Re: Laser question (long)

[brian whatcott <inet@INTELLISYS.NET>]

At 23:24 12/13/99 -0500, Tim Sullivan wrote:
> I hesitate to step in when a real expert might appear, but...

I know exactly what Tim means here - I recall mentioning interference
coatings some months ago on the list, in a way an engineer with some
background in transmission lines might write.

 But in following up on the topic, I came across a much better
article in EE Times that *doesn't* miss the point! (I'll quote it later)

> I worked with an Ar ion laser last year. There the mirrors where removable
> for cleaning. If you removed the rear cover, laser light shone out the back
> end as well./// while you would like the back mirror to be perfectly
> reflective, real interference mirrors are not.///
> Interference mirrors are still more reflective than
> silvered mirrors, but the non-reflected energy when using a silver mirror is
> taken up in heating the mirror and is thus not visible.///

> Tim Sullivan

I see that vendors are currently offering laser mirrors of two types:
the specular mirrors with aluminum, gold and silver surfaces with or
without hard coats, and the dielectric mirrors of the type Tim is referring
to.

The point about reflection at any angle, I completely missed earlier-
this might interest you (from Electronic Engineering Times).

------------------------------------------------------------------------

Photonic lattices stitch together new class of reflectors
=========================================================
http://www.eet.com/story/OEG19990520S0016

MINSK, Belarus — Materials research groups around the world are
building lattice crystals with properties of near-perfect reflection.
They are proving that dielectric stacks can be engineered to reflect
light effectively over all possible angles of incidence, enabling a
new class of nearly ideal reflectors. The novel materials systems
could be applied to a wide variety of technologies, since reflective
surfaces are crucial both to conventional optics and to such advanced
devices as laser diodes.

"In a simple planar geometry, the structures can be used as filters;
as mirrors to improve the performance of devices such as
vertical-cavity, surface-emitting lasers; or as optical switches and
shutters," said Dmitry Chigrin, a researcher at the University of
Essen (Essen, Germany). "By rolling the structures into hollow fibers
or tubes, the coatings can be used as inside walls of high-finesse
waveguides and microcavities."

Chigrin's research group, working with research teams at the National
Academy of Sciences and Belarusian State University (Minsk, Belarus),
has both theoretically and experimentally verified a
stacked-dielectric structure for optical frequencies. The reflective
capabilities of dielectric stacks have simultaneously been discovered
and explored by researchers at the Massachusetts Institute of
Technology (Cambridge, Mass.), where an infrared version of a device
has been demonstrated, as well as at the University of Bath
(England).

Sergey Gaponenko, head of the research group at the Institute of
Molecular and Atomic Physics of the Belarus National Academy of
Sciences, considers the new structures important for both optics and
optoelectronics. "This is an excellent example that new discoveries
can be found in the very classical and canonical field of wave
optics," he said.

The main advantage of the wide-angle reflectors, according to
Gaponenko, is that they do not dissipate energy, whereas metallic
mirrors both reflect and absorb light. "Metallic mirrors heat up and
are destroyed when exposed to some high-power fluxes," he explained.

The new approach to building reflective surfaces is similar to Bragg
reflectors, which similarly comprise dielectric stacks, in a
technique that has become essential for confining light in the
reflective cavities of microscopic diode lasers.

The new structures are composed of quarter-wavelength-thick layers
with differing refractive indices. Light at the design wavelength
enters the stack from above, at right angles to it. As it proceeds,
it is reflected from each of the layer interfaces as a result of the
refractive-index differential.

The reflection is reinforced by the optical-path differences among
the reflected beams. Generated by the different distances that each
beam travels before being reflected, as well as by phase changes
introduced at some of the layer boundaries, the path differences
cause the reflecting beamlets to interfere constructively. Thus, the
beam as a whole is reflected.

The quarter-wavelength geometry of the structures is vital for the
scheme to work properly, since the reflection drops off radically as
soon as the incident angle is changed. That has meant that engineers
have had limited options for applications where metal mirrors are not
suitable but wide-angle reflection is necessary.

The Belarusian/German team designed its dielectric stack as a
one-dimensional photonic crystal — a structure in which the
propagation of particular electromagnetic waves is not allowed,
because of forbidden bandgaps that are analogous to electron bandgaps
in semiconductors. The bandgaps can be opened or closed based on the
stack design. For light within the bandgaps, beams coming from any
possible angle of incidence will be reflected.

According to Chigrin, the first photonic crystals produced in Minsk
were accidents. "I was working with Andrei Lavrinenko at the
Belarusian State University when, at the very beginning of 1998, we
realized that we had found something new," he said. The colleagues
were working with anisotropic one-dimensional photonic crystals and,
occasionally, they would hit on the right parameters — just a
large enough index contrast between the layers — to create the
photonic-bandgap effect.

"I was really surprised to notice the direct evidence that such a
lattice can reflect light at all angles," said Chigrin. "It was
amazing and very confusing because the effect was long thought to be
impossible. Almost any review of work in the field claimed it was
impossible to achieve omnidirectional total reflection in
low-dimensional dielectric structures. So, at first, we just checked
the simplest case — a dielectric lattice of isotropic layers
— and it worked."

Over the next few months, he said, the researchers probed to find
holes in their theory, not believing that so straightforward an
effect could have been overlooked for so long.

Crystal demos

"In May we presented our work to Sergey Gaponenko's group seminar,"
said Chigrin. "He had agreed with our theory and proposed to carry
out an experiment to demonstrate it."

Several photonic crystals have subsequently been simulated and built.
One 19-layer device has demonstrated total omnidirectional reflection
in the low 600-nm (red) range.

According to Jonathan Dowling, senior research scientist at the Jet
Propulsion Laboratory (Pasadena, Calif.), the experiment is not as
straightforward to do as it sounds. "As a general rule, at shorter
wavelengths, it is harder to find materials with a large index," he
said. "Not just any two dielectrics will stick together to form
robust layers of uniform and controllable size. Here, there should be
a good match on the chemical properties, such as lattice structure
and dimensions.

"Index considerations first limit the choice of dielectrics, and then
the fabrication constraint is applied to the potential candidates."

As a rule, he said, "fabrication of photonic bandgap structures
increases in difficulty as the wavelength decreases. So the Belarus
group is to be congratulated on pushing the technology into the
visible regime."

======================================================================






brian whatcott <inet@intellisys.net>
Altus OK