Frequently Asked Questions
1. Hard coated fused silica optics
The coatings on the silica optics commonly used with high power Nd:YAG lasers are very durable and usually harder than the substrates themselves.
- Completely remove all excessive amounts of oil, grease, dirt and other contaminants. This may be done by washing in soapy warm water using a mild liquid detergent, or acetone. Gently flush the part with the solution being careful not to abrade the surface. A finger inside a latex glove is a useful tool for stubborn marks. When complete wipe with clean white tissue.
- After the above preparatory cleaning, (which is not always necessary), the optic is ready for a final cleaning. Acetone and the 'drop and drag' technique are recommended. Always hold the optic at its edge or place on a clean drywork surface on a lens tissue. Hold a piece of lens tissue above the mirror and place a few drops of acetone on the paper. Lower the lens tissue onto the optic and pull it over the surface. Note that the dry part of the tissue helps remove acetone residue. Repeat the above step until the optic is clean. Do not re-use the lens tissue.
Some people use ethyl or methyl alcohol in place of acetone. This is acceptable but care should be taken to prevent the solvent reaching the fingers as it may dissolve oils which can be deposited on the optic. In addition alcohol evaporates more slowly than acetone and generally leaves more residues on the surface.
2. Metal optics and metal coated optics
These generally come in three types; bare metal, coated metal (usually with gold) and enchanced coatings (dielectric coatings - e.g., phase retarders or circular polarising optics orsilicon mirrors).
Bare metal optics (usually copper) can be cleaned if in a very dirty condition using liquid proprietary metal polishers. Again a finger inside a latex glove is a suitable tool for removing stubborn marks. After this type of cleaning, washing with acetone is recommended to remove all traces of the liquid cleaner. Complete the procedure by wiping with a clean lens tissue as in 1b) above.
Soft gold coated mirrors have a very delicate surface and should not be cleaned using the above techniques. Non-contact methods such as flushing with acetone followed by applying dry nitrogen from a spary canister, are recommended.
Hard gold coated mirrors have a more resilient surface and should be cleaned using the techniques outlined in Section 1. Phase retarding mirrors (either with Cu or Si substrates) and silicon plane mirrors, all contain a dielectric layer(s) on the surface. This layer should not be allowed to come into contact with water, as this can lead to 'peeling' of the coatings. For cleaning these optics the procedures described below for ZnSe optics should be used.
3. Zinc selenide focusing lenses
- Surface dust or particles should be blown from the component using a rubber blower, ideally one with a non-return valve.
- Follow the procedures in 1b) using acetone and a clean lens tissue.
- Stubborn stick on particles may require an attempt to dislodge them by the local application of a cotton bud moistened with acetone.
- Repeat b) above.
- Small components should be handles with latex gloves or finger cots to avoid finger prints.
- Biological stains (breathing, sneezing etc.) cannot be removed with acetone. Distilled water should be applied and the optic dried before proceeding to step b) above.
Laser optics can and should be cleaned as often as necessary and the amount of cleaning is usually determined by the environment in which the components are used. Neglect in this area can be expensive in terms of performance and cost.