It should be fairly easy to generate a double-pass grin lens path.  After inserting the mirror, just make sure the return surface thicknesses are all negative, including the return grin thickness.  If any curved surfaces are involved, then the radii signs will need to be flipped too (for aspheres, the make double-path tool should do the trick).  Here is an example with a plano grin lens (lambda = 660 nm). 

Regards,

Jeff

1) I’ve only tried reverse propagation through radial gradient grin lenses, and for those none of the coefficients need to have their signs changed. Can’t comment on axial gradient lenses w/o spending a little more time.

2) The Seidel aberrations are not supported for grin lenses.  According to the help documentation:

However, any of the analysis features that rely on real ray tracing should be okay though, like the spot diagram, PSF, fiber coupling, etc.

Gradient index formulas where with a dependency in Z (gradium 3,4,5 and 7), are not simply reversed as other surfaces, as the origin of the index defining polynomial is changing from one face to another. You need to re-compute all coefficients containing Z after a change of coordinates  (a flip and a translation by the length of the GRIN). That should not change the maximal degree of the polynomial, but your length needs to be fixed.

In practice, the grin propagation steps may also not be aligned in both directions, as they start at the entrance, so you could accumulate errors along the way if the length is not exactly a multiple of the step size. If the steps are small enough, that is less of an issue. However, I remember an eye model that used a gradient index with a big step, so it was not possible to reverse it.

Hi Trevor,

There does not seem to be any z component, although the negative sign on the Nz1 & Nz3 may mean it’s just the numerical precision in the editor that makes it so. Radial terms do not need modification for flipping. If the Nz terms are all really zero, there is no problem with the step being as big as the lens thickness or bigger.