Cleaning Brewster Windows with Ammonium Bifluoride
CAUTION: only use ABF to clean quartz windows.
DO NOT TRY TO CLEAN COATED OPTICS WITH ABF - ABF will destroy the coating.
Some lasers, especially the air cooled variety, may have coated Brewster windows. You have been warned!
About ABF
Ammonium bifluoride (CAS 1341-49-7)
is an agent which may be used to clean the quartz Brewster windows in a
laser, when conventional methanol cleaning fails. Although it is corrosive
and a stronger cleaning agent than methanol,
its repeated use does not degrade the quartz windows. Tubes show no loss in
performance after hundreds of ABF treatments.
When to use ABF
Observe the Brewster window while the laser is lasing in the visible region.
If the window glows brightly where the beam strikes it, and the laser mode
or output power is not good, then it is necessary to clean the window.
First try to clean the window with methanol (procedure). If this is unsuccessful, try using ABF as detailed below.
Note that when viewing the window, if you can see bright areas inside,
it may be necessary to have the tube disassembled for cleaning or rebuilding.
Cathode sputtering may cause deposits inside the nearest window. The inside
of Brewster windows are not user serviceable!
Materials
You will need the following. Assemble everything ahead of time.
- Solution of 3 grams ABF crystals in 25 ml deoinized water in a plastic container.
- Methanol (reagent grade or better)
- Deionized water
- Lens tissues
- Wooden shafted cotton swabs ("Q tips")
- Hemostat(s)
- Paper towels
- Squirt bottle
- Safety equipment - rubber gloves, goggles, lab coat
Procedure
See the safety notes below.
CAUTION: only use ABF to clean quartz windows. DO NOT TRY TO CLEAN
COATED OPTICS - ABF will destroy the coating. You have been warned!
Do not allow ABF solution or water to come into contact with the
intracavity beam seal ("dust shield").
- Turn off laser system.
- Slide dust shield away to expose Brewster window.
- Place several paper towels beneath window to be cleaned.
- Put two to three drops of ABF solution on cotton swab. Do not soak.
- Apply the side of the cotton swab to the edge of the window closest
to the tube. Applying an even pressure, move swab across surface of window
while rotating swab in a direction opposite to that in which it would
naturally rotate (this is to lift deposits off the window and ensure that
a new area of swab is always in contact with the window).
- Limit exposure of window to ABF to 30 seconds. Do not allow ABF to dry
on the window.
- Squirt deionized water over the window, or apply with several cotton swabs
to remove all residual ABF solution
- Dry window carefully with lens tissue. Clean the stem with methanol.
- Clean the window with methanol using the standard cleaning procedure.
- Slide dust shield back into place
If there is excessive contamination on the Brewster window, it may be necessary
to repeat the above procedure several times.
Safety
See the MSDS for ammonium bifluoride. Wear gloves, eye protection and a lab coat. Do not
get ABF solution on yourself or your clothing. Wash hands thoroughly after procedure.
FIRST AID
- In case of contact: Flush eyes
or skin immediately with plenty of water for 15 minutes while removing contaminated clothing.
- If swallowed, do not induce vomiting. If conscious,
give tap water, milk or Milk of Magnesia.
- Call a physician.
This information was abstracted from a Coherent field service bulletin
(No. 136, June 28 1989).
jdc 960130; 961222
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