CLEANING PROCEDURES FOR OPTICS
 

Excerpts from Directed Light catalogue:
 

'Some good practice tips
1.    Reagent grade methanol is a high purity chemical. The dispensing container should be rinsed out with the
      methanol before filling. Never insert lens tissue, cotton swabs, fingers, etc., into the methanol. Always
      drip the methanol onto the material. This eliminates contamination of the methanol or its container.
2.    Lens tissue, when dry, is very abrasive to optical surfaces and will scratch them.
3.    Always hold optics by their edges.
4.    Once an optic is cleaned, install it or keep it covered. This will ensure that airborne particles will
      not have a chance to settle on the optic.
5.    Store your cleaning materials in a clean, covered area.'
 

Notes on the Cleaning of Laser Optics from Specta Physics 3900 CW Ti:Sapphire Laser Manual:
'Ion lasers are oscillators that operate with gain margins of a few percent. Losses due to unclean optics, which might be negligible in ordinary optical systems, can disable a laser. Dust on mirror surfaces can reduce output power or cause total failure. Cleanliness is, therefore, essential. The maintenance techniques used with laser optics must be applied with extreme care and with attention to detail. "Clean" is a relative description; nothing is ever perfectly clean, and no cleaning operation ever completely removes contaminants. Cleaning is a process of reducing objectionable materials to acceptable levels.

Since cleaning simply dilutes contamination to the limit set by solvent impurities, solvents must be as pure as possible and leave as little solvent on the surface as possible. As any solvent evaporates, it leaves impurities be hind in proportion to its volume. Avoid re-wiping a surface with the same lens tissue; a used tissue and solvent will redistribute contamination, they won't remove it.
 

Always use fresh solvent. Both methanol and acetone collect moisture during prolonged exposure to air. Avoid storage in bottles where a large volume of air is trapped above the solvent. Instead, store solvents in small glass bottles where either the solvents are used up quickly or the bottles are filled frequently from a fresh, uncontaminated source. Laser optics are made by vacuum depositing micro thin layers of materials of varying indices of refraction on glass substrates. If the surface is scratched to a depth as shallow as 0.01mm, the operating efficiency of the optical coating will be reduced significantly. Because an intra-cavity passive catalyst is used in the Chroma 5 laser, there should be little requirement for optical cleaning if the cavity is left closed and undisturbed. However, because optics are exchanged from time to time, there will be occasions when cleaning is necessary. Stick to the following principles whenever you clean any optical surface.
 

Model 3900S CW Ti:sapphire Laser
1.  Remove and clean one optic at a time, then replace it and optimize laser output power. If more than one
    optic is removed at time, all reference points will be lost, making realignment extremely difficult.
2.  Perform any maintenance in a clean environment, over an area covered by a soft cloth or pad, if possible.
3.  Wash your hands thoroughly with liquid detergent. Body oils and contaminants can render
    otherwise fastidious cleaning practices useless.
4.  Use dry nitrogen, canned air, or a rubber squeeze bulb to blow dust and lint from the optic surface before
    cleaning it with solvent. Permanent damage may occur if dust scratches the coating.
5.  Use spectrophotometric-grade (HPLC) solvents.
6.  Do not try to remove contamination with a cleaning solvent that may leave other impurities behind.
7.  Use powder-free, clean latex gloves or finger cots.
8,  Use Kodak Lens Cleaning Paper or equivalent to clean optics and plasma tube windows.
9.  Use each piece only once: a dirty tissue merely redistributes contamination, it does not remove it.
 

Equipment Required
Some of the following are supplied in the accessory kit.
a.  Dry nitrogen, canned air, or rubber squeeze bulb
b.  Plastic hemostat
c.  Clean (new) finger cots or powder-free latex gloves
d.  Kodak Lens Cleaning Paper or equivalent

Do not use lens tissue that is designated for cleaning eye glasses. Such tissue contains silicones. These molecules bind themselves to the mirror coatings and window quartz and can cause permanent damage. Also, do not use cotton swabs, e.g., "Q-Tips". Solvents dissolve the glue that is used to fasten the cotton to the stick, and the result is contaminated coatings. Only use photographic lens tissue to clean optical components.

General Procedures for Cleaning Optics
Cleaning Solutions Required: Spectrophotometric-grade (HPLC) acetone or methanol
The laser should be on and stable. Remove, clean, and replace one optic at a time, maximizing laser output power after each optic is cleaned. Use clean finger cots to protect all intracavity components, including the coated mirror surface. Do not allow the cavity to remain open for very long. The intracavity passive catalyst that reduces O3 in the cavity will become contaminated and, therefore, ineffective. Keep the output coupler and high reflector locked in place, and make sure the tubular cavity seals are in place. Following these simple rules will ensure a long catalyst life. The high reflector can be left in its holder for cleaning. However, the output coupler (OC) and the beam splitter must be removed from their holders to clean the second surface.
 

Warning! Methanol tends to clean better but, if not fresh, may deposit a water based film on the surface being cleaned. If this occurs, follow the methanol wipe with an acetone wipe to remove the film. As always, use fresh solvent from a bottle with little air in it.
 

1.  Use a squeeze bulb, dry nitrogen, or canned air to clean away any dust or grit before cleaning optics with
   solvent.
 

2.  Drop and Drag: Whenever possible, clean the optic using the 'drop and drag' method.
3.  Hold the optic horizontal with its coated surface up. Place a sheet of lens tissue over it and squeeze
    a drop or two of acetone or methanol onto it.
4.  Slowly draw the tissue across the surface to remove dissolved contaminants and to dry the surface.
5.  Pull the tissue slow enough so the solvent evaporation front immediately follows the tissue,
    i.e., the solvent dries only after leaving the optic surface.
6.  Only use spectrophotometric-grade acetone and/or methanol. Using lower grade reagents may lead
    to contamination of or damage to optical coatings.
7.  For stubborn contaminants and to access hard-to-reach places (such as the windows), use a tissue in a
    hemostat to clean the optic.

Lens Tissue Folded for Cleaning
a.  Fold a piece of tissue in half repeatedly until you have a pad about 1cm (0.5") square, and clamp it in a
    plastic hemostat
b.  If required, cut the paper with a solvent-cleaned tool to allow access to the optic.
c.  Saturate the tissue with acetone or methanol, shake off the excess, re-saturate, and shake again.
d.  Wipe the surface in a single motion. Be careful that the hemostat does not touch the optic surface, or the
    coating may be scratched.
e.  After placing the optic you just cleaned into the beam, inspect it to verify the optic actually
    got cleaner, i.e., you did not replace one contaminant with another. While folding, do not touch
    the surface of the tissue that will contact the optic, or you will contaminate the solvent.
f.  Don't Touch!
 

Steps for Cleaning Optics
1.  If necessary, remove the optic to be cleaned from its holder to gain access to optic surface(s).
2.  For easy access optics, hold the optic horizontal with its coated surface up, place a sheet of lens
    tissue over it, and squeeze a drop or two of acetone or methanol onto it. Gently draw the tissue across
    the surface to remove dissolved contaminants and to dry the surface.
3.  If stubborn contaminants remain, use the 'Tissue in Hemostat' procedure described above to clean
    the surface.
4.  If the optic is an output coupler, invert the optic and repeat the procedure to clean the second surface.
5.  For the Brewster windows, use the 'Tissue in Hemostat' procedure, but cut the tissue to fit within the
    Brewster window slot using clean scissors or wire cutters.
6.  Remove, clean and install mirrors one at a time to avoid accidental exchanges and misalignment.
    Close the pump laser shutter while optics are being changed. Further, adjust the laser for maximum output
    after cleaning each mirror.

Cleaning a Birefringent Filter
Typically, only the upward-facing surface of the birefringent filter tends to get dusty, and it may be cleaned without removing the filter from the system. If the lower surface requires cleaning, remove only the filter stack from its mount. Note which color dot is aligned with the Allen screw in the mount, loosen the screw using the wrench provided in the laser tool kit, and gently pull the filter out. Clean the surface(s) with a folded, acetone-saturated tissue clamped in a hemostat. Use very little pressure; the filter is made of thin material and can be very easily damaged. If you removed the filter from its holder, make sure to install it with the appropriate dot aligned with the setscrew. Do not over tighten screw. If the birefringent filter assembly is carefully aligned at the factory to orient the filter plates at Brewster's angle. Never remove this assembly from the baseplate. The filter stack should never be disassembled to attempt to clean the inner surfaces.'