How to distinguish between the guided mode and folded mode, in a big supercell?

Dear Prof. and MPB users

For simulating coupled cavities in photonic crystal slab, I have to 
choose a big supercell (e.g. 5a * 15a in x and y direction respectively).

In order to assure the correctness of the ctl file and simulation, I 
simulate a photonic crystal waveguide by removing a row of holes in 
trangular lattice of holes in PC. When I use 1a*15a supercell, I can see 
clearly the Band Gap and guided mode in the gap. But when I use the 5a * 
15a supercell, several modes folded in band gap.

How can I recognize the guided mode?

Seems, my question is a basic question in PBG and photonic crystal, but 
if any of the respected MPB user can help, it will be highly appreciated.

Thanks

Yingjun YL

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