MATERIALS AND PROCESSES FOR 1064nm MIRRORS WITH HIGH LIDT
Coating processes and materials that increase the Laser Induced Damage Thresholds (LIDT) of coated surfaces have been discussed frequently in Coating Materials News . The field is active with the latest research reported at the Laser Damage Symposium, a periodic international gathering of laser specialists for more than 50 years.
Many types of lasing media have been developed whose coated surfaces require durability and stability to the laser high energies that might be delivered as continuous irradiation (CW) or short pulses of very high energy density. The varying nature of damage created in the coating are functions of the peak energy, pulse width, spot area and repetition rate. CW exposure produces localized heating, resulting in melting and stress-induced cracking. Short pulses damage by ionizing the surface to produce a plasma that further erodes and vaporizes the coating. A common source for initiating damage is the presence of defects in the coating that might take the form of µm-sized inclusions from the deposition process, or other physical defects such as scratches and pits residuals of polishing/cleaning procedures. Absorption due to impurities contained in or on the surface of coated layers is also a source of failure, especially in the UV wavelengths [2, 3, 4].
Common Laser Sources
The Nd:YAG laser with fundamental wavelength at 1064 nm is used in many high-powered applications. It has received the most attention relative to LIDT applications. Coatings including high reflectors, anti-reflection, beam-dividers, polarizers, and filters on a variety of substrate materials are subjected to laser power densities as high as 250 W/cm^2 CW and 100 J/cm^2 for nano-second to femto-sec pulses. Surviving these energies and powers for various applications and environments challenges the state of the technology to increase LIDT development. Even with the rise of diode and fiber lasers where the fundamental wavelength can be further tuned for the application, the burden falls on coating technology at the most crucial surfaces during generation, delivery, and utilization of the beam.
The availability of different wavelengths from Nd:YAG lasers satisfies several application needs. The second, third, and fourth harmonic wavelengths at 532 nm, 355 nm, and 266 nm, respectively, are used for visible and UV applications including medical, communication, and power generation. Resonances are generated by second or third harmonic non-linear generation in crystals. Multi-layer frequency multiplier mirrors (for UV) using specific coating designs optimize efficiencies in selected wavelengths.
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