Re: mmw Optical Design

Helpful person wrote:

> Is it not possible to find two materials for mm wave optics so that an
> anti reflection coating can be applied?  Thermal expansion issues may
> be a problem, but at mm wavelengths one could lay the material down
> in sections, with gaps much less than the wavelength.
>
> Is chromatic aberration a problem?
> >

Interesting idea on vacuum forming 1/4 wave Delrin antireflection overcoats on each
side.  Probably couldn't stand any appreciable airspace between the Teflon and
Delrin.  Might have worked pretty well considering our application was strictly 95
GHz.

Teflon's dielectric constant is fairly constant over most of the microwave/MMW bands,
at least below 100 GHz.  Do a Google or Yahoo on "Teflon dielectric constant"  and
DuPont's website comes up http://www.dupont.com/teflon/films/H-55007-2part3.html
that shows dielectric values over temperature and frequency.  Probably equates to an
Abbe number in the thousands or millions, so chromatic aberration would not be an
issue.

Thanks to WCE for mentioning "Rexolite" - it's been a few years and I had forgotten
the name of that stuff.  The manufacturer is C-LEC with website at
http://www.rexolite.com/list.html.  From their website:  "Rexolite® is a unique cross
linked polystyrene microwave plastic made by C-Lec Plastics, Inc. Two different forms
are available; Rexolite® 1422 and Rexolite® 2200. (2200 is a fiberglass reinforced
version of 1422). "  Its Abbe number may be nearly infinite, as C-LEC's website
states: "Maintains a Dielectric Constant of 2.53 through 500Ghz".  I remember looking
into using it rather than Teflon, but chose Teflon because (1) we already had some
huge chunks of it available for free, and (2) Rexolite's higher index meant higher
near-field Fresnel reflections.  But, Elliot Burke's cute trick of using Delrin to AR
coat might have solved that problem, too, at least for our narrow frequency band.
Mike