Laser damage testing of ion beam sputtered (IBS) films for 1064 nm, Q-switched, low repetition rate
operation typically involves the measurement of small physical defects that can be detected visually or
photographically. For this kind of damage the small absorber model adequately describes most of the
observations. We discuss why the damage threshold for IBS anti-reflection (AR) coatings is lower than for
high reflectors and polarizers. We report on a new process technique that eliminates the physical damage to
IBS AR coatings for fluences up to 21 J/cm2, possibly higher. The success of this process and the lower
damage for AR's indicates that the small absorbers are at the substrate/coating interface or are in the
subsurface region of the substrate. We also discuss ongoing experiments to look for subtle forms of laser
damage involving changes of the optical parameters of the thin film materials, under long-term exposure to Q-switched
1064 nm radiation and continuous 355 nm radiation.
The automated laser damage testing system at REO has been in operation for over a year, providing quantitative and
detailed information on laser damage of ion beam sputtered (IBS) thin films in a production setting. Results have
accumulated in a database, which can be queried in complex ways. We present statistical analysis on event curves
(number vs. fluence) for various defect size groups. We examine the differences in event curves for high-threshold and
lower-threshold IBS optics. We also present results of experiments on laser conditioning of IBS thin films.
Research Electro-Optics Inc. (REO) has recently developed a new laser damage testing facility for the purpose of optimizing process parameters for fabrication and coating of high-damage optics. It also enables full or sample qualification of optics with laser damage specifications. The fully automated laser damage testing system uses microscope photography for detection of damage and a 3 ns pulse length 1064 nm laser for irradiation of the sample. It can test and statistically analyze damage events from a large number of shots, enabling large area testing for low probability events. The system measures and maps sizes and locations of damage sites down to a few microns in diameter. The results are not subject to variations due to the human operator. For coatings deposited by ion beam sputtering, small defects (less than 20 microns) are found to be most prevalent at the fluences specified for small optics for the National Ignition Facility. The ability to measure and characterize small defects has improved REO's ability to optimize their processes for making coated optics with high damage thresholds. In addition to qualifying particular parts, the periodic testing also assures that equipment and processes remain optimized.
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