Re: mmw Optical Design

In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] says...
> 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
> 
> 
> 
At 95 GHz, wavelength about 3 mm, a 1/4 wave thickness of teflon would 
be about 375 microns thick.  From your values for index of refraction, 
this could make a pretty good AR coating for the Delrin.

Perhaps it could be vacuformed to shape?  

regards-
Elliot