Cleaning germanium lenses elements improves performance, providing proper materials, techniques and handling procedures are used to minimize the risk of damage.
CVI LASER OPTICS
Optics can be contaminated in many ways. Contamination can be kept to a minimum by returning the optics to their case after use or by covering the optics 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.
Inspection of germanium lenses surfaces
During inspection, all optics must be handled in the cleanest area available (preferably a cleanroom or within a laminar flow bench). Proper equipment, such as powder-free cleanroom 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:
• 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.”
• 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.
Cleaning methods
1) Blowing method
2) Drop and drag method
3) Wipe method
4) Bath method
5) Soap solution method
6) Ultrasonic cleaning method
Solvents
Historically, germanium lenses components such as mirrors and beamsplitters have been cleaned by hand, using lint-free germanium lenses wipes and reagent-grade acetone or another liquid solvent such as methanol, ethanol, 97 percent 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 CO2 is a new technique that is used to remove oils and microscopic particles from germanium lenses waveguides, electro-germanium lenses 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 germanium lenses surface. Because CO2 is noncorrosive and relatively nontoxic, it is safer to use than many traditional solvents, but it requires nontraditional 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.
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 cleanroom environment. The room temperature should be kept between 15 and 25 °C (60 to 80 °F). Ideally, humidity should be controlled and kept below 30 percent.
CAUTION: Do not use a polypropylene box. Studies have shown that permanent outgassing of the storage box leads to adsorption of products detrimental to laser resistance of coated optics.
This article comes from photonics edit released