Curved waveguide band diagram

Dear Prof. Johnson and Meep users,

First, thanks for the Meep and MPB authors. Both open source software 
tools are very important for our research.
I have a question about how to use Meep (or MPB) to solve a problem 
about curved waveguides (ring structures).
I have used MPB to obtain the dispersion diagram of a straight waveguide 
in 3D (thanks to Prof. Johnson 2D waveguide examples). Now, I want to 
obtain the effective index and mode profile of curved waveguide (ring) 
in 3D, if possible with those tools.
 From the holey waveguide tutorial  in order to obtain the w(k) relation 
one should define a cell unit and  determine the direction of 
periodicity for the Bloch wavevector k. I was thinking to apply Meep for 
a ring structure in 3D but I am not sure on how to define the 
periodicity of this sort of waveguide. Correct me if I am wrong, but I 
presume that the wavevector should have two components in the x and y 
directions following a circular trajectory.
I will appreciate any comments or alternative approaches if any. Thanks 
for your time.
Regards,

-- 
German "Rick" Vargas

Florida International University