs with a limited optimization process starting from TiO2/SiO2 layer pairs with layer thicknesses in an opposite “chirp” arrangement. This approach proved to be successful, leading to a design with R > 99.5% from 801 – 999 nm and GDD < 20 fs2 from 843 – 949 nm (45° AOI, Ppol). The GDD behaves in a smooth way that lends itself to compensation of GDD effects. Also, the electric field intensities are favorable to high LIDT in that they quench rapidly into the outer coating layers or are of moderate strength, or they are located in the higher band gap SiO2 layers.
Proceedings of SPIE - The International Society for Optical Engineering
Xu, Yejia; Khabbazi, Amir; Day, Travis; Brown, Andrew; Emmert, Luke A.; Talghader, Joseph J.; Field, Ella S.; Kletecka, Damon E.; Bellum, John C.; Patel, Dinesh; Menoni, Carmen S.; Rudolph, Wolfgang
The laser damage behavior of high quality coatings under nanosecond pulse illumination is controlled by statistically distributed defects, whose physical nature and defect mechanisms are still largely unknown. Defect densities are often retrieved by modeling the fluence dependence of the damage probability measured by traditional damage test (TDT) methods, based on âdamage' or âño damage' observations. STEREO-LID (Spatioorally REsolved Optical LaserInduced Damage) allows the determination of the damage fluence (and intensity) in a single test by identifying the initiation of damage both temporally and spatially. The advantages of this test method over the TDT are discussed. In particular, its ability to retrieve detailed defect distribution functions is demonstrated by comparison of results from HfO2 films prepared by ion-assisted electron beam evaporation, ion-beam sputtering, and atomic layer deposition.
We have examined how different cleaning processes affect the laser-induced damage threshold of antireflection coatings for large dimension, Z-Backlighter laser optics at Sandia National Laboratories. Laser damage thresholds were measured after the coatings were created, and again 4 months later to determine which cleaning processes were most effective. There is a nearly twofold increase in laser-induced damage threshold between the antireflection coatings that were cleaned and those that were not cleaned. Aging of the coatings after 4 months resulted in even higher laser-induced damage thresholds. Also, the laser-induced damage threshold results revealed that every antireflection coating had a high defect density, despite the cleaning process used, which indicates that improvements to either the cleaning or deposition processes should provide even higher laser-induced damage thresholds.
High-reflection coatings with broad bandwidth can be achieved by pairing a low refractive index material, such as SiO2, with a high refractive index material, such as TiO2. To achieve high refractive index, low absorption TiO2films, we optimized the reactive, ion-assisted deposition process (O2levels, deposition rate, and ion beam settings) using e-beam evaporated Ti. TiO2high-index layers were then paired with SiO2low-index layers in a quarter-wave-type coating to achieve a broader high-reflection bandwidth compared to the same coating composed of HfO2/SiO2layer pairs. However, the improved bandwidth exhibited by the TiO2/SiO2coating is associated with lower laser damage threshold. To improve the laser damage resistance of the TiO2/SiO2coating, we also created four coatings where HfO2replaced some of the outer TiO2layers. We present the laser damage results of these coatings to understand the trade-offs between good laser damage resistance and high-reflection bandwidth using TiO2and HfO2.
We have examined how different cleaning processes affect the laser induced damage threshold of antireflection coatings for large dimension, Z-Backlighter laser optics at Sandia National Laboratories. Laser damage thresholds were measured after the coatings were created, and again 4 months later to determine which cleaning processes were most effective. There is a nearly twofold increase in laser induced damage threshold between the antireflection coatings that were cleaned and those that were not cleaned. The laser-induced damage threshold results also revealed that every antireflection coating had a high defect density, despite the cleaning process used, which indicates that improvements to either the cleaning or deposition processes should provide even higher laser induced damage thresholds.
When an optical coating is damaged, deposited incorrectly, or is otherwise unsuitable, the conventional method to restore the optic often entails repolishing the optic surface, which can incur a large cost and long lead time. We propose three alternative options to repolishing, including (i) burying the unsuitable coating under another optical coating, (ii) using ion milling to etch the unsuitable coating completely from the optic surface, and then recoating the optic, and (iii) using ion milling to etch through a number of unsuitable layers, leaving the rest of the coating intact, and then recoating the layers that were etched. Repairs were made on test optics with dielectric mirror coatings according to the above three options. The mirror coatings to be repaired were quarter wave stacks of HfO2 and SiO2 layers for high reflection at 1054 nm at 45° incidence in P-polarization. One of the coating layers was purposely deposited incorrectly as Hf metal instead of HfO2 to evaluate the ability of each repair method to restore the coating's high laser-induced damage threshold (LIDT) of 64 J/cm2. The repaired coating with the highest resistance to laser-induced damage was achieved using repair method (ii) with an LIDT of 49-61 J/cm2.