mal recovery period, damage initiated at a
small defect by a high CW fluence will
often spread to include surrounding areas
until the entire region within the high-fluence beam area has been damaged
catastrophically.
Fabrication issues
A variety of defects that can cause laser
damage are introduced at each stage of the
manufacturing process. Thus, it is worthwhile to review in detail the sources of the
defects, as well as the fabrication, cleaning
and coating methods that eliminate them.
One problem that occurs during compo-
nent fabrication is subsurface damage. This
consists of fractures and scratches caused
by the various grinding and polishing pro-
cesses, which become partially or totally
hidden during subsequent fabrication steps.
It has been shown that a thin layer of the
substrate material – the Beilby layer – can
reflow while the piece is being polished,
sealing these defects below the final sur-
face. Subsurface damage can reduce the
damage threshold by providing a place for
light-absorbing contaminants to reside, by
allowing atoms at or near the fractures to
be more easily ionized, and by causing
local intense modulations in the electro-
magnetic field.
Particulates are things like residue from
the polishing process and dust; these are
usually removed with detergent cleaning
chemistries and ultrasonic techniques. Organics are often airborne contaminants or
surface residuals from earlier cleaning and
blocking processes. Typical examples are
human sweat, finger oils, blocking wax,
adhesives and organic cleaning solvents.
These are usually removed with other organic solvents and detergents. The takeaway here is that a variety of cleaning techniques must be employed to thoroughly
prepare a surface for coating.
Control of the production environment to
eliminate sources of contamination is also
critical. The best practice is for the entire
cleaning and coating process to occur in a
cleanroom, rather than under a laminar
flow hood. Furthermore, it is highly advantageous to work with a vendor that can
fabricate and coat in a single facility, eliminating the potential contamination from
packing, shipping and unpacking steps.
High laser-induced-damage threshold optics are produced over a wide range of sizes and spectral ranges.
Coating considerations
Currently, high-damage-threshold coatings are made using a variety of deposition
processes, including various evaporative
techniques and sputtering methods. Most
coating manufacturers tend to employ a
single method and therefore try to convince
customers that their chosen approach is
uniquely suited to delivering high-damage-threshold thin films. At REO, we have ion
beam sputtering (IBS), ion-assisted deposition processes and traditional evaporative
methods at our disposal, so we can afford
to be unprejudiced in evaluating various
coating techniques.
Most of REO’s high-damage-threshold
coatings are produced using IBS because it
yields fully densified films that, after they
are formed, are essentially impervious to
the absorption of contaminants. Also, a
properly managed IBS process creates
films with relatively few internal defects.
Thus, IBS films will generally not damage
readily. However, because they do exhibit
relatively high internal stress, once damage
is initiated in a dense film, it is likely to be
catastrophic.
In contrast, the coatings produced with
evaporative techniques are relatively porous
and contain low internal stress, which
makes damage sites grow more slowly
than in a denser film. But this porosity also
allows higher levels of particulate contamination within the film, as well as entry of
water or volatile organics. A porous film
can absorb airborne molecular contaminants even during use. Furthermore, during
