UV laser optics tend to have a limited lifespan due to two main causes: laser-induced contamination (LIC) and UV fatigue.LIC is caused by the deposition of unwanted materials on the surface of the optic, while UV fatigue is caused by the cumulative exposure to UV light that results in damage to the optic. These two damage processes degrade the performance of the optical element over time until irreversible damage is caused.
Long-term experiments on 355 nm UV laser optics used in different environments have revealed key insights into the sources of contamination and fatigue, as well as mitigation strategies and cleaning techniques that may restore contaminated optics.
What is laser-induced contamination (LIC)
Contamination of optical elements can occur when UV laser light interacts with particles, water vapor, organics, and other contaminants in the system. These contaminants can come from ambient air, optomechanical equipment, and other materials in the system. Although mitigation methods such as aeration with dry nitrogen help, they can still lead to LIC.Any buildup of particulate matter can obscure the optical path, degrade component function, and potentially lower the laser damage threshold of the optics.
Condensation often occurs on optical surfaces due to low thermal conductivity. These condensed water molecules can then interact with the laser and surface materials to initiate LIC.In addition, gas emissions and other airborne molecular contaminants often lead to carbon-based deposits on optical surfaces. Tree-like growth of LIC can be observed in Figure 1.
Research conducted in 2005 described in detail the various laser interactions that lead to LIC. For example, light-induced pre-nucleation involves a molecular layer built up due to the direct interaction of UV light with the glass surface. After a sufficiently long exposure, the density of this buildup was shown to be at saturated levels.
Interaction with surrounding gases may also lead to deposition of contaminants. Photon energies at UV wavelengths smaller than 400 nanometers begin to approach the bond energies of common molecules (e.g., O2, CO2, CO, N2, etc.). This allows UV light to break down some of these bonds, creating other ions and molecules that can contaminate optical surfaces.
What is UV fatigue?
In addition to environmentally induced LIC, materials used for coatings and substrates are susceptible to degradation over time due to the process of optical fatigue, even if the intensity of the light source is below the Laser Induced Damage Threshold (LIDT).
The concept of UV fatigue can be likened to binding a book. Even light use can lead to wear and tear. UV fatigue experiments conducted by Edmund Optics have shown that under certain conditions, such as vacuum, UV laser irradiation can lead to UV fatigue effects.The distinguishing feature between LIC and UV fatigue is that LIC is a cumulative process, whereas fatigue is the destruction of the material, resulting in discoloration or other intrinsic changes, and possibly even the removal of the material.
Two phenomena that determine the conditions and mechanisms of this apparent reduction in optical performance are below the single-pulse damage threshold in the short-pulse laser regime.
The first mechanism is based on the modification of the refractive index, leading to a lensing effect that can increase the localized light intensity on the optical element.
The second mechanism involves the formation of optically induced defects through the formation of self-trapped excitons, which leads to the accumulation of absorption centers and loss of optical efficacy.
Both LIC and optical fatigue can occur in lasers at visible and infrared wavelengths, albeit to a lesser extent. However, the high energy of UV photons makes these effects more common in systems emitting in this spectral range.
The UV laser market has grown rapidly in recent years and is expected to have a CAGR of 5.4% between 2022 and 2028, according to research firm MarketWatch3. High-power UV lasers have become a key element in applications including printing, medicine, microfabrication, semiconductor processing, and additive manufacturing.LIC and UV fatigue cause the performance of these systems to degrade over time, requiring the periodic replacement of their optical components. This significantly increases the cost of maintaining a UV laser system and reduces system efficiency. A reduction in the system's LIDT may also increase the risk of catastrophic system failure caused by laser-induced damage (Figure 2).
Analyzing LIC and UV fatigue
Experiments help to simulate the degradation process of optical components in UV laser systems, investigate potential sources of contamination, and explore different corrective measures. In one such study, experiments were conducted to analyze changes in LIC and optical fatigue induced by UV laser irradiation using a 355-nm, 10- to 20-nanosecond pulsed laser emitting approximately 0.6 millijoules per pulse, with a beam diameter of 0.6 mm. The schematic diagram of this test bench is shown in Fig. 3.
The burn-box "combustion chamber" consists of several anti-reflective windows that simulate how a UV laser system is affected, such as a beam expander. The burn-box makes it possible to perform isolated experimental environments in parallel. A half-wave sheet and polarization beam splitter cube allowed control of the average power of each optical path in the experiment. A matched pair of energy meters measured the average laser power. This monitored the transmission degradation over the time of fatigue and/or contamination of the tested optics.
Fig. 3. Schematic of the UV exposure test bed developed to simulate degradation of optical elements in UV laser systems, to investigate potential sources of contamination and to explore different corrective measures. ar: anti-reflective window; fs: uncoated fused silica window; hr: highly reflective mirror; hwp: half-wave plate; pbc: polarization beam splitter cube.
Experiments were performed with daily and continuous measurements. Daily measurements involved opening the housing and placing an energy meter at each of the measurement positions shown in Fig. 3, including the position that typically contains the beam tipper, for a 3-minute measurement. Continuous measurements involved placing two energy meters at measurement positions other than the position that normally contains the beam tipper. The energy meters then recorded average power every 30 minutes until the next daily measurement. An environmental chamber allowed the investigation of discrete effects of various conditions, such as vacuum conditions or the presence of gas. At the end of each experiment, a differential interference contrast microscope allowed researchers to view contaminants on the window surface.
Figure 4.The opaque white contamination on previously transparent optics shown here is due to laser-induced contamination (LIC) after exposure to a UV laser. Image credit: Courtesy of Edmund Optics
General Experimental Results
The combustion chamber allowed for parallel isolation studies and a more realistic simulation of laser optics components such as the beam expander. Initial experiments showed that thread lubricants, anodized aluminum, and the new Viton O-rings are common sources of contamination in UV systems in many other types of optical assemblies. Removing these factors can improve the life of the optics tested.
Viton O-rings: With new, unopened O-rings, the transmittance of the combustion chamber window began to decrease four days into the test and became completely opaque after seven days. A milky white mist formed on the contaminated optical surfaces after the test (Figure 4). Baking the O-rings prior to use prevented some degree of outgassing, resulting in a 6% loss of transmission at five weeks rather than a complete loss of transmission after one week. Placing O-rings in a vacuum or letting them breathe freely in a clean environment is just as effective as baking them.
Anodized Aluminum: Anodized surfaces contain pores that trap contaminants that can be released during use. In addition, anodized materials may become reactive under UV exposure.
Stainless Steel: Experiments using cleaned stainless steel rather than anodized aluminum did not observe significant degradation after seven weeks.
Indium: Foil seals of indium provide higher resistance to UV fatigue compared to O-rings.
Additional experiments were conducted to test how the temperature of the optics promotes the growth of LIC, whether daily cleaning prevents the buildup of contaminants, and whether blowing dry air over the system has any positive effect. These new experiments are moving beyond 355 nm UV exposure to 266 nm wavelength testing.
Summarizing
Understanding and mitigating UV fatigue and LIC will become increasingly important as many laser systems tend to move to shorter wavelengths to utilize higher energy and higher resolution. Experimental results at 355 nm have shown that LIC can render UV laser optics completely opaque in as little as a week if conventional O-rings and anodized aluminum are used in the system. Fortunately, these effects can be significantly mitigated by replacing the O-rings with indium seals, replacing the anodized aluminum with stainless steel, and making the surroundings as clean as possible. When developing a UV laser system, talk to your optics supplier to find out how to make your system more resistant to LIC and UV fatigue as described in the article.
