The facets form a laser barís resonant cavity.
The light shall emit from the bar only at one facet,
with high efficiency. So it is necessary to confine
the light in the bar at one end and encourage
the light to emerge from the other, with minimal loss.
This is achieved by the addition a HR (high-reflection)
coating to retain the light in the bar on one facet and
an AR (anti-reflection) coating on the other facet that
efficiently lets the light out.
The coatings are different for each side of the bar,
so the bars need to be secured in a carrier that will
allow each side to be coated, and then turned over
for the second coating run on the reverse side, all the
while protecting the first side from further flux.