Caring for your optics
The goal of this article is to educate and inform the end user of the best methods to clean and maintain optics using industry proven best practices. Optics are considered as consumable items, but there is no reason why optical care should not be of up most importance. Following this guide is a fool proof way of ensuring maximum lifetime and optic health.
Consequences of contaminated mirrors & optics
Contaminated mirrors will reduce the reflectivity of mirror resulting in power loss. Secondary problems are caused by the mirror absorbing the laser energy, this causes the mirrors to heat up. In extreme cases this can cause the mirror to distort, warp or even crack.
Contaminated optics Contamination due to foreign materials on the optic’s surface includes dust, oil, grease, fingerprints, and hydrocarbons. These contaminants, if deposited on the optic’s surface, may lead to absorption and shorten optic lifespans and efficiency.Localized heating, caused by contamination, can lead to “thermal runaway” in high-power laser optics. High temperatures create an increase in free carriers within the bulk material which increases absorption. This process reaches an avalanche state, and thermal runaway commences at > 50° C for Ge, and > 200° C for ZnSe and GaAs.Surface imperfections also cause absorption and can include:
- Pits or digs
- Imbedded polishing abrasives
- Pinholes in coatings
- Inclusions in coatings
These surface defects act as damage sites which suffer degradation due to intense perturbations in the electric field surrounding the sites.
Absorption Effects in CO2 Lasers
The CO2 wavelength absorption level coupled with the optic thermal conduction characteristics and its mount are important in determining the laser system’s performance and optic’s lifespan.While the source and control of factors contributing to absorption are complex, the results are clear and include:
- Decreased output power
- Fluctuations in output power
- Mode instability
- Focal point drifting
- Coating failures
- External cavity optics failures (due to output coupler thermal lensing or beam delivery system contamination)
All these failure mechanisms are the result of thermal lensing (the actual change of an optic’s physical characteristics due to absorption).The thermal lensing effect on the beam mode is increased further by a change in the material’s refractive index due to temperature. This latter and more significant effect induces additional optical distortion in the transmitted beam.
Often overlooked when handling your optical components, contaminants to an optical surface will cause the optic to absorb more of the laser source energy, this will drastically shorten its life expectancy and power transmission characteristics’.
- Always use rubber gloves when handling optics, ensure clean and grease free.
- Use a valve lapping suction cup to handle len's, this prevents having to touch the component and risk exposure.
- Never touch optics on their coated surface, always handle edges.
- Optics should be placed on clean lens tissue when cleaning and inspecting.
- Do no breath, or speak over the optic.
The desired outcome is to remove contamination from the optics surface without damaging the coating. You should always start by using the least intrusive method and work up in intensity until the desired outcome is reached.
- Use an air bulb to remove loose contaminates and debris. Do not use compressed air as it contains water and oil which will contaminate the optic.
- Use a spectroscopy grade acetone; this will minimize the risk of a streak free finish. Dampen a cotton swab with the acetone, apply light pressure and a circular motion to the optic. Replace swab frequently to prevent scratching optic with abrasives removed from the optic. Finish by using parallel strokes to minimize streaking.
- If step 2 failed to remove all surface contamination the next step is acetic acid. This will dissolve some contaminants whist not harming the surface coating. Use lab grade acetic acid diluted to 50% strength). If you cannot obtain acetic acid, it can be found in household white vinegar (6% acetic acid). Follow the same procedure as step 2 using acetic acid, and finish with the same procedure as step 2 with the acetone solution to remove all traces of Acetic.
- If your optic still contains contaminants all hope is not lost. An optical polish can be performed using fine-grade 0.1 µm alumina polishing compound. Apply to a cotton ball, use a slow overlapping circular motion similar to polishing a car but without any pressure. This technique uses mechanical abrasion and performing for any longer than 30 seconds is not recommended.Rinse the lens with distilled water to remove traces of polish.Finish by following step 2 but with isopropyl alcohol as it pick up the remaining polish.Step 2 should then be followed using acetone.
Cleaning does not guarantee all contaminants to be removed, if you find your optics to still contain contaminants it is recommended that your optics should be replaced.
Following the advice given above in the handling section is advised when installing your optics.
The final focus lens should always be fitted with the convex surface facing the final mirror. The second surface is either concave or plano, this should face the work piece. When len's are incorrectly installed the focus spot size is larger, and the focal distance will change. This ultimately results in a large kerf and reduced cutting speeds.
When installing mirrors ensure the correct coated surface is facing the other mirrors or lens.
Following this guide will improve performance and extend your optics life.
Thinking about upgrading your optics?
Acetone, 99.6%, for spectroscopy ACS
Cotton swabs and balls
You should be able to obtain these locally.
Powder-Free Nitrile Exam Gloves
0.1 µm alumina polishing compound
Rubber suction cup heads
Warning: BEFORE PURCHASING STORING AND HANDLING CHEMICALS FAMILIARIZE YOURSELF WITH THE MSDS / SDS DATA SHEETS AND BE AWARE OF THE DANGERS INVOLVED