Dave wrote:
just because a source of photons is coherent doesn't mean the photons are
collimated.

One of the requirements of wave cancellation is
collimation and it happens automatically within
a coaxial transmission line. A laser beam is
also relatively easy to collimate. Because of
that, it makes a good example source for a
1/4WL thin-film anti-reflective coating.
--
73, Cecil http://www.w5dxp.com