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STUDIES TOWARD IMPROVING THE LASER DAMAGE RESISTANCE OF UV COATINGS

The shift in laser wavelength to UV is accompanied by the need to increase the Laser Induced Damage Threshold (LIDT) of the HR (high-reflective) and AR (anti-reflective) coatings on the optics that direct and manipulate the beam. Many previous Coating Materials News articles have touched upon topics that help us understand the process in creating the high-performance films necessary to unleash the full potential of photonics in industry. This article will outline, discuss and begin a dialog on a key aspect of optical coating lifetime and LIDT. It will address the roles that different materials and processes play in increasing the durability of coatings in their work environment.

Applications of Ultra Violet (UV) Lasers

Industrial applications of high-power lasers include welding, cutting and marking of various materials ranging from metals to polymers. The kW powers that were available until recently were provided by bulky CO2 lasers at wavelength 10.6 µm. Since all metals reflect highly at this wavelength, power levels needed to be very high to be efficient. In addition, as lasers became increasingly popular for cutting and welding, stray laser light presented an extreme workplace hazard. These factors posed a strict demand on the coatings that were used to steer the laser beam and anti-reflect the lenses.

These problems exist even at 1.06 µm, the wavelength of Nd:YAG lasers. There was a move to shorter wavelengths where material absorption was higher, thus increasing the efficiency of heating needed for welding and cutting. The third harmonic laser wavelength of Nd:YAG at 355 nm, and diode lasers with wavelengths <450 nm, have become the workhorses for the industry. Diode-pumped solid-state lasers are used for micro-machining, micro-lithography, micro-hole drilling, welding in micro-electronics and opto-electronic applications. In many cases, they replace the more expensive and exotic excimer lasers that operate at wavelengths below ~250 nm. High pulse repetition rates near 5 kHz are possible. Some applications require very high peak powers that only pulsed lasers can produce. Pulse widths in the nano-sec range are used.

UV lasers are especially effective for welding copper, aluminum and stainless steel. Copper is widely used to produce commercial electronic devices such as computers and cell phones. The high UV laser powers possible enable faster assembly than previously achievable. One reason for this is that the smaller wavelength permits concentration to a smaller spot-welding size.

Resisting Damage by High-power Lasers: Damage Mechanisms & Deposition Processes

A key objective is to increase the resistance of coatings to laser induced damage (LID). Laser damage is generally defined as the fluence at which materials will first exhibit damage in the form of melting, excavation of material or mechanical stress. (Photo Courtesy of LIDARIS.)

Over a half-century, researchers have presented their studies at the SPIE Boulder Damage Symposium, whose purpose is to disseminate the latest knowledge to the laser community with due emphasis on optical coatings. The fact that the Symposium has continued during the past 50 years attests to the ongoing complexity of the subject. The persistent goals in laser research fall into two categories: (1) identify and reduce the density of defects that pose limitations to LIDT and (2) understand the damage mechanisms such as pulse width and repetition rate for short pulse lasers and the effects of multiple shots on coating lifetime. Continue in download version

Click to view the full Technical Paper “Studies Toward Improving the Laser Damage Resistance of UV Coatings”