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In the UV, CaF2 begins to exhibit small amounts of intrinsic birefringence, which are non-negligible. In Code V there is a very simple way to set up a CaF2 crystal where a crystalline axis is simply established. When a retardance map is calculated for light refracted by such a CaF2 lens, birefringence shows up as “three-point” across the pupil if the light is diverging or converging through a CaF2 crystal with 111 orientation (see https://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/advanced/t6_6_1.html for more details). Or it can show up as a “cross” if the orientation is 001.

First of all, I’m not seeing CaF2 as a Birefringent In/Out material option. I understand I would need to define this in the Birefringent catalog. Second, I need to be able to vary the retardance magnitude and orientation depending on the crystal axis orientation and direction of propagation through the crystal. Any thoughts or suggestions?

Any thoughts on this? Is it possible to simply define a coating that imparts a spatially varying retardance signature?

You’ll need to define materials for the ordinary and extraordinary index, and then load them using a Birefringent In surface. The Birefringent In surface will also let you define the axis of the index ellipsoid so it is correctly located in the system.

In sequential mode you’ll need a configuration of the ordinary ray and another for the extraordinary ray. In non-sequential mode it’s simpler as NS mode can split the rays into e and o components automatically. I’d do it in NS mode if possible.

  • Mark 

That makes sense for a single ray direction through CaF2, but as soon as I have a ray going a slightly different direction through the CaF2 I would need a completely different material defined. If I was trying to model a plano-convex CaF2 element in the UV, I would need potentially 100s of materials defined…

At this point, I’m guessing this could only be modelled in Zemax with a very customized macro that doesn’t rely on Zemax’s birefringence calculation capabilities.

-Ryker

Hey Ryker,

 

I don’t think that’s correct, unless you are doing something you haven’t specified in this thread so far. You need to define n_0, n_e and the angle of the crystal axis vector just once. OpticStudio considers each ray’s angle wrt the crystal axis to compute the effective index seen by the ray.

Check out the many birefringence sample files in the samples folder, under both the sequential and non-sequential sub-folders, and search the Knowledge Base for Birefringence. A birefringent lens is an entirely feasible thing to model.

  • Mark

Hey Mark,

The thing about CaF2, n_o and n_e are a function of angle of propagation relative to the [111] axis of the crystal. Please see this paper: https://www.spiedigitallibrary.org/journals/Journal-of-MicroNanolithography-MEMS-and-MOEMS/volume-1/issue-3/0000/----Custom-HTML----Symmetry/10.1117/1.1503350.short#_=_

Thanks,

Ryker

OK, if it’s not uniaxial it won’t work. You might want to look at the Birefringent GRIN surface. It was written for liquid crystals but it might do what you need.

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