Re: mmw Optical Design

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 wqavelengths one could lay the material down
in sections, with gaps much less than the wavelength.

Is chromatic aberration a prioblem?
> 
> There is MUCH published on MMW and THZ optics, but here's my $0.02 worth.  The
> answer to your question is yes.  Ray optics to design, wave optics to
> accurately predict performance, and pay particular attention to Fresnel
> reflections.  I designed 14" aperture 48" EFL aspheric Teflon lenses for W-band
> with the big Z and they worked great, exactly as predicted by physical wave
> optics to within network analyzer accuracy.  Teflon is essentially lossless so
> its index of refraction is just the square root of its dielectric constant
> (about 2.0 at room temperature and 95 GHz).  One side was spherical and the
> other side was an even asphere with max departure from best-fit radius of about
> 0.03", or about 1/4 wave.  I had them locally machined at a shop with an NC
> lathe and CMM for surface profile verification.  We used Nylon screws to hold
> it to the ground plate aperture to minimize scattering.  The only problem was
> surface reflections, which we ended up isolating and removing with a 90º
> longitudinal phase shifter stage.
> 
> Teflon is HEAVY, and it cold flows over time, but has about the lowest
> dielectric constant and thus Fresnel reflection levels.  Delrin is
> dimensionally stable but has a dielectric constant of about 3.7, too high.
> 
> Mike