Re: mmw Optical Design

RLL wrote:

> I have been using Zemax to evaluate various mmw optical designs presuming
> that millimeter wave light behaves in a similar fashion to optical (or at
> least quasi-optical) wavelengths.  Does anyone know if this a reasonable
> assumption?
>
> Is there an optical tool that provides a more accurate method of predicting
> the behavior of millimeter and sub-millimeter light when used with optical
> lenses?
>
> I have used Zemax and added the proper refractive indices for the mmw
> optical materials into a new materials library.   I read of one tool called
> "MODTool" that was developed at JPL for mmw optical design.  Is anyone
> familiar with it?  It may only be for reflective surfaces, not for
> refractive.  I am interested in both.
>
> The empirical data I have collected seems to be in reasonable agreement (to
> a first order) with the Zemax predictions, but because the wavelengths are
> so large (relatively speaking), I cannot confirm exact agreement.
>
> Any assistance would be appreciated.
>
> Thanks,
>
>  - Russ in Santa Barbara

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