Cleaning optical components is about more than maintaining aesthetics. In high-precision systems, particularly those involving lasers, even minor contamination of the optics can lead to component failure. While contaminants — either on a coating or at the interface with the substrate — can absorb laser energy and degrade the coating or substrate surface, the presence of dust, streaks, and/or stains scatters light, which reduces the overall functionality of the laser optic. Numerous studies have shown that the cleaning process alone can vary the laser damage threshold of a coated optic by as much as 75%. In addition to ensuring optimal optical performance, cleaning helps to prevent permanent damage to the component. At the same time, the process of laser optics cleaning is multifaceted, requiring consideration of more than just the condition of the optic in question and the target application. Understanding a range of techniques, approaches to handling, and best practices for the many distinct optical materials in use today is essential to maintaining the superior performance of laser optics. There are four primary hand-cleaning techniques: the drop-and-drag method, the brush technique, the wipe method, and immersion cleaning. No matter which method is used, it is best to first clean the edges of the component to avoid drawing debris onto polished surfaces. Courtesy of Perkins Precision Developments. Handling optics Although standards across the photonics industry vary, the rule of “if it’s not dirty, don’t clean it” is widely accepted by the field. Optics are sensitive instruments, and since every interaction poses a risk of introducing damage or debris, they should be cleaned only when there is a clear need to do so. Unnecessary or aggressive cleaning can reduce the life of optical coatings, especially on high-performance components. To determine whether an optic requires cleaning, it should be carefully inspected using a strong visible light source. Holding the optic at different angles enables the person performing the inspection to more easily detect scattering caused by dust or residue. Table 1. Cleaning Methods and Applications Handling comes with its own set of best practices. It should always take place in a cleanroom environment where particulates and airborne dust are at a minimum. Additionally, to protect the surfaces of the optics from contamination, it is essential to wear powder-free gloves or finger cots that are resistant to solvents such as acetone. Bare skin should never come into contact with any coated surface, because natural oils or debris from hands can stain coatings and contribute to long-term degradation. Similarly, lens tissue should be used only once and then discarded. Reusing it can transfer contaminants back onto the optic and cause scratches. Given the cost of precision optical components, using a fresh lens tissue each time is a minor expense that helps prevent costly damage. Common cleaning practices and materials Historically, optical components such as mirrors, windows, and beamsplitters have been cleaned manually using lint-free wipes and solvent-based methods. However, the first step in cleaning, before using solvents, should be dusting. Wiping a dirty optic is analogous to cleaning it with sandpaper. If dust is the only contaminant present and no stains remain following dusting, additional cleaning is not required. A small optical component is cleaned with acetone, commonly used as a solvent for coated surfaces, and held and rotated using tweezers. Courtesy of Perkins Precision Developments. If large or loose particles are present, a bulb blower, camel-hair brush, or dry nitrogen gas should be used to remove them. It is not advisable to use commercial canned air because propellant residue can leave droplets on the surface. Liquid-based cleaning should be considered only after all dust or loose particles have been removed. Proceed with liquid-based cleaning only if necessary, using only high-purity reagent, analytical, or spectrophotometric-grade solvents with low-lint lens tissue, cleanroom wipes, or lint-free Q-tips for small optics. Commonly used solvents for coated surfaces include acetone, methanol, ethanol, 97%-pure isopropyl alcohol, methyl ethyl ketone, and methylene chloride. For uncoated silicon wafers, stronger inorganic acids such as trichloroethylene, hydrofluoric acid, and hydrochloric acid are suitable, and nitric acid may be used on uncoated germanium. However, acidic cleaners should never be applied to coated optics or to zinc sulfide and/or zinc selenide elements, since they can severely damage these optical materials. Each solvent offers its own benefits and drawbacks, though most dielectric multilayer coatings, including ion beam-sputtered coatings, which are particularly dense and durable, are unaffected by ethanol, isopropanol, and/or acetone. Acetone is effective at dissolving grease, but it evaporates quickly and requires users to wear acetone-resistant gloves. Methanol and strong acids, meanwhile, can be toxic or corrosive and must be used cautiously. A 60% acetone/40% methanol mixture is often recommended because it balances grease removal and speed of evaporation. Isopropyl alcohol is a generally safe and effective cleaning agent, but evaporates slowly, which may cause streaking. Also, because alcohol reacts with aluminum, isopropyl alcohol should never be used on bare or protected aluminum-coated mirrors. Spin cleaning, a semiautomated process, begins by spinning the optic slowly. The speed is eventually increased, and a high-purity solvent is applied a few drops at a time. Centrifugal force spreads the solvent and separates loose contamination from the optic. Courtesy of Perkins Precision Developments. Hand-cleaning techniques There are four main hand-cleaning techniques. Selecting the proper one depends on the optic type, size, and contamination level. With each of these methods, it is best to first clean the edges of the component to avoid drawing debris onto polished surfaces. Then wipe slowly to reduce streaking and ensure that the solvents have enough time to evaporate. The drop-and-drag method is the most common hand-cleaning technique. This approach is typically used for light cleaning of unmounted optics. A drop of methanol or acetone is placed on a lens tissue or microfiber cloth, and the wet portion of the wipe is gently dragged across the surface without pressure. This method allows the solvent to evaporate fully without streaks or wipe marks and helps prevent the transfer of polishing compounds or debris from the edges to the center. Importantly, the drop of methanol or acetone should never be deposited directly onto the optic. The brush technique is best for small optics, polarizers, and waveplates. A clean lens tissue is first folded into a “brush” that is wider than the optic being cleaned, and without touching the tissue areas that will contact the optic, the brush is held near the fold using tweezers or a hemostat. It is then dampened with the appropriate solvent solution. After shaking off any excess solvent, the brush is placed on the optic and drawn across the surface using consistent and gentle pressure. The brush should be discarded after one pass. The part is then rotated using tweezers, and the wiping process is repeated until the full surface is clean. The wipe method is often favored for moderate cleaning of more stubborn contaminants, including fingerprints and embedded residues. It is similar to the brush method, but in this technique, the lens tissue is hand-held. Synthetic lint-free swabs can be used for cleaning infrared optics, working from the center outward. For small-diameter or mounted optics, it may be more suitable to “paint” the perimeter and then sweep once across the center in a continuous motion to avoid drying marks. It is important that the swab or tissue be dampened with acetone or alcohol prior to use, and that it be discarded between swipes. Also, avoid back-and-forth scrubbing, as this increases the risk of surface scratches or damage to the coating. For uncoated surfaces with a sensitive or delicate coating on the back side, it is advisable to start with a light methanol wipe or acetone/methanol mixture to remove any loose contaminants that might be present from packaging or handling. Using a clean wipe or Q-tip, a light swab with polishing compound may be necessary to remove any stains or residual material on the surface, followed by a repeat wipe with methanol to remove any remaining compound. This method cleans the substrate surface and removes the polishing compound while drying fast enough to not affect the back of the substrate. A final hand-cleaning approach, immersion cleaning, is useful for the nonabrasive cleaning of protected metal coatings such as silver, aluminum, and gold, since these surfaces can be easily damaged by wiping with pressure. After initial dusting, the optic is immersed in acetone and then rinsed in fresh solvent until clean. Bare metal coatings, random antireflection nano-textured coatings, and other very soft coatings used for filters and deep-ultraviolet optics should not be cleaned or touched in any way. Alternate and emergent cleaning methods Ultrasonic cleaning is an automated, multistep aqueous process typically used prior to coating still-uncoated “hard” glass substrates with a Knoop coefficient ≥500, such as N-BK7, fused silica, and quartz. The procedure is performed in a cleanroom of class 100 or better and consists of a progression of rinses and washes in neutral detergents and high-purity deionized water, followed by a hot air dry. In some cases, a mechanical scrub or bromide degreaser step precedes the cleaning to optimize the efficiency of the ultrasonic cleaning cycle. Table 2. Method and Material Combinations Depending on the size and quantity of substrates, they may be held in stainless-steel basket containers or already mounted in the appropriate coating fixtures to prevent recontamination from excess handling prior to coating. The ultrasonic cleaning method is not generally recommended for use on coated optics or “soft” substrate materials such as calcium fluoride, magnesium fluoride, or flint glasses such as SF11. As with ultrasonic cleaning, spin cleaning is a semiautomated process that uses special equipment to facilitate cleaning. In this case, a motorized spin cleaning stage is used in a cleanroom environment or under a laminar flow hood. Once large particulates are removed, the optic is secured in the spin cleaner using a vacuum chuck or O-ring mount, ensuring it is well centered to prevent movement or wobble. The spin-clean process starts by spinning the optic slowly, for example, at a rate of 100 to 300 revolutions per minute (RPM), and using a filtered nitrogen blow to remove loose dust. The operator then increases the speed to 500 to 1000 RPM while high-purity solvent — typically acetone, methanol, or isopropyl alcohol — is applied a few drops at a time using a pipette or syringe. In this process, centrifugal force spreads the solvent and removes loose contamination. If deeper cleaning is required, the spin cycle can be stopped so that the optical surface can be hand-wiped, then resumed and re-rinsed with solvent. Finally, the optic is spun for 10 to 20 s at ~2000 to 3000 RPM to whirl off any remaining solvent and completely dry the optic. FirstContact represents another approach. This surface contact-based polymer strip cleaner dissolves organic contaminants and encapsulates particulates as it dries to a robust film. Peeling off the polymer film renders the surface clean without residue or nanoparticulates. Preliminary test studies have shown FirstContact to be suitable for use on optics, mirrors, and diffraction gratings, and it can also be used to protect and shield optical surfaces during handling/packaging and transport. Finally, liquid CO2 spray cleaning is a preferred solution for the removal of oils and microscopic debris. Here, purified CO2 is sprayed, followed by warm-air drying. The technique can be used on sensitive surfaces, including optical waveguides, biomedical parts, semiconductor components, and electro-optical devices. Although the initial setup is expensive and requires a moisture-free, controlled environment, the process is nontoxic, noncorrosive, and suitable for high-damage-threshold coatings. Post-cleaning care Surface inspection should be performed during and after cleaning using bright fiber lighting and at various angles to determine whether any streaks or particulates remain. Once clean, avoid additional handling, as this can lead to recontamination. The optic or uncoated substrate should be immediately placed in its final mount or wrapped in fresh, lint-free lens tissue and stored in a sealed, dust-free container. Ideally, store and package coated optics in non-outgassing, noncontact containers such as molded PET-G inserts or clamshells. Proper cleaning of optics is not a one-size-fits-all task. Each substrate material and coating type has specific tolerances, and even seemingly minor mistakes — such as using the wrong solvent or applying too much pressure — can result in permanent damage. By following the correct techniques and minimizing unnecessary handling, technicians can significantly extend the functional life of optical components while preserving their optical integrity.