Be the first on your block to own a laser polisher!
By Scott Wilson
Sound a bit crazy? Think again! With the proliferation of laser-this and
laser-that widgets that seem to pervade our lives, who in their right mind
would consider laser polishing? Well, it really is not as far out as you might
think. This topic appears to have a reasonably well established body of literature
in the scientific community, where it has been applied to the final polishing
process for precision optical components made from diamond, fused quartz,
and other materials.
The earliest work I found was done in our own backyard by scientists at
Kirtland Air Force Base in Albuquerque. Studies examined the effect of
illuminating a pre-polished surface with a high-powered carbon dioxide laser.
It was found that if the laser intensity was high enough, then sufficient
power could be absorbed in the surface to cause partial melting and a resultant
smoothing of the surface. This achieves much the same effect as standard
polishing. It was also unfortunately found that, by doing this, sufficient
enough stress was “frozen” into the sample that would allow fracture sometime
in the future, as the stress drove the propagation of minute internal cracks.
Although these results were a mixed bag, science did not stand still. More
work has been published on this topic, as additional materials have been studied
for laser polishing and other strategies tried.
The smoothing or polishing of diamond films and substrates  appears to
have attracted some degree of interest, mostly from the aerospace electronics
community. Diamond is vastly more resistant to thermal damage than typical
optical materials, and the laser polishing approach appears to have worked
quite well. In this case referenced, scientists in Switzerland and from the
General Physics Institute in Moscow used an ultraviolet laser at a glancing
angle of incidence from different directions to process pre-polished samples.
Surface quality was apparently nearly as good as the best mechanical/abrasive
polish. I cannot say if laser polishing is being used in commercial diamond
processing. If it is not, then I would expect to see at least some use of
it in the coming years, particularly for large volume production, where it
would provide a significant economic advantage.
Other work in this area has addressed the fracture issue that plagued early
work. The approach taken was to raise the temperature of the sample sufficiently
that the stress induced by the laser polishing was immediately relieved. This
technique was applied, with some success, to materials similar to many gem
materials that exhibit a high temperature coefficient of expansion. Surface
roughness was found to decrease from an initial value of 500 nm RMS to 1 nm
RMS, considered to be a very high quality surface.
I would NOT want to heat up a prize tourmaline or emerald to the temperature
required for laser polishing for fear of damaging the crystal. Mineral inclusions
within a crystal expand upon heating and may explode, and water inclusions
generate steam explosions when heated. Many other gem species would fall in
this same category. However, it just might work for gems like sapphire, ruby,
and other high temperature oxides, particularly those that commonly undergo
serious heat treatment to enhance color.
Feasible laser polishing of gem materials will take some time, but someone
will eventually accomplish it. Given the rapidly falling price of laser technology,
someday we gemcutters will be able to purchase a tabletop laser polishing
attachment to accompany our faceting machines. What a remarkable concept!
1. Durn. Cannot find the reference. Sometime around 1987. Alan Stewart and
others, Boulder Laser Damage Symposium. I still have some of the samples.
2. A.V. Khomich, V.V. Kononenko, S.M. Pimenov, V.I. Konev, S. Gloor, W.A.
Luethy, H.P. Weber, “Optical properties of laser-modified diamond surfaces”,
Proc. SPIE Vol. 3484 (1998), p166-174. ISBN 0-8194-2942-2
3. F.L. Laguarta, N.B. Lupon, F. Vega, J. Armengol, “Laser application for
glass polishing”, Proc SPIE Vol 2775 (1996), p603-610. ISBN 0-8194-2160-X