White Paper Cleaning and Storage Procedure for Optics and Objectives CLEANING PROCEDURE Optics can be contaminated easily in many ways, however, it can be kept to a minimum by returning the optics to their case or by covering the facing side for protection from the outside environment. However, even with all these precautions, the optic will eventually accumulate dust, stains or some other form of contamination while in use. INSPECTION OF OPTICAL SURFACES During inspection, all optics must be handled in the cleanest area available (preferably a clean room or within a laminar flow bench). Proper equipment, like powder-free clean room gloves or finger cots must be worn at all times to avoid grease and oils from being transferred to the optic. Lens tissue paper, dust free blowers, hemostats, cotton swabs, cotton tips, and reagent-grade acetone and methanol, will all be needed for cleaning optics. The acetone and methanol must be fairly fresh to avoid leaving any marks on the optics. Reagent-grade Isopropyl alcohol can also be used instead of acetone. Pro Tip: Clean optics against a dark background so dust can be seen and eliminated more efficiently. There are two ways in which an optic can be evaluated: i.) If the optic is being used in a laser based system, contamination on the optic might cause the optic to scatter the laser light, thus reducing power and making the optic glow. ii.) An optic can also be visually inspected by holding it below a bright light source and carefully viewing it at different angles. This will cause the light to scatter off the contamination enabling the viewer to see the various stains and dust particles. Scratches can result from improper cleaning methods Precision optics should be handled properly while cleaning. Here, the prism is held on the frosted surfaces.
CLEANING METHODS 1- Blowing Method Avoid touching the surface of the optic with unprotected fingers! Always wear powder-free gloves or finger cots. The first step in cleaning any kind of optic is to remove dust or loosely held particles by blowing them off the surface using a dust free blower (use a dry nitrogen or CO 2 blower) or a camel hair brush. This reduces the chance of scratching the optic during the actual cleaning. Other pressurized gas products are not recommended because the propellant could be released onto the surface in the form of liquid droplets. NOTE: This is the only cleaning method allowed for bare metal, soft coatings, and pellicles. These types of surfaces should never be touched. 2- Drop and Drag Method For this procedure the optic will have to be removed from any mount it is in, and placed in a clean room environment. In many cases it is beneficial to clean the edges of the optic first to prevent dirt or excess polishing compound from being drawn onto the polished surface. 1. Use the Blowing Method first. 2. Place a drop of methanol or reagentgrade acetone on the side of a lens tissue paper. With the other hand, hold the optic by its edges. 3. Gently place the wet area of the tissue onto the optic. 4. Slowly drag the lens tissue across the surface applying equal pressure until the lens and tissue are dry. 3- Wipe Method If the optic cannot be taken out of its mount, or if more intensive cleaning is required, then the next step will be to wipe using lens tissue paper along with methanol and acetone. 1. Use the Blowing Method first. 2. Fold a piece of lens tissue paper to create a folded edge that is a little longer than the size of the optic and hold securely with a hemostat. 3. Wet the folded edge of the tissue with acetone. 4. Wipe the optic with the lens tissue paper with one continuous motion. Apply some pressure on the wipe to remove stubborn stains. CAUTION: It is important that the tissue paper be dampened with acetone or alcohol before use and that the surface not be scrubbed back and forth, which would increase the likelihood of surface scratches or damage to a coating. NOTE: Repeat this process, always with new lens tissue paper, to eliminate redepositing any contamination on the optic. A final wipe with methanol is recommended since methanol does not leave streaks on the surface. It is usually better for cleaning laser optics that are exposed to particles in an uncontrolled environment or outside of a vacuum chamber. If the size of the optic is very small, cotton tips can be used instead of lens tissue paper with the same procedure. 5. If required, repeat method with a fresh lens tissue paper. CAUTION: Be careful not to drag a piece of dirt across the surface as this could scratch the surface. 2 Prototyping to Production
4- Bath Method Small optics or optics that require milder cleaning than the wipe method can be cleaned by the immersion method. 1. In this method a Petri dish is lined with lens tissue paper and filled with methanol. 2. Once in the Petri dish, clean the optic gently with a piece of soaked cotton ball. 3. Once wiped, agitate the Petri dish for a few minutes. 4. Repeat steps 1 through 3 with acetone instead of methanol. 5. Next, remove the optic from the Petri dish and blow dry. 6. If stains occur because of the blow-drying use the drop and drag method described above. CAUTION: Do not use this method with cemented or mounted optics. Metallic coatings vary in hardness and durability and so should be treated with extreme caution. 5- Soap Solution Method If the optic contains contamination in the form of grease (fingerprints, etc.), methanol and acetone will tend to simply re-distribute the grease. In this case, the optic will need to be cleaned with a soap solution. The soap solution should be a nonabrasive, mild solution without any additives (green soap works well for this). 1. Use the Blowing Method first. 2. Use a soap solution and clean with either the Wipe Method or the Bath Method. 3. Rinse thoroughly with de-ionized water in a Petri dish. 4. Cleaning with methanol and acetone (Wipe Method or Bath Method) should then be next. 5. After rinsing, blow dry with a dust free blower to remove all water. 6. Ultrasonic Cleaning Method One other cleaning option is to use an ultrasonic bath. This method is the same as the Bath Method, except all the Petri dishes are placed in an ultrasonic bath for about one minute. Please note, that even with dielectric coatings, the use of the ultrasonic bath may cause the coating to flake at the sides where it has the weakest bond. Ultrasonic cleaning is an automated multistep aqueous process for the cleaning of substrates prior to coating. CVI Laser Optics 3
SOLVENTS Historically, optical components such as mirrors and beamsplitters have been cleaned by hand, using lint-free optical wipes and reagent-grade acetone or another liquid solvent such as methanol, ethanol, 97% pure isopropyl alcohol, methyl ethyl ketone (MEK) or methylene chloride (MEC). Some inorganic acids such as trichloroethylene (TCE), hydrofluoric acid (HF) and hydrochloric acid (HCl) may be used on uncoated silicon wafers, and nitric acid may be used on germanium substrates. Acidic solutions, however, should never be used on coated or uncoated zinc sulphide (ZnS) or zinc selenide (ZnSe) components. Acetone is very good at dissolving grease, but it dries very quickly and always should be handled with acetone-impenetrable gloves. In general, isopropyl alcohol is a safe and effective cleaner except for cleaning aluminium coatings. Because alcohol reacts with aluminium, it should never be used on protected or bare aluminium-coated mirrors. Methanol and most acidic solutions can be toxic or damaging to optics or coatings if misused, so care should be taken to follow the instructions provided by the manufacturer. Liquid CO 2 is a new technic that is used to remove oils and microscopic particles from optical waveguides, electro-optical devices, silicon wafers and a variety of biomedical, aerospace and semiconductor components. This process delivers a precisely controlled and purified spray alternated with warm air cycles to the optical surface. Because CO 2 is noncorrosive and relatively nontoxic, it is safer to use than many traditional solvents, but it requires non-traditional procedures and a controlled, moisture-free work environment and so may incur additional expenses. In the long term, however, it may turn out to be a less expensive and a more effective means of achieving ultraclean surfaces, possibly resulting in coatings with higher damage thresholds. COMBINATIONS TO AVOID Solvent Alcohol Acids (e.g. HF, HCl) MEC, MEK, TCE All Ultrasonic cleaning Optic Type Aluminium coating or substrate Coated optics ZnS, ZnSe VIS and UV optical glasses (e.g. N-BK7, Fused Silica, CaF2, MgF2) Coated optics Bare metal coatings (e.g. gold, copper, aluminium) Metal coatings Soft optical glasses (e.g. CaF2, MgF2, N-SF11) METHODS AND APPLICATIONS Method 1-Brush or Air Blower 1-CO2 clearing 2-Drop and Drag Method 3-Wipe Method 4-5-Immersion Cleaning 6-Ultrasonic Cleaning Application Large, loose dust particles on most coated or uncoated optics Spray cleaning on uncoated optics, fibers, crystals and polymers Light cleaning of dust and lint on most coated or uncoated optics Moderate cleaning of fingerprints or stains on substrates or hard coatings Nonabrasive cleaning of soft and protected metal coatings Multistep cleaning cycle on hard glass types (prior to coating) 4 Prototyping to Production
WHICH CLEANING METHOD FOR WHICH OPTIC? IDEX Optics and Photonics supply various types of substrates and coatings to our customers. Below is a list of these types, along with the recommended type of cleaning. Optic / Method 1- Blow Bare metallic substrates and deep-uv coatings 2- Drop and Drag 3- Wipe 4- Bath 5- Soap Solution 6- Ultrasonic Other bare substrates Spherical Ball Lenses and Gradient Index Lenses Cemented optics Single layer MgF2 coatings MAX Brite coatings Laser coatings (AR, HR) Protected Aluminium coatings Protected Silver and Protected Gold coatings Pellicle Beamsplitters * ** Plate Beamsplitters Cube Beamsplitters Retardation Plates Plate Polarizers Ultrafast coatings Interference Filters Color Glass Filters Neutral Density Filters (metallic) Neutral Density Filters (absorptive) High Energy coatings Etalons * Use cotton tips instead of lens tissue paper ** Blow gently, do not touch pellicle surfaces! STORAGE CONDITIONS Once you have cleaned the optic, place it in the mount it will be used in or wrap it in lens tissue and place it in its container right away. The proper container to use is a polycarbonate/ptfe/pet-g box, in a clean room environment. The room temperature should be kept between 15 C and 25 C (60 F to 80 F). Ideally, humidity should be controlled and kept below 30%. CAUTION: Do not use polypropylene boxes. Studies have shown that permanent outgassing of the storage box lead to adsorption of products detrimental to laser resistance of coated optics. For more information please contact CVI Laser Optics www.cvilaseroptics.com ACKNOWLEDGEMENT: Thank you to Floriane Teofanescu, Technical Sales Representative, for her contributions to this article CVI Laser Optics 5
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